AN2281 datasheet(4/38 Pages) STMICROELECTRONICS

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Theory of operation

AN2281

4/38

Figure 2 provides a direct comparison of ideal current excitation waveforms for typical three-

phase sinusoidal and trapezoidal PM motors.

Figure 2.

Sinusoidal (PMAC) and trapezoidal (PMDC or 6-step) current excitation

PMDC motors are specifically designed to develop nearly constant output torque when

excited with a six-step switched current waveform. Their stator windings are concentrated

into narrow phase belts. The resulting back-EMF voltage, induced in each stator phase

winding during rotation, can be modeled quite accurately as a trapezoidal waveform.

PMAC motors are, on the contrary, specifically designed to be excited with a sinusoidal

current waveform. Their stator windings are typically distributed over multiple slots in order

to approximate a sinusoidal distribution so that the resulting back-EMF waveforms

generated are sinusoidal shaped.

Except for the intrinsic characteristics of stator windings, a PM machine can be excited with

both drive methods without any great loss of efficiency. The main difference between the two

types of excitation consists of the acoustic noise generated. The abrupt variation of the

trapezoidal phase current, in fact, generally introduces a great amount of acoustic and

electronic noise in comparison to the sinusoidal phase current.

In the 6-step PMDC method, one of the three phases is always unexcited, making it possible

to access back-EMF zero-crossing (i.e. rotor position) information, while in a PMAC motor

drive the three phases are always excited during the electrical period, making it necessary

to use at least one rotor position sensor.

Nevertheless, the relatively reduced amount of noise when a PM motor is excited with

sinusoidal current in comparison to 6-step excitation makes it the preferred choice for all

applications in which audible noise is a critical issue.

Actually, some complex algorithms for driving PMAC sensorless motors have been

developed, but they require more computational power than would be available from an 8-bit

microcontroller.

Phase A

Phase B

Phase C

120°

240°

(a) Sinusoidal

(b) Trapezoidal

240°

120°

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