Amps or HP:

This topic for the “Did You Know” article series is Amps or HP, which one is important?

Almost all drives manufactures that sell in the U.S. will rate their drives in HP at least from a marketing standpoint.

Go overseas and you will see kW ratings instead.  Both mean essentially the same thing, but can get a salesman (or saleslady) in trouble pretty quick.

First  1 HP = .75kW
So if you have an OEM that sells both to the US and overseas, he might ask for a 40HP/30kW drive.  Just jot down the conversion. 1HP = .75kW

But do drives really have “HP” in them?  No, drives are segregated into voltage categories and then by amperage. Most commonly 230V/460V/575V.   The “convenient” HP rating is to help you hone in on the right drive.  And for the most part if you have a standard off the shelf motor built in the last 30 years you are probably ok using HP.  However, if the motor is custom designed, then the amps might not align with the ratings published for standard motors.  And remember from our last “Did You Know” article, Constant Torque vs. Variable Torque, drives produce AMPS, not HP.

So, a 480V, 100HP standard motor might be rated for about 132A.  But a 100HP custom motor used on a crane or hoist might be rated upwards of 155A. 

You will find a 100HP drive is rated for about 150A.  If we didn’t ask for the motor amp rating, this one would be undersized.

Just remember that 95% of the time, you will be okay with the HP rating, but the drive is really rated to handle xxx Amps and the drive must be rated equal or greater to that amperage.

Special thanks to National Power Quality and
Consulting for providing these Did You Know articles.

For electrical consulting needs, NPQC can be contacted at www.natpowerquality.com