[TheForge] 5HP electric motor advice sought

[Paul Novorolsky]

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
>
>