I have connected two oil-free motors in parallel, but they won't start, they appear to try starting but they jam (so I turn them off before they burn).
If I connect motors individually, they start and keep working, so I think the problem is in starting process. (these are single phase motors btw, 110V)
From what I have read these motors don't need starting capacitor, only running capacitor, because the motors have 4 pole winding (though this information isn't 100% reliable).
I don't think I have short circuit somewhere, I think two motors might have both capacitors interfering with each other and something prevents them from starting correctly. This is my hypothesis, but I am not sure, so I would like to ask,is there something special I need to do to wire two induction motors in parallel? Right now I am testing with 2, but in the future I want to connect 4 motors in parallel.

 

Likely the voltage drop from the wiring resistance is such that the voltage is not enough to start both motors at once (motors can take up to 10 time their running current to start).
Either run larger wire from the power outlet, or add delay relays so only one starts at a time.
One AC outlet may also not have the capacity to start two motors at once.

How much operating current do the motors require?

 

Likely the voltage drop from the wiring resistance is such that the voltage is not enough to start both motors at once (motors can take up to 10 time their running current to start).
Either run larger wire from the power outlet, or add delay relays so only one starts at a time.
One AC outlet may also not have the capacity to start two motors at once.

How much operating current do the motors require?

thanks for your comments, I am using 16 AWG wire, and the motors are 750Watt each, so it should be enough current imho. Ok, I will try adding the relays.

 

Opinions don’t count, measurements and reading the specs do.

One thing is the running motor amps, another is the starting motor amps. Which can be from 3 to 5 times the running amps.

 

Another thing, you mentioned 750 watts, but you have to divide that by the power factor, which usually is around 0.85 for small motors, which would give a load of 880 VA.

 

so it should be enough current imho

IMHO it's apparently not.

 

One last thing before I sign out from this thread. No NEMA 5-15 receptacle will allow enough current for 4 motors. A NEMA 5-30 would barely meet the running capacity, but could also have startup issues even with a staged startup.
If you have a 220 volt split-phase supply, it would be far better.

 

so it should be enough current imho.

All things electrical or electronic don't really care about opinions, how you think it should work, how you want it work, or what some YouTube video claims.

 

What is not clear because it is not shown is any portion of the connection scheme. Are the two compressor modules only connected together at the mains power connection? or is it more than that??.
And a second question is about the pressure lines? Are they tied to each other so that the second compressor has to start against a pressure head?
And then what about the power cord? Is it one of two cords, where originally each compressor had it's own cord and now you are running twice as much power thru one cord?? Try using a separate cord for each assembly and see if that works. Voltage drops in power wiring can be responsible for a lot of problems. Often simple to fix but difficult to spot.

 

IMHO it's apparently not.

exactly, after adding relay switch and moving both motors to individual power source everything worked perfectly well. Thank you very much for all the help!

 

And a second question is about the pressure lines? Are they tied to each other so that the second compressor has to start against a pressure head?

I connected the pressure switch to the relay, so if pressure gets to maximum it will power off both motors. And the relay powers on both motors instantly, no delay is needed since I have enough current (I replaced cables with thicker ones)

 

Given the symptoms, the problem had to be an excessive voltage drop during starting.