Anovel Single-phase electronic starter circuit meant for 0.5 HP and 1 HP motors, is presented. It incorporates both over-load and short-circuit protections. A specially made current sensing device has been added in this starter to sense the current being drawn by the motor.

If the motor jams due to bearing failure or defect in the pump or any other reason, then it would draw much higher current than its normal rated current. This will be sensed by the current sensing device, which will trip the circuit and protect the motor. The other reasons resulting in motor drawing higher current include:

(a) Windings damaged or short-circuit existing between windings.
(b) Shorting of motor terminals by mistake.
(c) If under voltage or single phasing occurs in the mains supply source (normally a 440V AC, 3 phase with neutral - four wire system).
The main components used in the circuit comprise a specially wound sensing transformer X1, another locally available step-down transformer X2, one single change-over relay RL1, two 2-change-over relays (RL2 and RL3) and other discrete components as shown in the figure. The mains supply to the motor is routed in series with primary of transformer T1 via normally open contacts of relay RL3. The primary of transformer X1 is connected in the neutral line. To switch on the supply to the motor, momentarily switch S1 is to be pressed, which causes the supply path to transformer X2 primary to be completed via N/C contacts of relay RL1. The relay RL2 gets energised due to the DC voltage developed across capacitor C2 via the bridge rectifier. Once relay RL2 energises, the N/O contacts RL2(a) provide a short across switch S1 and hence supply to the primary of transformer X2 becomes continuous and hence relay RL2 latches even if switchS1 is subsequently open. The other N/O contacts RL2(b) of relay RL2, on energisation, connects the voltage developed across capacitor C2, to relay RL3, which thus energises and completes the supply to the motor as long as current passing through primary of transformer X1 is within limits (for a 1-hp motor). When the current drawn by motor exceeds the limit, the voltage developed across secondary of transformer T2 is sufficient to energise relay RL1 and trip the supply to relays RL2 and RL3 which was passing via the N/C contact of relay RL1. As a result the supply to the motor also trips

The contact rating for relays RL1 and RL2 should be 5 amperes, while contact ratings of relay RL3 should be 10 to 15 amperes. The transformer X1 can be wound using any suitable size CRGO core. (One can use a burnt out transformer core as well). The primary comprises of 30 to 31 turns for use with 1-HP motor and additional 8turns, if you are using a 0.5 HP motor. The fuses F1 and F2 are kit-Kat type. The ON push button is normally ‘off’ type, while off push button S2 is of normally ‘on’ type. Capacitors C1 and C2, apart from smoothing the rectified output, provide necessary delay during energisation and de-energisation of relays. Diodes across relays are used for protection as free wheeling diodes.

Starters for 0.5 HP and 1 HP motors are not easily available in the market. Users are therefore compelled to use 10 Amp rated circuit breaker for such motors. A mechanical starter or auto starter would turn out to be costlier than the circuit given here, which works very reliably. The author has been using such an electronic starter with 1 HP pump set for nearly a year without any problem. Parts used in this circuit are easily available in most of the local markets.