Commutation timing is initialized when the rotor (in a three-phase, four-pole brushless dc motor) is at a position where the mechanical and electrical angles coincide.
Select figure to enlarge.

Electrical angle alignment of brushless dc (BLDC) motors is the process of determining how to apply electric current to the motor. In essence, it is determining where the magnets are with respect to the windings, not unlike determining the spark plug timing of an automobile engine.

In many precision positioning systems, particularly those that commutate in a sinusoidal fashion, alignment of the motor electrical angle is required – even when absolute position is available. The method used to accomplish this process affects system cost, complexity, and performance.

Introduction

Before the advent of sinusoidally driven brushless dc (BLDC) motors, most servo-positioning systems relied on dc motors, employing brush-type commutators to channel electrical current to the appropriate winding to develop torque. Today, the job of commutation has moved from the mechanical realm (brushes and sliding contacts) to that of electronics.

Brushless motors, by definition, do not have an integral means of commutation. Instead, they must be commutated electronically; solidstate switches apply current to the motor windings in a manner that produces useful torque or force.

One way to commutate BLDC motors is with the “six-step” or “trapezoidal” technique, which relies on Hall-effect sensors. Inexpensive, the method is widely used in applications that don’t require smooth torque for positioning. For precision positioning, an alternative approach, sine drive commutation, is usually the method of choice.

Trapezoidal commutation lacks precision because of inaccuracies associated with Hall-effect sensors and torque disturbances stemming from the discontinuous nature of six-step drive currents. Sinusoidal commutation, on the other hand, is quite precise; but it has drawbacks too.

In addition to requiring more sophisticated electronics, sinusoidal commutation won’t work right if the electrical angle of the rotor with respect to the stator is unknown. In systems where the control and drive functions are performed in separate platforms, this can add a lot of needless complexity.

Initialization

Torque T delivered by a sinedriven BLDC motor is:

T =I_lineK_t cosα

where α is the angle of misalignment between the magnets and the windings.

Brushless dc servomotors, such as these from BEI Kimco Magnetics, provide high torque density and outstanding response, but they also require electronic commutation.

As long as the angle of misalignment is constant, output torque will remain smooth, without “ripple.” And although misalignment reduces torque (actually K_t), it is usually by very little (<2%) for reasonable values of α (<10°). Note: misalignment is usually specified in degrees “electrical.” To convert to mechanical degrees, the angle must be divided by the number of pole pairs.

Despite the fact that torque rolloff due to electrical angle misalignment is a cosine function – and therefore insignificant for small angles – it is still necessary to align the electrical angle with the motor angle. Any amount of misalignment wastes energy as motor heat, energy that would otherwise (in a properly aligned system) produce torque.

In perhaps the worst case (cosα<0), the motor will produce torque (or force in the case of a linear motor) in a direction opposite of that which is expected. This translates to positive feedback (in the position loop) and can have catastrophic consequences.

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