In this article we will about the methods used for starting squirrel-cage induction motors.
1. Direct-on-Line Starting or Full-Voltage Starting:
Squirrel-cage induction motors, as a general rule, up to 4 or 5 kW are started by this method. As the name suggests, this method involves direct switching of polyphase squirrel cage induction motor to supply mains, as illustrated in Fig. 1.67(a).
The push-button type direct-on-line starter, which is very common in use, is shown in Fig. 1.67(b). It is simple, inexpensive and easy to install and maintain. It consists of a set of ‘start’ and ‘stop’ push buttons, a contactor (an electromagnet) with its associated contacts and usually overload and under-voltage protection devices. The start button is momentary contact that is held normally open by a spring. The stop button is held normally closed by a spring. Thermal overload relays are commonly used for motor overload protection.
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Under-voltage protection is provided by the contactor (that is controlled by a three-wire control circuit) maintains the interruption of circuit even after the supply is restored.
Torque Developed on Starting of Induction Motor By Direct Switching:
Input power to rotor = Tω
Rotor copper loss = s x input power to rotor
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Where, I2 is the rotor current per phase, R2 is rotor resistance per phase, T is the torque developed, s is the slip and ω is the supply angular velocity.
Or Torque developed, T = 3I2 R2/sω
if rotor resistance is constant.
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Since rotor current is proportional to stator current I1, so-
At starting slip, s = 1
So starting torque, Tst = K(Ist)2 where Ist is the starting current
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Full-load torque,
Where If is the full-load motor current and sf is the full-load slip.
So,
When the motor is connected directly across the supply mains, the starting current is equal to the short-circuit current, Isc
2. Primary Resistor (or Reactor) Starting:
This method of starting of a polyphase cage 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. 1.68, 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 short circuited 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 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.
The advantages and disadvantages of this method of starting of cage motors are given below:
Advantages:
1. Smooth acceleration.
2. High power factor during start.
3. Less expensive than 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.
Circuit diagram of a magnetically operated primary resistance starter is illustrated in Fig. 1.68 (b).
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 is the short-circuit current. Starting torque,
= K2 x Torque obtained by switching the motor directly
This method finds its application in some cases where only very low starting torques are required. A curve plotted between the starting-current/full-load current and the starting-torque/full- load torque in case of stator resistance method of starting of induction motor is depicted in Fig. 1.69.
3. Auto-Transformer Starting:
In auto-transformer starting method the reduced voltage is obtained by taking tappings at suitable points from a three phase auto-transformer, as shown in Fig. 1.70(a). The auto-transformers are generally tapped at the 50, 60 and 80 per cent tapping points, so that adjustment at these voltages may be made for proper starting torque requirements. Since the contacts frequently break large values of current, arcing is sometimes quenched effectively by having them assembled to operate in an oil bath.
Auto-transformer starters may be either manually or magnetically operated. The wiring diagram of an auto- transformer starter is shown in Fig. 1.70(b).
Let the motor be started by an auto-transformer having transformation ratio, K. If Isc is the starting current when normal voltage is applied, and applied voltage to stator winding at starting = KV then motor input current,
Ist =KIsc ….(1.73)
Supply current,
= Primary current of auto-transformer
= K x secondary current of auto-transformer = K2 Isc
Starting torque,
= K2 x Torque obtained by direct switching. …(1.74)
Hence the line current and the starting torque are reduced in the square ratio.
The advantages of this method of starting of cage induction motors lies in the fact that the voltage is reduced by transformation and not by dropping the voltage in the resistors (or reactors) and, therefore, the current and power drawn from the supply mains are also reduced in comparison to primary resistor (or reactor) starting. The internal losses of the starter itself are small during long starting periods.
Other advantages of this method are:
(i) Availability of highest torque per ampere of supply current
(ii) Adjustment of starting voltage by selection of proper tap on the auto-transformer
(iii) Suitability for long starting periods
(iv) Closed transition starting and
(v) Motor current larger than supply current.
The drawbacks of such starters are low power factor and higher cost in case of lower output rating motors.
This method can be employed for starting of star-connected as well as delta-connected motors.
For starting of large cage motors (of output rating exceeding 20 kW) this method of starting is often used.
The values of starting torque available, together with the corresponding values of the voltage applied to the stator and the motor and line currents expressed as percentages of the normal full-load values, for different tap positions are shown in Fig. 1.70(c).
4. Star-Delta Starting:
This method of starting of cage induction motors is based upon the principle that with 3 windings connected in star, the voltage across each winding is 1/√3 i.e. 57.7 % of the line-to-line voltage whereas the same winding connected in delta will have full line-to-line voltage across each.
The star-delta starter connects the three stator windings in star across the rated supply voltage at the starting instant. After the motor attains speed the same windings, through a change-over switch, are reconnected in delta across the same supply voltage.
The basic diagram of connection is shown in Fig. 1.71(a). An actual starter incorporates undervoltage and overvoltage coils, as shown in Fig. 1.71 (b). The starter is also provided with a mechanical interlocking device to prevent the handle from being put in the ‘Run’ position first.
Since at starting instant, the stator windings are connected in star, voltage across each phase winding is reduced to 1/√3 of line voltage and, therefore, starting current per phase becomes equal to Isc/√3.
Starting line current by connecting the stator windings in star at the starting instant = Starting motor current per phase = 3.
Starting line current by direct switching with stator windings connected in delta = √3 Isc.
Hence by star-delta starting line current is reduced to one-third of line current with direct switching.
Starting torque,
Hence with star-delta switching, the starting torque is also reduced to one-third of starting torque obtained with direct switching.
This method of starting of cage motors is simple, cheap, effective and efficient since no power is lost in auxiliary components. This method is also suitable for high inertia and long acceleration loads.
This method needs a motor to be delta-connected for normal operation and all the six terminals of the 3-phase stator windings are to be brought out. The reduction in voltage is fixed and starting torque is also low. So this method is limited to application where high starting torque is not the essential requirement e.g. machine tools, pumps, motor-generator sets etc. This method is unsuitable for line voltage exceeding 3,000 V, because of excess number of stator turns required for delta connection.
Such starters are employed for starting 3-phase squirrel cage induction motors of rating between 4 and 20 kW.
Precaution with Star-Delta Starting:
The initial current flowing when the motor is started in star is 57.7% of the short- circuit current in delta together with a transient in each phase. The transient currents decay rapidly but the steady state is not reached until the motor has attained 70 per cent of its synchronous speed. The change-over from star to delta connection should not be made until the motor attains about 90 per cent of synchronous speed, otherwise there will be a current surge considerably greater than full-load current which may even be greater than the standstill current with star-connection.