Starting methods of Squirrel cage Induction motors:
1. Direct on line starting
2. Primary Resistor (or Reactor) Starting
3. Autotransformer Starting
4. Star delta starting
1. Direct-on-Line Starting or Full-Voltage Starting:
Direct-on-line (DOL) starting is a method of starting an induction motor by connecting it directly to the power supply. This method is typically used for small squirrel-cage induction motors with a power rating of up to 5 kW.
The DOL starter consists of a set of start and stops buttons, a contactor (an electromagnet), and often overload and Undervoltage protection devices. The start button is a momentary contact that is held open by a spring, while the stop button is held closed by a spring. When the start button is pressed, the contactor is energized, closing its contacts and allowing the motor to start. When the stop button is pressed, the contactor is de-energized, opening its contacts and stopping the motor.
Thermal overload relays are commonly used to protect the motor from overheating due to an overload. They work by sensing the temperature of the motor and interrupting the circuit if the temperature gets too high. Undervoltage protection is provided by the contactor, which is controlled by a three-wire control circuit. This circuit maintains the interruption of the circuit even after the supply is restored, ensuring that the motor does not start until the voltage has reached an acceptable level.
2. Primary Resistor (or Reactor) Starting:
The primary resistor (or reactor) starting method is a way of starting a three-phase squirrel cage induction motor by using resistors (or reactors) in series with each stator lead. When the motor is started, the resistors (or reactors) create a voltage drop across the motor terminals, resulting in a reduced voltage applied to the stator. As the motor accelerates, the resistors (or reactors) are gradually removed and finally short circuited when the motor reaches its operating speed.
This method of starting is similar to the dc shunt motor starter, which also uses a series resistor to provide a voltage drop between the power supply and the motor. However, there are some important differences between the two. In a dc shunt motor, the starting torque is directly proportional to the starting armature current, while in a three-phase induction motor, the torque varies as the square of the motor terminal voltage. This means that an equivalent reduction of starting current will result in a much greater reduction of starting torque for an induction motor compared to a dc shunt motor.
Despite the initial higher cost of reactors compared to resistors, reactor starting is often preferred for three-phase induction motors because it results in smaller power losses and is more effective at reducing the voltage applied to the stator at starting. This can be beneficial for reducing the stress on the motor and improving its starting performance.
Advantages of resistor Starter:
- Smooth acceleration: The resistor starter allows the motor to gradually increase in speed, rather than starting at full speed, which can help extend the life of the motor and reduce stress on the system.
- High power factor during start: The power factor, or ratio of real power to apparent power, is higher during the starting process with a resistor starter, which can help improve the efficiency of the system.
- Less expensive than auto-transformer starter in lower output ratings: An auto-transformer starter is a type of starter that uses a transformer to reduce the voltage applied to the motor during starting. In lower output ratings, a resistor starter may be less expensive than an auto-transformer starter.
- Closed transition starting: Closed transition starting refers to a starting method where the motor is brought up to speed without any interruption in power. This can help prevent damage to the motor and the system.
- Available with as many as 7 accelerating points: Some resistor starters have multiple accelerating points, which allows the motor to be ramped up to speed in stages, rather than all at once. This can help improve the smoothness of the starting process.
Disadvantages of using a resistor starter for cage motors include:
- Resistors give off heat: The resistors in a resistor starter generate heat as they dissipate energy. This can be a disadvantage if the system does not have adequate cooling or if the resistors are not properly sized for the application.
- Low torque efficiency: The torque, or turning force, produced by a motor during starting is lower with a resistor starter than with some other starting methods. This can make it more difficult to start the motor, particularly in applications where a high torque is required.
- Starting duration usually exceeds 5 seconds: The starting process with a resistor starter can take longer than with some other starting methods, which can be a disadvantage in applications where a quick start is required.
- Starting voltage is difficult to adjust: The starting voltage of a motor with a resistor starter is difficult to adjust to meet varying conditions, which can be a disadvantage if the system needs to operate under a wide range of load conditions.
3. Autotransformer Starter:
An auto-transformer starter is a type of starter that uses a transformer to reduce the voltage applied to an electric motor during the starting process. The transformer is connected in series with the motor and is used to lower the voltage applied to the motor, which reduces the starting current and helps protect the motor and the system.
The reduced voltage is obtained by taking tappings, or connections, at specific points on the transformer. These tappings are typically located at the 50%, 60%, and 80% points on the transformer, and can be used to adjust the starting voltage to meet the torque requirements of the motor.
There are two types of auto-transformer starters: manually operated and magnetically operated.
A manually operated auto-transformer starter is operated by a person using a switch or a lever to control the voltage applied to the motor. The operator selects the appropriate tapping on the transformer based on the load conditions and the starting torque requirements of the motor.
A magnetically operated auto-transformer starter uses a magnet to automatically switch the tappings on the transformer. The magnet is energized when the motor is started, and it moves to select the appropriate tapping on the transformer based on the load conditions and the starting torque requirements of the motor.
Advantages of Autotransformer Starter:
- Availability of highest torque per ampere of supply current: The auto-transformer starter provides the highest torque per ampere of supply current, which can help improve the starting performance of the motor.
- Adjustment of starting voltage by selection of proper tap on the autotransformer: The auto-transformer has multiple tappings, or connections, that can be used to adjust the starting voltage to meet the torque requirements of the motor. This allows the operator to customize the starting process to meet the specific needs of the application.
- Suitability for long starting periods: The auto-transformer starter is suitable for use in applications where the starting process may take a long time, as it can provide a smooth and controlled starting process.
- Closed transition starting: The auto-transformer starter allows for closed transition starting, which means that the motor is brought up to speed without any interruption in power. This can help prevent damage to the motor and the system.
- Motor current larger than supply current: The motor current with an auto-transformer starter is typically larger than the supply current, which can help improve the starting performance of the motor.
Disadvantages of Autotransformer Starter:
- Low power factor: The power factor, or ratio of real power to apparent power, is lower during the starting process with an auto-transformer starter than with some other starting methods. This can reduce the efficiency of the system.
- Higher cost in case of lower output rating motors: An auto-transformer starter may be more expensive than some other starting methods, particularly for lower output rating motors.
This method of starting is often used for large cage motors (of output rating exceeding 20 kW) because it can provide a smooth and controlled starting process that helps protect the motor and the system. It can be used for both star-connected and delta-connected motors.
4. Star delta starter:
Star-delta starting is a method of starting cage induction motors that involves connecting the stator windings of the motor in a specific configuration to reduce the starting current and protect the motor and the system.
At the starting instant, the stator windings of the motor are connected in a star configuration, which reduces the voltage applied to each winding to 1/√3, or about 57.7%, of the line-to-line voltage. This reduces the starting current and helps protect the motor and the system.
After the motor has reached a certain speed, typically about 75% of full load speed, the stator windings are reconnected in a delta configuration using a change-over switch. In the delta configuration, each winding is connected across the full line-to-line voltage, which allows the motor to operate at full power.
The advantage of star-delta starting is that it allows the motor to be started with a reduced starting current, which helps protect the motor and the system. It is also a relatively simple and inexpensive starting method.
However, there are also some drawbacks to using star-delta starting. The starting process can be slower than with some other starting methods, and the motor may not develop full torque until it is reconnected in the delta configuration. Additionally, the change-over switch can be a point of failure in the system, and the motor may experience a momentary power interruption when the switch is activated.