Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - Anyone know how to figure out DC motor specs by measurements?

kennybobby · 10-20-2012, 12:14 AM

Oh yeah i see that i did make a big assumption---that it actually was a motor! lol

A generator will make a lousy motor--it may seem to run like a motor when you connect the power, but just because you can doesn't mean you should...

Nevertheless, for dc motors the speed is determined by the voltage and the torque by the current.
We use motor constants that are linear characteristics determined from dynamometer test data. Kt is the motor torque constant: how many ft-lbs of torque are generated per amp. Kb is the motor voltage constant (back-emf constant): how many volts per rpm. When using these units of measurement the Kt is equal to 7.040 times the Kb, so the rule of thumb is 7x .

You made some test measurements using 12 volts such as the no-load speed and current, and the zero-rpm stall current, which is the same sort of data that we measure on the dyno. From your data i made calculations of those motor constants and winding resistances, etc.

At the 21 hp (15kW) with 120 volts we got the 130 amps. From the 12 volts 770 rpm no-load speed we got the Kb, from there got the Kt. You did a stall test and got 200 amps at 12 volts. Scale it up to 120 volts and the current goes up x10 = 2000 amps, then use the Kt to calculate the corresponding torque, ~200 ft-lbs. Of course it may melt the wires at 2000 amps, but that is how much current it will draw at 120 volts.

A motor would be able to handle that current but i doubt the generator would. If you have no nameplate spec data then all bets are off--best to start at low voltage and work your way up. You need some sort of motor controller to adjust, regulate and limit the current into the motor, otherwise it may melt down, even with 12 volts, 200 amps is serious current and can do some damage.

10-20-2012, 12:14 AM	#4 (permalink)
kennybobby Southern Squidbillie Join Date: Aug 2012 Location: Heart of Dixie Posts: 97 Thanks: 50 Thanked 26 Times in 22 Posts	Oh yeah i see that i did make a big assumption---that it actually was a motor! lol A generator will make a lousy motor--it may seem to run like a motor when you connect the power, but just because you can doesn't mean you should... Nevertheless, for dc motors the speed is determined by the voltage and the torque by the current. We use motor constants that are linear characteristics determined from dynamometer test data. Kt is the motor torque constant: how many ft-lbs of torque are generated per amp. Kb is the motor voltage constant (back-emf constant): how many volts per rpm. When using these units of measurement the Kt is equal to 7.040 times the Kb, so the rule of thumb is 7x . You made some test measurements using 12 volts such as the no-load speed and current, and the zero-rpm stall current, which is the same sort of data that we measure on the dyno. From your data i made calculations of those motor constants and winding resistances, etc. At the 21 hp (15kW) with 120 volts we got the 130 amps. From the 12 volts 770 rpm no-load speed we got the Kb, from there got the Kt. You did a stall test and got 200 amps at 12 volts. Scale it up to 120 volts and the current goes up x10 = 2000 amps, then use the Kt to calculate the corresponding torque, ~200 ft-lbs. Of course it may melt the wires at 2000 amps, but that is how much current it will draw at 120 volts. A motor would be able to handle that current but i doubt the generator would. If you have no nameplate spec data then all bets are off--best to start at low voltage and work your way up. You need some sort of motor controller to adjust, regulate and limit the current into the motor, otherwise it may melt down, even with 12 volts, 200 amps is serious current and can do some damage.