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I am looking for specific information regarding VSD Driven electric motor as per NEMA or IEC Standard. I have got on site one 315 k W 400 V squirrel cage electric motor but I doubt if it is designed to be driven by a variable speed drive. This motor is heating like a *beep* and I am afraid it is going to fail sooner or later. The temperature of the casing measured with a Fluke laser thermometer is around 90 Deg C at around 80 % load.

Is there any specific information on the motor name plate which indicates that it is inverter driven motor ?





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Unless they've added to the standards since I last looked, there is NO REQUIREMENT to state that a motor is rated for Inverter Duty on the Nameplate (U.S.) / Rating Plate (IEC) ... In the U.S. a manufacturer can ONLY state Inverter Rated or Inverter Duty per the applicable section of NEMS MG1 -31 IF, and only IF it complies with ALL of the requirements of that section. The rub is that one requirement in that section is that for a given phase rotation of the applied voltage, that the motor rotate in a specific direction. (IEC motors do so)...... but there is no NEMA requirement for specific supply phase rotation and direction of motor rotation.


What you have neglected to provide for us to consider are:


1. complete N/P info


2. Mains Voltage (input voltage) to the VFD


3. What speed / Frequency you are trying to operate at .....


4. What the Output Volts and Amps are at that operating point.


5. What is the AMBIENT temperature surrounding the motor ?


6. Is the motor nameplated for Class B or Class F Temperature rise ?


I suspect that the motor internally is getting hotter than your measurement indicates.

Check the DE (Drive End) bearing housing temperature..... if you can..... scan it at the bottom of the bearing housing, in case the mtr design causes air flow to be directed downward over the bearing housing.


Your answers to the above will help us all to understand and evaluate your case and advise accordingly.


Kind regards,


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Some additional things to consider, ....


I will assusme that the motor is an IEC type


As such, is it dual voltage rated ?


If it is dual-voltage rated, can we safely assume that you have checked and verified that is is connected correctly and that the jumpers (or connections) are set for the higher voltage ?


Can we also assume that the VFD is set for the correct output V/Hz ? (400/50 = 8.0 V/Hz)


(question #2 of previous post is most important in this consideration, particularly if mains voltage to VFD is LESS THAN 400v....... How much less? )


Is the motor operating at a greater SLIP than what is posted on the Rating Plate (N/P) ...

i.e., a 4-pole, 50 Hz motor might post 1485 rpm, indicating that rated slip is 15 rpm ....

so if you are operating at 40 Hz, actual motor rpm at full load should be 1200 - 15 = 1185 rpm


If the motor were to be operating at 40 Hz, for example, and the actual shaft rpm were to be measured as 1182 rpm ....or less..... then you'd know that the motor was operating at a slip of 18 rpm ..... or more..... which would certainly overheat the motor.....


Or .... another scenario ..... if you are operating at or near rated load current, at a speed less than 1/3 to 1/2 motor rated speed .... the motor might not be developing sufficient cooling to get rid of the heat.



So, here are some things to check and verify.

  • check for correct motor connections
  • check that VFD is set for correct output V?Hz
  • Check slip speed at which motor is operating
  • Check that motor cooling (air flow, etc.) is not obstructed
  • Check VFD Current Limit Setting to assure that VFD is not operating in Current Limit below 100% rating.

The last item would prevent sufficient torque to support the mechanical load, resulting in overheating of the motor.


Another contributor would be if you are operating at 50 Hz (or above) and the VFD is not capable of developing 98-100% rated FUNDAMENTAL RMS output voltage) ..... in which case the motor will operate voltage starved, and slip more, and produce more heat. If this is the case, you could either reduce the MAX SPEED (Max Freq.) setting of the VFD a few Hz ..... OR .... raise the input voltage to the VFD from 400 v to 415v. This will increase the dc bus voltage proportionally with a consequent increase in the Fundamental RMS output voltage at 50 Hz and above....

(I am assuming that you are not trying to operate the motor above 50 Hz. Advise if this assumtion is not correct, and please provide correet detail.)


Of course, the motor could be defective, but I think you would have other indications of a motor failure.


Kind regards,

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Hi JOhmega,


Thanks for your very lenghty, sharp and accurate comments. The motor is driving a fan and the operating speed is between 80-90 % nominal speed. It is a 315 k W 589 A 744 rpm 400 V 50 Hz 0.81 PF IMB3 40 Deg C Motor.

The current displayed on the VSD is well below FLC of the motor around 490 Amps. The thermal cooling model displayed on the drive is between 70-80 %. The ambient temperature is 35 Deg C. It is evident from the thermal model of the VSD , the motor is not heating . Nevertheless it is heating.

The drive has got auto tuning features and the parameters such as Stator, rotor resistances and inductances have been self-adjusted by the drive itself. There is no obstruction on the cooling fan of the motor.





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Dear bob,

probably you are using a motor at full load, this means that at 50 Hz output frequency the requested power will be something like 315 kW.

At 90% of speed, a 490 A current is OK; at 80% of speed, a 490 A current is too high and the application must be checked.

Standard fan are designed for nominal speed. An 8-pole motor at reduced speed could be ot enough cooled. Usually for such types of applications it is recommended to install a separate fan. I agree that the requested torque is lower than nominal, but other losses are still the same.


Another possibility. The iron sheets are not the best (there are difficulties on the market to get the right sheets!)

and their losses are high. Can you reduce the carrier frequency to reduce losses?


What about insulation class declared? F class ? Class B overheating on F class ?

What about efficiency?




Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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Hi Bob,


Excellent!. Thank you for your reply with the data.


Here are my observations based upon the data you provided.


Contrary to Mario's comment about needing an additional source of cooling because it is an 8-pole motor and can be operated at low speeds; the laws of physics don't support his claim. Because it is a centrifugal fan load, the torque load drops off as the square of the speed reduction.... and the power drops off as the cube of the speed reduction. Ergo, load is very minimal at low speeds. For example, at 50% speed, the load is nominally 25% of rated and the power is 12.5% of rated.


I do agree with Mario that at 80% speed, 490 amps is too much .... I would hope that the 490 amp load you reported is at 90% speed or very close to 90% speed.


The motor has very low slip (0.8%) which means that is is a low impedance motor. Also, being an 8-pole design, it has low leakage inductance.


There is a possibility that if the current limt is set too low, that it is clipping the consequent peak-currents .... which could add heat.... (Use an oscilloscope with an AC current probe to see if clipping is occurring)


But now, let's get to the issue of HEAT.


WHY do you feel that the motor is overheating ?


Consider that you have an elevated ambient ....35°C.


If the motor is designed for a class B temperature rise ..... i.e., 80°C above a 40°C ambient (for a 1.0 S.F. motor) ..... for a total temperature of 120°C


(for a 1.15 S.F. motor, Class B rise is 90°C above a 40°C ambient)


Is your motor rated at a 1.0 or 1.15 S.F. (Service Factor) on inverter power ? Check with the manufacturer....


You reported that your temperature scan of the mtr. housing revealed a temp. of 90°C. Allowing a 10°C differential between the outer housing and the internal core ..... you are still within the thermal rating of the motor on a Class B temp. rise for a 1.0 S.F. motor.


If you subtract out the ambient temperature from your temp. scan...... you get 90 - 35 = 65°C. Allowing for a +10°C differential between the housing and the core ..... you'd have an internal core temp of 75°C which, as explained, is well below the 80°C Class B temperature rise...


You don't state (and I neglected to ask) the approx. age of the motor. If it was manufactured within the last 10 years or so .... the insulation of the windings is Class F .... (115°C) ....


So, with the above considerations, I don't think you have a motor thermal problem.


Would you agree ?


Kind regards,




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Dear jOmega,

I agree totally with you.

Usually I don't recommend to add an additional fan to cool the motor.

I agree that this motor should have no thermal problems.

In this specific case I understood that the motor seems to be too hot, probably this motor was not designed for inverter applications, or the application is done in a way where the cooling by conduction is too low (i.e.: transmission by belts, attachments by rubber for vibration dumping). In this situation running at 90% of nominal speed - and 81% of nominal torque - should be a problem if the self-cooling fan is not in position to supply enough air pressure to reach the cooling fins.

Also external painting of motor could be involved in this problem. An old motor could be painted and re-painted, the paint thickness becomes high, and the resistance in °K/W stator-to-air will increase, this means that the internal temperature will be higher.

Also the infrared emissivity of such painting could change some values: a black motor is better that a white or silver motor, due to the fact that black painting could emit more infrared power per unit surface.






Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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Ciao, Mario.


As I said in my previous post, I don't think the motor is being thermally stressed beyond its design limits. I think, and offered rational, that the motor is operating below its thermal design limits.


I also asked Bob Why he thinks there is a thermal problem, and await his answer.


Over the years, I've had more than one person allude to a motor thermal problem where none existed.


Consider, if you will, that most people do not realize just how hot a motor operating at a Class B temp. rise will feel when they put their hand on the housing. 80°C above an ambient of 35° or 40° C. is very hot to touch.


Many years ago, an associate at a motor manufacturer showed me that to get a better sense of the internal core temperature, measure the temperature at the DE (Drive End) or front (shaft end) bearing housing. and preferably at the bottom surface (as opposed to the top surface) of the bearing housing.


The highest single temperature in a motor assembly will be in the Rotor. The heat from the rotor is convected to the stator where it combines with the heat (copper loss, etc.) in the stator windings before being conducted to the atmosphere by the interface of the stator laminations and the exterior housing... frame..... of the motor.


So, let us both await further information/comments from Bob.


Kind regards,

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Hi Mario & JOhmega ,


Again thank you for your comments. This motor was installed in 1996.

I honestly think that the motor is heating abnormally because the temperature mesured on the casing of he motor near the DE is around 90-95 Deg C and the temperature of the DE bearing casing is around 80 Deg C. I also opened the connection box where the winding could be seen and scanned there, the temperature measured is around 110-112 Deg C.

I trust the insulation class should be F, however why did you state that at 80 % loading, that is at 20 % slip ,the current is too high~490 amps which is well below the FLC of the motor~589 amps.

I have installed a force cooling fan on the motor and shall keep you posted on any improvement.



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Hi Mario & JOhmega ,

.............however why did you state that at 80 % loading, that is at 20 % slip ,the current is too high~490 amps



Time to clarify, Bob.


Mario's statement is :

"At 90% of speed, a 490 A current is OK; at
80% of speed
, a 490 A current is too high and the application must be checked.


He was referring to 80% speed; not 80% Load.


Q. Where does the 20% slip come from that you stated ? Even though you haven't identified the specific Manufacturer and model number of the motor, the rated slip of your motor is 0.8% and 1.0% of synchronous speed (120 x F) ÷ PP ....

where F = Frequency and PP = No. Motor Pole Pairs (i.e. 8 for your motor) = 750 rpm


((750 - 744) ÷ 750) x 100 = 0.8% Rated SLIP


Again, to clarify ...

  • motor load is a centrifugal fan
  • Centrifugal loads obey the Square-Law loadiing; i.e., load is proportional to the square of the speed
At 80% speed, based upon the Square-Law Loading principle, the motor load should be around 376 amps ..... and the actual motor SLIP should be something less than the rated FULL LOAD Slip. You stated your motor is listed as 744 rpm on the rating plate (N/P) ... This equates to full load, rated SLIP of 6 rpm or..... 0.8% of synchronous speed.


At 40 Hz, synchronous speed is 600 rpm and if you were at full load .... with full load slip of 6 rpm, your shaft speed would be 600 - 6 = 594 rpm .... at 40 Hz, using the Square Law calculation, you should be, 56% load ..... and your slip should be something greater than 3 rpm but much less than 6 rpm ..... which would translate to a shaft rpm of 597 to less than 594 rpm.


Check motor speed and motor current at 40 hz and if the fan has louvers or dampers, make sure they are fully open ... rpm should be between 594 and less than 6oo rpm and motor current should be around 376 amps. If the values you observe are not as stated above, then I suggest you check the application because something isn't right .... probably the brake HP of the fan at the this speed is higher than the motor is capable of supporting. ....... UNLESS....... and I've seen this many times before..... the fan is not intended to be operated to full speed of the motor .... but rather ..... some lesser speed as a maximum. In that case ... full load is intended to occur at a lower speed.


If you find that to be the case .... please advise us accordingly because we are assuming that the fan and motor are both matched for rated power/load at 50 Hz.


Some questions about the fan ...

  • Is the fan direct driven by the motor ....or is the fan belted to the motor ..? Has it always been this way?
  • Was this fan designed to be driven by this motor ?
  • Was it designed to be direct driven or belted to the motor ?
  • Do you have the Fan curves; hopefully one showing BHP versus rpm ?
There seems to be a mis-match here





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