TXT Field Voltage?

[scarbone3]

I am troubleshooting failures of a couple 1206MXM speed controllers on my 36V TXT PDS cart with motor part # 73445G02. The mofsets failed twice in the speed controller so I believe it is associated with the high amp motor side of the circuit. I am starting to troubleshoot now.

I have the golf cart on stands and I am running the motor to see the voltages. The armature volts seems good and goes from 2VDC to 36VDC when the pedal is depressed, approximately. The field voltage goes to about 2.5-3VDC with the pedal halfway depressed, then starts climbing and goes up to 7VDC fully depressed. The manual I'm working from says 2.7VDC +/- .5 is acceptable. I'm not sure if this manual is applicable to my motor though.

Should I expect the field voltage to go up to 7VDC or is that excessive? Appreciate anyone's help!

[JohnnieB]

The 2.7V +/- 0.5V is with the rear tires off the ground and the pedal on the floor. Field voltages for less that WOT or with the tires on the ground is different, higher since the motor would be at the RPM limit imposed by the 1206MX controller with the tires off the ground.

You get max field voltage at low RPM and minimum field voltage at max RPM.

More correctly, field amps are what is actually being adjusted, but field volts are easier to measure than field amps.

Field amps tend to be roughly 1/10 of the armature amps and field volts is whatever it takes to get that number of amps to flow through the field windings.

For a stock cart motor the field amps tend to vary from about 4A to about 20A while aftermarket motors can be 3A to 30A (like my motor) or more.

Here's a couple field weakening curves (Field maps) for a CC stock sepex motor and for my D&D ES-76-51 motor.

[scarbone3]

Originally Posted by JohnnieB

The 2.7V +/- 0.5V is with the rear tires off the ground and the pedal on the floor. Field voltages for less that WOT or with the tires on the ground is different, higher since the motor would be at the RPM limit imposed by the 1206MX controller with the tires off the ground.

You get max field voltage at low RPM and minimum field voltage at max RPM.

More correctly, field amps are what is actually being adjusted, but field volts are easier to measure than field amps.

Field amps tend to be roughly 1/10 of the armature amps and field volts is whatever it takes to get that number of amps to flow through the field windings.

For a stock cart motor the field amps tend to vary from about 4A to about 20A while aftermarket motors can be 3A to 30A (like my motor) or more.

Here's a couple field weakening curves (Field maps) for a CC stock sepex motor and for my D&D ES-76-51 motor.

Thank you for that information!

So my testing is with the tires off the ground. With the pedal on the floor, I am seeing 7 volts at the field terminals of my controller. So it sounds like I have opposite of what I should expect. With armature amps increasing as the pedal is depressed to the floor, my field volts are increasing also from 2.5-7. Would that indicate a problem with the motor?

[JohnnieB]

I'm pretty sure field amps track armature amps, so field amps (and field volts) decrease as armature amps decrease, and armature amps decrease as motor RPM increases. Therefore, it might be the motor not spinning up as fast or as far as it should be doing, indicating excessive mechanical load, or the motor is sick.

Perhaps new brushes and bearing ins the motor as well as a thorough cleaning to get rid of the carbon dust.

Also, I'd install an Alltrax XCT48400-PDS user programmable controller that has protected outputs.