We’re often asked to talk about brushed v brushless motors so here goes…..[we’ll put some pictures in here in due course but for now it’s just words].
Brushed motors have been around for ages, they are;
- Simple – put a voltage across them and off they go.
- Less efficient – typically around 75%.
- Noiser, both physically and electrically.
- Brushes will wear out over time – typically 1000 – 3000 hours.
Brushless motors on the other hand tend to be;
- More efficient due to less friction – around 90%+.
- Quieter, both physically and electrically.
- Longer lasting – typically 10 000 hours.
- Able to run at higher speeds.
- But – they need a more sophisticated controller to make them move.
So why do brushless motors need a more complicated controller? In a brushed motor as the armature rotates, the brushes and commutator switch the voltage between the successive windings, making in effect a mechanical switch. In a brushless motor this mechanical switch is not there and thus the switching has to be done electronically by the controller. To do this switching the controller needs to know about the position and speed of the armature, it can get this information either from Hall sensors on the outside of the motor or by sensing the voltage induced in the stator coils [sensorless mode].
Multiple motors. A controller for a brushed motor does not care how many motors are connected to it so long as the overall current rating is not exceeded, our current record is a single controller driving eight motors. But this does not apply for brushless controllers, because the controller needs to know about the position of the armature a single controller can only drive a single brushless motor. Note, there are dual channel brushless controllers that can drive two motors, but the general principle that you cannot just add motors in parallel to an existing controller still holds true.
Motor types. A brushless controller needs to be told quite a lot about the motor it is working with e.g. inductance, resistance, pole pairs, threshold speed etc. This is not the case with brushed.
Regenerative braking. For a brushed motor regen braking is relatively easy and done by quite straightforward switching within the controller. But with brushless motors more sophisticated measurement and switching is required, and our experience is that regen is difficult to do well without significant investment in software.
Why do speed controllers sometimes fail?
Here at 4QD we go to a lot of trouble to make sure our controllers are reliable. We make sure we have enough capacitance, we use the best mosfets we can find, we fit substantial heatsinks, we minimise the susceptibility to radiated and conducted noise, and we conduct extended life tests by running controllers on an automated test rig.
And yet very occasionally a customer will get a seemingly random failure. Actually there’s a clue in that sentence, in five years we’ve not had a single mosfet failure on our test rigs, the reason for that is that we take a lot of care with electrical noise suppression. Motors are electrically noisy things, just look at the back of an old electric drill to see the sparks. We’ve written a full article in our knowledgebase about motor noise and how to prevent it, it also describes other sources of noise, and some detail on mosfet failure modes. It’s well worth 10 minutes of your time reading this and taking the necessary steps to make sure that your controller doesn’t suffer an untimely end. If you just want the short “what do I need to do?” version have a look here.
We’re now shameless about plugging our suppression components. They don’t cost a lot but since we introduced them we have seen the field failure rate of our controllers drop significantly.