Motor temp on VFD

[rlundsrud]

I have a question about a 3 phase 1 HP motor connected to a Danfoss VFD. The case of the motor is about 150-160°f when the motor is run at 60hz, at lower speeds it runs much cooler. I know the motor is within its design specs, but I wonder if I should add line reactors to limit harmonics (and thereby lower temp) to the motor. Would adding line reactors actually effect a change or should this motor have a higher NEMA design or insulation class? The application is a circulation pump, the pump pretty much runs all the time. Any advice would be appreciated, here are the specs on the motor.

230v 3 phase 3.4amp
Code-L
SF-1.25
Insul. class-B3
NEMA design-B
Type-TS
Time rating-cont.
Ambient-40°c

 

[ptonsparky]

I would expect it to run hot. The higher the RPM, the more work it is doing.

 

[victor.cherkashi]

I believe a poor quality of VFD heats motor more than good one. NEC don't take this into account, in my opinion derating should be in this kind of situation.

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[petersonra]

Class B insulation is rated for 266 degree F. I don't see an issue.

When a motor runs close to full load, it gets warm. Is this motor running at close to full load, or maybe just slightly over?

A lot of people are used to motors that run well under full load so think a barely warm motor is normal.

Line reactors won't do anything as they only affect the wave form going into the drive.

I doubt that any kind of load reactor would change the temperature of the motor all that much.

 

[ptonsparky]

I believe a poor quality of VFD heats motor more than good one. NEC don't take this into account, in my opinion derating should be in this kind of situation.

Sent from my Nexus 5 using Tapatalk

A Danfoss is not in the realm of poor quality.

 

[victor.cherkashi]

In any case overload protection should disconnect the motor in case of overcarent including harmonics

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[kwired]

In any case overload protection should disconnect the motor in case of overcarent including harmonics

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Is harmonics even an issue on the output side of the drive?

The raw output of the drive is not very sinusoidal, but simply connecting a motor does smooth it out some and the addition of a load side reactor smooths what the motor sees even more.

 

[Besoeker]

A Danfoss is not in the realm of poor quality.

I'm sure Danfoss would agree............

 

[Besoeker]

Is harmonics even an issue on the output side of the drive?

Not in my experience.

The raw output of the drive is not very sinusoidal, but simply connecting a motor does smooth it out some and the addition of a load side reactor smooths what the motor sees even more.

The voltage isn't but the current usually is with a sinwave weighted PWM pattern which almost all are. Output reactors are commonly used the limit the dv/dt of the switching edges to protect motor winding insulation.

 

[rlundsrud]

The motor isn't overloaded, it's running within its load specs. I know it's within the heat tolerance of the insulation, I was just worried that it was running hotter than it really should given its current draw. If the cause of this higher temperature was harmonics, load side line reactors are not very expensive so I thought that might be a solution. I just wasn't sure if line reactors might reduce the temperature.

 

[LMAO]

The motor isn't overloaded, it's running within its load specs. I know it's within the heat tolerance of the insulation, I was just worried that it was running hotter than it really should given its current draw. If the cause of this higher temperature was harmonics, load side line reactors are not very expensive so I thought that might be a solution. I just wasn't sure if line reactors might reduce the temperature.

Line reactor does not reduce the motor temperature; its purpose is to reduce/isolate "line" harmonics from source, and to protect the VFD diodes in case of voltage fluctuations.
A load reactor however somewhat smooths out the VFD output PWM, but it also drops the voltage a little bit so you'd need higher current thus possibly higher temperature.
A dv/dt filter has nothing to do with motor temperature; it just helps protect motor insulation against high dv/dt due to PWM and motor long leads.
A sinewave filter turns VFD PWM into an almost perfect sinewave, it is usually used on non-VFD rated motors.

I'd say if you don't have an overheating problem leave it alone, if it ain't broke don't fix it. You can install an RTD on motor and connect it back to VFD to monitor the temp.

 

[Jraef]

... I was just worried that it was running hotter than it really should given its current draw. ...

How do you know if this is the case? Did you measure the case temperature when the same pump is running Across-The-Line to compare? 160F is not that unusual for a fully loaded motor.

Besides, contrary to popular belief, case temperature means nothing in a motor, unless you are tracking it and you see a CHANGE. If you have RTDs embedded in the WINDINGS, that's meaningful information, but the outside case temperature varies by design, cooling fins, air flow, air gap, etc. etc. Bottom line, there are no generalizations you can make.

Measure the current, or in this case, read it from the VFD display. As long as you are within the FLA rating of the motor, you are fine. Now, one thing that IS true is that running a motor from an inverter DOES increase the heating in the motor slightly. For this reason, 99% of motor mfrs will tell you that if you have a motor with a Service Factor (i.e. 1.15 or 1.25 in your case), the SF is consumed in the harmonic heating effect, so you should ALWAYS treat any motor run from an inverter as having a 1.0SF. Conversely, you should NOT use a motor that is rated for 1.0FS on an inverter drive unless the motor mfr. explicitly says that you can (and backs that up with a warranty).

Now, separate issue:
Being that this motor only has Class B insulation, I'd hazard a guess that this is NOT an "inverter duty" motor. For THAT reason, I would put a dv/dt filter on the output of the drive. Forget heating, there are other more real and damaging issues with running off-the-shelf general purpose motors from a VFD, many of which can be attenuated by using a dv/dt filter, or if there is over 3,000ft from the VFD to the motor, a Sine Wave filter.