Fuses and Circuit Breakers

Choosing fuses and circuit breakers is a difficult subject……

• Difficult because motor currents in most applications are very peaky.
• Difficult because the controller’s battery current is not the same as the motor current.
• Difficult because fuses ratings are often the carrying current: the blowing current and time is often unpredictable and badly specified.

The three things you could protect with a fuse are the wiring, the motor, and the controller.

Wiring

Wiring is the most appropriate part of an installation to protect with a fuse. The fuse current rating should be chosen to be below the rating of the cable in the circuit being considered.

Motor

It is not a good idea to have a fuse in the motor wiring: if the controller is correct for the motors and both are correct for the mechanical load, the motors should not get overloaded. Also, if a motor fuse blows, the resultant arc can damage the controller because of the voltage spike generated.

If you want to protect the motor against a stall current this should be done by the controller.

Controller

A fuse in the battery line is not guaranteed to protect the motor or controller. Fuses come in many different types and their rupture speed can vary significantly. In general terms a 100A fuse will withstand a current of 200A for two seconds or more. Yes there are quick blowing types, but these will still take a few hundred milliseconds to blow. Damage to the mosfets inside a controller can happen in microseconds, much faster than a fuse can react.

A rule of thumb

1. Start with finding out your motors nominal maximum current.
2. Choose your motor cable size to be rated higher than the maximum motor current.
3. Choose your battery cable size to be rated higher than the sum of all the motor currents.
4. Choose your battery fuse to be below the rating of your battery cable.
5. Set your controller to limit the motor current to 2 times the sum of all the motor currents*.
6. If you get nuisance blowing, use a higher rated fuse, or reduce the controller current limit.

* If this limits torque too much then increase to 3 times, but check temperature of the battery cable.