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Motor Overheating
Sep 27th, 2000



Our new Recreation Association used a pool contractor that installed a 10HP electric motor on a single-phase current. The motor seems to run very hot, too hot to touch. They insist this is okay. I have concerns about the life of the motor as well as this being a safety risk (burns) and a fire hazard. I am used to this size motor being hooked up to 3-phase electrical. Do you think the motor will last and should two smaller pumps have been used since only a single-phase was available? Any advice would be helpful. Thanks.

Please send all responses to Mike@mikeholt.com

From: Sherman, Steve

Response No. 1:

The first thing I would check for is a properly installed ground rod and the resistance of your ground. Next I would check to see that the neutral is not being used as the ground. After that I would ensure that all of the individual components of the pump system are properly grounded.† Next I would check for possible water intrusion into junction boxes and outlets etc., and that the all-electrical connections are watertight. GB makes a silcone-filled wire nut thatís great for this purpose or you can fill your own.

From: Roginn


Response No. 2:

If your motor is running hotter than rated it needs to be corrected or it will shorten the motor life.† Here are a few considerations to take into account:

1. Is the motor under-rated for the job; e.g., is the load too great?† If so, the motor should kick out on overload. Ergo, is the overload protection correct?

2. Is there enough air circulating around the motor to keep it cool?

3. Are you supplying enough voltage to the motor; e.g. are the feed conductors large enough; is the power supply too far away; or is the power supply not capable of delivering the necessary power?

4. Maybe nothing is wrong and you need to label the motor as hot, limit access and/or cage the motor.

A good qualified electrician should be able to determine the problem using the manufacturer specs and some basic test equipment. Also, asking the motor manufacturerís advice is always a good practice! Hope this helps.

From: Mario


Response No. 3:

I was the head of the electrical division for Arizona Pool Builders for two years and while a 10 HP on a single-phase service is unusual, I have seen it installed a couple of times. As long as the motor is sized properly for the pump it should operate fine. However, there are several tests that should be performed. The first would be a simple test with an amp meter to determine if the motor is being overloaded. Second, I would check the voltage at the motor terminals with the motor running to see if there is excess voltage drop. The third is to check the temperature of the motor after it has been running awhile. The nameplate should give a temperature rating for the motor, and then you can test with any accurate temperature probe to make sure that it is operating within the normal range. If your contractor has not done this then he can't tell you for sure that the motor is okay.

From: White, Michael


Response No. 4:

This is in reference to the above captioned subject. In my opinion, if the motor is very hot, the possibilities are:

1. The motor is rated to operate at high temperature (rare possibility).

2. The motor is overloaded, in which case the life of the motor will appreciably decrease.

∑       Check the rating of the motor.

∑       Check if there is any clogging in the pipe of the water flow to the pool due to any solids in the pipe or any air locking.

∑       Check the cooling of the motor and whether there is any insulating material covering.

∑       Check if the capacitor used is shorted or under-rated.†

∑       Check if the voltage rating is proper.

∑       Check the split phase winding, if any.

∑       Check the voltage drop in the supply wires to the motor as the current drawn by the motor is appreciable.

The solution can be two smaller motors if the drop is high and no 3-phase supply exists. As the utility has allowed the use of single-phase, there is no unbalancing in the supply. It is better for both the power supplier and consumer to have a 3-phase connection. But, for the size under consideration, a 3-phase motor might be a little bit costlier. In my opinion, no electrical equipment should be run hot. Thanks.

From: Shrestha, Soorya B.

Response No. 5:

A motor this big on a single-phase?† What is it drawing - 60-80 amp? Something is very wrong here. He might have the windings set up for a 3-phase, and running single-phase. In 3-phase, wired correctly, it should draw about 28 amp. @ 240V. Thanks.

From: Ogle, Dave & Gia


Response No. 6:

There are some factors affecting the surface temperature of a motor:

1. Overload of the motor. Compare nameplate data to measured current. There is a good chance that the pump impeller is oversized. Check the torque curves of both the motor and the pump.

2. Ambient temperature. The higher the ambient temperature, the lower the natural cooling of the motor.

3. Exposure to sun radiation. This will add to the evolved heating produced by the temperature.† Consider shading the motor if possible.

4.Temperature of the fluid pumped. The higher the fluid pumped the higher the motor temperature.

5. Voltage drop. As the voltage at motor terminal gets lower, the current required to drive the load will increase leading to higher power losses (P (loss) * I^2) in the feeder and the motor windings. Try a larger feeder. A transformer might be helpful in increasing the voltage at the motor terminals.

If you choose a good insulation class of your winding, this will elongate the life of your motor.† For example, choose the insulation class to be class F while specifying the temperature rise to be class B. Check the motor nameplate for this information. Moreover, a service factor of 1.15 is better than 1.0 in your case.

Three-phase or single-phase? If I have a choice I will go for a 3-phase motor. But if the motor is designed for a single-phase, it should work well provided you specify your environment and power supply conditions to the vendor before purchasing.

From: Qahtani, Mohammad S.

Response No. 7:

Well, amps are amps and volts are volts and times each other equals kW (multiplied by power factor of course). The motor has a nameplate current rating and it is below, at, or above that rating. If the motor is running above nameplate current then you have the source of the overheating. Pool pumps can be particularly tricky as motor load varies as the cube of the flow.† A minor change in flow will result in a big change in current. Poolrooms can be quite hot, higher ambient temps will require either de-rating the motor or the need to supply outside air for cooling purposes. My old rule of thumb is that if you can hold your hand on the motor the temperature is okay. There are, of course, special high temperature cases but I don't think that this is one of them. Another rule of thumb is that for every ten degrees over the design rating of the insulation, the life of the insulation is halved. I would investigate the over temperature source by checking the motor running current with a calibrated true rms. voltmeter. Recording the motor current over a period of time would provide a better idea of what is going on over time. Second, I would check and make sure the fan is correct, I have seen where over time and over temperature the plastic cooling fan falls apart and the air flow over the motor is way off of spec. Thirdly, I would check out the ambient temperature of the pump room and make sure this falls within spec. Once the motor is running there is very little difference between a single-phase motor and a 3-phase motor other than the line current is 73% higher for a given HP with a single-phase supply. I donít think that two motors would make much difference except it would be less efficient.

From: Strand, Roger I.

Response No. 8:

There is an error in my epistle, the percentage difference in line current between single and three-phase is 57.8%, and the multiplier is 1.73. I thought about this after I had sent my reply off.†

From: Strand, Roger I.

Response No. 9:

We can evaluate the problem if the motor model number and manufacturer are known. Also need similar information relative to the pump.

From: Blake, Jim


Response No. 10:

Just as a quick answer for Steve Sherman, I think that he should first of all turn the motor off, then:

1. Inspect the motor's nameplate and the number of wires going into it. If it has more than three wires, and if the nameplate shows a delta or wye-shaped symbol, or mentions "phases," then replace the motor with a single-phase type.

2. Find the motor's circuit breaker (in the building). See to it that the breaker feeds only that motor, and that it is sized to 1.25 times the motor's nameplate full-load current. This current, after applying the multiplier, would figure to be at least 78 amperes at 120 VAC, 45 amperes at 208 VAC, or 40 amperes at 240 VAC for 10 HP.

Since the motor is getting so hot, it is dissipating perhaps 2,000 watts in heat in addition to its theoretical 7,460 watts full-load shaft output. The motor clearly is not suited to its input voltage, or its starter winding, if any, is not cutting out. Also, its internal thermal breaker is not working.† Lastly, the circuit breaker in its branch feeder is probably oversized. As Mike writes so nicely, I hope this note helps Steve in some way.

From: DiBugnara, Frank R., P.E.

Scottsdale, AZ

Response No. 11:

10 HP is a big motor, and will run hot. Three-phase voltage will not make it run any cooler. If the motor is making a loud noise, there is a problem. Have a second electrician check it out. It may be improperly connected. Shielding the motor from direct sunlight is a good way to extend the motor life. If itís too hot to the touch, it sounds like it is working too hard.

From: MARK.


Response No. 12:

It is obvious. If he uses a 3-phase motor, it will be better than a single-phase because of the following:

∑       The motor is going to be small

∑       Less heat, which is good for the winding.

∑       The sound of 3ph motor is going to be smooth.

∑       Easy to maintain and install

∑       Continuously running without any heat

∑       It will consume less energy.


From: Al-Tisan, Abdalla A.

Response No. 13:

Need to verify the motor temperature. Most is a 40-degree rise over ambient. If this is the case and it is located in a hot area, the housing would be extremely hot to the touch. Another area to look at is voltage drop. If running below rated voltage, the temperature would rise above the 40 degrees.

From: bnharfla

Response No. 14:

The 10 HP motor was probably a 3-phase motor. They might be available single-phase, but as the member stated most motors this size are 3-phase. Three-phase motors and generators can be connected in a "zigzag" configuration to operate on single-phase. However, they must be de- rated to do so.

Did the member think to record the nameplate data off the motor? This would tell instantly if it were single-phase.

I agree that the motor should not run hot enough to cause concerns of physical burn risks to arise in the member's mind. Even NEMA Class H insulated motors and generators shouldn't run that hot on the surface. Is it still running or has it failed already?

From: Batchelor, Wesley


Response No. 15:

It has been my experience that any time a motor is running too hot to touch there is an issue that needs to be addressed. Is this a continuous duty motor? Is it even large enough to handle the load? I would take some current readings to be sure this motor is running within the limits specified in Table 430-148 of NFPA 70. Run this motor at the highest voltage possible. Make sure the feeder is large enough for the current involved plus 125% and take voltage drop into account if this is a long run.

From: Clement, David


Response No. 16:

I would do two things right off the bat:

1. Check the load with an amp meter, and

2. Take the rmo reading of the motor case and compare the two to the motor specs. The next thing would be to check and make sure the motor is sized for the pump load, it may be under- sized for the amount of work.

From: White, Mike

A&M Electric.

Response No. 17:

Is the pump primed correctly? The heat you are talking about just may be friction due to insufficient water in the pump chamber. Now about having to add more pumps.† You just have to ask yourself how many GPM gallons per minute you want to move. Now about the single-phase, 3-phase thing. Well, you now have a 10 HP motor. Each HP is, I believe, 745 watts per horse.† So, with that said you have about 7450 watts on a single-phase, while on the 3-phase each phase is roughly around a third per phase. I would have stuck with the 3-phase motor. This is strictly an opinion based on my experience and training. If I am incorrect please let me know the true answer. Thanks.

From: Zapari, Mel

Response No. 18:

Need to know the voltage, E=? @ Single-phase with this 10 HP distance from source?

From: FRANCOJFZ@aol.com

Response No. 19:

Without knowing whether the pump is hydraulically sized properly (the correct size water pipe in and out for the distances involved so that it is circulating enough water for cooling), a good way to test if the wiring is adequate is to measure the single-phase voltage with the pump not running at the motor, and then to measure the voltage at the pump when it is running. Note the difference. If it is excessive, then he has a problem. The fact that it is 10 HP single-phase should not be a problem if all other conditions are adequate.

From: Jaco, Joe


Response No. 20:

A 10 HP motor is the maximum size listed on Table 430-148, of the 1999 NEC, for 230 volt, single-phase. The key to determining the load on a motor is to accurately measure the amperes while in normal operation. The amount, as indicated on the nameplate, should not be exceeded. I question the use of this large a motor on a single-phase system unless it is used for a short period of time. A high current reading can be a voltage drop on the circuit, due to the high-running current. A motor must have a circuit to provide the rated voltage at its terminals. The motor running protection should be checked to determine if proper size is in the starter. This sounds like a design problem, and should be reviewed by a motor and pump specialist.

From: BP6769@aol.com

Response No. 21:

Assuming one starts with a single-phase motor and that motor is hooked up within the manufacturerís recommendations and local code requirements, there is nothing wrong with using a large single-phase motor. The fact is that motors do create a lot of heat.

A 10 HP motor at 230 V single-phase uses 50 amps, which is approximately 12.8 kW at 90% PF.† If this motor is 95% efficient, it will create about 639 watts of heat and noise (mostly heat). So that motor is putting off about as much heat as six 100 watt incandescent lamps. That's a lot of heat.

From: Faber, Neil

Response No. 22

the motor could be in backwards or bent and not pushing enough air through the motor.† Therefore the motor would heat up and should trip the thermals.† If it does not trip the thermals the thermals are bad or it's still not hot enough.† I have encountered this only twice in my career but it did happen.†

From Charlie Poppe, Cpopinc@aol.com

Response No. 23.

The one possibility I have not seen addressed is that the motor is rated at 208/230 volt input and it is being used on a 120/208 service. If so the motor should have a 200 volt nameplate. A 208/230 volt motor at full load operates well from 198 volts to 260 volts. A 208 service can be as low as 182 volts (including 5% votage drop internal to the customer. A 200 volt motor operates from 180 to 220 with no problem.

From John, john_mccomb@fpl.com

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