Construction And Working:
This method of starting of a motor is very simple and provides smooth acceleration of the motor.
In this method of starting of a 3-phase squirrel cage induction motor reduced voltage is obtained by means of resistors (or reactors ) that are connected in series with each stator lead, as shown in fig, during the starting period.
The voltage drop in resistors (or reactors) causes a reduced voltage across the motor terminals. As the motor picks up the speed, the resistors (or reactors) are cut out in steps and finally shortcircuited when the motor attains the operating speed.The primary resistor method is quite similar to the dc shunt motor starter with its series armature resistor because both provide a voltage drop between the voltage supply and the motor terminals.
There are, however, some very important differences. In a dc shunt motor the starting torque is directly proportional to the starting armature current whereas, in the case of a polyphase induction motor the torque varies as the square of the motor terminal voltage i.e., there is a far greater reduction of starting torque for an equivalent reduction of starting current.
Although the initial cost of reactors is high in comparison to that of resistors, reactor starting is preferred because this method incurs small power losses and is more effective in reducing the voltage applied to the stator at starting
A circuit diagram of a magnetically operated primary resistance starter is illustrated in fig.
It consists of three-line resistors, two sets of three-line contacts. two contactors, over-load protection, a set of start and stop buttons, and several auxiliary contacts.
When the start button is pressed, the main contactor is energized through the overload relay contacts OL (normally closed), which in turn closes the three main contacts M and one auxiliary contact MC.
Thus the motor is connected to the line through the starting resistors. At the same time, a mechanical escapement timer is started.
After a definite time has elapsed during which the motor has speeded up, the time delay contacts TC close.
This energizes the accelerating contactor A, which in turn closes the three contacts A and short-circuiting the starting resistors applies full voltage to the motor.
Overload and under-voltage protections are provided in the same manner as in the direct-on-line starter.
In the starter described above, the starting resistors are cut out in one step. Starters are also available,
in which arrangement is to cut out starting resistances in several steps and thus to give smooth acceleration with less line disturbance.
If the normal supply phase voltage =V volts and by using line resistance starter the voltage is reduced to kV volts, then starting current is also reduced in the same ratio i.e. starting current Ist = k Isc where Isc the short-circuit current.
= K2 X Torque obtained by switching the motor directly.
Advantages:
1. Smooth acceleration.
2. High power factor during start.
3. Less expensive than an auto-transformer starter in lower output ratings.
4. Closed transition starting.
5. Available with as many as 7 accelerating points.
Disadvantages:
1. Resistors give off heat.
2. Low torque efficiency.
3. Starting duration usually exceeds 5 seconds, so needs expensive resistors.
4. Starting voltage is difficult to adjust to meet varying conditions