1800 RPM otors versus 3600 RPM

[SparkyJunior]

Hi guys,

I have a 150HP motor being controlled with a 200 HP VFD. The motor is rated for 3600 RPM. The client now wants to go with a 200 HP motor. Will the VFD need to be increased as well? I know the current ratings of VFD's are based on 4 pole 1800 RPM motors. How does having a 3600 RPM motor affect this?

Thanks for the insight.

 

[CJCPE]

VFD manufacturers select current ratings corresponding to nominal HP ratings so that their products are suitable a reasonable percentage of the 4 pole motors that customers are likely to select. It is best to obtain current ratings from the VFD and motor suppliers before purchasing the equipment. Two-pole motors typically have full load current ratings that are the same or less than four-pole ratings. Six-pole motors typically have ratings that are a little higher and current ratings generally increase as the number of poles increases.

It should not be difficult to find a new 200 HP inverter duty motor that is suitable for use with an existing 200 HP VFD.

 

[VxA]

You may want to consider the duty of the motor. An Allen-Bradley PowerFlex 700 Drive rated for 200hp "Normal Duty", is only rated for 150hp "Heavy Duty." You may want to make sure that the original drive was not oversized on purpose due to the harsh life the motor and drive will be exposed to. If this is the case you may want to up the drive size. If the motor needs to start and stop often, and it requires high torque on startup, you may want to use a Heavy Duty rating.

http://www.ab.com/en/epub/catalogs/36265/1323285/1323309/tab5.html

 

[CJCPE]

VxA has a good point. Some manufacturers provide "standard duty," "normal duty" or "variable torque" drives with little or no short term overload capacity to handle hard-starting or other brief overload requirements. "Heavy duty" and "constant torque" drives are often designed with a substantial overload capacity such as 150% of rated current for 1 minute.

You should determine why the client wants to go to a larger motor and check to see if both the continuous and intermittant capacity of the VFD will be adequate to provide the expected performance.

 

[DickDV]

What the above posts are indicating is to get the FLA of the new 200hp motor. Also get the continuous amps, 1 minute rated overload amps and 5 second rated overload amps on the drive and compare the for overload capacity. Drives and motors know nothing about horsepower, only volts and amps.

For example, your motor nameplate says 220 amps. The drive continuous rating at 10% overload for one minute is 230 amps. The drive 5 second rating is 265amps.

The system will be able to operate at 230/220 of motor nameplate continuous, at (230+23)/220 for 10 seconds, and at 265/220 for 5 seconds. All of this assumes the motor stays within its thermal limits as well.

Hope that clears things up.