unusual motor design

Some time ago, I saw a motor, which I don't understand. It is installed in a building in London, which used to be a pumping station,
and is now a museum, The Kew Bridge Steam Museum http://www.kbsm.org
The motor is used to drive a compressor, which supplies air for starting Diesel engines. The compressor is not the original one, but was installed by the museum; it was obtained from, if I remember correctly, a sewage works, where it served a similar function.
It is a three phase machine, running on the standard British supply voltage of 415V (pre recent change in nominal Voltage to 400) and the plate declares it to be an inducton motor. So far so good, but through the ventilation slots can be seen at one end of the shaft three slip rings. Why would an induction motor have slip rings? It's a relativly old machine, 1930s maybe, I can't remember the power rating, but from the size I would guess maybe 20HP.
The motor is started by a manual device, operated by a lever which swings in an arc, similar to the controller on an old style tram car. The lever cannot be swung over too quickly, as it has to be alternatly pushed in and pulled out, to clear a number of stops. Are the slip rings part of the starting system?
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As others have mentioned, this is most likely a wound-rotor motor. The rotor windings are connected to the slip rings so an external resistor can be inserted into the circuit during starting. The cumbersome manual starter you describe probably has several circular contacts spaced at different distances out from the pivot point. Some apply line power to the stator windings, others connect the rotor circuit to various steps on the starting resistor bank.
Compressors, conveyers and crane/hoists were a common application for this type of motor. Pumps and blowers, whose torque requirements vary with the speed squared and have relatively little inertia. So ordinary squirrel cage motors are used for those applications. But compressors and conveyers have torque requirements that can be nearly constant (unless the compressor is designed to 'unload' for starting) regardless of speed. Large inertia loads also used wound-rotors because the large amount of heat generated when accelerating slowly can be dissipated in the external resistor bank with fans and such, not burn out the motor.
daestrom
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wrote:
| Compressors, conveyers and crane/hoists were a common application for this | type of motor. Pumps and blowers, whose torque requirements vary with the | speed squared and have relatively little inertia. So ordinary squirrel cage | motors are used for those applications. But compressors and conveyers have | torque requirements that can be nearly constant (unless the compressor is | designed to 'unload' for starting) regardless of speed. Large inertia loads | also used wound-rotors because the large amount of heat generated when | accelerating slowly can be dissipated in the external resistor bank with | fans and such, not burn out the motor.
Why can't that energy coming off the rotor to be dissipated instead be used in some way in the stator (separate windings) to further boost the starting torque? Or would that just be way too much heat?
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in article snipped-for-privacy@enews4.newsguy.com, snipped-for-privacy@ipal.net at snipped-for-privacy@ipal.net wrote on 2/6/04 11:51 PM:

There are all kinds of things that can be done with enough development and hardware to make motors more efficient when they are not run at their design conditions.
The wound rotor induction motor is designed to run at rated speed with the slip rings shorted. The resistors are just used for starting at high torque against loads that have high torques at low speed. Such loads include positive displacement pump but not fans and blowers. At rated speed, with the slip rings shorted, the motor acts very similarly to that of a squirrel cage motor. Current in the rotor is at the slip frequency.
Bill
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wrote:

this
the
cage
have
is
loads
Actually, *modern* wound-rotor controls do exactly that!!!
Modern wound-rotor controllers use solid-state electronics to take the power from the rotor and convert it back to 60HZ AC and apply it back into the line. So the characteristics of the motor are as if you had a high rotor resistance (good torque when starting), but the energy isn't dissipated as heat.
The OP did say something about this unit being in a museum or something. Before the current state of solid-state electronics, it was common to just 'dump' the energy into a large external resistor bank.
daestrom
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Close to where I am, the National Railway is still operating three large 3 ph Synchronous motors (circa 1934) driving a huge single 'lung' compressor, these motors are about 5 ft high, but very narrow. They are run up as a regular induction motors and then when close to sychronism the DC is switched in to the the armature coils via the slip rings and they then run as a syncronous motor, Some years ago I assisted in converting the DC supply which was a separate induction-motor-driven DC generator over to DC Solid State supply. In those days the slip frequency was detected in a non-electronic fashion, it is interesting to see the way things were done back then. Al I
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in article snipped-for-privacy@posting.google.com, Stephen Furley at snipped-for-privacy@mail.croydon.ac.uk wrote on 2/5/04 9:57 AM:

A typical squirrel cage motor or shorted wound rotor induction motor has low torque when starting or blocked. That is, the torque drops very rapidly when slip increases beyond a certain point. The blocked torque may be insufficient to start against a high torque load. Adding resistance to the secondary (an induction motor is like a transformer in many ways) increases the starting torque. Once the motor reaches speed, the resistance can be shorted out. This shorting may be done step-wise.
Bill
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the stator is connected to the mains. the slip rings are probably connected to a resistor setup to start the motor. this gives a soft start so the maind are not too heavily loaded. the motor may be then run by shorting the slip rings or, the slip rings may be connected to direct current supplies to make the motor synchronous. later motors are similar but the starting function is done automatically. sammm

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message

It is

pumping station,

http://www.kbsm.org
air for

original one, but

remember
function.
British supply

400) and the

good, but

the shaft

rings? It's

power
lever which

style tram car.

be alternatly

the slip

Wound rotor induction machines w/ slip rings allow you to insert resistance in the rotor circuit. Well, you could insert whatever you like in the rotor circuit, but normally it is resistance. Inserting resistance in the rotor circuit gives more torque at low speed (shifts the torque/speed curve left without decreasing max torque), great for faster acceleration and/or loads that need a lot of starting torque. Removing the external rotor resistance in stages steps the torque curve back to the right, allowing all acceleration to be done at near-maximum torque. Low rotor resistance allows more efficient operation, so all the external resistance is usually removed for full speed operation. Similarly, external rotor resistance can be used for speed control. Shifting the torque curve gives a new operating point at a different speed. 'Squirrel cage' rotor induction machines do not allow you to insert resistance in the rotor circuit. Squirrel cage IMs are cheap and rugged and the most popular. Some squirrel cage IMs are designed with shaped rotor bars to effectively increase R at low speed but come back to normal R at full speed. This gives better low speed torque. j
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Aside from the "dial a torque" advantage, the wound rotor causes the heat from the rotor "slip" to be generator in the resistor bank rather than within the rotor.
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Hello Steven
This is almost definitely a slip ring induction motor with a manual secondary resistance starter.
Best regards, Mark. http://www.lmphotonics.com
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