What should I set the Overload at for this Motor?

[sdkokx]

Hello,

I have a motor running off of a VFD and we keep getting an Overload Fault on the inverter, what can I safely set the Thermal Overload to on the VFD for this motor? It's a 25hp motor on a 480v circuit. It is currently set at 38 Amps by default. I know standard FLA is 34 Amps but I also know that running at a lower frequency can result in higher current. This unit never goes above 51hz.

 

[GeorgeB]

Nameplate gives you 37.6 @50hz, 1.0 SF. That current is really close to that expected at 1.15 SF and 60hz. It's my opinion the motor is undersized or is uncomfortable with the drive. I'd TRY a change in chopper frequency but doubt it will help.

 

[Jraef]

You have to set the overload at 30.3A, exactly what it says on the motor nameplate. Anything other than that and you cut the motor life exponentially.

If the VFD is tripping on motor overload, it is most likely because the motor is over loaded. Running at low frequency does NOT increase the current by the way. Current = torque, torque relates to load. Too much load with not enough torque means the load slows the motor down, the slip increases and the motor draws more current, leading to the condition we call overloading. That's what the overload protection setting is there for.

But just to be thorough, you have to remember that in a VFD, there are two "overload" conditions: the motor can overload, and the DRIVE can overload, separately from the motor. The VFD should give you an overload display or code that differentiates those conditions. If it is a VFD overload, it might be something other than the motor, i.e. a loss or unbalance in one of the incoming phases.

 

[Smart $]

Under the NEC, where 125% is insufficient to start the motor or run it under normal load, you can incrementally increase the setting, but not greater than 140% of nameplate FLC, on a motor having a SF of 1.15 or greater: 140% ? 30.3 = 42.4A

 

[Smart $]

You have to set the overload at 30.3A, exactly what it says on the motor nameplate. ...

That's not what the NEC says...

For motor with an SF of 1.15 or greater, the setting can initially be as high as 125% of namplate FLC.

ETA: 430.32(A)(1) and re: my last post 430.32(C), and both are not superseded by 430 Part X.

 

[GoldDigger]

Under the NEC those rules allow you to increase the rating of the branch short circuit and ground fault protection to a point where it hopefully will not trip during starting. The branch breaker is mainly protecting wires.
It does not tell you anything about the setting of the required overload protection, which is a characteristic of the motor and protects the motor.

Tapatalk!

 

[Smart $]

Under the NEC those rules allow you to increase the rating of the branch short circuit and ground fault protection to a point where it hopefully will not trip during starting. The branch breaker is mainly protecting wires.
It does not tell you anything about the setting of the required overload protection, which is a characteristic of the motor and protects the motor.

Tapatalk!

Ummm.... 430 Part III covers overload protection.

 

[Jraef]

Under the NEC, where 125% is insufficient to start the motor or run it under normal load, you can incrementally increase the setting, but not greater than 140% of nameplate FLC, on a motor having a SF of 1.15 or greater: 140% ? 30.3 = 42.4A

Yes, but most motor manufacturers will tell you that you cannot run a motor from a VFD at anything other than a 1.0 service factor. The additional heat design incorporated into a 1.15 SF motor gets consumed in the high harmonic content of the PWM output of the VFD. I guarantee that the VFD mfr instructions will tell him to set the OL at the exact motor nameplate FLA, no more. The 115% pick-up point (for a 1.0 SF) in the thermal damage curve of the motor is ALREADY factored into the VFD overload programming, meaning that when you set it for the motor FLA, the countdown on tripping on OL does not start until AFTER you already get to 115% of that value. Adding it AGAIN will cost you the motor.

A lot of people read that section of the NEC and apply it too liberally. Because IEC motors have no "service factor", IEC overloads (and all VFDs) are all pre-calibrated for the pick-up point and must be set AT the FLA of the nameplate. Some NEMA overloads do this as well, you must RTFM before arbitrarily making that decision. The NEC allows for it but that does not trump the manufactures instructions. "Intended use" trumps all.

 

[kwired]

That's not what the NEC says...

For motor with an SF of 1.15 or greater, the setting can initially be as high as 125% of namplate FLC.

ETA: 430.32(A)(1) and re: my last post 430.32(C), and both are not superseded by 430 Part X.

I wasn't quite sure how to respond for the VFD but Jraef covered it. Otherwise with thermal overload devices, read the instructions, you select units from 100% of the current rating, the tables already have 125% current built into those selections. Instructions will likely tell you to derate your FLA if you have a motor marked with a 1.0 SF before making selections from the chart. Probably similar with solid state overload units, can't say I recall reading this information on any instructions though.

 

[kwired]

Hello,

I have a motor running off of a VFD and we keep getting an Overload Fault on the inverter, what can I safely set the Thermal Overload to on the VFD for this motor? It's a 25hp motor on a 480v circuit. It is currently set at 38 Amps by default. I know standard FLA is 34 Amps but I also know that running at a lower frequency can result in higher current. This unit never goes above 51hz.

View attachment 10218

In general the voltage to frequency ratio must be maintained. The current at 10 Hz can still be the same as it is at 60 Hz though. You have decreased the speed so you are doing less work meaning less power is applied. The voltage at 10 Hz should be right about 1/6 of what it is at 60 Hz though.

Many constant torque applications the current will remain fairly constant no matter what the speed is. That is the actual motor current not the drive input current.

 

[sdkokx]

Thank You everyone for your help, I fortunately did not have to adjust the Overload at all. I was given a little more time to investigate the unit before it was going to be sent back and as it turns out the starting frequency parameter was set too high resulting in the eventual overload. I was originally hoping for a quick and safe band-aid for the problem till I could figure out what was really going on. Thanks again.

 

[Jraef]

Thank You everyone for your help, I fortunately did not have to adjust the Overload at all. I was given a little more time to investigate the unit before it was going to be sent back and as it turns out the starting frequency parameter was set too high resulting in the eventual overload. I was originally hoping for a quick and safe band-aid for the problem till I could figure out what was really going on. Thanks again.

Oh yeah, sounds like someone turned on the "torque boost" feature to overcome friction at starting, without understanding that you cannot use that if the motor will run too long at those low speeds. Must be an older VFD that does not have Sensorless Vector Control.

You should still heed our warnings about your OL setting though.

 

[john m.]

Im Having almost the same problem. Replaced a dual speed motor of a cooling tower with an inverter duty motor with the same 30Hp rating. We set the VFD at 110% of the 69 FLA, 230Vac @ 60Hz. We went manual from 20hz -30hz -40hz and 50hz then VFD trips above 50Hz. Display shows around 78-80amps peak. Do you think its overloaded? Harmonics?

 

[kwired]

Im Having almost the same problem. Replaced a dual speed motor of a cooling tower with an inverter duty motor with the same 30Hp rating. We set the VFD at 110% of the 69 FLA, 230Vac @ 60Hz. We went manual from 20hz -30hz -40hz and 50hz then VFD trips above 50Hz. Display shows around 78-80amps peak. Do you think its overloaded? Harmonics?

I think your voltage to frequency ratio is probably off and some paramater tweaking needs to be done. - This is assuming the load is not demanding too much. If possible have you tried running it directly across the line to see what it draws then? If too high that way then there is too much mechanical load for some reason.

V/F ratio should remain constant no matter what the frequency reference is. Exceptions are for short duration during accel/decel but at constant speed should be 230V @ 60Hz, 115V, @ 30Hz, 76.66V @ 20Hz ... basically the same ratio as 230:60 no matter what frequency is selected.