20.1. Squirrel-Cage Motors.
20.1.1. Stator rheostat starter. The connection diagram is shown in Fig. 30.
Fig. 30. Stator rheostat starter.
Reduced voltage is impressed on each stator phase due to the resistance R of the rheostat.
Hence the initial current drawn from the supply mains will be less than if the machine were to be switched directly on to the supply mains.
If x = fraction of voltage (V) reduced by the stator resistors
- This method is suitable for starting of small machines only.
- High power factor during start.
- Smooth acceleration.
- Less expensive than auto-transformer starter in lower output ratings.
- Closed transition starting.
- Heat is given of T by the resistors.
- Expensive resistors are required because starting duration usually exceeds 5 seconds.
- Low torque efficiency.
20.1.2. Auto-transformers. Fig. 31 shows the connection diagram for autotransformer starting of squirrel-cage induction motors.
In this method the reduced voltage is obtained by taking tappings at suitable points from a three-phase auto-transformer (Fig. 31). The all to-transformers are generally tapped at the 50, 60 and 80 per cent 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-transformers may be either normally, or magnetically operated.
Fig. 31. Auto-transformer starter.
Relation between starting (Tst) and full-load (Tf) torques. Let the motor be started by
an auto-transformer having transformation ratio K. If t; is the starting current when normal voltage is applied and applied voltage to stator winding at starting is KV then,
Motor input current, Ist =KIsc
Supply current = Primary current of auto-transformer
It may be noted that this expression is similar to eqn. (32) except that x has been replaced by the transformation ratio K.
- Voltage is reduced by transformation and not by dropping the voltage in resistors, and therefore, the current and power drawn from the supply mains are also reduced in comparison to resistor starting.
- Availability of highest torque per ampere of supply current.
- Adjustment of starting voltage by selection of proper tap on the auto-transformer.
- The method is suitable for long starting periods.
- Motor current larger than supply current.
- Closed transition starting.
- Low power factor.
- Higher cost ill case of lower output rating motors.
- This method can be used for starting of star-connected as well as delta-connected motors.
- This method is often employed for starting of large cage motors (rating exceeding 20 kW).
20.1.3. Star-Delta Starter. Star-delta switching method is based upon the principle that with three windings connected in star, voltage across each winding is 1/ i.e., 57.7% of the line to line voltage whereas the same windings 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 re-connected in delta across the same supply voltage.
The basic connection diagram of a star-delta starter is shown in Fig. 32. An actual starter incorporates under-voltage and over-load coils. The starter is also provided with a mechanical inter-locking device to prevent the handle from being put in the ‘Run’ position first. Such starters are employed for starting 3-phase squirrel-cage induction motors of rating between 4 kW and 15 kW.
When star-connected, the applied voltage over each motor phase is reduced by a factor of 1/ and hence the torque becomes 1/3 of that which would have been developed if motor were directly connected in delta. The line current is reduced to 1/3. Hence, during starting period when motor is star-connected, it takes 1/3rd as much starting current and develops 1/3rd as much torque as would have been developed were it directly connected in delta.
Fig. 32. Star-delta starter
Relation between Tst and Tf:
where Isc is the current per phase which delta-connected motor would have taken if switched on to the supply directly; however, line current at start is equal to 1/3 of line Isc.
Here, 1st and Isc represent phase values.
- This method reduces the starting line current to one-third but the starting torque is also reduced by the same amount.
- This method is cheap but limited to applications where high starting torque is not necessary e.g., machine tools, pumps, motor-generator sets etc.
- The method is unsuitable for motors for voltage exceeding 3000 V because of the excessive number of stator turns needed for delta connection.
- Such starters are employed for starting 3-phase squirrel cage induction motors of rating between 4 and 20 k W.
Precaution with star-delta starting. When the motor is started in star the initial current flowing 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% of its synchronous speed. The change-over from star to delta connection should not be made until the motor attains 90% 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.