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 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.
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.
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
- Start with finding out your motors nominal maximum current.
- Choose your motor cable size to be rated higher than the maximum motor current.
- Choose your battery cable size to be rated higher than the sum of all the motor currents.
- Choose your battery fuse to be below the rating of your battery cable.
- Set your controller to limit the motor current to 2 times the sum of all the motor currents*.
- 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.