Other Machines

This rather ambiguously named section of these notes is concerned with machines other than the traditional three machine types (traditional DC machines, three-phase wound field synchronous machines and three-phase induction machines). The machines in this section can often be found in fractional horsepower applications (under 746W), though they are not limited to this power range. These machines include permanent magnet synchrouns machines, brushless DC machiens, single phase induction machines, switched reluctance machines and stepper motors.

Synchronous Reluctance Machines

Synchronous reluctance machines make use of a reluctance rotor design but use a standard 3-phase ac winding. An induction cage on the rotor provides start torque, but once the machine is close to synchronous speed, the reluctance torque will synchronise the machine with the supply.

Although synchronous reluctance motors can have a low torque densuty, they traditionally fond use in a number of specific applications, including clocks and also textile applications, where precise speed control is required to spin constant thickness fibres. (This also applies to optical fibre production). Modern developments with synchronus reluctance machines focus on high efficiency motor drive systems, where a power electronic converter is used to provide a variable frequency supply and control is applied to maintain synchronous operation.

Permanent Magnet Synchronous Machines (PMSM)

PMSM are usually three-phase machines, though many new designs have other phase numbers. Operation is essentially similar to large wound-field synchronous machines with one important exception:

PMSM are usually operated from variable speed power electronics and do not operate at a single fixed speed.

PMSM were traditionally found in sizes up to one or two horsepower. This limitation existed due to the cost of permanent magnet material causing larger sized machines to be prohibitively expensive. However, this is no longer the case: PMSM motors are the electrical machine of choice for hybrid electric cars, at power up to 100-200 kW; the US navy has contracted the test construction of multi-MW PMSM machines for ship propulsion; some transportation companies are considering the use of multi-MW PMSM for high speed rail applications.

The primary advantage of PM motors is high torque per unit volume, due to the capability of the magnets to produce a very high flux density.

Due to their operation and control depending on the use of variable speed drives, synchronous reluctance and PMSM are considered to be beyond the scope of an introdcutory machines course.