Formula for change in Hz to HP

[Jessie00721]

Hello, I was wondering how the Hz actually effect the hp of a motor using a VFD? For example, lets say I have a 10hp 460v 60hz motor and dropped the hz using a VFD down to 40. How would I calculate the actual hp drop? Thanks you all for your time!

 

[Skogsgurra]

The HP is proportional to the speed - and thus to frequency.

Reducing frequency from 60 Hz to 40 Hz reduces the shaft power from 10 HP to 10*40/60 = 6.67 HP.

There are details that may influence the outcome but they usually mean less than a few percent and can usually be forgotten.

Gunnar Englund

http://www.gke.org

 

[electricpete]

skogs is right. One of the few details he mentioned is probably the reduction in cooling that occurs as speed decreases further which may require reduction by a larger factor than the ratio of speeds.

I'm not sure if it was part of your question but above rated speed, the horsepower limit is roughly constant (doesn't increase with speed). That is up until some point in the neighborhood of 150% where ratio of breadown torque over full load torque becomes too low.

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[aolalde]

Below the base speed the torque is constant and the power is proportional to the motor speed and then proportional to the frequency too. Above the base speed the torque reduces with the frequency and the power stays almost constant up to the motor Overspeed limit.

The base speed is normally that corresponding to the driver maximum voltage output.

 

[Jessie00721]

Thanks for the great info! You guys are great!