VFD rated motor, where is the threshold for a external fan on motor?

[controlsdude]

I remember seeing this on here and looked at the faq and did a quick search.

general conveyor use
Motor 480vac, 3.1fla 60 hz 2 hp rated for vfd use.
VFD at 46 hz due to speed changes, runs all day at this this speed. stops only on flow control or motor upstream stopping

Is there a limit to how low in hertz you can go on the vfd before you put an external fan on the motor?

I thought it was if the vfd hertz was 2/3 below or <40 hz that you would need an external fan?

 

[waross]

It depends on the load on the motor. Given that some manufacturers use the same frames for 1800 base motors and 1200 base motors you should be safe down to 1200 RPM in all but exceptional installations.
Discolored paint on the motor is often a good indication that you should have had more cooling air flow.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[controlsdude]

It is a 1800 rpm motor

 

[DickDV]

controldude, this is something best answered by the motor manufacturer. Most have a graph in their motor catalog showing the full-torque speed range and a derate for operation at slower speeds.

As a general rule, at 2hp you can load the motor to full torque down to around 1/4 speed so that would be 15hz. Below that, the motor will overheat or you will need to reduce the load torque somewhat.

By the time you reach 200hp, that speed range is reduced to only 2/1 so 30hz would be the minimum without derating.

Better check with your motor manufacturer for specifics. It may make a small difference whether the motor is ODP or TEFC also.

 

[controlsdude]

your exactly right DickDV, checked with the manufacturer and at this low of horsepower it is around 15 hz.