PM DC Servomotor

[rgroves]

There's a motor in my garage, been lugging it around for years.  It's a Peerless-Winsmith PM servomotor, rated at 230 VDC and up to 150A.  The plate shows a continuous duty cycle.   I bought it with a notion of making a diesel powered DC generator to extend the range of my 144 volt electric pickup.  Then I got tired of the electric pickup, got rid of it, and the motor went into the archives.

Do any of you electric motor guys know anything about this type of motor?  Is a "servomotor" able to work as a drive motor, or as a generator?
Looking forward to hearing what you can tell me.

Thanks

Russell Groves

[europachris]

That thing must be a monster!  I work at a company that manufactures PM DC motors up to 3hp or so, from 12V to over 200VDC.

Typically, the 'servo' motor application for PMDC motors is dead.  It has been replaced by either stepper motors or brushless AC servo motors.  All it takes to convert our PMDC motors to servo operation is to assemble an encoder or tach to the motor and drive it with a suitable drive that reads the output of the encoder or tach and drives the motor accordingly.

A PMDC motor uses permanent magnets for the field.  They can be turned into a generator simply by driving the motor with a power source.  As a motor, they are usually well into the 80% range on efficiency.  Not sure about going the other way.

The other issue is that the voltage output is totally dependent on the rpm of the motor.  There is absolutely no self regulation of any sort, so I'm guessing that the output voltage vs. load curve will be pretty ugly.  But, then, there might be a simple way to make a voltage controller that varies the rpm of the engine to keep the output voltage of the system where you need it.

Chris

[kyradawg]

Peace&Love , Darren

[Andre Blanchard]

I have a dc pm servo motor as well.

Would be neat to use one as a variable amperage battery charger with one of those stepper motors that Chris was talking about to control rack position/RPM/charging amperage.

Does anyone know of gizmo that can sense/use genny output amperage and battery bank voltage to put out a liniar 0-5V referance signal to drive the stepper?

Then the rpm's of the motor would vary to maintain the rate of charge you choose say c10 and as the bank reaches capacity the listeroid would turn slower and slower until finally shuting off when full capacity is meet.

Peace&Love , Darren

I have/do use a PM generator and engine to charge batteries.  The simplest way I have found to do it is like this.

  1:  Big diode between generator and the battery to prevent reverse currents, it just makes things go a lot smoother.  Use a big momentary switch to short the diode if you want to use the motor for starting the engine.
  2:  Rework the engine governor so that it works only to limit RPM to some safe value.
  3:  Now just set the throttle so that you have the current you want going into the batteries.  As the battery voltage goes up the RPM will increase a little and the current will drop a little, what you have is (within reason) a constant power charger.
  4:  If you want to get fancy, add a voltage relay to kill the engine when the battery gets to the voltage you set.

Since, for efficiency reasons, I consider and engine powered generator to be only good for bulk charging a battery pack I have never considered the need for any more control then that.  I would let the PV panels do the finish charge or if doing a equalizing charge just set the current at a reasonable level for the size battery pack and let it go for a while.   Actually I have never even connected up the voltage relay since I have only used the engine generator to charge the large house pack for the inverter, when the pack voltage gets to the set point on the diversion regulator the engine starts surging in time with the diversion load being turned on and off.  At that that point I go and shut the engine off.

  5:  If you wanted more you could connect a little geared motor to the throttle such that when the voltage relay pulled in the motor would run and pull the throttle back causing the voltage to drop and the voltage relay to drop out.
      This would give you a two stage charger, first stage constant power and second stage constant voltage.  Add a time delay relay that is starts the first time the voltage relay trips and you have automatic control over how long it holds the voltage constant before shutting the engine down.

______________
Andre' B.

[kyradawg]

Peace&Love , Darren

[kyradawg]

Peace&Love , Darren

[Andre Blanchard]

Andre, Killer post I too see the main use of the lister as being a prime mover for battery bank charging.

If you dont mind I've got a couple of questions.

1. Is the output amperage a function of RPM only regardless of battery voltage?

It should be a function of the difference in no load voltages of the battery and the generator, and the series resistance of the generator, wiring, and batteries.

Ohm's Law  I = V / R
V = Gen voltage - Bat voltage
   Voltages measured at zero current.
R = Gen resistance + Wiring resistance + Bat resistance
   Wiring resistance includes switches, fuses, ammeters, connectors, etc..

2. I plan to have a large enough bank (1560@24v or 780@48v) to allow for a maximun of 25% discharges. What is the best charging rate/rates to top off from a 25% discharge that will allow for the longest bank life?

Keep it under 100 to 200 amps and you should be fine.
With lead batteries you can charge at the same currents that you can discharge at as long as no cell voltage is greater then 2.4 volts and the internal battery temperature is less then 110°F.  Both of which are difficult to accurately measure in real time so it is not often done.  Also the faster you are charging the faster things can turn to shit when something goes wrong.

3. Where does the diversion regulator dump the load and is it a homemaid devise?

It is the regulator that came with my Whisper 1000 wind generator.  It is a brute force way of regulating battery voltage, just a metal box with 1400 watts of resistors wired for 12 volts turns on at about 14.5 volts and off at about 12.5 volts.  Which is not as bad as it sounds because when the load turns off the voltage jumps up to about 13.5 volts.  But it does make the lights blink as the inverter tries to keep its output voltage steady.
I have been going to rig up three or four more 100 watt diversion regs set at progressively lower voltages so that the system operates a little smoother but I never seem to get to it.

Peace&Love , Darren

The motor was an E-tek.
http://www.greenspeed.us/electric_scooter_e-tek_motor.htm

The engine was a single cylinder Wisconsin 3" bore 3.25 stroke.
The battery pack was 8 6 volt golf cart batteries wired for 12 volts, I believe it was running about 70 or 80 amps.
____________
Andre' B.