Electric Motor Starters

Toolguy · Dec 31, 2006

I recently found a deal on a used 3HP heavy-duty air compressor. I have it hooked up in my shop now and it works OK buts it really draws the amps when starting. I may not have enough electrical capacity to run it the way it should. While running it draws about 18 amps on both legs and the voltage is good at 115 to 118. However, when it starts up it draws 68 amps and the voltage drops to 104 momentarily. The 20A breakers do not trip. This seems excessive to me and I am worried that this might hurt the motor over the long haul. I know that magnetic or capacitor starters are needed on some motors. Does anyone know if a starter of some type would help? I have 125ft of 8ga wire running from my main electrical panel to the breaker box. From there it is another 50ft to the compressor. I only have 12ga wire from the breaker box to the compressor but I ran a parrallel set of 12ga wire to the compressor as a test to see if it made a difference with more wire capacity. It did not change the amp and voltage readings. I am stumped at this point. Any help would be greatly appreciated. Thanks.

P.S. The compressor has an unloader so it is not starting under pressure. It does have a huge cast iron pulley/fan on the pump. I am thinking it probably draws a lot of power to get that thing turning.

SPIKER · Dec 31, 2006

inrush currents are normally 2~3 time the running current on any motors. SO I'd say you are probably OK if it is only a 3~5 seconds of start up. Otherwise a start capacitor may be needed. that does not mean that the motor was designed with this in mind. most motors that are designed for them have end switches that spin up and disengage the cap others use 2 caps a start and a run cap. these are heavy duty applications like compressors which need a LOT of torque. this is the application you should probably have. anyhow if it was one you had no big deal go with it & hope it lives a long & happy life...
mark M

BobRip · Dec 31, 2006

The voltage drop and current while starting is OK. However, a circuit should not exceed 80% of its rated current for continuous operation. For a 20 amp circuit this would be 16 amps. I would change to 10 guage wire and a 30 amp breaker for that unit. You cannot use paralleled wire permanently. This should also help with voltage drop while starting at least a little. The short time that the high current and low voltage occur will not damage the motor.

What else are you running on this 8 guage wire. If you have a lot of other loads then the voltage will be lower to the compressor while they are running. You should test running voltages while running your other loads if you plan to run them while running the compressor.

mudcat · Dec 31, 2006

Continuious duty is 3hrs +. The 12ga wire is fine.

Toolguy · Jan 1, 2007

The only other draw on the 8ga wire is a few flourescent lights and my wall outlets. When all the lights are on there is a 2 volt drop. I rarely have them all on at the same time and the wall outlets are used very little as well. When I get around to it I think I will upgrade the wire to 10ga and change the breakers to 30a just to have a little margin of capacity.

This compressor will not get used very much. I had a single stage, light duty 4hp compressor for years. The main reason I changed was because it took forever to put air in my truck tires which require 110psi. That is near the upper limit for a single stage compressor. The regulator was set at 95psi on and 120psi off. In order to get 110psi in the tires the compressor would have to be running. So for every tire I would have to bleed the air off to get the compressor to turn on and then hold the inflator on the valve stem until the compressor shut off. Not a very time efficient way of doing things.

The other reason I changed is because I stumbled across the new compressor for only $300. I have already sold my old one for $265. The new (to me) compressor is old but is in good condition. The motor seems to be newer than the compressor as it is a different color. The compressor shows some signs of having been apart before so perhaps it was rebuilt at some point in it's life. When I got it it was set to turn on at 155psi and shut off at 175psi. I have it turned down to 120-140. That is more than enough pressure for my needs. Running it higher is just a waste of electricity.

Thanks for all the help, guys.

kmdigital · Jan 2, 2007

Ok, dumb question! I am almost certain that the motor can be wired for 230 volts, so why don't you change it for the higher voltage and put it on a double pole breaker? To the best of my knowledge, all single phase motors rated a 1/2 hp or greater are dual voltage, so this should be your first option.

This will literally drop your amps in half, and the motor will also start faster. Plus, although you don't run it that much, it will cost less to run because the load on you meter will be balanced.

Changing the voltage should be as easy as switching two or possibly four wires, and there should be a diagram either on the motor plate itself, or under the cap that covers the wire terminals.

caver · Jan 2, 2007

Up that 50 foot run to 10AWG should help.

Inspector507 · Jan 3, 2007

I'm thinking it already is wired 240V. A single phase 3HP motor connected at 120V will pull 34A. Besides he said it draws about 18A on both legs.

kmdigital · Jan 3, 2007

Inspector507 said:
I'm thinking it already is wired 240V. A single phase 3HP motor connected at 120V will pull 34A. Besides he said it draws about 18A on both legs.

You may well be right, though that 34 amp rating assumes full load which he isn't under. He said he has 120 volt, that drops down as the motor starts, so that is what I based it on. I assumed he was measuing it across both terminals, not one terminal to ground. And a 120 volt motor will draw the same amps on both legs also, they will just be twice as much as for a 240 volt motor.

Based on what he wrote, it sounds like his motor is 120 volt, and lightly loaded at 18 amps, probably because he is set the pressures way down from their possible maximum and possibly because the motor may be larger than necessary or geared down to reduce the load.