208v 3 phase motor problems 2

[scubasteve]

rhatcher thanks for your response.I have isolated the motors as one of my trouble shooting steps.And if i get them to run the startup is slow and gradually increases.
Most of the time i hear the relays energize and the motors hum.the motors are 3 phase 208v but only have 3 conductors
to each.I cant seem to grasp that.They were running fine
until yesterday and i belive i have eliminated loose
connections,shorts,and the typical good stuff.I hope this
info helps;and again i appretiate yours.

 

[rhatcher]

For 3 phase motors only 3 conductors are required (with a ground wire to make 4). You may be experiencing an intermittent 'single phase' condition when the motors are humming and a high resistance connection on one phase when they are running slow.

If you have determined that all 4 motors are affected then I would go to the common point. If a single contactor starts all four motors then I would go straight to that. First, measure the available voltage at the line side of the contactor. If the voltages are balanced, then (only allow the motors to 'hum' for a few seconds at a time to avoid damage) start the motors and measure the voltage across each phase of the contactor from the line side to the load side. A good contact set will exhibit 0V and a bad set will show a measurable voltage up to 208V.

Speak up if this is not the answer....

 

[jbartos]

Suggestion: If you prefer to have a peace of mind, you may measure the line current in each motor line conductor and compare it to the nameplate Full-Load Amp (FLA) current by using a clamp-on ammeter. The line current should not exceed the FLA. If it does, then some troubleshooting is needed. Else, you may also check the power supply quality by a harmonic analyzer to make sure that the Total Harmonic Distortion (THD) of voltage is less than 5%. The voltage unbalance shall be less than 3%. Then, you may almost rest in peace.

 

[wireman1]

scubasteve

I agree with rhatcher it sounds like you have you have a single phase problem. sometimes a 3 phase motor will run when it is single phased but with a reduced output speed or torque. It could be any number of things from a blown fuse or malfunctioning circuit breaker, a bad contact on a motor starter or a malfunctioning overload, a loose or coroded termination. According to you post you have tried most of that and rhatcher is right on in his advice. Sometimes it helps to start from the beginning and work your way to the motors. Good luck and be carefull. If you have any doubts you might be better off and money ahead to hire an electician.

 

[electricpete]

Based on the other post it looks like rhatcher and wireman were right on and have helped scubasteve solve his problems. Stars for both. In particular, watching rhatcher's responses over a period of time, it's obvious he has outstanding knowledge of motors on both the practical and theoretical side.

Thanks to both you guys for sharing your expertise.