As long as your controller can handle the voltage, the small difference between 72 volts and 80 volts probably isn't causing the problem. I'd leave the pack as it is (I'm jealous, BTW... still on lead here!)
As far as the controller goes, it is unlikely to be the problem as well. Stock controllers put out about 350 to 400 amps for a short time (starting from a stop, floored) but motors this size generally taper off how much power they can use as they accelerate. At top speed, it is likely you are using less than 130 to 150 amps. Hot, but probably not enough to cause problems.
The more likely problem is that the motor is overspeeding. The GEM transaxle has an overall ratio of about 8.9:1, so at 35 MPH indicated the motor is turning about 4,500 RPM - about the fastest I'd recommend turning these motors for long durations of time. For reference, at 45 MPH, the motor is turning about 5,800 RPM - Way too fast for these small stock brushed motors.
Many GEMs have been modified to operate over 35 MPH indicated (they came from the factory with a 25 MPH limit). At higher speeds, the brushes in DC motors tend to 'float' which causes an arc to form between the brush and the commutator. This arcing causes a tremendous amount of heat, and is likely the cause of your motor(s) burning out.
One possible way to help with the brushes arcing too much at high speed is to increase spring tension. The other is to reduce the speed of the motor - keep it under 35 or so.
Most of these motors are somewhat under-sprung for traction applications at about 2 or 3 PSI tension. Brush pressure should be 6 to 9 PSI for EV applications. It seems counter intuitive that higher spring pressure would help things last longer, but one of the main killers of brushes and commutators is arcing, not excessive tension.
Hope this helps!
--- On Fri, 9/30/11, art maurice <amaurice@...> wrote:
From: art maurice <amaurice@...>
Subject: [eaasv] New motor
Date: Friday, September 30, 2011, 3:19 PM
I bought a GEM EV and then went online to get an upgrade kit. I also had Richard Hatfield's company install Li batteries up to 80 volts. I burnt out the original 5 HP motor but a 7.5 HP motor came witht he upgrade kit. After about 6 months I burnt out the 7.5 HP motor. The GEM service guy said my batteries were too powerful for the motor and suggested I shrink the battery pack.
Then I was telling this story to a friend who electrically inclined and he said a 10 HP motor should be able to handle that battery pack. He qualified it by saying not to "punch it" when I start the car because I could damage the transmission.
My question to you all is: Will a 10 HP motor solve my problem or should I shrink the battery pack?
Thanks in advance,