PM DC MOTOR AS GENERATOR

[FredRosse]

I have made a home generating system (Diesel Cogeneration) where a small Diesel engine directly drives a simple single phase AC induction motor. When the Diesel engine drives the motor above synchronous speed, it becomes a generator, delivering power to the grid or my house, and when the generation exceeds the house load, my electric meter spins backward, giving credit for the excess generation. This unit ran for 9 seasons, with my electric bill a few dollars a month, rather than several hundred dollars per month. The electric company warned me to never get a negative meter reading, as this automatically kicks that info to the police, under suspicion of electric theft. It was easy to manipulate loads such that this never happened.

I have a similar question relating to using a PM DC MOTOR AS GENERATOR. In my electric boat, the PMDC motor draws 30 amps, but with a tailwind, reducing the propeller load, amps tend to go downward. Continuing this process (lets say I get a tow from another boat, which spins my propeller faster) will the amps eventually go to zero, and with further increase of driving speed will the PM DC motor reverse the amp flow, and become a generator, charging the boat battery?

If so, will the efficiency as a generator show characteristics virtually the same as the motor duty efficiency, or am I missing something?

Thanks in advance for reply

 

[Skogsgurra]

I think you are right. Just test it.
It is called regenerating when the DC motor is used as a brake and fed via a thyristor (SCR) rectifier. In your case, it is even simpler. As speed goes up, the motors brakes and the brake energy goes to the battery. There may be a few "practical details" to care about if there is a speed controller between battery and motor, but the idea is sound.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[FredRosse]

The motor is rated at 24 VDC 30 Amps, and I have two deep cycle 12 Volt batteries in the setup. A simple relay system (two DPDT relays) switches to have the two batteries in parallel for 12 Volt operation (low speed), or arranging the two batteries in series for 24 Volts (hi speed). With the two relays I have forward slow, forward fast, reverse slow, reverse fast. While not as fancy as the variable speed setups for electric boats, the cost is of this control system is low, and performance is fully acceptable for use on the waterways.

 

[Skogsgurra]

It also simplifies generator operation. Just do it!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.