System Design Method for a Load Commutated Inverter-Synchronous Motor Drive

In a thyristorized load commutated inverter synchronous motor drive system, the inverter relies on the back EMF of the synchronous motor to provide the reverse voltage for commutation. The level of the back EMF is a function of both the synchronous motor's field current and rotational speed. The inverter operating frequency is determined by the motor speed and number of pole pairs. For a fixed thyristor firing angle, an increase inmthe level of armature current decreases the available thyristor turn-off time due to overlap of the incoming and outgoing thyristor. This overlap time is a function of the commutating reactance of the synchronous motor. These factors, field current, motor speed, inverter frequency, and armature current, together with a thyristor turnoff time specification, set a maximum limit on the value of motor commutating reactance. The motor designer and inverter designer must perform a tradeoff study of these factors to arrive at a satisfactory drive system. The results are presented of an analysis that allows the synchronous motor reactance to be specified based on the other given parameters of the inverter-synchronous motor drive system. Test data are provided from a 20-kVA 500-1000 Hz load commutated inverter/ inductor type synchronous motor drive system. The effects of motor speed and field excitation on the operating condition of the drive are discussed.