Motor Thermal Overload Fault from VFD

[TM519]

Hi everyone, I have a VFD-driven motor that is thermally protected by both stator and bearing RTD's, 2 per phase. The VFD that will be driving the motor has an option for motor thermal overload fault protection to be turned on and the plant doesn't want to activate it because they say they have gotten nuisance trips from this setting at other plants in the past.

The VFD manual states "A Motor Thermal Overload Alarm 1 of an impending overload fault is issued as a warning, when the motor current exceeds the overload pending parameter. A Motor Thermal Overload Alarm 2 is issued an a thermal trip timer is started, when the drive current exceeds the overload setting. If the this condition is present for a period greater than the time set in the Overload timeout parameter, the drive will trip and annunciate the even as Motor Thermal Overload Fault"

While I can agree that the RTD's will protect the motor itself from any thermal issues (RTD's are hooked into the DCS system) is there anything that the motor thermal overload fault setting on the VFD does to protect the VFD itself that we would be putting the VFD in harms way if we deactivated the setting?

 

[kwired]

Most drives these days will protect themselves even if you don't have proper parameters set to protect the motor.

 

[petersonra]

I am not sure that having RTDs hooked into the DCS to shut down the motor qualifies as a proper overcurrent device as required by code.

I can tell you that I have NEVER run across a legitimate case where the overload setting in a VFD caused nuisance trips when the parameter is set correctly. It is like claiming regular thermal overloads cause nuisance trips. They trip when they are set to trip to protect the motor and wiring. That is not a "nuisance". That is doing exactly what they are supposed to do.

 

[Jraef]

I am not sure that having RTDs hooked into the DCS to shut down the motor qualifies as a proper overcurrent device as required by code.

I can tell you that I have NEVER run across a legitimate case where the overload setting in a VFD caused nuisance trips when the parameter is set correctly. It is like claiming regular thermal overloads cause nuisance trips. They trip when they are set to trip to protect the motor and wiring. That is not a "nuisance". That is doing exactly what they are supposed to do.

I couldn't agree more.

A commonly misunderstood aspect of the thermal OL protection provided by a VFD is that in some of the better ones, the thermal OL protection curve is biased by the speed and the known cooling ability of the motor at that reduced speed. For example in the motor setup, you tell the VFD that it is a TEFC motor, so it then KNOWS that if you run it at 10% speed, the cooling fan is not doing anything so it LOWERS the amount of current that the motor can take without being damaged.

As to the RTDs going into the DCS, it is not the RTDs that would have to be listed as a motor thermal protection device, it is the thing that the RTDs connect to that would. It is all but guaranteed than the DCS mfr has not had their entire DCS listed as a "Motor thermal protective device".

You can SUPPLEMENT the motor thermal OL protection of the VFD with the RTDs, but you cannot replace it. You can also get a Motor Protection Relay that accepts RTDs and is listed as a protective device, or you can run the RTDs into the VFD in some cases and have them supplement / bias the thermal OL protection provided by the VFD. There are viable options, but not the one they have chosen.