WRT using RC Model style Brushless motors in this application:
Tim Asked:
But now I have a further question – just confirmation if you like. If I connect the three wires of two identical drone motors, colour to colour, and spin one of them, the other will spin at the same speed – won't it? And swapping two of the pairs would reverse the drive? This would give us a further option, in effect replacing a too-convoluted cable run with three wires, driving a genuine vintage mechanical rev-counter head.
Just say Yes, it will work, or no, and explain. Thanks
Regards, Tim
Robert Atkinson Answered:
Your proposed "back to back" brushless DC motors driving a mechanical tachometer will work just fine. This is EXACTLY how aircraft remote tachometers work.
And also:
So if you have a Brushless DC motor without a controller you can use it as either a pemanent magnet 3 phase AC generator or a permanent magnet 3 phase AC motor. So connect 2 "back to back" , turn one fast and the other will turn at the same speed.
And IanP said:
A brushless motor needs a power supply to run (to generate the rotating field) and it would not works as a generator.
So, a lot of stories about..
Tim, Connecting a NORMAL RC Brushless motors back to back does not seem to work reliably.
Those motors have very powerful magnets working against a metal ( magnetic) stator, and the magnetically induced mechanical detent is immense. To overcome this, the electrically generated magnetic field needed must be greater that the permanent magnetic field. That requires a large ampere-turns ration of the rotor. These motors typically have from 4 to 50 turns of wire per winding – a 4 turn winding will need many amps to generate an opposing magnetic field able to repel the permanent magnets. A 50 turn will require less amps, but many more volts. Whatever the turns, the motor needs to spin quite fast ( a few 100 to a few thousand RPM) to generate enough voltage to drive sufficient current into the coupled slave motor. The resulting magnetic field in the slave may be sufficient to overcome the permanent magnet field, but the slave will simply sit there vibrating – it CANNOT catch up in RPM to the driven motor. And at low drive speed, the voltage is to low for the slave to be bothered. It does not work well at all. The slave motor cogs, sometimes starts, and generally jerks about. At higher drive motor speed the slave motor 'sometimes' gets going and then keeps pace – not deterministic I fear.
Generally, the type of master/slave generators have the slave as a coreless motor, that is, the rotor has magnets, but the stator has only copper coils, no metal stator, so there is NO magnetic detent.
The simple permanent magnet attraction between metal rotor/stator keeps the motor from being able to rotate at low voltage inputs, and when the field is strong enough, the rotor cannot catch up in speed, ie, it loses steps!
Robert:
The slave motor does not seem to start reliably at all…I know the A/C Tacho's well – we repaired many on old Harvard's at the flying school neat where I lived – The slave motor exhibited hardly any magnetic detent at all.
IanP:
See my post in the very beginning – These motors make excellent AC generators – a 200KV (RPM/Volt) motor of 70mm diameter, 70mm long, easily delivers 900watts at 5000 RPM.
I am trying to do a video that does not make us all sea-sick – using a few different RC brushless motors..Will post it soon.
Joe
edit – the usual darn typo's…
Edited By Joseph Noci 1 on 05/09/2018 13:51:40
Edited By Joseph Noci 1 on 05/09/2018 13:52:01
Edited By Joseph Noci 1 on 05/09/2018 14:17:58