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 is often quite unpredictable and badly specified.

The two things you could protect with a fuse are the motor, and the controller. But you must first decide what sort of fault condition you are likely to encounter and what effect this will have on motor and battery current.

Motor

It is generally not a good idea to fuse the motors: if the controller is correct for the motors and both are correct for the mechanical load, the motors should not get overloaded. Also it has been known for a blowing motor fuse to damage the controller because of the arc when a fuse blows.

Let’s say you want to protect the motor against a stall current. What is the motor’s stall current? Is this higher than the controller can give? It should be – part of the controller’s job is to protect the motor against stall currents. If it doesn’t then you have an over-size controller!

So it’s not the stall current you need to protect – so exactly what current  /time rating fuse or trip will give you protection where the controller won’t? There’s no simple answer – you need to understand the problem first. But selecting a motor fuse rating is not straightforward. In practise, it is often simplest to rate the fuse empirically: if you get nuisance blowing, use a higher rated fuse.

For relevant information, see:

  • Current Limit
  • Current requirements
  • Motor ratings
  • PWM controllers – how they work.

Controller

So how about a battery fuse, to protect both motor and controller? Again – what do you want to protect against?

Remember – motor and controller currents are not the same. Since the controller is very efficient, the motor power if very nearly equal to the power supplied by the battery. So motor volts times motor amps equals battery volts times battery amps.

So to define the maximum likely controller current, you need to guess the maximum motor voltage which will occur under the likely fault conditions. That is not easy. Probably your best choice is to measure the battery current under different conditions and chose a fuse which won’t blow under normal conditions.

As an empirical method, try a fuse or breaker in the battery rated at the same current as the motor’s nameplate (continuous rating) current. If it blows under real conditions, increase it until nuisance tripping stops. But we cannot guarantee that it will necessarily catch all faults.

Battery reversal

This is best protected against as explained in the section Reverse polarity protection. However, a fuse can, in most setups, give adequate and simple protection.

If the battery is reversed, current will be limited by the battery impedance, the battery wiring and the voltage drop in the MOSFETs of the controller and the voltage drop in the fuse or breaker you are fitting. In lots of set-ups, the battery wiring is sufficiently thin to restrict current so that a fuse will probably blow before the MOSFETs, and this is probably the most significant reason for fitting a supply fuse. It needs to be as small as possible but large enough so that nuisance tripping does not occur, but its exact value is impossible to state as it depends on your wiring and on your battery.

As a rule of thumb, a 25A fuse is about right for DNO-5, Porter 5 and similar controllers. For Porter 10, and DNO-10 a 40 amp fuse is indicated (mainly because higher value blade fuses are difficult to get).