phase resistors

[upinsmoke]

I recently ran across a motor that had been wired with a resistance wire from phase to phase on the T terminals of the mag starter. The motor is a 25hp 1750 rpm motor that was having a "slow start" issue. A 14ga wire was installed between T1-T2 and then from T2-T3. What is it's purpose and why would that solve a voltage drop situation?

 

[Skogsgurra]

Not quite sure how that is wired. It sounds like it could be the old KUSA-starter. But, then you need a switch also to isolate the starter from the grid.

If it is a KUSA, it works by connecting two motor phases to the grid and one via the resistor. That reduces current because stator voltage is reduced and at the same time a (reduced) rotating field is produced by the resistive path to the third winding. When the motor is running, the third phase is connected directly (resistor shorted).

It cannot be used on constant torque loads (conveyor and such) but works very well on fans and other high-inertia loads.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Is this the shorting contactor on a two speed motor? Is this the only contactor or are there some other contactors involved with this motor.

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

 

[upinsmoke]

The starter has no switch, these 14ga wires are installed as a permenant part of the run and start. It starts across the line and has no other contactors. The load is constant, it's an air compressor motor. It is single speed.

 

[burnt2x]

How many power terminals did you see on the contactor?
Are there 6 leads out of the motor?
When I see three terminals jumpered, I would think its the wye point of the motor winding.

 

[upinsmoke]

There are three wires that feed into the "T" terminals on the starter. There are three power leads running to the "L" terminals for incoming power. Three lines and a ground. I'm thinking that whoever put the jumpers in was applying some old hearsay fix to a problem of low voltage. ???

 

[Skogsgurra]

Probably picked some misunderstanding up on the Eng-Tips

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...