Quote:
Originally Posted by kennybobby
Does it say if the 21 Hp is a continuous rating or for just a limited duration?
let's assume it is continuous. 21 Hp is about 15,700 watts, so the 120 VDC rating let's us calculate the current as 130 Amps continuous for the armature.
From your measurements at 12 VDC the field resistance is about 1.7 ohms, so at 120 VDC it will draw about 70 amps. i would not want to energize the field all the time, only when the pedal is pressed.
A brush-type DC motor has a fairly linear response, so scaling up the voltage to 120 from 12 will increase the no-load speed x10, so the motor voltage constant is 7700/120 ~ 64 rpm/volt. The motor torque constant is 1/7th of the voltage constant, or 9.2 amps per ft-lb. Dividing the 130 amps by the torque constant indicates that the motor would make about 14 ft-lbs of torque at 7700 rpm on a 120VDC power supply.
As the motor is loaded the current will increase and the speed will decrease. When the motor is stalled at 120 VDC the armature currents could go up to 2000 amps and generate 200 ft-lbs of torque. My guess is that it is an aircraft starter motor, not intended for continuous operation. As such it is probably not reversible, and probably not really suitable for an electric vehicle. Does it have any sort of cooling fans/vents/ducts--that would be another clue?
It may still be possible to run this motor in an EV by using lower voltages with high capacity batteries, e.g. Lithium cells, depending upon the transmission. Good luck to you.
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Thanks for the response! Your posting uses some assumptions, but I think it will help me (and I'm sure others) to estimate some parameters for used DC motors.
The motor has no markings. I'm told (on the DIYelectricCar forum) that it is a 'jack & heinz #g23 aircraft generator'. I'm having trouble finding out the specs for that model online.
So I don't know if it is actually 120VDC. But given that assumption, I follow some of your calculations.
Where did you get the 1/7 of the amps per foot-lb of torque come from? Is that a rule of thumb? Does it use the motor diameter? Did I miss the link to some of the numbers in your post?
2000 amps at stall with 200 foot-lbs is great, but again I don't understand where those numbers came from. I'd like to, though.
There is no cooling fan. No ducts that are visible. I'm told that the jet engine (in the plane that used this motor/generator) produces a BUNCH of compressed air and that the air can be 'bled' to cool the motor. If it is run hard, a *LOT* of air would be required. At EVCON 2012 I was told that some of the used aircraft starter/generators can require so much air for cooling that the compressor or blower required to keep it from melting has more horsepower than the electric motor. I have not been able to verify this information so far.
I'll be monitoring the motor temperature as well as the armature temperature quite closely.
Yes, I have been told that this motor is not likely a great candidate for an electric car. It's likely good enough for me to use for a bit of comparative testing against a surplus AC motor.