Battery current and motor current [tour 4]

The battery current and motor current in a PWM controller can be very different.  Many people do not realise is that a PWM controller operates rather like a transformer (using the motor’s inductance) to deliver all the power it takes from the battery to the motor.

PWM controllers are not far short of 100% efficient – they cannot dissipate a huge amount of heat – so very little of the power drawn from the battery is lost in the controller.

The controller varies the motor speed by varying the voltage fed to the motor.

The motor draws whatever current is demanded of it by the mechanical loading. You can use our motor current calculator to work out how mechanical load affects motor current.

The effect of the above is that the motor current into a sufficiently loaded motor can be far higher than the current drawn from the battery: at half full speed, motor current can be nearly double the battery current.

This is why, even is a fairly low value circuit breaker is fitted in the battery, motor current can still be very high, and why all controllers manufactured by 4QD have motor current limiting, to protect the MOSFETs.


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