question:AC motor with rotor brushes?

Hi all. I have a diesel pump motor sitting on my workbench. This is a 115V single phase (no capacitor) 1/2 hp motor. On the Stator enclosure, thier are two steel caps covering a set of spring mounted brushes. These brushes are simply shorting the rotor commutator to the stator housing. The motor runs just fine. The nameplate is quite worn, the only legible info is the voltage, and hp. I am wondering if anyone has seen this type of motor?, and what are the brushes for? Any Thoughts?

Thanks all.

Reply to
chris
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in article d%OOd.40059$tU6.27300@edtnps91, chris at snipped-for-privacy@telusplanet.net wrote on 2/10/05 11:55 AM:

It is impossible to tell from your description just what your motor might be. My guess is that it is a repulsion-induction motor. It uses just one of the many ways of creating a rotating field during start up.

Bill

Reply to
Repeating Rifle

a 115V single

set of spring

commutator to

voltage, and hp.

what are the

its a common motor, built mainly in the 1920's through 50's. before run capacitors were developed and in use to provide phase shift in motors (giving them the angular rotation).

You saw them on high torque applications in the size range you talk about ...in the refrigeration business they were quite common... I worked on hundreds of them as late as the 70's amd

80's... they are much less common now.

The brushes provide a complete circuit (to ground) in the rotor segment windings that is made*physically slightly out of phase with the rotating stator field (60hz supplied AC power).. by the positioning of the brushes... so that the rotor segments will be drawn around by the rotating stator field.

These type motors were heavier and more costly to build than the currently available motors in that smaller size range that use capacitors to generate the phase shift that provide rotation.

The commutator type motor you describe though is still built in the larger sizes... although not exclusively anymore.. capacitor motors are dominating primarily because the capacitance can be tuned to optimize speed and motor efficiency under variable loads.. on say a 300 hp motor, the capacitor bank can be 3' tall, 3' long and 2' deep...and the electronics package half again as large.

Phil Scott

Reply to
Phil Scott

Repulsion-induction motors typically had a wound rotor with two brushes on opposite sides of the commutator. The brushes were shorted together, but they did not need to be grounded. By rotating the brush holder, the rotation direction could be reversed. In this configuration, the motor ran in the repulsion mode. Starting-torque typically was quite high. When the motor speed came close to the synchronous speed, a centrifugal mechanism lifted the brushes and shorted all of the commutator segments together, and the motor ran in the induction mode, typically using a squirrel-cage built into the rotor under the repulsion windings. Thus the formal name was "repulsion-start-induction-run", commonly called simply "repulsion- induction". When such a motor started, the sound was a whine, with rising pitch, of the brushes on the commutator segments; then there was a loud click, and then there was just the hum of an induction motor.

When large electrolytic capacitors became practical, capacitor-start motors replaced repulsion-induction motors. The stator was wound as a two-phase motor, and the rotor was of the relatively simple squirrel-cage type, without windings, commutator, or brushes. The capacitor provided a phase shift in one of the two stator windings, to provide the rotating magnetic field required for starting the motor. Starting torque was fairly high. Usually, but not always, the capacitor and its associated winding were switched out when the rotor neared synchronous speed. On some such motors, the large starting capacitors were switched out, but smaller run-capacitors were left in.

The repulsion mode, and starting-capacitors, are used to provide starting torque for single-phase motors. Three-phase motors are much simpler because the three-phase power provides a rotating magnetic field to start the motor.

The stator field does not rotate; it just alternates. It induces alternating current in the rotor, and the brush position causes repulsion between the alternating stator and rotor magnetic fields - hence the name "repulsion motor".

Reply to
Dick Alvarez

with two

were

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In this

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was

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again as

First...thanks for filling me in on some details I had not gotten involved with.

Variable speed drives for large centrifical chillers (carrier, trane, york) etc..and large variable speed blowers predominantly, but there are many other uses...I think in some types of very high speed elevators so they can ramp the acceleration mode.

Do they have wound rotors?

Not as far as Ive seen.. these are 3 ph squirrel cage motors, laminated rotors, wound stators.

What

I am not at the expert level on that issue.. there are different types... some called variable *frequency drives..VFD's... by changin the frequency the speed of the motor changes obviously.

I am not totally sure exactly how the VFD is done.. those with capacitors are primarily to improve motor efficiency though by means of improving the power factor ... back emf balance with the power supply etc.. some or maybe most VFD's also incorporate a capacitor bank to correct the PF under various load and speed conditions.

To indicate complexity etc... I was quoted 20,000 dollars for a VFD with power factor correction for a 200 hp chiller drive ..that was 14 years ago.. prices and VFD strategy has evolved since then. Maybe now with much less in the way of capacitor banks.

Are these motors single-phase or

There are single phase VFD's for home AC units..I think its VFD.. some sort of variable speed motor capablity done with electronics. $150 for a 1/2hp motor of that type... 110v or

230 v single phase.. fairly new on the market.

Mostly though VFD has been for larger 3 phase motors in the HVAC industry..but also in many industrial applications. Its expensive. There may be other forms of electronic variable speed drives that Im not familiar with.

Phil Scott

Are single-phase motors made that big? Sounds

edu

Reply to
Phil Scott

You would know that yourself... single phase is not seen much above 5 hp.

Most VFD's until just lately, have been for very large 3 ph motors. 50 hp would be a small vfd application in the HVAC business...in machine tool controls there are other strategies and motor types as you probably know with all sorts of different ways to control speed.

Phil Scott

edu

Reply to
Phil Scott

elevators is where i see them as in old elevators

Reply to
PCK

message

(carrier,

some

what do the newest high speed elevators use to ramp the acceleration and deceleration of the motor?

Phil Scott

Reply to
Phil Scott

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