Contactless Electric Drive Control System

an electromechanical automatic control system that has no closing and opening contacts in the electric circuits that supply the electric drive. In control systems for an electric drive attempts are made to avoid closing and opening electrical circuits by means of contacts because they reduce the reliability and technical-economic ratings of the electric drives. Electrical contacts wear, burn out, sometimes weld, and spark, thus creating noise and radio interference. The primary merits of a contactless electric drive control system are reliability; long life; a reduction of the fire hazard, noise, and radio interference; increased speed of response; and lowered maintenance effort.

The most widely used systems have contactless electric apparatus that use thyristors, transistors, and magnetic amplifiers operating in a switching mode as the primary components. Switching the current on and off in the main control circuits of high-powered electric motors is frequently done by “control converter-motor” units (systems). In this case, the electric circuits connecting the converter and motor are not opened. The converter receives electric control signals and adjusts the electric voltage and current strength of the motor. A contactless electric drive control system that performs control, protective, counting, and other operations consists of low-powered contactless converters, relays, sensors, and logic units.

If the voltage at the input terminals of the contact (a) and contactless (b) units of electric drive control systems (Figure 1) are equal or close to 0, then contacts 1″, 2″, and 3″ of relays 1, 2, and 3 are closed, but the transistor T is cut off and current from a power supply U_s flows through the control winding CW. If a signal in the form of a negative potential is available at one or several of the inputs, the relays are de-energized and their contacts are opened, but the transistor is turned on and CW is either cut off or is shunted by the low internal resistance of the gated transistor. In this case the strength of the electric current is practically reduced to zero, but the flow through CW is not completely stopped.

Figure 1. Elements of electric drive control systems: (a) contact type: 1, 2, and 3 are relays; (b) contactless type: r is a resistance, T is a transistor, U, is a power supply, and CW is a control winding.