Motorized Generator

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lectricpower said:
How does a generator become motorized.
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Limiting the discussion to 3phase synchronous alternators - most generators:
Lock out the reverse power relay so the main won't open and the stator is stays connected to the line. Shut off the prime mover. As long as there is field current, the rotor will keep spinning in sync with the stator. The line will pump enough power back into the stator to keep the prime mover spinning.

In principle, an alternator isn't much different than a synchronous motor.

Single phase house generators?? I couldn't tell you anything about them.

carl
 
another "point-of-view"
in the early part of my carrer, many of the industrial facilities had "M-G" sets, or "motor-generators" to generate DC for motor control.
These M-Gs were basically a/c motors & d/c generators built on a common shaft and in a common housing.
No doubt there are a few still in operation.
 
Motor generators were some of the first PQ power supplies, some had battery back up, (Pillar was a major manufacture of MG UPS's). Today MG's are in use with different set ups with diesel engines for a battery less UPS.
 
augie47 said:
another "point-of-view"
in the early part of my carrer, many of the industrial facilities had "M-G" sets, or "motor-generators" to generate DC for motor control.
These M-Gs were basically a/c motors & d/c generators built on a common shaft and in a common housing.
No doubt there are a few still in operation.
Click to expand...

They are still in use worked on one Friday. Had a brush hung up.
 
I replaced one for an Air Force base, they needed 400 HZ to test equipment, so it set there running all day even though they didn't use it a whole lot. If I remember correctly it was around 50 HP, Don't remember the KW rating. The new one was electronic with no motor, and used very little power in standby mode.
 
I saw a really nice Siemens electric motor generator flywheel UPS a while back. That flywheel was big enough to probably run a couple hours if there was no load on the genset. Quiet, too. You could easily hold a normal conversation since there was no diesel running.
 
augie47 said:
another "point-of-view"
in the early part of my carrer, many of the industrial facilities had "M-G" sets, or "motor-generators" to generate DC for motor control.
These M-Gs were basically a/c motors & d/c generators built on a common shaft and in a common housing.
No doubt there are a few still in operation.
Click to expand...

Every US Submarine has them and I have seen quite a few in large industrial, and generation plants.
 
Some of the better ambulances have a 12vdc x 120vac motor generator to make true sinewave power for certain medical instruments on board. I think Redline is one manufacturer of those.

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Last edited:
081116-1142 EST

lectricpower:

Consider a permanent magnet DC motor.

This is simultaneously a motor and a generator. With no electrical connections to the armature you will develop a voltage across the armature when you rotate the armature.

This is based on
v = N * d(phi)/dt
where v is the output voltage, N a constant including the number of turns, and d(phi)/dt is the rate of change of the flux linking the coil. d(phi)/dt is directly proportional to speed in a linear system or RPM in a rotational system.

If the device (motor or generator) is being rotated, then this voltage is generated.

When it is a generator the internally generated voltage will be slightly greater than the terminal voltage, and in motor mode the internal voltage will be slightly less than the terminal voltage.

From this you can see that motor RPM as a first order approximation is directly proportional to the applied armature voltage. Or that generator output voltage is directly proportional to armature RPM.

.
 
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