I haven't tried using a treadmill motor controller and cannot be specific but hope this is of some help.
Lets start with some calculations from your motor DC specs:
Power = 120 volt x 21 A = 2,520 watts
1 HP = 745 Watts so divide by 745 gives 3.38 HP?
Calculating backwards from 2.5 HP
2.5 x 745 =?1,862.5 Watts
Divide by 120 gives 15.5 amps at full output.
This discrepancy probably means that the motor can handle up to 21A when under load.
When running at its maximum speed of 7000 RPM the armature is acting like a generator creating a 'back-emf' ?or voltage of -120 volts opposing the 120 volt input and that is why it can't run any faster than 7000 RPM. (otherwise it could be a recommended limitation depending on the centrifugal/centripetal forces that the armature can handle without flying apart.)
My Pulse Width Modulator circuit displays amps and volts and I have a tachometer as well. The data from that may help answer your question. Under heavy load the motor slows down, the back EMF decreases at lower RPM and input voltage is able to push through more amps. Eventually as the amperage increases the driver circuit will cut off the power supply to avoid burning out its components. ?In addition to that, the PWM circuit tries to maintain power output by increasing average amps as it switches on and off.?
I think the solution is to alter the gearing/pulleys to allow the motor to run at higher RPM.
Note: the potentiometer used in these circuits operates on 0 to 5 or 0-10 volts, so is quite safe. It is still a good idea to have it grounded and/or insulated well.
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Evan
Lathe: 1955 Boxford Model A with screw cutting gearbox, power feed with several accessories, hand tools and a pillar drill press.
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Evan