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Rev up efficiency of motor-control based designs

Posted: 18 Feb 2009 ?? ?Print Version ?Bookmark and Share

Keywords:tutorial? motor brushless DC? BLDC PSMM?

By Ee Beng Lam

Brushless DC (BLDC) and Permanent Magnet Synchronous (PMSM) motors are technologies that have seen an increase in demand, because of high efficiency and increasingly cheaper production cost. These types of motors are expected to gain market importance in the low power range 750W to 5kW particularly.

There is pressure therefore on designers to cut the cost of motor installation, including the control systems, and this is where more efficient low-cost MCUs can help. Understanding the requirements however can help illustrate what features control circuit designers need to focus on to get the best performance for the smallest investment in silicon.

Furthermore modular software blocks and hardware reference designs offer fast motor control solutions. The core motor control software routines are proven and remain the same, independent of the motor size, so applications including white goods through to major industrial installations can be driven by the same core device.

Understanding motors

A motor has two primary parts; the non-moving part is called the stator and the moving part, typically inside the stator, is called the rotor. Depending on the motor type, the stator and rotor can consist of coil windings or permanent magnets.

In order to enable a motor to rotate, two magnetic fluxes are required, one from the stator and the other from the rotor. By controlling the current applied, a rotating magnetic field can be generated. The motor rotates because of the interaction of the rotating magnetic fields, as the magnetic field from the rotor attempts to align with that of the stator.

Brushed DC motors depend on a mechanical system to transfer current. The brushed motors have a wound rotor attached to the centre with a permanent magnet stator bonded to a steel ring surrounding the rotor. A commutator provides a means for connecting a stationary power source to the rotating coils, typically via conductive brushes that ride on smooth conductive plates. As the brushes come into contact with the commutator, current passes through to the rotor coil. The uneven torque that results from a single coil rotor can be smoothed by adding additional coils and commutator segments.

AC induction motors, on the other hand, do not depend upon the mechanical system to control current, but instead pass current through the stator which is connected to an electrical supply directly or via a solid-state circuit.

The motor stator has a number of coil windings, that when driven by an alternating current, operates as a set of electromagnets to generate the required flux. It typically has a squirrel-cage rotor, consisting of a ring at either end of the rotor, with bars connecting the rings running the length of the rotor. In a 3-phase motor, the stator coils energize and de-energize sequentially, creating a rotating magnetic field. This induces current to flow in the bars of the squirrel-cage rotor, which in turn creates another magnetic field.

Motor specifics

The term 'BLDC' is a marketing driven label to promote the idea that a BLDC motor with the appropriate drive is a suitable drop-in replacement for a brushed DC motor and its drive. Technically, a BLDC motor is still an AC motor; that is, alternating current inputs are required in order for the motor to move.

A BLDC motor is a rotating electric machine where the stator is a classic 3-phase wound stator, like that of an induction motor. Rather than inducing the rotor, the BLDC rotor has surface-mounted permanent magnets which provide the steady-state magnetic field.

When the wound stator is energized by a 3-phase alternating current, it creates a rotating magnetic field that causes the rotor to rotate synchronously with it. The BLDC motor, with trapezoidally distributed windings, is driven by trapezoidal currents coupled with the given rotor position.

Similar to the BLDC motor, the rotor of a PMSM consists of permanent magnets. The stator of a PMSM has its 3-phase windings distributed sinusoidally, as opposed to the trapezoidal distribution found in a BLDC motor. It operates in the same way as a BLDC motor, when the wound stator is energized by a 3-phase alternating current; it creates a rotating magnetic field that causes the rotor to rotate synchronously with it.

The PMSM, with sinusoidally distributed windings, is driven by sinusoidal currents coupled with the given rotor position. A motor will draw only as much power and consume only as much energy as it takes to satisfy the load, thus to save energy, the options are to reduce the load, reduce the operating time, or increase efficiency.

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