Describe any two methods of starting synchronous motors.
2 Answers
Starting synchronous motors can be a bit tricky because they require the rotor to be brought up to near synchronous speed before they can be synchronized with the stator magnetic field. Here are two common methods for starting synchronous motors:
### 1. **Using an Induction Motor Start (Direct-On-Line Start)**
In this method, the synchronous motor is initially started as an induction motor. The rotor is typically equipped with damper bars or a squirrel-cage winding. Here's how it works:
- **Initial Start-Up**: When power is applied to the motor, the stator produces a rotating magnetic field. The rotor, which is initially stationary, induces currents in the damper bars. This induced current generates its own magnetic field, causing the rotor to rotate.
- **Acceleration to Near Synchronous Speed**: The motor accelerates to about 70-80% of the synchronous speed. This is crucial because the rotor must be close to the synchronous speed to avoid stalling when synchronized.
- **Field Excitation**: Once the rotor reaches the required speed, the DC field winding of the synchronous motor is energized, and the rotor locks into synchronization with the stator's rotating magnetic field.
**Advantages**:
- Simple and cost-effective for large motors.
- Allows for smooth acceleration without requiring complex starting equipment.
**Disadvantages**:
- High inrush currents during start-up.
- Potential for rotor overheating if not monitored properly.
### 2. **Using a Variable Frequency Drive (VFD)**
A Variable Frequency Drive can also be used to start a synchronous motor smoothly. This method provides greater control over the motor's acceleration and speed:
- **Frequency Control**: The VFD varies the frequency of the supply voltage, which allows the motor to start at a lower speed. This enables the rotor to accelerate gradually without the risk of stalling.
- **Synchronization**: As the motor approaches synchronous speed, the VFD can adjust the frequency to precisely match the synchronous speed of the motor. Once this is achieved, the DC field excitation is applied, allowing the motor to synchronize.
- **Soft Start**: This method provides a "soft start" feature that limits the starting current and reduces mechanical stress on the motor and the connected load.
**Advantages**:
- Improved control over the starting process.
- Reduced starting currents and mechanical stresses.
- Ability to operate at variable speeds beyond the synchronous speed.
**Disadvantages**:
- Higher initial cost due to the VFD.
- Requires more complex control systems.
### Conclusion
Both methods have their own advantages and disadvantages, and the choice depends on the specific application, the size of the motor, and operational requirements. Direct-On-Line starting is simpler and more cost-effective for larger motors, while VFDs offer more precise control and reduced stress on the motor system.
### 1. **Using an Induction Motor Start (Direct-On-Line Start)**
In this method, the synchronous motor is initially started as an induction motor. The rotor is typically equipped with damper bars or a squirrel-cage winding. Here's how it works:
- **Initial Start-Up**: When power is applied to the motor, the stator produces a rotating magnetic field. The rotor, which is initially stationary, induces currents in the damper bars. This induced current generates its own magnetic field, causing the rotor to rotate.
- **Acceleration to Near Synchronous Speed**: The motor accelerates to about 70-80% of the synchronous speed. This is crucial because the rotor must be close to the synchronous speed to avoid stalling when synchronized.
- **Field Excitation**: Once the rotor reaches the required speed, the DC field winding of the synchronous motor is energized, and the rotor locks into synchronization with the stator's rotating magnetic field.
**Advantages**:
- Simple and cost-effective for large motors.
- Allows for smooth acceleration without requiring complex starting equipment.
**Disadvantages**:
- High inrush currents during start-up.
- Potential for rotor overheating if not monitored properly.
### 2. **Using a Variable Frequency Drive (VFD)**
A Variable Frequency Drive can also be used to start a synchronous motor smoothly. This method provides greater control over the motor's acceleration and speed:
- **Frequency Control**: The VFD varies the frequency of the supply voltage, which allows the motor to start at a lower speed. This enables the rotor to accelerate gradually without the risk of stalling.
- **Synchronization**: As the motor approaches synchronous speed, the VFD can adjust the frequency to precisely match the synchronous speed of the motor. Once this is achieved, the DC field excitation is applied, allowing the motor to synchronize.
- **Soft Start**: This method provides a "soft start" feature that limits the starting current and reduces mechanical stress on the motor and the connected load.
**Advantages**:
- Improved control over the starting process.
- Reduced starting currents and mechanical stresses.
- Ability to operate at variable speeds beyond the synchronous speed.
**Disadvantages**:
- Higher initial cost due to the VFD.
- Requires more complex control systems.
### Conclusion
Both methods have their own advantages and disadvantages, and the choice depends on the specific application, the size of the motor, and operational requirements. Direct-On-Line starting is simpler and more cost-effective for larger motors, while VFDs offer more precise control and reduced stress on the motor system.
Starting a synchronous motor can be challenging because, unlike induction motors, synchronous motors are not self-starting. Here are two common methods used to start synchronous motors:
### 1. **Using an Auxiliary Motor (Pony Motor)**
This method involves using a separate, smaller motor known as an auxiliary motor or pony motor to bring the synchronous motor up to speed. The process is as follows:
- **Initial Start**: The pony motor is connected to the shaft of the synchronous motor. When the pony motor is started, it drives the synchronous motor up to near synchronous speed.
- **Synchronization**: Once the synchronous motor reaches near synchronous speed, the excitation of the synchronous motor's rotor is activated. This creates a magnetic field in the rotor, allowing it to lock in with the rotating magnetic field of the stator.
- **Disconnection**: After synchronization, the pony motor is disconnected, and the synchronous motor continues to operate at synchronous speed.
**Advantages**:
- Simple and effective, especially for large synchronous motors.
- The pony motor can be relatively small, as it only needs to bring the synchronous motor up to speed.
**Disadvantages**:
- Requires additional equipment (the pony motor).
- The process can be time-consuming.
### 2. **Damper Windings (Amortisseur Windings)**
Damper windings, also known as amortisseur windings, are special short-circuited windings placed on the rotor of the synchronous motor. This method works as follows:
- **Rotor Construction**: The rotor of the synchronous motor is equipped with damper windings, which are similar to the squirrel cage of an induction motor.
- **Starting**: When power is applied, the motor initially behaves like an induction motor due to the damper windings. The motor starts to accelerate as an induction motor because the damper windings create a torque that causes the rotor to spin.
- **Synchronization**: As the rotor approaches synchronous speed, the excitation is applied to the main field winding, and the motor transitions from induction motor operation to synchronous motor operation. The rotor locks into synchronization with the stator's magnetic field.
- **Damper Windings Function**: After synchronization, the damper windings do not contribute to the motor’s operation, as the rotor is locked in synchronously.
**Advantages**:
- No need for external starting equipment like a pony motor.
- The motor starts automatically when power is applied.
**Disadvantages**:
- Higher starting current compared to other methods.
- The damper windings may add to the cost and complexity of the motor design.
These methods are widely used based on the specific requirements of the application, such as the size of the motor, the available power supply, and the need for starting torque.
### 1. **Using an Auxiliary Motor (Pony Motor)**
This method involves using a separate, smaller motor known as an auxiliary motor or pony motor to bring the synchronous motor up to speed. The process is as follows:
- **Initial Start**: The pony motor is connected to the shaft of the synchronous motor. When the pony motor is started, it drives the synchronous motor up to near synchronous speed.
- **Synchronization**: Once the synchronous motor reaches near synchronous speed, the excitation of the synchronous motor's rotor is activated. This creates a magnetic field in the rotor, allowing it to lock in with the rotating magnetic field of the stator.
- **Disconnection**: After synchronization, the pony motor is disconnected, and the synchronous motor continues to operate at synchronous speed.
**Advantages**:
- Simple and effective, especially for large synchronous motors.
- The pony motor can be relatively small, as it only needs to bring the synchronous motor up to speed.
**Disadvantages**:
- Requires additional equipment (the pony motor).
- The process can be time-consuming.
### 2. **Damper Windings (Amortisseur Windings)**
Damper windings, also known as amortisseur windings, are special short-circuited windings placed on the rotor of the synchronous motor. This method works as follows:
- **Rotor Construction**: The rotor of the synchronous motor is equipped with damper windings, which are similar to the squirrel cage of an induction motor.
- **Starting**: When power is applied, the motor initially behaves like an induction motor due to the damper windings. The motor starts to accelerate as an induction motor because the damper windings create a torque that causes the rotor to spin.
- **Synchronization**: As the rotor approaches synchronous speed, the excitation is applied to the main field winding, and the motor transitions from induction motor operation to synchronous motor operation. The rotor locks into synchronization with the stator's magnetic field.
- **Damper Windings Function**: After synchronization, the damper windings do not contribute to the motor’s operation, as the rotor is locked in synchronously.
**Advantages**:
- No need for external starting equipment like a pony motor.
- The motor starts automatically when power is applied.
**Disadvantages**:
- Higher starting current compared to other methods.
- The damper windings may add to the cost and complexity of the motor design.
These methods are widely used based on the specific requirements of the application, such as the size of the motor, the available power supply, and the need for starting torque.