Electric Motor as Generator - Not Electrical Eng. So speak slowly 1

[pipelyer]

Curious on the output of an electric motor if used as a generator - is it basically a 1/1 equation. Is there a way to boost the output or other options out there to utilize the same motor to generate electricity?

 

[waross]

Yes, no, maybe, maybe not, probably, probably not, marginal results, fairly good results, don't try this at home;
It depends.
Many industrial motors (AC-asynchronous or asynchronous and DC motors) will generate and supply power back into the grid when driven over-speed.
If the motor is driven by an electronic drive or VFD, the drive must have provisions to regenerate.
The old Motor-generator DC drives regenerated.
Largest example that I have seen.
four 1300 HP DC motors being driven over-speed.
The motors the regenerated power caused the DC generators to drive the AC drive motors over-speed.
The AC drive motors pushed a combined 3 or 4 Mega-Watts into the grid.
It was impressive.
In your case, it depends on the motor and the configuration.
Some will work islanded, some only work when grid connected.

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

 

[davidbeach]

An electrical machine (aka "motor") connected to the electrical power system and to a rotating mechanical system will be a motor when the electrical system supplies energy and the mechanical system absorbs energy and will be a generator when the mechanical system supplies energy and the electrical system absorbs energy.

Think of an elevator with a counterweight. If the elevator/counterweight system is moving in the direction gravity wants it to move, the energy is from the mechanical system to the electrical system - a generator. If the elevator/counterweight system is moving against gravity, the energy comes from the electrical system and goes into the mechanical system - a motor.

 

[jraef]

You have to first define "motor". AC or DC? If AC, is it single phase or 3 phase? If 3 phase, is it induction or synchronous? If induction, is it wound rotor or squirrel cage? If it's DC, is it permanent magnet or brush type? Etc. etc. etc. There is no simple generic answer to your question without further definition of the device in question.

But to answer the second part of your question about "boosting the output", that's a NO. A motor is rated for a certain amount of mechanical output; torque at a given speed. In creating that torque, the motor consumes energy and in that process, uses current. Current use creates heat, so the "rating" of a motor is mostly based upon the amount of current it can handle safely, i.e. without damage to itself. When working as a generator, the torque will come from the external source, referred to as the "prime mover", but the POWER OUTPUT of the motor is still limited by the amount of current that can go through it safely. Therefore the maximum power it can produce as a generator is never more than the maximum power it is rated for as a motor. No "boost" in power (other than temporary overload situations).


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Rodmcm]

I have put in over 8 MW of motors as generators ,usually in the range of 500kw to 1MW, including synchronous motors into hydro schemes. However there are some issues that you need to consider

a) Any induction motor running as a generator does so with leading power factor. That is to say the VARs are 'absorbed' by the motor and kW exported. This means that if the generator is connected to some plant that is connected to the grid then the kW absorbed from the grid will decrease by the generator output but the VArs will increase as the load and generator both 'absorb' VArs. Hence a lousy pF on the apparent load from the grid that may cost you on tariff charges.

b) Induction generators do not need a governor as their speed is set by the grid. The energy is applied till the induction generator gets up to grid frequency and then the breaker is closed. There will be a current inrush almost as large as a motor staring DOL but for less time as the grid does not need to accelerate the load from stop as with a motor.

c) Usually the energy in is set by a simple actuator to the water inlet in the case of a hydro. This means that if the grid is lost the generator will seriously overspeed until the energy is dropped off. Motors usually are not constructed for this overspeed for any length of time and thus can be damaged.

d) Deflector plates on Pelton wheels is the most effective way of getting rid of the energy

e) When buying an induction generator new the mechanic issues around speed are addressed in the rotor and bearing designs

f) You cannot easily supply an isolated load from an induction generator. All of the ones I did supplied to a grid

 

[Rodmcm]

Now to answer your question. Usually the kW output is near or the same as as the motor nameplate kW, usually the issue is the efficiency of the turbine it is connected to. I had one client who ran the hydro to the limits of the temperature rise in the stator, which gave him another 5% over nameplate rating on cold nights

 

[cranky108]

I once did a study of an oil field, where the pumpjacks were not counter balanced correctly, and during the up stroke the AC induction motors (three phase) were providing power to the jack, and during the down stroke the motors were providing power back to the grid.
It was hard to get a good reading on the power flow as it kept changing (I had an analog meter), and the same with the voltage, as for some reason several pumpjacks seemed to be syncronized that made it that much harder.

So yes some motore can generate back, but the winding were designed for providing power, and may overheat if used as a generator near the limits (ratings).
It was not made to be a generator, so it will have different heat limits (has anyone look at that).

 

[waross]

Typical full load speed 1760 RPM (1800 RPM minus 40 RPM).
As the motor is over-driven at an increasing speed, the line current will drop until at 1800 RPM the only current will be magnetizing current.
As the speed is increased further, the line current will increase until at 1840 RPM (1800 RPM plus 40 RPM) the line current will be about full load current.
When motoring the magnetizing current is 90 degrees displaced from the real current.
When regenerating the magnetizing current is 90 degrees displaced from the real current.
The parameter driving the real current is the slip RPM or slip frequency. Plus 40 RPM slip is almost the same as minus 40 RPM slip.


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

 

[cranky108]

At the top of the thread it said to speak slowly. But what most of us do when someone does not understand, we just speak louder (like that is suppose to help).

It's hard to speak at a lower level, after all the years of doing this profession. So excuse us if we fail.

 

[emilygrey]

A motor–generator (an M–G set) is a device for converting electrical power to another form. ..so using it as a generator can be given a try.