Variable-speed-drive range utilising brushless synchronous motors

AC drive systems are taking an increasing proportion of the UK electric variable-speed-drives market. Studies indicate that the proportion taken by AC drives has risen from approximately 20% of the market in 1984, to 25% in 1986. The most common AC variable-speed drive utilises a cage induction motor in combination with a force-commutated inverter. Over the power range from, say, 20 kW to 1000 kW this type of AC drive is almost invariably more expensive than its DC counterpart. The authors describe a variable-speed-drive range utilising brushless synchronous motors, in combination with a naturally-commutated inverter. They show that this drive type goes a long way to offering the benefits of the more common AC drive systems but at a lower cost for drive powers in excess of 50 kW. The brushless synchronous motor-drive system is describes in detail and its characteristics compared with other drive types