Overvolting brushless motor
 

Can anybody point me to some documents explaining if, how and why I can provide a BLDC motor with higher voltage than rated without damaging it? I know it IS possible, but I want to understand what actually max voltage depends on.

thanks.

What motor do you have?
A motor has to turn DC in to AC at some point, that is part of what the brushes do, in a brushless "DC" motor there is often an inverter or switch of some sort that switches the polls of the electro-magnet, that circuitry can only handle so much voltage before it burns out, so that is what is going to fail first.
As with most electronics that you want to run more power into, the limiting factor is often heat, run to much power in and at some point something will melt or burn up, with brushed motors you can run them faster as long as you keep them cool, up to the point where they rip them selves apart.

What I need to know is how much voltage my motor controller can send to the motor without burning it, to figure out if I can allow my electric scooter to go faster than 30 mph.
The scooter currently has a 1800W/48V motor and a 48V/50A controller: could I for example replace the controller with a 60V/50A? How can I know IF I can and WHY I can?

Thats kind of like saying my current 4 cylinder puts out 50 hp and I need it to put out 75 hp.

Temperature is your main problem, as long as it stays cool the only problem you could face is wiping the charge from the magnets, bearing damage from increased rpms or throwing the windings. :eek:

No, I think the first thing that is going to happen is you could fry the switch that the motor has instead of brushes, the windings are the most durable part of the motor, so you shouldn't be at risk of blowing the motor up unless you are running it at higher then normal voltage without a load on the motor and even then permanent magnet motors are harder to over speed then some other types of motors.

But if you are only going up to 60v then you should be fine, unless you are using lithium batteries, lead acid batteries will have some decent voltage sage, so at full power with lead acid batteries the motor might only be seeing 40v, but bump the voltage of the battery up and at full power the motor will be seeing closer to 48v, in theory you could have a 200v battery pack and your speed controller is going to be what limits the voltage going to the motor, so the point that the motor would burn out would depend on how much throttle you gave it and in turn how high the voltage got that was going to the motor.

What about raising controller voltage output from 48 V to 60V, but limiting current by an additional resistor?
Current is what actually "burns up things", so if I could provide 60V/25A rather than 48V/25A, maybe I could obtain much speed (at obvious cost of less power/torque).

How do I determine the internal resistance of a motor?

If you use a resistor, you will drop current AND voltage. The resistance of all motors is very low and can be measure with a high precision multimeter. You don't want to use a resistor to lower voltage because doing that will waste battery power by turning electricity to heat in the resistor. If you use a resistor to do what you want, the current will also drop so the net effect will be NO INCREASE in range or speed. I've overvolted scooters before so I can tell you that you can probably overvolt by just adding 1 more battery without even using a new controller. Make sure your motor and controller are properly cooled. Sometimes, the manufacturers enclose the motor and controller in the vehicle body thus preventing air flow. You will want to put some holes in the body paneling which face the direction of air flow if this is the case.

Current is NOT what actually "burns up things". It is power. Power = current x voltage. P=IE=IR^2

"at obvious cost of less power/torque" - here you are referring to the trade off between torque and speed. This trade off only happens when power use is kept constant. In your scenario, you are raising power usage by 25%. You will see an increase in both speed and torque if you raise power.

Brushless motors have permanent magnets inside. If too much current through the motor, the permanent magnets will be demagnetized. When the magnets are demagnetized, the motor is junk. It happens fast, magnets can be demagnetized in microseconds.

If you want more out of a brushless motor, give it more volts *and* modify the drive train gear ratio so it spins faster compared to the output. That way the amps will stay within limits, and everything will be fine, untill you reach the cooling limits..

theres 4 limits to a brushless motor:
-rpm, too much, and bits will fly off. this is typically much much higher than most brushes motors, because no brushes to worry about. bigger diameter motor means lower speeds though.
-magnets, too many amps, and you'll kill the magnetic force. but generally, try to stay far away from that region, as the motor efficiency drops off a lot even before then.
-heat, too hot, and you'll kill the magnets and/or fry the windings, or both. Cooling is required if you push more power through the engine, even if its very efficient.
-volts in absolute sense: the wire isolation might not stand up if you go really far with overvolting, it might arc through. But for a couple volts extra, that shouldn't be a problem.

Endless-sphere is a good forum for electric bikes.
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Endless-sphere.com • Index page
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Many guys there are running 72v in order to go faster. Most of the controllers have 100v mosfets in them. If you increase the voltage you have to program a lower max current. Power fries a motor and higher voltage makes it easier to draw power.