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How do Hall Effect brushless motor controllers work?

How do Hall Effect brushless motor controllers work?

March 28, 2017 MotorsTechnical

Most electric bike motors use hall sensors to determine the motor position and help the controller to drive the motor correctly.

This blog gives a simplified explanation as to how this motor control works, and is a follow up to our previous blog about what is a hall sensor and why do electric bikes need them?

 

Almost all electric bike motors are three-phase brushless motors.

“Brushless” means that there are no physical brushes like in old style DC motors that change where the current goes in the motor to keep it spinning around. This old technique automatically moved the current around the motor to keep it spinning.

“Three-phase” means that there are 3 sets of coils inside the motor. The controller has to put current into each of these coils in sequence as the motor turns.

brushless motor
hall effect motor diagram

To make the explanation easier, let’s use this simplified diagram of a 3-phase motor.

In the middle is the rotor, the bit that spins. It has magnets on it and it works the same as just a simple 2-pole magnet.

Around the outside there are the coils that electrical current flows through to make a magnetic field, which the rotor tries to align with.

The simplest model we can make is of a motor with 3 single coil pairs, and one single poled magnet in the middle representing the rotor.

Real motors have more poles on the rotor, and more coil pairs, but the essence of how it works is the same. Also eBike motors usually have the coils on the inside and rotor and magnets on the outside, but it is easier to explain it this way.

Control sequence for a 3-phase motor

The controller keeps moving which coils the current goes through just as the motor turns, to keep pulling it around and around.

Step 1: Current through phase A

hall effect motor diagram

Step 2: Current through phase B

hall effect motor diagram

Step 3: Current through phase C

hall effect motor diagram

The motor turns because current in one of the coils near to where the motor magnets are pointing, pulls the motor around to align the magnets with the coil.

E.g. if the motor is stopped just near position 1, the controller puts current into coil 1, and the motor turns to align with coil 1.

But then, when it reaches coil 1, the current now needs to move to coil 2 in order to keep the motor turning around.

IF the controller doesn’t switch the current like this, then the motor will stop.

The motor will stop if the current doesn't move

Putting it all together...

hall effect motor diagram

When you run a brushless motor continually, the current looks like the graph in the picture above. The current in each phase goes up and down to keep the motor running smoothly around.

Where to hall sensors fit into this?

It is the hall sensors that tell the controller what is the current position of the motor. They sense when the rotor magnets are near, and this tell sthe controller where the motor is.

There are usually 3 hall sensors, one hall sensor for phase A, on for phase B and one for phase C.

E.g. in this graphic, the hall sensor for phase B is activated because the rotor has just arrived near it, this tells the controller to put current into phase B.

The motor will stop if the current doesn't move

hall effect motor diagram

What if a hall sensor breaks?

A common issue with hall effect motors is that a hall sensor breaks, due to water damage, overheating, short circuiting, over-voltage, or some other reason.

If this happens, the motor won’t work with a hall effect controller, because the controller will get confused.

In that instance, you need a no hall controller , these controllers use a clever trick of electrical engineering to detect motor position without the need for hall sensors.

But that is the subject for another blog post!

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