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Squirrel Cage Motor As A Generator?


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#1 bnichols04

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Posted 11 March 2009 - 10:59 PM

helllo this is my first post to this site i hope this is the right place for this question. i have acquired a huge collection of squirrel cage electric motors and i was thinking that i could make a generator out of these but i need more information about how to go about it. I have about 40 of them ranging from 110v-430v motors. rmp rating from 730-3400. HP ranging from 1/8-3 HP. I have read that i can use them as a generator as long as they are spinning faster than their "rated" speed. How much faster can i spin them before i damage the motor? Do i need to add more "running" capacitors to the motors to use them as a generator? If I use a 230v motor is that the voltage it will output? thanks for any input!


#2 marke

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Posted 21 March 2009 - 08:10 PM

Hello bnichols04

Welcome to the forum.

If you connect an induction motor to an electrical supply, it will spin at close to synchronous speed. (The synchronous speed is a function of the frequency of the supply and the number of poles of the motor)
If you apply a load to the motor shaft, it will slow down slightly and develop torque to spin the load, and draw power from the supply.
If you feed torque back into the motor, it will still try to run at synchronous speed, it will speed up slightly and will generate power back into the supply.
For example, if you connect your motor to the supply and attach a windmill to the shaft, it will drive the windmill at close to synchronous speed and act as a fan. If the windmill is placed in a strong wind which tries to make the windmill run faster, it will prevent the windmill from speeding up by transferring energy from the windmill back into the supply.
If the torque developed by the windmill is greater than the torque capacity of the motor, it will not be able to hold the windmill and the windmill will accelerate.

The torque must be limited to the maximum torque capacity of the motor.

Best regards,
Mark.

#3 bnichols04

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Posted 22 March 2009 - 11:55 PM

ok i think i understand so as long as i keep the motor running higher than the rated speed it will produce electricity? will that have problems with overheating the bearings if it is running that fast for long periods of time? so my motor is a 110/220v motor rated at 750 rpm if i run it at 800-850 then it will produce 110v or 220v electricity? do i need to wire capacitors to the motor to use it as a generator? thanks for the help

#4 marke

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Posted 23 March 2009 - 05:59 PM

  1. You need to supply the motor with torque rather than speed, that way it will find it's own speed which will be just above synchronous speed.
    For example, if you have a four pole 50Hz motor with a synchronous speed of 1490 RPM, it has a full load slip of 10RPM. If you drive this motor with the equivilent of full load torque, it will over speed by the same slip. Therefore it will run at 1500 + 10RPM = 1510RPM
  2. You need to have the motor connected to the supply and you do not need to have capacitors connected. There are schemes where you run as a standalone with the motor not connected to the supply and capacitors used to provide the VARs. There are varying reports on the effectiveness of doing this. If this was really reliable and effective, generators would be commercially made this way.
Best regards,
Mark.

#5 Clayts

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Posted 26 March 2009 - 05:08 AM

Now with the tech stuff taken care of.. If your worried about bearings, pop the shields and put in some polyrex ex (high temp grease) or replace bearings with C3 type. It's because of questions like this that I frequent this site...good luck. It would be great to see a progress report.


#6 marke

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Posted 26 March 2009 - 05:45 AM

If the motor is connected to the supply, then the speed will only be marginally higher than full load speed and there will be no impact on the bearings.
If the motor is not connected to the supply, then the VARS required must be supplied by capacitors and there is no control over the output voltage and frequency.

Best regards,
Mark.

#7 Clayts

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Posted 26 March 2009 - 07:24 AM

You lost me on the VARS?

If you lose residual in rotor how can you flash ..or remagnetize.


#8 Clayts

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Posted 26 March 2009 - 07:40 AM

Sorry, yes..I understand VARS, but can you explain in more detail? ie "if motor not connected to supply"
and I guess you would simply run as motor to re-induce magnetic field into rotor!!


#9 foontoon

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Posted 25 July 2010 - 10:28 AM

This forum and thread came up on a search I did so I joined and this is my first post.

I too am interested in the feasability of using a squirrel cage fan and motor to prouduce enough electrcity to run a roof top A/C on a motor home. I think it might draw as much as 18amps on start up. So I may need a heavy motor or more than one.

The motor home is a Prevost bus that is as aerodynamtic as a brick and get's 7 mpg. The idea is do make 110 power while on the Hwy w/o running the generator. I'm not worried one bit about the added drag on his already flat front bus that weighs 45,000 pounds. I'm not an electrician to know how big a unit I would need. At Hwy speed, their should be more than enough air flow to get the motor turning. I need to know if there is a way to limit the speed of the RPM's in some fashion to prevent it from overspeeding and burning up. I fire on the roof would be difficult to exthinguish.

Any ideas on how doable this would be ?

Jeff




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