Can a brushless motor controller be used to control a 3 phase Induction Motor?

Question

I've been seeing lots of BLDC motor controllers for dirt cheap. From my limited understanding, BLDC motors are almost the same as induction motors, except they use permanent magnets. I was thinking about taking the output and stepping up the voltage to drive an induction motor. Is this possible?

Answer 1

BLDC motors are 'almost the same' as induction motors in the same way a gas turbine engine is 'almost the same' as a piston engine.

A BLDC motor is a DC motor with electronic commutation. The controller replaces the function of the commutator and brushes in a brushed DC motor, using Hall sensors or back-emf detection to switch power to each winding in exact synchronization with the rotor position.

Induction motors work quite differently. The stator induces current into the rotor to produce a magnetic field in it, which then interacts with the stator field to produce torque. Under load the rotor can 'slip behind' the stator field and still produce torque while running at lower rpm, unlike a synchronous motor which must stay in lock step or it will stop rotating.

So a BLDC (or brushed DC) motor is like a piston engine which uses a cam shaft and valves to synchronize combustion and exhaust with piston movement, whereas an induction motor is like a gas turbine engine which just burns fuel continuously and lets the turbine spin at whatever speed it can manage.

'Dirt cheap' BLDC controllers are designed to control synchronous permanent magnet brushless motors only. They will not work with induction motors unless you replace the firmware. Open source firmware is available for ATmega based Brushless ESCs. You could try modifying this firmware to produce sine wave PWM, drop the back-emf detection so it runs open loop, and control the frequency directly.

Most induction motors are designed to work on mains voltage and frequency. Stepping up the voltage from a low voltage BLDC controller is certainly possible, but the result may no longer be 'dirt cheap'. The simplest way would be to use a 3 phase transformer with appropriate primary and secondary windings. It would probably have to be custom built to match the voltage, power, and desired frequency range of your controller.

Answer 2

I've been seeing lots of BLDC motor controllers for dirt cheap. From my limited understanding, BLDC motors are almost the same as induction motors, except they use permanent magnets. I was thinking about taking the output and stepping up the voltage to drive an induction motor. Is this possible?

Summary: It's notionally possible to get a not very good result if you understand the differences and allow for them, but it is extremely unlikely to be worthwhile.

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In the following I'll refer to a BLDCM rotor coils as "field coils" although they are strictly "power coils".

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Because:

An induction motor 'follows' a 3 phase sinusoidal or pseudo sinusoidal waveform set. The motor rotates at a slightly slower rate than the rotating 3 phase waveform set. The difference frequency induces voltages in the rotor "bars" which produce currents and magnetic fields which provide the field which follows the stator field. When operated at fixed mains frequency these motors are almost always "open loop" with rotor speed being 'slightly lower than mains frequency "rotation" divided by the number of pole pairs.
A pseudo sine wave can be used (usually when using cheap variable frequency controllers).

Whereas

A BLDCM (brushless DC motor) has a usually 3 phase permanent magnet rotor set which follow (usually) 3 phase square wave signals without slip. In model motor applications and many others speed is controlled by tachometer speed detection (via a range of means) which then may regulate supply voltage on/off or may vary supply voltage analogly.
However, the pole by pole switching of the drive coils is carried out so as to be in synchronism with the rotating magnets. To do this the rotor position must be determined so that each magnet is properly attracted or repelled at the right time. Early BLDCMs usually did this with some form of pole position sensors - often a Hall cell per phase. With time the major method of switching time determination has moved to using the back EMF waveforms /voltages generated in the stator coils by the rotor magnets. This is good enough for most purposes. But a stationary BLDCM needs to be "poked with a stick" (o ra randomish field coil pulse) to get it moving so back emf feedback can be generated. BLDCMs may at startup briefly "shake their heads and turn briefly in the wrong direction. Where rapid startup and positive initial direction control is vital hardware sensors may still be used.

SO - an "ESC" (electronic speed controller) usually implements a sensorless back-emf phase switching system. The required hardware apart from the controller IC is minimal. There is no generated rotating magnetic field fed to the motor - rather the motor arranges the generation of its own field.

SO - to use an ESC with an induction motor a 3 phase position sensor would be required, the controller would have to accept these signals (some will) and the voltage and current must match motor needs. Also, the square wave waveform generated is not optimum for use with the (usually sine wave driven) induction motor and extra losses and noise can be expected.