Three lead motor
Three lead motor
(OP)
I normally work the instrumentation/DCS side of things motor related so I dont deal with motors/ motor starting that often.
I was at a WWTP the other day tying some vibration probes into the DCS and was working near the MCC where a plant operator was changing a motor controller on a 100 HP 480v Hoffman blower motor. I noticed in the cubicle 2 controllers, one w/ a pneumatic TD relay. As I passed by the motor I noticed that the motor was only had three leads. Just my own curiosity I looked at the wiring and the power leads at the controllers are in parallel (line and load side-line sides fed directly from the CB), with 3 leads from each controller heading to the motor.
I must be missing something obvious. There are 3 blowers all w/ identical cubicles. I didnt check contactor ratings to see if one alone was too small to carry motor current....... why the controllers in parallel?
The blowers are 40 yrs old (and the motor probably not much younger) and I know that the cubicles were upgraded 10 yrs ago so I dont know if originally the motors were wye delta.
I was at a WWTP the other day tying some vibration probes into the DCS and was working near the MCC where a plant operator was changing a motor controller on a 100 HP 480v Hoffman blower motor. I noticed in the cubicle 2 controllers, one w/ a pneumatic TD relay. As I passed by the motor I noticed that the motor was only had three leads. Just my own curiosity I looked at the wiring and the power leads at the controllers are in parallel (line and load side-line sides fed directly from the CB), with 3 leads from each controller heading to the motor.
I must be missing something obvious. There are 3 blowers all w/ identical cubicles. I didnt check contactor ratings to see if one alone was too small to carry motor current....... why the controllers in parallel?
The blowers are 40 yrs old (and the motor probably not much younger) and I know that the cubicles were upgraded 10 yrs ago so I dont know if originally the motors were wye delta.





RE: Three lead motor
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Three lead motor
RE: Three lead motor
Two controllers with a time delay on one sounds like a part winding starter. A part winding start motor has six leads. In some instances a part winding start motor was used to reduce the start current below an fixed limit. This limit may have been arbitrary or it may have been to reduce voltage dip somewhat.
In many instances a part winding start motor is capable of starting DOL, but with increased voltage dip on the supply system.
An older motor may have been rewound at some time in the past and only three leads supplied instead of the original six leads.
If three are six leads running from the starter to the motor it is almost certainly a part winding start motor.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Three lead motor
RE: Three lead motor
Bill
--------------------
"Why not the best?"
Jimmy Carter