Marathon series motor hot

[JB in PA]

Seems like my marathon motor is getting pretty hot but I don't have much reference or history to go by. Technically I've got an 84' Marathon but all that's left is the GE perpendicular to rear end motor and rear differential. All electronics are swapped out with CartsUnlimited conversion kit with 400 amp All trax controller, 48v with new batteries.

Its been nothing but super short rides which I felt around for temperature on and didn't feel hardly anything to note at first even with full speed short drives.
I just did a 30-45 minutes super slow speed "driving test" afternoon with my young kids letting them get the hang of driving it and the motor really heated up.
It was hot to the touch, temp showed up towards 150f on the main body of the motor. I assume that's not normal?

What causes a series motor to heat up? Amps I assume?


This drive that heated it up was all the kids driving super slow. The Alltrax is turned down for the kids on "user 1" in the software which works great but maybe I turned it down the wrong way?
I've got max speed at 30% (and reverse speed down). That certainly slows it down but should I turn the max amps down also?

Guess the question is what causes a motor to heat up in general?

Any reason slow speeds would cause a motor to heat up more?

Thoughts? Thanks guys

[JohnnieB]

Motors convert amps into motion and into heat. The percentage of amps converted to motion versus the percentage converted to heat is the motor's efficiency. The efficiency range of the DC traction motors used in golf carts varies from about 50% up to about 85% depending on the RPM and a multitude of other factors.

The faster a motor spins, the fewer amps is is capable of drawing, so the motor in a cart going slow can craw more amps than the motor in a cart traveling fast.

The heat generated by the amps increases exponentially while the change in amps is linear. (W = I² x R). In other words, when the amps are doubled, the heat generated is quadrupled.

You'll have to look at your motor's label, but I believe the GE grade Class B insulating, so the max temp rise is 80°C above an 40°C ambient. (144°F rise over 104°F ambient)

That is a max temperature of 248°F, but it is measured at the windings and has to be prorated down when measured at the case. I don't know what the prorating is for Class B insulation, but my guess is max case temperature should not exceed 200°F.

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For the Kid's setting, turn to max amps down to 250A or less. Maybe even lower since you are running a 48V system.

If you can, record a data log (continuous sampling rate) and find out what the motor's amp draw is when driving around at slow speeds.

[JB in PA]

Awesome, thanks Johnnie, I always appreciate your explanations with the background info (it's the engineer in me).

This is good info and all makes sense. My motor doesn't have any kind of tag (and I've had it totally apart and all over it) but sounds like I probably haven't done anything totally abusive to it but at least it makes sense why the slow speed crawling was hard in it.

Thanks for the 250 amp suggestion, I'll get it changed (I just slowed it down for kids with max speed settings and kept amps where they were).

I turned up their speed a bit also, it was really crawling and they can handle more, just didn't want to overdue it at first (I think they only had 28%)

Out of curiosity, what's the difference in the Alltrax software between Motor amps and Battery amps? I assume Motor amps is what it will give to the motor but what's Battery amps then?

[JohnnieB]

Most of the cart motors have Class H insulation and the case temp can go up to about 115°C (239°F), but there were a few early carts with Class B motors.

Of course overheating just shortens the lifespan and the motors will last twenty years or more.

Crazy as it sounds, motor amps can be higher than battery amps. The output of the controller is PWM DC (Pulse Width Modulated) and the pulses go from 0V to full battery pack voltage at about 18KHz for Alltrax controllers. (MOSFETs are either fully off or full on) The amps the motor is allowed to draw is regulated buy the duty cycle of the PWM and at anything except 0% duty cycle or 100% duty cycle, the motor is running on pulsed DC. When the pulse goes from pack voltage down to 0V, (MOSFETs turn off) a diode in the controller allows the collapsing magnetic field to generate amps and current flow continues in the winding in the same direction until the magnetic field is collapsed, or the next DC pulse occurs. The net result is that the average motor current is higher than the average battery current when the duty cycle is above 0% or below 100%. At 100% throttle, battery and motor amps tend to be the same, as does battery and motor voltages.

Here is a page from the Curtis 1204/5 controller manual that explains the phenomenon pretty well. It took me a while to get my head around how different segments of a series loop can have different amp flows. (The secret is there are two series loops sharing some common paths)

Also attach is another page that explains it also. Plus the entire manual in PDF form.

[coosa]

JohnnieB can give you way better info than me, but I can tell you that a motor case temp of 150 is not unusual on my PDS carts. I use a laser thermometer and after a 30 minute slow ride like you describe I would expect mine to be that hot, and wouldn't be surprised if it was a little hotter.

My wife burnt up the motor in my 84 Marathon by driving it very slow when gathering pine straw, but it still had the resistor system in it. She not only burnt the motor up, but caught the woods on fire and I came home to find the forestry department plowing a fire lane around my house. I expect she got it far over 150. :)

[JB in PA]

Great info,thanks again guys. Johnnie, great background as usual.

coosa, thanks for the real world backup that this isn't unusual. Hopefully my kids don't cause a fire though. :) I'm just hoping maybe my Alltrax settings can keep it as cool as possible to extend the life a bit on this old and very hard to replace motor. Everything else is working great but still feels weird that I replaced everything except the motor...my own fault for not realizing how hard these motors are to replace until I was knee deep in the project though.
Thanks again guys. updating alltrax now for kids tryouts later today.

[bronsonj]

If it were me and I was worried I'd do as Johnnie said and reduce the max amps and also increase the speed. When you are crawling it's tough on the drive system of an electric cart.

The other option if you can't do those two is to add active cooling... I pull a parade float during the annual Christmas parade. It's a heavy load and it's slow, so I added an array of 12v computer fans above the motor aiming down. I'm 48V so I put 4 in series and it made a HUGE difference in the case temp. I put them above the motor so as to keep them further away from splashing and debris.