# induction motor as generator



## Duncan (Dec 8, 2008)

Hi Guys
Another silly question
If I spin an AC induction motor will I get voltage?

This is for my friends model train
We intend driving the train using 24v DC battery, motors and controllers

He wants to "make it like a real one" by having a small Honda engine (4Hp) which we will use to drive a generator and charge the battery

Can I simply use a normal 230v single phase induction motor? - Do I need to do something to "energize" to start or can I simply spin it up with the engine (using the engine's governor to control speed) and draw power to charge the battery?


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## major (Apr 4, 2008)

Induction motors will generate when slip goes negative. That is when the RPM exceeds the synchronous frequency set by the line. The line is providing the excitation. This is accomplished in vehicles using the battery and inverter. 

I have seen some induction generators which use an exciter winding or capacitor on one phase. These are able to run without grid tie. I don't think anybody does it with single phase.

Can't you use an alternator? 

major


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## gunnarhs (Apr 24, 2012)

Duncan said:


> Hi Guys
> Another silly question
> If I spin an AC induction motor will I get voltage?
> 
> ...


Hi as Major has explained you need (negative) slip. So you would "energize" the induction motor by applying a excitation voltage at frequency so that the forced rotor spin will be slightly higher (low negative slip for maximum efficiency). This is being used a lot in cheap windmill-parks but usually with 3 phase motors, the advantage of this setup is a very simple control that only follows the actual spin frequency.
When little or no wind, the excitation Voltage/frequency is switched off.
So you would use induction motors for variable or extreme load (wind) conditions.

In a case of a generator with almost constant frequency (?) it would not make any sense to use an (asynchronous)induction motor, you should use a (synchronous) Permanent magnet motor/generator or (synchronous) alternator with a stationary field


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## bigmotherwhale (Apr 15, 2011)

http://www.redrok.com/cimtext.pdf

i think this should help


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## dcb (Dec 5, 2009)

Duncan said:


> "make it like a real one"


FWIW I wondered what a "real" one does and as near as I can tell they use an alternator connected to the engine (like most generators already do) then they rectify that and feed it to an inverter and run multiple polyphase traction motors with it. 

The traction motors are often induction motors. This *may* be a case where a single controller can drive multiple induction motors without much fuss, or possibly one controller per side would work if offset torque was a problem in turns.

An alternator could possibly be rewound/rewired (or purchased) as 3 phase, and fed directly to the motor bank, and load controlled with the field and rpm with the throttle, if you are looking for quick and dirty. Induction traction motors would be the most forgiving in such a setup. I wouldn't try it with a pm alternator though, basically the "engineer" gets a throttle control and a field control and various current meters (some temperature meters too) and tries not to blow anything up. Except for the field buck controller, it would all be very analog.


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## electro wrks (Mar 5, 2012)

major said:


> Induction motors will generate when slip goes negative. That is when the RPM exceeds the synchronous frequency set by the line. The line is providing the excitation. This is accomplished in vehicles using the battery and inverter.
> 
> I have seen some induction generators which use an exciter winding or capacitor on one phase. These are able to run without grid tie. I don't think anybody does it with single phase.
> 
> ...


The only single phase application I've seen is in a brushless 5kW consumer motor generator:


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