Mike, I am a Lead Electrician at Tinker Air Force Base, Midwest City, Oklahoma.
I have a crew of electricians including myself that are interested in seeking some training in 400
Hz to enable us to work on 400-cycle converters. We have searched the Internet for information and
keep getting dead ends. Can you help us find a training source?
From: Dustin Manicom
Response No. 1:
I worked for Northrop Grummnan and had training on 400 Hz inverters from the Unitron
Corporation in Dallas, Texas. They have a great week course on their product.
From: Harry Tiedemann
Response No. 2:
Go to: http://www.ftcnorfolk.navy.mil
This is a Navy Training site in Norfolk, Virginia.
General Description: The 400 Hz Motor Generator Electrical System Maintenance COI
provides technical corrective maintenance training on 400 Hz motor generator regulators as well as
theory of operation and troubleshooting on the voltage and frequency regulator. A review of solid
state circuitry and magnetic amplifiers is provided along with test equipment. Several representative
400 Hz motor generator sets are used in troubleshooting.
From: James J. Linley, SSPA
Response No. 3:
After 15 years in the Air Force, I know that 400-cycle equipment belongs to the AGE
shop (Aerospace Ground Equipment shop) in the EMS or FMS squadrons. See those people for the tech
data on this type of equipment.
From: EAPRENGER@ aol.com
Response No. 4:
The U.S. Navy has many resources on 400 Hz applications. The Gyros for the ships’
compasses are 400 Hz and aircraft also use 400 Hz. Some important safety info – special cables are
required and it causes skin effect when the current travels in the cable. This causes electric shock
that results in burns on the surface of the skin.
From: Dieter Franiok, Electrical Contractor
Response No. 5:
They might try “crossing over” and looking in the Army TMs (Tech Manuals). I know
the Army use(d) 400 Hz equipment when I was in during Desert Shield/Storm. I’m not sure of any exact
TM numbers, but it may be in the TM-11-xxxx series.
From: Shawn Coppel
Response No. 6:
Please have Dustin contact me with questions. I worked on 400 Hz designs and installations
for the Dept. of the Navy through a civilian contractor Vitro/Tracor/Marconi and now its called BAE
Systems. Today I am employed by a major CNC machine manufacturer. I have experience with both MG sets
and Solid State Frequency Converters. I’ll try to help. The most important thing to learn is the difference
in voltage drop calculations and remembering to use non-ferrous conduits like aluminum better SCH
From: Paul Bunbury/EE
Response No. 7:
You might be able to receive training from the manufacture of the equipment that you
need to work on or through the military service school system. I am a Federal employee at a Naval
Training Center and there has been times where other Federal employees have received training here.
Check with the military training officer or your training specialist. Good luck!
From: Patrick Ward
FIRE HYDRANT CORROSION
Mike, I have a problem with fire hydrants corroding
in an unusual manner. It seems that the outside of the bolt heads on the fire hydrants is disappearing;
that is, they just shrink away. This isn’t a city-wide problem but it occurs intermittently here and
there. The City Water Supervisor believes that it may be due to voltage potential on the fire hydrants.
He suggested that bonding jumpers to the water pipes in the homes should be removed. I don’t think
that is the answer.
Do you know of a way that we can test the fire hydrant and its general proximity
to see if there is a problem with voltage at the hydrants? Or, do you think that there may be another
problem that is causing the bolts to disappear? This is a rather perplexing situation that I haven’t
been able to get any concrete answers to. I hope that you may have some suggestions for me or perhaps
you can send me to someone that may have the answers. Thank you for any information you may have
and, keep up the good work!
From: Joe Stammeyer
Response No. 1:
My company has a similar problem that I would like help
with. At one of our convenience stores, the copper water piping entering the building has rapidly
corroded. The building grounding system is bonded to this copper pipe. Do you know what the cause
and cure for the problems are? My guess was that current was being impressed onto the copper pipe
causing it to corrode. I was going to suggest installing an anode bed in the ground to sacrifice itself
over the copper pipe. What do you think?
From: Michael ChowMHChow@MAPLLC.com
Phone: (614) 274-1125, x227
Fax: (614) 274-0983
Response No. 2:
I suggest you look into cathodic corrosion protection
at the location this occurs. I doubt this is caused by house grounds. If it were, it would occur
everywhere there are houses or other buildings with that type of grounding (all buildings with power
and metal water pipes).
Response No. 3:
I think the Fire Marshal
just got a batch of cheap fire hydrant bolts and the heads are simply rusting away!
From: Earl C. Dean
Response No. 4:
Have the fire department check for dissimilar metals between the
bolts and the hydrant bodies. This sounds like a classic case of electrolysis.
Response No. 5:
As a former Chevron
engineer, I can tell you that pipeline corrosion is very complex. I suggest that you obtain a good
corrosion protection manual or consult an engineer who specializes in corrosion problems.
From: J. & N. SystemsJesse Kray, PE
Kray Cabling, Inc., Richmond, CA
Response No. 6:
I don’t believe grounding
is any part of the problem. As an industrial electrician, it would seem the better you are grounded
the less opportunity there is for any cathodic action. In Alaska here, we do use a ground grid under
the oil tanks in our tank farms consisting of titanium and another sacrificial anode tied in with
sodium bromide bags, at an outrageous cost. Have you contacted other fire departments around the country
with a typical climate such as yours in seeking solutions?
I have no personal experience
here but would give serious consideration to contacting the U.S. Navy about your metal corrosion problem.
They must have a team of experts in metallurgical problems. The problem may be if dogs are relieving
themselves that their system is rich in hydrochloric acid, thus creating problems.
From: J. & N.
Response No. 7:
This almost sounds like
a cathodic protection problem. Major pipelines use this method to keep their pipelines from disappearing
prematurely. I know that there must be some corrosion analysts in your area. I would contact them.
Also, see if other utilities in the immediate area are using cathodic protection. Your problem could
be one of induction. Good luck! Corrosion problems are never easily solved.
From: D. & N.
(559) 248-8969 ext.
Response No. 8:
It could be a weird
case of electrolytic action, either due to the already stated potential differences causing highly
rapid oxidation, or perhaps the bolts are of a dissimilar metal or alloy relative to the hydrant body.
This alone would create metal destruction. The services of a competent metallurgical lab are required
for this one.
From: Allan Soifer,
Response No. 9:
jumpers should not be done. One thing you may try would be to get a special alloy bolt that will not
deteriorate by electrolysis, one that is the same strength as the steel bolt.
From: Charlie POPPE
Response No. 10:
My name is Ed Chavern and I teach a seminar that covers this topic. <A HREF=”http://www.angelfire.com/pa3/cmf/index.html”Electromagnetic
Field (EMF) Electrical Inspection Training</A or www.angelfire.com/pa3/emf. With the correct
meter, which costs about $100, the current and the source of the problem can be detected. Don’t disconnect
any jumpers. This is a problem that is not too difficult to correct.
From: Ed Chavern
Response No. 11:
I believe the problem
is something else; however, one way to test if there is voltage potential on the hydrants is to drive
an independent ground rod (a short one) about 18 inches into the soil 10 or 15 feet away. Then take
a voltmeter and read AC and DC voltages from the hydrant to the independent ‘rod’. If more than a
couple of volts are measured, then some electric circuit is not returning the current back to its
source. This can be a bad connection or an open circuit. Investigation must take place by the Utility
and/or nearby electric users. If higher voltages are detected, then underground corrosion is a strong
possibility and can be very dangerous. The problem can be cathodic or anodic. Any metal part touching
the earth will deteriorate where the current “gets off.”
From: Ron Nelson
Response No. 12
I would suggest looking
for a source of DC, not AC current. The only source I could think of would be electrified trains.
I worked for the Chicago Transit Authority for 10 years. This kind of thing was a common problem in
and around rail terminals. If anyone still thinks home ground jumpers could be the problem (I doubt
it myself), a piece of plastic pipe should be inserted by the city in new water main hookups OUTSIDE
the home, at least 10 feet from the building. This will allow for the grounding properties to remain,
while removing the possibility of current flow through the pipe.
It has been my experience
that if there is a problem with AC current in a water main, it is the fault of the Power Company,
who has loose neutral wires. Green scum (copper from the pipes) can sometimes be seen in wash basins
and toilets. Dimming and brightening of lights is also a symptom.
From: Mark Elkins
Mike, Where do you place
a capacitor (line side, load side of motor disconnect, short-circuit protection device, overload)
for a 400 hp motor three-phase power factor correction capacitor? Also, why do capacitors lower the
From: Steve Dodd
Response No. 1:
In regard to the PF correction question: Place the capacitors on the LOAD
side of the motor starter, as near as possible to the motor [s]. Give them overcurrent protection
also [fuses]. Also place a "Draining Resistor" in parallel with them, with a value of something
like 22K. This will assure that there is no stored charge left in them after the circuit is opened.
The motor will drain them, but if the motor is disconnected from the circuit and the circuit is energized,
this will charge the capacitors and leave them charged after the circuit is re-opened.
The capacitors will offer Capacitive Reactance [Xc] to the circuit, while the typical
loaded induction motor will offer Inductive Reactance [Xl] to the circuit. Xl causes a lagging power
factor and the CURRENT will lag behind the voltage. Xc causes a leading power factor and the VOLTAGE
will lag behind the current. Low [50%] lagging power factor will cause the circuit [and load] to draw
twice the current, which is the total KVA of the load. The KVA equals the TRUE POWER in Watts, plus
the REACTIVE POWER, in Volt-Amps Reactive [or KVARs]. The KVA is called the APPARENT POWER. An example
1 horsepower motor - 100 volt AC Squirrel Cage - nominal power factor [50% PF]. When
motor is developing 1 full horsepower at the shaft, the branch circuit will be delivering 1,492 Volt-Amps
[1.49 KVA], which is 14.92 amps at 100 volts. Half of this KVA load is True power - in Watts, which
is 746 watts. The other half is Reactive Power - in VARs, which is 746 VARs. The total is 1,492 KVA.
By adding capacitors [Xc] in which the Capacitance - in MicroFarads - will equal somewhere near 746
VARs, the circuit's total KVA and Power Factor changes. At the transformer, up to where the capacitors
are connected, the Power Factor is near unity [close to 100%]. This causes the total current on the
branch circuit to be almost 7.46 amps. The reason why the capacitors will cause a lower current flow
is because they "Tune" the circuit and correct the power factor. Therefore, there is a lower
current flowing because the overall change results in a lower KVA load on the circuit.
I tried to keep the answers concise and much shorter than the typical reply I give,
so I sure hope this makes some sense plus explains what you were asking. It's a simple thing to understand
once you grasp the basic concept behind power factor [which is not something that can be explained
on the back of a box of Cocoa Puffs.]. So, once again - here is my favorite slogan: "I suggest
that you get at least two good books that cover basic AC theory" in order to fully grasp the
reasons why this phenomena occurs. The books will also answer other pending questions that will come
up. Good luck!
From: Scott E. Thompson
PC # 2
Response No. 2:
Please review this and pass it on to Steve. I am not a college professor when it comes
to power factor correction but I believe I can answer Steve's question in a way that does not use
the heavy math. Please also forward other answers you have received to me so I can share that with
First: Place the capacitor on the line side if you wish to extend the life of the disconnect contacts.
If, however, you desire better "power factor correction," connect capacitors to the load
side of the disconnect as close to the motor as possible. Next: Capacitors do not lower the current;
they shift the current or allow it to lead to compensate for the lagging current caused by any inductive
load. They do raise the current.
I teach my students as was taught to me in Tech school. Just remember the phrase "ELI the ICE
man. "E = electromotive force or voltage measured in Volts. I = Current or Induced current measured
in Amps. The L stands for inductance which occurs with coils or motors and is measured in Henrys'.
The C stands for capacitance which occurs with capacitors and is measured in Farads'. The ELI means
that in an inductive circuit the Voltage leads the current (Amps.) The ICE means that in a capacitive
circuit the Current (Amps) leads the Voltage.
Think of it in terms of DC instead of AC. When you hook a battery up to a coil the current does not
flow at the moment of connection but must build up the flux or magnetic field and as the flux field
builds the current grows to maximum. The voltage, on the other hand, is applied directly across the
coil and battery voltage is measured on the coil from the moment of connection.
On the other hand, when you hook a battery up to a capacitor the current flows immediately but the
voltage requires time to build to the maximum (battery) level. The current flows full strength inhibited
only by the resistance of the source, wire and reactance of the capacitor while the voltage may be
applied it does not appear across the capacitor until full capacitor charge occurs.
In a pure capacitive -- non-resistive or inductive -- AC circuit, the phase shift from the leading
current to the lagging voltage is equal to 90 degrees. While in a pure inductive -- non-resistive
or capacitive -- AC circuit, the phase shift from the leading voltage to the lagging current is equal
to 90 degrees. If you need to see the oscilloscope graph for that, I could show it on an MS word document
but I recommend you look in any basic electronics or industrial electricity book.
From: Richard A. Melcher
Response No. 3:
Your E-mail of 11/1 regarding sizing of a capacitor to a 400 hp three-phase induction
motor: You do not specify either the rated voltage nor the RPM of the motor, but based on a typical
installation (1800 RPM synchronous speed/probably 1750 full load speed), I would suggest a 60 kVAR
capacitor would provide correction to approximately 95% and a reduction of about 6% in line current.
If the capacitor is installed after the OL relays, the short circuit protection for the motor provides
protection also for the capacitor conductors and no additional disconnect device is needed. Conductor
size can be calculated but is also available from charts: based on a 480 volt installation, 60 kVAR
draws about 72 amps, will require a #2 conductor, probably THWN [don't forget to apply 110-14(c)(1)].
This is not only the most economical method but also provides the best P.F. correction without the
problem of voltage rise when the motor is not running.
If you want to install the capacitor ahead of the OL's (and possibly ahead of the motor short circuit
protection if a combination starter is used), a separate disconnect must be used to provide this protection.
Again, based on the above, a 200-amp device with a 125-amp fuse is suggested. This does not provide
the best protection and can lead to serious overvoltages during curtailed plant operations (weekends).
Article 460 of the NEC contains all of the relevant details and the handbook (fig 460.1) gives a picture
of both of these installations. The OL's must be reduced in value since the line current is reduced
(only when the cap is on the load side of the OL's) Article 460-9 discusses this issue.
Regarding reduction in line current: induction motors are essentially inductors and, as such, have
a lagging power factor (the current in the circuit lags the voltage) and therefore, the input is measured
in volt-amperes, not watts. For an example, using name plate data, convert the horsepower to watts
(746 w/hp) and compare to the current times the rated voltage. In an AC circuit, volts times amps
are not watts as in a DC circuit. The motor requires lagging vars to create the magnetic energy in
the motor. The capacitor has the opposite electrical effect; the current leads in the circuit and
provides leading vars. Properly selected values of capacitance (a capacitor) will, in effect, offset
the lagging requirements of the motor by providing leading vars. It is important to size the capacitor
on the basis of the motor's RPM since when the motor is turned off, the inertia of the
load causes the motor to become an induction generator for a few seconds (capacitor providing the
excitation) and the induced voltage only has the impedance of capacitor to discharge in (bad situation).
There is also the possibility that should the motor be restarted while it is coasting (and functioning
as a generator) the phase angle of the motor's voltage and the reconnected line voltage probably are
not in phase and could cause serious mechanical and electrical damage to the motor.
Most capacitor manufacturers will provide all of this information and calculate the amount needed
for the entire plant. DO NOT forget, if the electrical system contains nonlinear loads (VSD's, etc.)
the electrical system will contain various harmonics, probably 3rd and 5th. The impedance of a capacitor
inverts to the apply frequency; i.e., as the frequency goes up, the impedance (or better, reactance)
goes down, meaning the current will increase often well beyond the rated value. Capacitor suppliers
evaluate your system for these harmonics.
Hope this provides enough information (maybe too much), more as an engineer and not an installer.
From: Bill Ramage
Response No. 4:
In response to some of your teacher questions:
1. I know of a couple companies that sell electrical
labs and support materials. The best one that I have seen for electricians (have not personally
used them) is sold by Hampden Engineering Corp.-P.O. Box 563, East Longmeadow, MA. 01028—800- 253-2133.
They have several different models depending on the subject. They also have dissectible motor labs
for single and three-phase motors. They also have transformers, power supplies and other equipment
2. You might also try Mid West Tech--206 West
2nd Street Madrid, IA. 50156—800- 358-5702 www.midwestech.com They sell Lab-Volt lab equipment
(which I have used). Lab-Volt's address is P.O. Box 686, Farmingdale, New Jersey. 07727---800- 522-8658.
Not sure if you can buy direct or not!
3. I would be interested in your idea about an
instructors’ group. Not sure how much time I could put into it as I changed employers recently and
that has kept me quite busy so far. I also belong to and have belonged to several incorporated clubs.
I think most any lawyer that does incorporations should be able to help you with the details. I have
one locally but you should be able to find one in your area. You can call me if you wish. 515-964-6838
4. Hope this helps-- Sorry I haven't been E-mailing
more but I lost my dad last summer and am executor to his estate and that has tied me up along with
my new job and single-parent duties.
Keep up the good work! I use your materials quite oftenFrom: Terry Crowe
In response to Steve Dodd's questions about capacitors: Article 460 states that capacitors
may be installed on either the line or load side of the disconnect provided that the capacitor(s)
can also be disconnected from the energy source for maintenance etc. I have installed them at the
motor side of the motor starter so that they would be out of the circuit whenever the motor is not
running. The whole idea is to provide phase correction for poor power factor situations and when the
motor is off there is no need for that. Also to leave them energized would cause the power factor
to be over corrected and most power companies don't want that either. I don't know if the contacts
last longer or shorter on the starter because of inrush current to the capacitor. In theory it should
Another way is to disconnect the capacitor(s) with a contactor (relay) at
the same time the motor is turned off. No overload protection is needed for just the capacitor but
over current protection is required along with some kind of discharge device as capacitors can (and
often do) store electricity. This can be quite lethal. Always check for AC & DC
voltage before working on them! NEC requires an automatic discharge within 1 minute. Never assume
that it is working!! Do NOT short the terminals with a screw driver or similar device. Use
a resister or maybe a light bulb if voltage isn't too high. It is typical to use a contactor with
a capacitor bank (several in parallel) to correct several motors at the same time. Works especially
well when they all start/stop at the same time (a lot easier to install).
The reason the circuit uses less amps with the capacitor in the circuit
resides in the phase shift between voltage and current in the circuit. An inductive circuit causes
a 90-degree shift with voltage leading current. The net effect makes the circuit appear to use more
energy than what the load requires (we call this apparent power). A capacitive circuit does a phase
shift in the opposite direction (current leads voltage). If the right balance of capacitance is added
to an inductive load (motor) then the phase shifts cancel and the current and voltage peak and zero
at the same time (we call this true power). This causes the supply to only see the load and maybe
a little circuit resistance (wiring, contacts, terminations, etc.). Net result reduces the amount
of current needed and allows the generating part of the system to run cooler. If too much capacitance
is in the circuit (not disconnected with motor, wrong value, etc.), then the circuit will appear to
use more power again. Hope this helps!
From: Terry L. Crowe
Electrical Maintenance Instructor
Des Moines Area Community College
Response No. 5:
Art.460-8 (b) and (c) exceptions allow connection
to the load side of the motor controller or disconnect. That's where I would put it, otherwise you
will need a disconnect and separate overcurrent protection for the capacitor which could lead to other
The reason they lower RMS current is that they increase RMS voltage. The motor and capacitor are basically
in a dance together. The capacitor stores electrons on the upward travel of the AC sine wave. When
the sine wave falls is when the capacitor discharges and puts voltage into the motor. The motors part
of the dance is that it "borrows" energy to build the magnetic field on the upward travel
and puts it back into the circuit as the sine wave falls (inductance), just like a transformer does
when its field collapses. This
is known as reactance in a circuit. This portion of borrowed energy that does not get consumed and
the amount of energy (Watts) that's consumed makes up your power factor. Power companies know that
industrial users have a lot of inductive loads and charge a power factor penalty because they have
to have big enough wires to supply the demand. The capacitor closes the distance between the borrowed
and consumed loads.
If you need more, let me know.
From: Rick Hart
Response No. 6:
Without looking it up,
I would say on the line side (to provide a means of isolating the motor from the capacitor for motor
servicing). Capacitors would lower the current cause after they discharge their storage and they must
recharge for the next cycle, thus creating a drop in current.
Response No. 7:
In answer to the question asked by Steve Dodd, Dodd Electric, you would place the
capacitor on the load side of the motor. Capacitors lower the current with magnetizing. This magnetized
current will result in considerable savings in the amount of current used. Hope this helps.
I also like your idea of a National Association for NEC Instructors but one change
should be considered. You might consider making it an International Association, as there are a considerable
number of electricians in other countries that adhere to the NEC. We here in the Turks and Caicos
buy all of our supplies from the United States and therefore must use the NEC in order to make things
uniform. I will get back to you later on other ideas about the Instructors Association.
From: Claridge Wilson
Response No. 8:
Electrical Motor Repair
by Robert Rosenberg and August Hand (Third Edition) is full of these types of motors and others.
From: Dennis Carlson
Response No. 9:
The phase angle of the capacitor in an AC circuit
is opposite to the phase angle of the inductive motor load in the AC circuit (when the correct size
capacitor is selected for the motor). For example, in a parallel circuit, if there were 15 amps in
the inductive branch and 15 amps through the capacitor branch they would be 180 out of phase with
each other. As a result, the currents cancel each other out. If plotted as vectors, they will cancel
each other out creating a resonance circuit. (Resonance Circuit? when the line currents and line voltages
are in phase).
Placement of the capacitor should be in parallel with the 400 HP motor. If at all possible, place
the capacitor as close to the motor as possible to avoid other electrical issues. See Article 460
for code compliance.
Response No. 10:
Capacitors for power
factor correction of individual motors should be placed on the load side of the motor controller.
Once the motor is on-line the effects of the capacitor would be in place. When the motor is off-line,
the capacitor would also be disconnected. The overloads may need to be adjusted for the improved power
factor and lower operating current of the motor. This also exempts you from having a separate disconnecting
means for the capacitor bank. Persons working on this system should be aware of the possible stored
energy contained in capacitors. Almost all correction capacitors that I have seen do have a method
for discharging when switched off. No.2 capacitors will provide the stored electrical energy needed
to magnetize the windings during its discharge cycle and allow the incoming line currents to provide
the actual energy needed to produce the work required by the motor. The phase shift allowed by the
insertion of a capacitor more closely aligns the current and voltage waveforms to allow a greater
portion of input volt-amps to be used as work (watts).
Compare it to the timing
of your cars' engine. If the timing is off enough, that spark is provided prior to proper fuel mixture
delivery and we have low efficiency. If we can time the spark at the exact moment the mixture is provided
to the cylinder, we have a greater combustion and complete use of our available resource. The same
is true if the current and voltage waveforms are timed correctly, or corrected by use of capacitors.
The closer the timing, the more improved use of our resource, in this case, volt amp input. Line currents
won't have to be so high to account for the out-of-phase losses we often experience with large inductive
loads, such as motors.
From: Phil Burke
Response No. 11:
To answer the questions raised, I will briefly and simply describe the actions and
reactions. The capacitor should be connected on the load side of the motor disconnect
which will activate the capacitor only when the motor is running. Capacitor protection, wiring and
discharge requirements must be in compliance with NEC Article 460.
Motor current to the motor consists of two component (vectors) of currents added vectorially to form
a "total" current to the motor which can be read by a current meter. One component (vector)
current is "reactive" and for an induction motor it is "inductive.” The second component
(vector) of this current is "real" the current which is useful current that produces torque,
work and other motor requirements.
The capacitor connected in line with the motor produces "capacitive" reactance which cancels
part of the "inductive" reactance of the motor. Since the "total" current to the
motor equals the sum of the "reactive" and "real" currents, the "total"
current will be less due to "capacitive" reactance canceling part of the "inductive"
reactance of the motor.
From: Alexander I.Orloff, P.E.
Orloff Computer Services
Pioneers in Electrical Software for PCs
Web Site for Electrical Software:
Web Site for Electrical Engineering:
Address: Orloff Computer Services
1820 E. Garry Ave., Suite 117
Santa Ana, CA 92705
Response No. 12:
"Where do you place a capacitor?" Common Wealth Sprague (CWS) Capacitors,
Inc provides a publication with a good description of where and how to install capacitors. You can
call them at 413-664-4466 and ask for "Power Factor Correction - A guide for the Plant Engineer".
In the meantime, here are some general guidelines. It would be best to call CWS with the exact system
description before proceeding though.
If the motor has an across-the-line starter and will not be reversed or jogged and is not multi-speed,
then the capacitor can be installed between the starter and the overloads. If the motor is reversed
or jogged or is multi-speed, then the cap should be installed between the disconnect and the starter.
If the motor starter is a Reduced Voltage Autotransformer (RVAT), you may also need an additional
contactor to keep the capacitor off-line until the motor is up to full speed. The CWS publication
Is the 400-hp motor started with an Adjustable Speed Drive or a "Soft start"? If so, then
someone should look at the harmonics generated by the drive and insure that the harmonic frequencies
don't match the system resonance frequency. There is a section in the guide that talks about this
also. It is possible that an additional contactor might be needed to keep the capacitor off-line until
the motor is started and/or a filter might be
required for the harmonics.
In addition, the CWS guide gives the recommended sizing for capacitors so that the PF is not corrected
beyond .95, which could result in damage to the motor.
"Why do capacitors lower current?" This is a hard topic to explain without all the mathematical
analysis that goes along with it. There are many explanations out there and
the CWS guide does a very good job and includes all the pictures and formulas. I hope this make sense.
When you increase the power factor of a motor load by installing power factor correction capacitors,
you reduce the KVA requirements between the utility and the point at which the capacitor is connected
to the system. The motor still requires the same KW and KVARs but now the capacitor is supplying the
KVARs instead of the utility. The current between the motor and the capacitor hasn't changed. But,
the current between the utility and the capacitor is reduced.
I wanted to get you an answer as soon as possible. There are a lot of things to think
about regarding PF capacitors. Hopefully, I've answered your immediate questions. If you need a clarification
or have any further questions don't hesitate to call or E-mail.
From: H. (Scott) Gibson
Senior Energy Utilization Engineer, Energy Services
Snohomish County PUD
Desktop Fax: 425-267-6361
Toll free: 1-877-783-1000, ext. 1702
Response No. 13:
In my limited experience a KVAR bank generally is installed depending on what it serves.
If more than one motor, then on the line side of each motor and with a disconnect to allow the KVAR
bank to be isolated from the system. When serving just one motor it may be installed load side but
again with a disconnect to allow for isolation of the KVAR bank. I have also seen KVAR banks installed
so that they can be taken out of a system but allow the motors to continue to operate so a plant or
system shut down is not necessary. One big thing to remember when working on these systems is to be
bleed off the charge the capacitor has before doing any work on these systems.
Capacitors change the Voltage-Current Relationship by changing the phasing of this relationship. If
the capacitor is effective in correcting the power factor situation the system becomes more efficient
and the current demand is lower because there is less waste (more efficient). The Capacitor offsets/counteracts
the inductive signature of the motor or system and helps make the complete system more efficient.
Do not know if this helps or just confused this more.
From: Brian Cox
Response No. 14:
The basic answer to the question is, capacitors (sized for the specific motor characteristics)
should be installed at the motor, where the KVAR is required. Installing capacitors will improve the
power factor relationship, thus reducing the current flow to the motor. KVA is required to operate
an induction motor, which includes the reactive requirements, which is not read by a standard amp
meter. By improving the power factor to 95% the total current flow is reduced through all equipment
components between the serving transformer and the motor terminals (if the capacitors are installed
terminals of the motor). Therefore, it is important to locate the capacitor at a point so when the
motor is operated the capacitor is in operation with it. If the capacitor is connected to the motor
circuit before the motor starter, then the capacitor will need to be turned on and off manually with
the operation of the motor.
Capacitors lower the reactive current needed from the transformers to operate inductive loads. However,
it is important to remember the capacitor only corrects that situation between the (power source)
transformer(s), and the capacitor. That's why its important to locate the capacitance as close to
the motor terminals as possible. Usually one will record the operating characteristics of the motor
operation then size and install the capacitors
needed. The motor installation should be designed for the installation of capacitors even if they
will be added later.
Another option is Synchronous motors generate reactive power as they operate so they can actually
help the entire plant power factor problem. Since I work at a Utility, power factor is usually part
of the electric rate structure, so it is also important to operate the plant at the power factor level,
which does not penalize the customer on the electric bill. I hope this helps answer the question.
From: Ron Nelson
Response No. 15:
The vars that will be
provided by the capacitor does not care what side of the line you place them; however, you need to
make sure you protect the capacitor with short circuit protection. The current is only lower on the
upstream side of the capacitor because the var current component of the total current is now being
provided by the capacitor and not the line. Try to place the capacitor as close to the load as possible
to take advantage of line loss characteristics.
From: Karl Hauer
Response No. 16:
We usually start motors at full line voltage regardless
of how large the motor is. I would think the capacitor would be needed as near as possible to the
motor terminal because that is where you really need it.
Capacitors at motor terminal serve two objectives:
1. It will try to maintain the voltage level during
motor startup. Remember that it is charged before the motor is started.
2. It supplies the motor with reactive power.
That is, it will feed the motor with some of its magnetizing current. That is why the current supplied
to the motor from the utility will be decreased accordingly.
From: Mohammad S. Qahtani
Response No. 17:
The capacitor shall be connected on the motor side of the overload relay and the relay
shall be set for the reduced line current which will be lower than the motor current at all loads.
Also, care shall be taken while sizing the capacitor to prevent overvoltages due to self-excitation
and transient torques.
Since the magnetizing reactive current that is required by the motor is supplied by the capacitor,
the line current drawn from the source reduces.
From: T.Prasada Reddy
Response No. 18:
See NEC 460-9. Capacitors
are most generally used on the load side of the overload relays. They affect the sizing of the overload
relays because the relay sees less total "imaginary" current. Normal motor load is somewhat
inductive. That means that there is some imaginary current and some "real" current. A capacitor
also uses imaginary current, but it is of opposite polarity, so it can cancel out the imaginary current
drawn by the motor. Overload relays must be sized for all the current, both the real current and the
imaginary current. The addition of a capacitor cancels out some of the imaginary current, so the overload
relay needs to be sized smaller so as to provide the same level of protection for the motor. Hope
that this helps!
From: Vince Saporita
Response No. 19:
The power factor capacitors
must be connected ahead of the overloads in order for the overloads to function properly. They lower
the current by bringing the power factor to unity, which eliminates excess current draw.
From: Paul E. KellerPek@tlc-engineers.com
Response No. 20:
It is beneficial to install capacitors associated with large motors in a manner that
they turn on and off with the motor. This will make sure the capacitor is off when the motor is off
and keep the capacitor from causing a problem with leading power factor when the motor is off.
A capacitor reduces amperage by offsetting the inductive VARs necessary to magnetize
the motor. Example, if a motor with no capacitor correction has a 70% power factor, it uses 400 KW,
400 KVAR and 565 KVA. The KW is the useful work, the KVAR is the energy needed to magnetize the windings,
and KVA, apparent power, is the vector sum of these two. If a capacitor is installed that is about
270 KVAR, it will provide that much of the magnetizing current. Leaving 400 KW. 130 KVAR (which is
400 KVAR minus the 270 KVAR of the capacitor) and 420 KVA (the square root of 400 KW squared plus
130 KVAR squared). and the new power factor is 95%. The reduction in KVA is proportional to the reduction
in amperage. So, in this example, the amperage is reduced by 26%. Note: the amperage is only reduced
from the point at which the capacitor is installed, back towards the source. So, if you put the capacitor
at the main service, only the main breaker and the electric utility see the benefit. If you put the
capacitor at the motor, you will reduce the amperage in the feeder to the motor also.
Always have a person that is trained in sizing and installing capacitors look at the design for the
installation. Variable frequency drives and other equipment may have problems when capacitors are
From: John McComb
Response No. 21:
You would place the capacitor on the load side of the motor disconnect, short-circuit
protection device, overload. Capacitors lowering the current, not sure what he is asking here.
From: Gary Gilbert
Response No. 22:
Most electrical loads are inductive in nature and have a lagging power factor (i.e.
motors, light ballasts, etc.). This means their current draw is made up of two mathematical components:
resistance and inductance. Most electricians know these terms as kWs and kvars, the combination of
the two being kVA. One must keep in mind that the two components are actually two-dimensional vectors
and cannot be directly added to each other. Nor can the kVA of one load be directly added to the kVA
of another without first breaking the value into its individual components.
But without getting into the math side of this issue, just understand that a motor
load is comprised of two components: a resistance (due to the copper conductors in the windings) and
an inductance (due to the magnetic fields set up in the motor which causes it to rotate in the first
place) which, when combined, form what is known as impedance. Therefore, for any given supply voltage
the motor is going to draw a current from the power distribution system that is comprised of both
a resistive component and an inductive component.
To reduce the current the power distribution system must supply to the motor, we need
to supply the motor requirements by other means. To reduce the resistive current the motor draws from
the system, we could locate a real-power generator right at the motor to supply its load. However,
this is generally expensive and impractical as the generator must be synchronized with the rest of
the power distribution system, and it has on-going operational costs. Or, we could reduce the inductive
component the motor draws from the power distribution system by locating a reactive-power generator
at the motor to supply its inductive load. A capacitor just happens to be a type of reactive-power
generator. Capacitors are compact and relatively inexpensive, and can be located just about anywhere.
By connecting a capacitor near the motor terminals, the inductive requirements of the motor are supplied
by the capacitor and the power distribution system only needs to supply the resistive component of
the load. The net result is a reduction in the current flowing in the power distribution system, a
lowering of the ratio of the real current to the inductive current flowing in the system (i.e., an
increase in the overall power factor of the system), a reduction in the heat loss a power distribution
system generates, a reduction in the HVAC required to cool the area, and a reduction in the size of
the power distribution system required to supply the load. All good things.
However, one needs to be careful when applying capacitors for power factor correction
or for any other purpose. Power distribution systems are not the simple things they used to be. There
has been a proliferation of electronic loads, DC drives, and other components out there known as non-linear
loads. Their complex impedances, when coupled with the capacitance of power factor correction capacitors
can cause a system resonance that is both dangerous and destructive. The addition of power factor
correction capacitors should be reviewed by a professional engineer competent in the evaluation of
harmonic impedance studies to assure a resonance condition is not created while trying to solve a
As for the location
of the power factor correction capacitor, they should be located as electrically close to the load
as possible (i.e., load-side of the starter). One should review the requirements of ANSI/NFPA.70,
Article 460 for details. If the starter is located in a MCC, the capacitor should be mounted nearby
and the associated protective equipment should be located in the MCC. This makes connection of the
capacitor terminals to the load-side of the starter simple. If the starter is self contained and stand
alone, locate the capacitor and associated protective equipment adjacent to the starter and add an
extra set of load-side lugs in the starter for connection of the capacitor. In either case, the components
of the combination starter will operate properly and protect the motor for both overload and overcurrent
regardless of the status of the capacitor.
In all cases the motor feeder, branch circuit, and starter need to be sized for worst-case
operation (i.e., assuming the capacitor is out of operation). The benefits of multiple power factor
correction capacitors is reaped upstream in the reduction in size of the larger supply equipment where
the loss of one capacitor does not significantly impact the system.
I hope this answers Mr.Dodd's question.
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