AN2281 datasheet(3/38 Pages) STMICROELECTRONICS

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AN2281

Theory of operation

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Theory of operation

Standard induction motors, normally designed to run at base speeds between 850 to 3500

rpm, are not particularly well suited to low-speed operation, as their efficiency drops with the

reduction in speed. They may also be unable to deliver sufficient smooth torque at low

speeds.

The use of a gearbox is the traditional mechanical solution to this problem. However, the

gearbox is a complicated piece of machinery that takes up space, reduces efficiency, and

needs both maintenance and significant quantities of oil. Replacing the gearbox with

permanent magnet motors/drive configurations saves space and installation costs, energy

and maintenance, and provides more flexibility in production and facility design. These

motors use magnets to produce the magnetic rotor field rather than the magnetizing

component of the stator current like in the induction motor.

Figure 1 shows a cross section of a typical permanent magnet (PM) motor. The rotor has an

iron core on the surface of which is mounted a thin permanent magnet. An alternating

magnet of opposing magnetization produces radial directed flux density across the air gap.

This flux then reacts with currents in the stator windings to produce torque.

The two most common types of brushless PM motors are classified as:

●

Synchronous, with a uniformly rotating stator field as an induction motor. This type is

also referred to as PMAC (BLAC)

●

Switched or trapezoidal, with stator fields that are switched in discrete steps. This type

is also referred to as PMDC (BLDC)

Figure 1.

Cross-section of PM motors

1 pole pair

3 pole pairs

-c

-a

-b

-c

-a

-b

AN2281 Scheda tecnica(PDF) 3 Page - STMicroelectronics