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Generators & Electric Motors General Discussion Antique Generators and Old Electric Motors: Questions and answers about restoring and showing old power generation systems.

Generators & Electric Motors General Discussion

Question about Generator Speed and Motor Speed


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  #1  
Old 05-12-2010, 02:47:25 PM
iceman iceman is offline
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Default Question about Generator Speed and Motor Speed

I'm Woundering why if a generator that is a four pole spins at 1,800 rpm at 60 Hz, then a Electric Motor thats a Four Pole spins at 1,725 rpm. Why the lost in rpm's.
Just woundering
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Kieth aka iceman:
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Old 05-12-2010, 03:13:24 PM
armandh armandh is offline
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Default Re: Question about Generator Speed and Motor Speed

in the generator it is the stator coils and field polls that determine HZ vs speed.

in a motor it is the stator coils and number of rotating induction [bars] that determine HZ vs speed.
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Old 05-12-2010, 06:10:13 PM
phabib phabib is offline
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Default Re: Question about Generator Speed and Motor Speed

I won't argue with the above answer, but I don't think it really answers the question.

The motor is rated for horsepower produced at a given RPM. You've seen those 6 horsepower vaccum cleaners that somehow defy the laws of physics to produce 6 HP on only 900W of input power. They do it by measuring the horsepower developed when the motor is on the verge of being stopped and drawing max current. As load is applied and the motor slows, it will draw more amps and make more power (until the smoke escapes if you do it for too long)

In order to have a consistent measure, it was decided to measure HP at 1725 RPM instead of the full 1800 that a motor might produce when it isn't doing any work.

At least that's what remember reading. I'd be glad to be corrected if I'm wrong.
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Old 05-12-2010, 06:41:47 PM
Ed Wright Ed Wright is offline
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Default Re: Question about Generator Speed and Motor Speed

A couple of good answers above. I might add - The 4 pole motor synchronous speed @ 60 Hz is 1800 rpm. An induction motor at synchronous speed delivers zero torque. The induction motor has to "slip" below this rpm in order to develop currents in the rotor to produce torque and deliver useful work. The amount of load that results in 1725 rpm is where most 4 pole motors are rated for power output.
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Old 05-12-2010, 08:10:57 PM
Lloyd H Lloyd H is offline
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Default Re: Question about Generator Speed and Motor Speed

Ed has got it right Lloyd H
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Old 05-12-2010, 10:14:29 PM
KEB KEB is offline
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Default Re: Question about Generator Speed and Motor Speed

Just add a little more explanation. Think of the motor as two magnets, one turning and trying to pull the other around with it. The "turning" magnet is the result of the varying magnetic field in the stator windings, and the one being "pulled" is the magnetic field in the rotor.

If there were absolutely no friction or load, the two magnets would stay aligned and the motor would turn at 1800 (or 3600) RPM. However, if I apply a load to the rotor, the second magnet "slips" with respect to the first one, and the rotor turns a little slower. The further one field gets ahead of the other, the more torque is created The more load, the more slip, and the slower the rotor turns.

In a single phase motor, the fields don't really rotate, but the phase (timing) relationship between the stator and rotor fields has the same effect. In a three phase motor, the field in the stator actually does rotate, and drags the rotor field along with it.

Keith
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Old 06-09-2010, 07:52:59 PM
David K David K is offline
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Default Re: Question about Generator Speed and Motor Speed

Keith,

Actually a 4 pole induction generator needs to spin at 1880 rpm (a 2 pole 3760 rpm) in order to make 60 Hz electricity.

Ed gave the most accurate answer. In an INDUCTION motor, the rotor must "slip" (fall behind) with respect to the rotating field in the stator. There are bars in a rotor. A magnetic field is induced (thus the name "induction motor") into these bars from the stator. The bars in the rotor must break the magnetic field ("slip") in order for them to become magnetized. (It's a science thing.) If the rotor were to spin at exactly the same rpm as the magnetic field, no magnetism would be induced into the rotor and thus no power would be produced. The intended purpose of the motor determines how much slip a motor manufacturer designs into a motor. That's why some motor nameplates read 1725 rpm while others read 1740 or 1750 rpm.

In a SYNCHRONOUS motor, the rotor magnetism is not induced into the rotor. The rotor is magnetized thru its windings. The windings are energized with electricity delivered thru the brushes. Thus a synchronous motor runs at synchronous speed (900 rpm, 1200 rpm, 1800 rpm, 3600 rpm, etc). As with an induction motor, the number of poles in the motor still determines the rpm.

The reason an induction motor must spin slower than its synchronous speed is the same reason why an induction generator must spin faster (1880 rpm) than the synchronous speed.

As with synchronous motors, a synchronous generator needs to spins at synchronous speed (1800 rpm, 3600 rpm, or whatever).

As with everything in life, there are variables. Thus when you lug a motor down, it can and will run slower than nameplate rpm. Which is a great thing about an electric motor..... Lug an induction motor down, the rpm's drop thus the amount of slip increases. Increased slip induces a stronger magnetic field into the rotor. A stronger rotor allows the motor to (temporarily) produce more torque and pull thru the increased load.

If you think that was confusing to read, try typing it!!

Take care,
Dave
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Old 06-12-2010, 02:08:08 PM
snowcountryfarmer snowcountryfarmer is offline
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Default Re: Question about Generator Speed and Motor Speed

The best explanation of electrical machines (motors and generators that is)that I have read, at least the easiest for the layman to understand, is the Navy Neets module.

The navy published a series of educational materials dealing with electricity and electronics theory, that was made available to sailors so that they could read and learn on their own, at their own pace. If you can get through the whole series, you will probably be close to having an electrical engineering degree, in practical terms. The materials have lots of good theory, as well as real world examples.

Just google "navy neets" or look here:

http://www.rarmy.com/coleman/neets/index.html
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