Designing a MCU-driven permanent magnet BLDC motor controller: Part 1

David Swanson and Kurt Perski, STMicroelectronics

July 13, 2008

David Swanson and Kurt Perski, STMicroelectronicsJuly 13, 2008

BLDC motors have a lot of advantages in automotive applications. They are lighter weight, more durable, and consume less current than their brushed cousins. They have an excellent low end torque capability and can be quite capable of a very wide speed range.

This translates to an optimum solution for many applications within the nasty automotive environment we all know and love. Applications such as fuel pump, HVAC blower motor, seat cooling fan, engine cooling fan, AC compressor motor, and water pump, to name a few, can benefit greatly from BLDC technology.

Anything with a continuous or high duty and/or in need of variable speed control could easily benefit both fiscally and durability-wise from BLDC motor technology.

It wasn't so long ago when BLDC motor controllers were cumbersome and expensive. Their difficulty to implement, inability to start reliably (without extra position sensing help), and overall cost kept the BLDC option off of the table. The inverters used to drive the windings alone were prohibitive in cost and in size.

Thanks to Moore's law (and the Power MOSFET derivative of it), the cost and size of these components have dropped dramatically. So much so that folks are looking seriously at those aforementioned applications and thinking positively. When performance and reliability go up, at the same time that costs comes down, the folks that count the pennies begin to smile. I think that we're there.

There are many microcontrollers that can easily control a brushless motor. Some are more capable than others at doing this task. A lot has to do with their peripherals. Some have the right kind of analog to digital converter controls allowing synchronous sampling of an analog voltage.

Others have dedicated motor control peripherals allowing for efficient Back EMF sensing, proper timing of different key moments in a brushless control algorithm. Since there are several different micros to choose from, we will begin by focusing on the tasks at hand, controlling a brushless motor, and then start getting a bit specific as we get into the design of the controller itself.

Typical motor configurations
A brush-type permanent magnet motor has the magnets in the stator, or the stationary part of the motor, and the windings in the rotor. As the rotor rotates between the brushes, the commutation, or switching from one phase to the next, naturally happens. It's a mechanical thing. It is a simple and somewhat reliable solution although brushes are the weakest link in the durability "chain."

Brushless motors are just that, brushless. They have to commutate the windings by some other means other than brushes and commutator bars. We hope to do it using semiconductors. Things are a bit switched around in a brushless motor. In a permanent magnet brushless DC motor, the magnets are in the rotor and the switched, or commutated, windings are in the stator.

Figure 1. Wye and Delta winding configurations

This takes a fairly simple design, brushes and commutator bars, and makes it much more complex (resistors, MOSFETs, capacitors, a microcontroller, Voltage regulator), more reliable, easier to use, cheaper and better " we know it will be better... It has to be.

Typical brushless motors are three phase machines with either wye or delta wound stators, with the vast majority of them being wye wound. The driving mechanism is the same for both.

The difference is that a wye wound machine has one end of all of the phases connected together in the middle. If there are three phases then this looks like a "Y"; hence the "wye" nomenclature. A delta wound machine ties the ends together such that, for a three phase machine, the configuration looks like a triangle, or a delta.

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