Variable Frequency Driven Motors (Inverter Duty motors) for Class 1, Div. 2 Hazardous Locations

[Shahzad]

Hello,

As we know normal induction motor name plate has fixed frequency mentioned on it so a calculated, designed heat will be produced by it. However, if we have a inverter duty motor or a motor driven by VFD and we want to use that motor in Hazardous classified area, then being inverter duty motor, frequency will be changing as per PWM VFD technique. Is there any special requirement for inverter duty motors? As we know that a motor and VFD combination changes the characteristics of the motor in terms of amount of heat generated by it which may create a safety concern from explosion point.

How should we design an application of Motor + VFD for Class 1, Division 2 areas?

 

[kwired]

Hello,

As we know normal induction motor name plate has fixed frequency mentioned on it so a calculated, designed heat will be produced by it. However, if we have a inverter duty motor or a motor driven by VFD and we want to use that motor in Hazardous classified area, then being inverter duty motor, frequency will be changing as per PWM VFD technique. Is there any special requirement for inverter duty motors? As we know that a motor and VFD combination changes the characteristics of the motor in terms of amount of heat generated by it which may create a safety concern from explosion point.

How should we design an application of Motor + VFD for Class 1, Division 2 areas?

For hazardous locations surface temperatures can come in to play - you might either need motors designed with more cooling surfaces to allow more power at lower operating temperatures, or even have aux cooling fans to cool the motor instead of a cooling fan that is mounted on the shaft of the motor to be cooled - especially when reducing speed. Control methods may or may not need thermistors to assure surface temps don't get too high in hazardous location applications.

 

[Shahzad]

For hazardous locations surface temperatures can come in to play - you might either need motors designed with more cooling surfaces to allow more power at lower operating temperatures, or even have aux cooling fans to cool the motor instead of a cooling fan that is mounted on the shaft of the motor to be cooled - especially when reducing speed. Control methods may or may not need thermistors to assure surface temps don't get too high in hazardous location applications.

When you say " need motor designed with more cooling surfaces" can you elaborate.

for instance, for VFD driven motor if I were to use Class F insulation rated with NEMA Design B motor, TEFC, is it ok for Class 1, Div2? or what type of motor design I should choose?

 

[kwired]

When you say " need motor designed with more cooling surfaces" can you elaborate.

for instance, for VFD driven motor if I were to use Class F insulation rated with NEMA Design B motor, TEFC, is it ok for Class 1, Div2? or what type of motor design I should choose?

larger cooling fins is one possibility - just some way to dissipate heat and result in lower surface temperatures at rated loading. Temp that may be targeted for a particular environment - ignition point of different materials factors in here and will be designed not just for hazardous location but possibly for a specific hazardous material being handled.

If you run a typical "fan cooled" motor at a lower than nameplate speed, it gets less cooling air from it's fan and likely increased surface temp if still loaded to full load (at the new speed) variable torque loads will have less load at lower speeds though so that is also a consideration.

Different design to improve heat dissipation can give that motor higher rating without higher surface temps.

 

[retirede]

The allowable temperature would be determined by the gas - the group comes into play, not just class and division.
For group D, you likely have no issue.

 

[Shahzad]

The allowable temperature would be determined by the gas - the group comes into play, not just class and division.
For group D, you likely have no issue.

Should I use Class F, TEFC motor? or should I use, Class B, TENV motor or should I use explosion proof class F motor? Any guidelines or document to refer to?

 

[rbalex]

From NEMA MG-1:

14.32 OPERATION OF ALTERNATING CURRENT MOTORS FROM VARIABLE-FREQUENCYOR VARIABLE-VOLTAGE POWER SUPPLIES, OR BOTH

14.32.1 Performance

Alternating-current motors to be operated from solid-state or other types of variable-frequency or variable-voltage power supplies, or both, for adjustable-speed-drive applications, may require individual consideration to provide satisfactory performance. Especially for operation below rated speed, it may be necessary to reduce the motor torque load below the rated full-load torque to avoid overheating the motors. The motor manufacturer should be consulted before selecting a motor for such applications (see Parts 30 and 31).

WARNING: Motors operated from variable frequency or variable voltage power supplies, or both, should not be used in any Division 1 hazardous (classified)locations unless:

a. The motor is identified on the nameplate as acceptable for variable speed operation when used in Division 1 hazardous (classified) locations.

b. The actual operating speed range is not outside of the permissible operating speed range marked on the motor nameplate.

c. The actual power supply is consistent with the type of power supply identified in information which is supplied by the motor manufacturer.

For motors to be used in any Division2 hazardous (classified) locations, the motor manufacturer should be consulted.

High frequency harmonics of inverters can cause an increase in the level of leakage current in the motor. Therefore, users are cautioned to follow established grounding practices for the motor frame.

Failure to comply with this warning could result in an unsafe installation that could cause damage to property, serious injury or death to personnel, or both.

 

[retirede]

Should I use Class F, TEFC motor? or should I use, Class B, TENV motor or should I use explosion proof class F motor? Any guidelines or document to refer to?

For division 2, you would not need an explosion proof motor unless it contained a spark-making device (such as a temp cutout).

Refer to rbalex’s post for further guidance. Some motor manufacturers will limit the application of their motors to fixed-speed, sine wave only power, in hazardous locations. I’ve seen this from Baldor. Coordination with the motor supplier is key.

 

[Shahzad]

From NEMA MG-1:

Thanks. so that mean if a motor has to be used with VFD in any classified area, both motor and VFD as an application be certified, tested and name plate of motor must say that motor is suitable for this application?

 

[rbalex]

Thanks. so that mean if a motor has to be used with VFD in any classified area, both motor and VFD as an application be certified, tested and name plate of motor must say that motor is suitable for this application?

Not Exactly.

For Division I, a motor must be specifically identified on the nameplate for ASD applications. I have seen a few cases where the nameplate specifically prohibited use for ASD applications.

In Division 2, the motor manufacturer should be consulted. Specific ASD nameplate markings will be unlikely.

I didn't emphasize it before, but grounding/bonding is critical because the ASD will create harmonic content. It is mentioned in the NEMA standard above. When properly installed per Section 501.30, that issue is pretty well covered for either Division.

 

[jdsmith]

A good reference on this topic is IEEE 1349 - Guide for the Application of Electric Machines in Zone 2 and Class I, Division 2 Hazardous (Classified) Locations.


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[paulengr]

A simple solution in many cases is locate both outside the boundary and drive a belt or line shaft.