Three Phase Converter trouble Shooting

[cutonce1]

I just built a 10hp rotary phase converter. I used 2-50MFD and 3-45 MFD run capacitors connected in parallel mode. The one end of the parallel connection is connected to L3, and the other end of the parallel connection is connected to L2. I used a potential relay to monentary turn of starter capacitors. These starter capacitors are 1-130 to 156 MFD and 2-189 to 227 MFD. The startor capacitors are conencted in parallel mode and connected to the contactors.

My problem is: when the phase converter is turned on, the motor starts in a full speed right way. However, the motor gets hot and start smelling. Ichecked voltage on L1, L2, and L3. L1 and L2 read 115v each, but L3 does not read as I expected 120v plus app. 60v (+/-). I may have connected too many run capacitors. The idel motor is a high efficiency fan cooled and used as CNC motor previously. I appreciate your suggestions or solution to get L3 proper voltage.

 

[busbar]

Search this board for discussions of operating 3ø devices from 1ø sources.

If you have time to experiment and don't require high reliability, see the rec.hobby.metalworking newsgroup. There may be some suggestions on capacitor sizing and switching.

Voltages of interest are line-to-line, and if unbalanced, require derating {unloading} of the driven motor. It sounds like you are measuring/reporting line-to-ground or line-to-neutral values. [FWIW, the third voltage to neutral/ground should be about 200V.] The driven motor generally does not care about voltage-to-neutral or –ground, for there should be no driven-motor connections to either [outside of code grounding.] Line-to-line voltage with 5% imbalance require the driven motor to be derated to ¾ of nameplate horsepower. Voltages L1-L2, L2-L3 and L3-L1 are of concern for a small 3ø motor.

 

[SteveKW]

Busbar has a good point about the voltage but it sounds more like your trying to run the whole thing off of 115vac. Nowhere do you say that you have 230vac powering the converter on 1 & 2 like you should (making #3 the "wild" leg).

Also you need to make the run capacitors equal and the start capacitors equal in microfarads so they all share equally in the work.

You need to add more RUN caps to bring up the voltage on the wild leg and then less to lower it. Voltage is of course checked with the three phase motor and converter running and loaded.

You would have a very unique three phase motor if it was wound for 115 volts. The standard low voltage for three phase is 230.

 

[jbartos]

Suggestion to the original posting: This kind of design is nowadays aided by sofware simulation/calculations. It saves your time and hardware.