Electric motor voltage/current questions

[rottie]

Engineering questions/ golf cart electric motor design....
It has got me thinking as I am applying ~60VDC to a motor (AMD7124) that is only "rated" for 36-48Vdc.
1. At what point is too much of a good thing? 60V/72V/90V/144V/???
Yes, I know it will spin faster with a higher voltage, in my case 2800-3000rpm so we should be good there. From other threads 6000rpm is a max safe? limit or something less????
2. I will be limiting the current to 400-500 amps. What is the max current of the AMD7124 say continuous and 30 seconds? or is it more based on armature/field winding temps or brush capabilities?
3. Should I limit input power i.e. higher voltage/less current or lower voltage /higher current????
4. What is the efficiency of these series motors, If I had a data sheet like the AC conversion motors, it would answer a lot of my questions.

[JohnnieB]

1. DC traction motors convert Watts (Volts x Amps) into torque, so they will operate over a fairly broad range of voltages, but there are limits.

I suspect running a motor rated at 36V motor at 72V, might be pushing the envelope a bit, but I might be wrong.

As for Max RPM, it depends on how well the armature is constructed and balanced and the RPM limit of the bearings used. That said, 6000RPM is probably a safe max for a stock golf cart motor and likely conservative for a high quality aftermarket motor. Your AMD7124 is a high torque motor, which sacrifices RPM for low end torque, so RPM shouldn't be an issue.

FWIW: I have tiny tires (17" tall) and my motor spins at about 7,000RPM at my cart's max speed (~28.5MPH), but I seldom drive faster than 25MPH (~6200RPM).

2. Heat kills motors and their heat rating is based on the thermal characteristics of the insulation on the wires in field and armature windings. The max sustained temperature for Class-H insulation is 356°F, but most motor don't have temperature sensors embedded in the windings, so you have to monitor the temperature at the motor's case and the max recommended temperature when measured at the motor case is 239°F.

Thermal damage is cumulative, so the hotter you run the motor, the quicker it dies.

3. High voltage/less current. Heat is generated by the square of the amps passing through the motor that are not converted into torque (W = I² x R).

4. The efficiency of motors varies over the RPM range they are operating at with an efficiency as low as 50% at the upper and lower extremes and up to about 80% to 85% in the mid-RPM spectrum.

Good luck find RPM vs Torque data on motors.

[rottie]

Thanks JonnyB.
That give me some more insite into these motors.

2. I might add thermal couples just to monitor.

4. That what I was afraid of on the low side, thus lots of heat.



Woohoo!!! UPS just arrived with my goodies.
The motor, controller, fuse, contactor and relay reverser.
now time to get to work.....

[JohnnieB]

2. That is want I did. Here are a couple pictures of my stock PDS motor.

There is a time lag between winding temp and case temp and I have another DMM that has a Min/Max function, so I let it keep track of the max temperature reached.

4. Calculating your tire height from the numbers in your signature block, I get about 20.5", try to keep the speed above 4.9 MPH (1,000RPM) and it should run reasonably cool.

Also, running at ~60V, it will run cooler than at 48V or 36V.

Also, with Li batteries, you'll be a couple hundred pounds lighter, so the workload on the motor will be less, which in turn will run cooler.

When you get it together, please post your measure tire height, max seed and motor temperature.

[Volt_Ampere]

Heat in a DC motor is proportional to the RMS of the Armature Current. Using higher voltage will give you more current capability and higher motor speed capability but if you limit the current, the motor will not get any hotter.