1 ph motor

[ptonsparky]

Pump installer called & asked a few questions on a 15 hp submersible well. 1 PH. He had determined that the contactor had some loose connections and repaired those. He wanted to replace it but finding a 1ph contactor rated at 75 amps inductive is tough. Locally anyway. He started it up and noticed the current continued to climb even though pressure rose. Run capacitors out. He wanted to know why, not why they failed but why the current continued to rise. My motor theory isn't good enough to explain it to him. Any takers?

 

[BJ Conner]

Check the water level. As water level falls the pump will do more work and the current will go up. The water level could be drawn down as the pump runs.

 

[dkarst]

Could be that the run cap is gradually failing and you are just seeing the increase in current resulting from poorer power factor. Say you have a motor with good run cap drawing (making numbers up) 35A at a power factor of 0.95. Under the same mechanical load, as the run caps fails, motor will draw say 45A to supply same mechanical power to the output shaft and power factor will drop to 0.85. This is the primary reason to correct power factor, less current drawn through all the lines/transformers from source to load that is not contributing to real power. These are NOT exact numbers, just tossing a possible theory out there.

 

[Jraef]

Are the run caps "out" or does the start cap never get switched out of the circuit? Being a submersible, are they using a voltage relay in the cap box to do the switching? Is that working right?

By the way, a "75A contactor" couldn't care less if it's 1 phase or single phase feeding it, you just need to use 2 of the 3 poles.

But I would avoid using amps alone as the size criteria, motor starting ratings are based on a lot more than just running amps. Since 15HP 1 phase is somewhat unusual, nobody lists that as a 1 phase motor rating, they stop at 10HP 230V. So I would use a 3 phase equivalency to be safe. 15HP 1 phase x 1.732 = 26HP 3 phase or 35kW. I would use a NEMA size 3 contactor, (or Siemens / Furnas makes a NEMA size 2-1/2). If you like IEC contactors, use one that has a motor starting rating of 35kW or more. Looking at one IEC manufacturer's charts, that equates to a contactor with a contact rating (AC1) of 115A. A 75A rated IEC contactor would only be capable of starting about 9HP at 230V 1 phase so it eventually smoke trying to start a 15HP 1 phase motor.