Anovel single-phase electronic starter circuit meant for 0.5HP and 1HP motors is presented here. It incorporates both overload and short-circuit protections. A special cur-rent-sensing device has been added in this starter to sense the current being drawn by the motor. If the motor jams due to bearing fail-ure or defect in the pump or any other reason, it would draw much higher cur-rent than its normal rated current. This will be sensed by the current-sensing device, which will trip the circuit and protect the motor. Some other reasons for the motor drawing higher current are as follows:
Circuit diagram :
Electronic Starter For Single-Phase Motor Circuit Diagram
- Windings damaged or short-circuit between them.
- Shorting of motor terminals by mistake.
- Under voltage or single phasing occurring 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, single-changeover relay RL1, two double-changeover relays (RL2 and RL3), and other discrete components shown in the figure. The mains supply to the motor is routed in series with the primary of transformer X1 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 mo-tor, switch S1 is to be pressed momentarily, which causes the supply path to the primary of transformer X2 to be completed via N/C contacts of relay RL1. Relay RL2 gets energised due to the DC voltage developed across capacitor C2 via the bridge rectifier. Once the relay energises, its N/O contacts RL2(a) provide a short across switch S1 and supply to the primary of transformer X2 becomes continuous, and hence relay RL2 latches even if switch S1 is subsequently opened. The other N/O contacts RL2(b) of relay RL2, on energisation, connect 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 1HP motor).
When the current drawn by motor exceeds the limit (approx. 5A), the volt-age developed across the secondary of transformer X2 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.
Transformer X1 can be wound us-ing any suitable size CRGO core. (One can use a burntout transformer core as well.) The primary comprises 30 to 31 turns for use with 1HP motor and additional eight turns, if you are using a 0.5HP motor. Fuses F1 and F2 are kit-kat type. The ‘on’ pushbutton is normally-‘off’ type, while ‘off’ pushbutton S2 is of normally-‘on’ type. Capacitors C1 and C2, apart from smoothing the rectified output, provide necessary de-lay during energisation and de-energisation of relays. Diodes across re-lays are used for protection as free-wheeling diodes.
Starters for 0.5HP and 1HP 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. Parts used in this circuit are easily available in most of the local markets.
Author : Sarat Chandra Das- Copyright : EFY
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