Discussion in 'Electronic Basics' started by Jon Danniken, Jul 28, 2005.

1. ### Jon DannikenGuest

Hello, I have a question about contactor ratings. On a contactor I am
looking at, it is rated at 60A (for 2240VAC), but also at only 10 HP (for
240VAC). From my understanding, ten horesepower is ~7460VA, which is only
~31A at 240V.

Why the discrepancy between the HP and A ratings? I thought it could be due
to resistive vs. inductive load, but the same contactor is also rated 75A
resistive.

Thanks for any insight into this,

Jon

2. ### JazzManGuest

I suspect that it has something to do with the amount of
current the contacts will break, as well as what they
will carry closed. Someone with more knowledge should be
along shortly.

JazzMan
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3. ### John PopelishGuest

Keep in mind that no motor is 100 % efficient, but look up the locked
rotor current for a few 10 HP 240 VAC motors. (Hint: depending on the
class of motor, this can be something between 4 and 20 times full load
rated current.) Each time you start one of these, they draw this
current for at least a fraction of a second as they accelerate from a
standstill. If they are actually locked, they draw this current till
some protective device opens the line or turns off the relay. So if
driving a 10 HP motor, not only do the contacts have to handle this
current at the moment of contact closure, they may have to break this
high and inductive current if the motor is ever overloaded.

4. ### Jon DannikenGuest

Thanks, John, that makes absolute perfect sense; thanks for the information.

Jon