Drives and Controls Magazine
motors `could replace induction machines`
Published:  01 December, 2005

Permanent magnet motors `could replace induction machines`

Yaskawa has developed a range of synchronous permanent magnet motors which, it claims, are smaller, lighter and more efficient than induction motors of a similar rating. The company suggests that the IPM (internal permanent magnet) motors could eventually supersede induction motors in many applications.

The motors use permanent magnets embedded in the rotor to improve flux density and distribution. In a conventional induction motor, torque is generated when secondary currents flow in the rotor. These currents result in losses and the generation of heat.

In an IPM motor (shown above), torque is generated by the magnets, thus avoiding the secondary copper losses. The result is a machine with an efficiency of around 94.5% at full speed (compared to around 92.4% for a high-efficiency induction motor) and 92.1% at 40% speed (compared to around 87.1%). The lower losses are also said to improve precision.

Yaskawa says that IPM machines are up to 55% smaller and 40% lighter than induction machines of a similar rating. Other claimed benefits include higher power factors, and reduced maintenance because there is no copper wear on the rotor, limiting shaft heating and doubling bearing grease life. The high efficiencies and low heat levels mean that there is less need for cooling which, in turn, reduces noise levels.

The IPM motors are controlled by special AC inverters. For applications such as fans, pumps and compressors, the inverters can be used without sensors. For applications such as machine tools, extruders and lifts, they can be used with encoders and magnetic pole sensors to provide a faster response with a lower speed error.

Initial applications are expected to focus on:

machine tools, where the IPM motors could be used for space-saving, low-vibration direct spindle drives which could regenerate energy during deceleration;

lifts, where the compact, efficient, low-noise motors could be incorporated in lift shafts, avoiding the need for dedicated machine rooms;

cranes, where the low weight, small size, high efficiency and low maintenance attributes would be key attractions; and

fans, pumps and compressors, especially those in continuous use.

The high power density of the IPM motors, has allowed Yaskawa to produce low-profile versions (shown above) that look like large pancake motors. These are targeted at applications such as gearless drives for lift shafts.

According to Yaskawa, the efficiency of a direct-drive IPM in a high-speed lift application would typically be around 90%, compared to 77-86% for a conventional four-pole induction motor with a helical gearbox. In lower-speed lift applications, the IPM would operate at around 82% efficiency, compared to 60-68% for an induction motor and wormgear combination.

The IPM motors are available initially in ratings from 400W, 200V to 160kW, 400V.

Yaskawa`s pioneering matrix inverter drive is taking longer to reach the market than planned. Field trials have been extended and the first deliveries will take place in Spring 2006 at the earliest. The 5.5kW and 11kW drives are likely to cost about twice as much as standard drives.