Counter-emf Starters

Author: E.E. Kimberly

As a motor accelerates, its counter emf increases and the voltage E_a across the armature increases almost in proportion to the increase in motor speed. In Fig. 15-6 is shown the wiring diagram of a counter-emf type of motor starter. The starting relay SR closes the main starting switch M when the starting contacts are closed. The holding contacts HC short-circuit the starting-button contacts and hold the circuit closed through the coil of SR after the starting button is released. The motor starts and accelerates until the voltage E_a across the armature rises to the voltage at which the relay R₁ is designed to close. Relay R₁ is usually designed to close at about one-half the rated voltage of the motor. This voltage occurs at about one-half the rated speed. Relays R_z and R_s are designed to close at about 80 per cent and 90 per cent, respectively, of rated voltage and rated speed. The relays R₁, R₂, and R₃ are usually made adjustable in order that the motor speeds at which they close may be selectively chosen for the particular characteristics of the service for which the motor is installed.

Fig 15-6. Wiring Diagram of Counter-emf Automatic D-C Motor Starter

Counter-emf starters are unsatisfactory in sizes above about 3 hp because the closing pressures on the contacts of the relays are small and the - large currents taken by larger motors are likely to cause the contacts to weld together so that they cannot be opened.