Battery Current and Motor Current

The battery current and motor current in a PWM controller can be very different.

The PWM voltage output from the controller is a square wave, shown in red below. This voltage is applied to the motor, but remember that the motor is actually an inductor and this means that the current through it cannot change instantly, rather it will increase at a rate determined by the inductance of the windings, in the diagram we have shown the current through the motor in black [it’s actually a bit of a curve but we’ll skip that for now].

Now also remember that the controller has a large smoothing capacitor in it, for the sole purpose of smoothing out these sharp current spikes. Energy from the capacitor fills in the gaps and averages out the current that is drawn from the battery, we have shown the battery current in blue. As a rule of thumb, the battery current can be considered to be the peak motor current multiplied by the duty cycle percentage.

The effect of the above is that the motor current into a loaded motor can be many times higher than the current drawn from the battery.

This is why, even if 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. In the Pro-100, 160, and 360 controllers it is the motor current value that is monitored and which will trigger a C2 alert if exceeded. We are evaluating introducing a battery current alert in future software, as this may work better with the battery management systems of LiFePo4 batteries.

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