In this topic, you study Autotransformer Starter – Working & Diagram.
A three phase, star-connected auto-transformer along with a suitable change-over switch forms an auto-transformer starter (Fig. 1). When the switch is in the starting position, the Stator of an induction motor is supplied with reduced voltage through the auto-transformer using suitable tap. This limits the starting current to a safe value. Usually three tappings per phase are provided to give 50, 65 or 80 per cent of normal line voltage across the motor terminals. When the motor attains about 80% of its normal speed, the switch is thrown to run position, which connects the motor directly across the supply and cuts out the autotransformer from the circuit. These actions may be carried out automatically by time-delay relay operated magnetic contactors.
Fig. 1: Auto-transformer starter
The loading of the system during starting and the power loss involved are small with this type of starter. Moreover, it can be used with Star as well as delta connected motors. The provision of several taps on the auto-transformer makes the adjustment possible to suit the local conditions. But it is more expensive than a star-delta starter.
Autotransformer Starter Working Principle
In this method, an autotransformer with suitable tappings is employed for starting star as well as delta connected induction motors. The reduced voltage at starting is obtained by tappings provided on the autotransformer. Usually most of the auto transformers are provided with 50, 65 and 80 percent tappings. When the motor is started, a reduced voltage is applied to the circuit. As the motor picks up speed, the auto transformer connections are cut out gradually and full supply voltage is impressed across the motor terminals. The autotransformer starters may be operated manually or automatically depending on the requirement. Figure shows the circuit diagram of an autotransformer starter operated manually for star connected induction motor.
It consists of an autotransformer which is connected to the stator of induction motor. The other terminals are connected to main through a multi-pole double throw switch. As it is operated manually, it consists of a handle to change the position of the autotransformer from off to start position. Whenever the handle is moved from off to start position, the autotransformer gets activated and a reduced voltage is applied across the motor terminals. As the motor picks up the speed almost to 80%, the autotransformer terminals are cut out and full supply voltage is applied across the motor.
Let,
Is – Starting current
K – Transformation ratio of autotransformer
Ifl – Full-load current
Tst – Starting torque
Tft – Full-load torque
Sft – Full-load slip.
The motor input current i.e., secondary current of autotransformer,
\[{{I}_{2}}=K{{I}_{s}}\]
Supply current = K x Secondary current of autotransformer
\[=K\times K{{I}_{s}}={{K}^{2}}{{I}_{s}}\]
Starting torque,
\[{{T}_{st}}={{T}_{fl}}{{\left( \frac{{{I}_{st}}}{{{I}_{fl}}} \right)}^{2}}\times {{S}_{ft}}\]
\[={{T}_{fl}}{{\left( \frac{K{{I}_{st}}}{{{I}_{fl}}} \right)}^{2}}\times {{S}_{ft}}\]
\[{{T}_{st}}={{K}^{2}}{{T}_{fl}}{{\left( \frac{{{I}_{s}}}{{{I}_{fl}}} \right)}^{2}}\times {{S}_{ft}}\]
Advantages of Autotransformer Starter
- Acceleration is high.
- Line current is reduced.
Disadvantages of Autotransformer Starter
- Uneconomical.
- Used only for large motor.
Summary of Autotransformer Starter
1. The starting current of autotransformer starter is x2 times less than that produced by DOL-sta1ter i.e., Ist = x21sc
2. The starting torque of the autotransformer starter is reduced to x2 times of the starting torque obtained with the DOL starter i.e.,
\[\frac{{{T}_{st}}}{{{T}_{fl}}}={{x}^{2}}{{\left( \frac{{{I}_{st}}}{{{I}_{fl}}} \right)}^{2}}{{s}_{fl}}.\]
3. Autotransformer starter has better flexibility and good starting characteristics when compared to DOL and star-delta starter.
4. Autotransformer starter has better efficiency but it is more expensive when compared to DOL and star-delta starter.
