Ohm's Law only refers to pure resistance (conductor at constant temperature in its original form). It certainly doesn't apply to a motor.

What you can do with them very much depends on the spec. of the controller and what sort of motors they are.

Russell

DC electric motors act as generators. The speed increases until the generated Voltage is the same as the applied Voltage.

Starting current is usually enormous.

Once up to speed, power consumption is down to load and making it hot.

Cheap motor controllers depend on turning it on/off many times a second and varying the on/off ratio.

That introduces an interesting 4th factor to the equasion, Inductance. This is the thing that blows up your electronics.

If you want 18 Volts, why not use three 6 Volt batteries?

Protection by limiting current is usually achieved with a fuse.

A 100A controller can handle 100A before it starts to go wrong. It doesn't insist on 100A flowing through the motor.

Volts times Amps equals Watts

746 Watts = 1 horse power

So 18V x 100A = 1800W = 2.4hp

Ron you're thinking about this from the wrong end. Your controller will control the VOLTAGE to your motors which will then draw whatever CURRENT they need at that voltage setting. If the motors are running light – ie not driving anything and you set the voltage to 24 volts then they will draw little current. If however you were to lock the shafts and then apply the 24 VOLTS they will try and draw a lot more CURRENT. What blows up in the latter case could be the controller if the motors try to draw more than 100 amp or if could be the motors burning out if the controller can supply all that the motors demand.

Without knowing what current your motors will draw at full load it's pretty much impossible to say what size controller you need, you need to do more homework and supply more information. If you're worried about the motors demanding too much current from the controller then stick a fuse in the lead to each motor and a master fuse in the lead from the controller. With car batteries as the power supply then fuses are essential in any case – have you seen what happens when you short out a car battery?

I have answered this many times, I find it best to start uncomplicated and then progress.

I'm betting Ron has a wicked idea for a killer, arena robot and has bought a bunch of Amp-Flow, rare earth motors to play with.

+1 to that. At least something to go on, even a photo would be useful.

The problem is there are many different types of motor and many different controllers. You need to identify what you've got. The description 'a non adjustable 100 amp motor controller' doesn't help at all.

'Ideally I would want the voltage to be 18 volts and have the amps adjustable or if its simpler set to a maximum of 60 amps.' – yes, no and maybe! A better way is to control the motor by pulsing power into it at different rates. Rather than controlling volts or amps directly, the controller reduces or increases the amount of time power is made available to the motor. This has many advantages, not least efficiency.

If you can describe what you've got, and what it's for, I'm sure the motor experts can advise and explain.

Dave