Three lead motor

[gtwy]

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.

 

[waross]

Probably a part winding start motor starter. The original motor may have been replaced with a full winding start motor at some time in the past.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[gtwy]

That's what I was thinking- Although what I can't reconcile is why go to the trouble of rewiring the cubicle and paralleling the controllers. I know that all the cubicles were upgraded (there's 3 200 hp and 3 100 hp cubicles and the 200 hp's got soft starts vs. the old w-d configuration). The expense of a 2nd controller etc. for the 100 hp motors when one would do.......... Seems like at best, install a power block at the bottom and drop the 3 controller leads to that and wire the 6 cables accordingly- hard to tell what went on- I'm familiar w/ the engineering firm that did the upgrade- top notch/do it right kind of outfit.

 

[waross]

I'm confused. Does the motor have three leads or six leads?
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

 

[gtwy]

It's definitely a 3 lead motor- that's why I was confused too. I'm going back next week to finish the vibration hook up, If I get a chance I'm going take another look- maybe a tap xfmr hidden around the corner that I didnt see?

 

[waross]

How many cores from the starter to the motor?

Bill
--------------------
"Why not the best?"
Jimmy Carter