[TheForge] 5HP electric motor advice sought

[Paul Novorolsky]

Ooops, made a typo. Below I said OK to put 10ga wire on a 30amp circuit. 
What I meant  was it's OK to put 10ga wire on a 20 amp circuit.
(Actually, I've done that for longer runs, to reduce the voltage loss)

Paul Novorolsky wrote:
> Mike,
> I only have a partial response, and I'm not very motor literate, but I 
> do have electrical experience, so I only addressed the circuit 
> capacity. (and probably in more detail than you wanted anyway)
>
> Horse power can be converted directly to watts (and therefore current 
> capacity)
>
> A 5 HP motor is 3728 watts. (About 745.7 watts/hp). However, that is 
> OUTPUT power, not input. The motor gives off heat, so it will require 
> more than that to drive it.
>
> (I often use a 750 watt / hp for a quick "in my head" calc for motor 
> ratings. A motor that is advertised as 1hp, but is only rated for 5 
> Amps at 120VAC is NOT a 1 HP motor.)
>
> So at 230 V (3738 / 230 ) that's about 16 Amps, so the motor draws at 
> least that much.
>
> So the 23 Amps may include a startup surge, or perhaps that is the 
> steady draw. (I'm not sure how the switch was rated)
>
> At any rate, 12ga wire is rated for 20Amps. So you may either exceed 
> the rating briefly during startup, or it may be dangerous to run it.
>
> The key here will be your breaker. Even if the current only exceeds 20 
> amps briefly, it may trip some of the time.
>
> One thing you don't want to do is combine a  bigger breaker than the 
> capacity of the circuit (wiring) that it is to protect. Otherwise, the 
> wiring may get hot enough to start a fire. So my advice for this piece 
> is go to a 30 Amp breaker and 10 ga wire. (You can put 10 ga wire on a 
> 30 amp circuit, but never put an under-rated wire on a smaller circuit 
> breaker. That is a 30 Amp breaker with 12ga wire is dangerous)
>
> Motor starters (at least "in the old says") were basically centrifugal 
> switches. They engage the starter winding, and once the motor spins 
> up, the centrifugal force operates the switch, and the motor switches 
> off the starter winding and runs on the main winding alone. That's 
> probably all I know about it.
>
> **paul
>
> Mike Spencer wrote:
>> I need some ignorance-removal re. a 5 HP motor.  If that's nothing you
>> know about,  you can stop here and skip the boring details. 
>> I bought a 5 HP, capacitor start, 230V, (marked "23 amps" and 1.15
>> service factor) motor.  It has a big junction box with 4 capacitors of
>> one kind, two of another kind and some sort of magnetic switch.  Shaft
>> is 1-3/8".  I replaced a capacitor that was arcing, swapped wires
>> around to get the right direction of rotation.  For testing purposes,
>> it's hooked up:
>>
>>    230V welder outlet
>>           |
>>    20' of 12 ga. wire
>>           |
>>    Heavy spring-loaded manual switch
>>           |
>>    Motor line cables
>>
>> In that configuration, it starts smoothly and runs fine with no load.
>> But I have questions before I stick it on my compressor:
>>
>> 1.  Is my existing 12 ga. wire okay for 5 HP/23 amps in a 30' run?  Or
>>     should I have 10 ga. or even 8 ga.? The piece of line cable dangling
>>     out of the motor is 8 ga.
>>
>> 2.  Is my Square-D 9013GHG pressure switch going to work? Fuzzy
>>     markings inside (apparently in Spanish) seem to indicate that it's
>>     only rated for 3HP at 230V single-phase and 5HP 3-phase (if that
>>     makes any sense.)
>>
>> 3.  There's some kind of thing called a "starter". Do I need one?
>>     What does it do?  If I need one, how do I wire it up so that the
>>     pressure switch takes advantage of whatever the "starter" does?
>>     Or do I need a pressure switch that is also a "starter"?
>>
>>
>> The compressor is rated 400 PSI.  I only run up to 150 PSI and the
>> pressure switch cuts in again at -- I forget, 90 PSI?.  Original 10HP,
>> 3ph motor was 3250 RPM with a 4" sheave. Present motor is 2 HP, 3250
>> RPM with the same 4" sheave. This new 5 HP motor is 1725 RPM and I was
>> planning to put an 8" sheave on it to get the same RPM at the
>> compressor.  This will, I'm thinking, increase the starting current in
>> the motor but I don't know if it's enough to matter.
>>
>> I downloaded a 4.5 meg PDF from Square-D that gives every imaginable
>> mechanical engineering spec for 9013-series pressure switches *but
>> not* their current-carrying capacities or electrical specs. Feh.
>>
>> Any advice or answers welcome.
>>
>> Thanks,
>> - Mike
>>
>>   
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