A mill motor inching drive using a three pulse cycloconverter with double integral phase control

This paper describes the first commercial installation of a three pulse cycloconverter drive using a new type of phase control modulation method developed by the authors called double integral control. The new control method gives the three pulse cycloconverter similar performance in both harmonic output and speed range to a six pulse cycloconverter, but at a much reduced cost and much higher efficiency. The particular application is an inching drive for the maintenance of various mills at a large copper mine in Canada. There are 6 mills at the site, each of which is driven by one or two synchronous motors rated at 4160 V 60 Hz 4000 hp (3000 kW). The motors are run direct on line but started as wound rotor induction motors using their damper windings, which are brought out via slip rings. The cycloconverter is rated at 0-570 V, 0-9 Hz, 1500 A with a supply input of 600 V, 60 Hz. A mill to be turned for maintenance is connected to the cycloconverter with its motors configured as synchronous machines with their damper windings shorted. The cycloconverter, with its adjustable low frequency output, is then used to turn the mill as required. Starting torque for the mills can be up to 1.5 times the rated torque which dictates the high current rating of the cycloconverter. The cycloconverter is rated for continuous duty to allow for possible future applications at the site. With an efficiency of greater than 99.5%, only fan cooling is required.