Should I use both a MOV & a flyback diode for a very high power DC motor (7560W)?
That MOV won't protect other circuitry from back EMF from the motor.  The diode will practically short the MOV. (The diode will start conducting at about 0.7 volts while the MOV can only start conducting above the 84 volts supplied to the motor.)

I wouldn't suggest using a MOV to protect against back EMF, because a: they're non-polarized (the -84V vs -0.7V issue) and b: they wear out and will eventually fail.



The diode will be conducting very often and stealing 0.7V*90A=63W from the motor when the PWM level is zero.  It may sound like a lot of power, but it's below 1% of what is fed to the motor.

That diode would probably need to have a fan for cooling it, or at least a heat sink.

And it also needs to be rated for at least 100 Amperes. The inductance of the motor "wants" to keep conducting the same current and when the voltage is removed from the motor, it turns into a generator that pumps out the same 90 amps, but through the flyback diode.