What are the different methods for the starting of a synchronous motor ?
2 Answers
Starting a synchronous motor is a bit more complex than starting an induction motor because synchronous motors are not self-starting. They require special methods to bring them up to synchronous speed before they can lock into the synchronous operation. The different methods for starting synchronous motors are as follows:
### 1. **Using a Damper Winding (Amortisseur Windings)**
- **Description**: Damper windings are copper bars embedded in the rotor pole faces. These act like the squirrel-cage winding of an induction motor.
- **How it works**: When three-phase power is applied, the motor starts as an induction motor because of the induced currents in the damper windings. Once the motor reaches a speed close to synchronous speed, the excitation (DC supply) is applied to the rotor windings, and the motor is synchronized to the mains.
- **Advantages**: Simple and commonly used method in synchronous motors.
- **Disadvantages**: The motor must reach near-synchronous speed before synchronization, and damper windings can cause losses during steady-state operation.
### 2. **Using a Separate Small Motor (Pony Motor)**
- **Description**: A small auxiliary motor, known as a pony motor, is used to drive the synchronous motor up to near-synchronous speed.
- **How it works**: The pony motor is connected to the shaft of the synchronous motor and drives it up to synchronous speed. Once the speed is nearly synchronous, the field winding of the synchronous motor is energized, and the pony motor is disconnected.
- **Advantages**: Simple and independent of load conditions.
- **Disadvantages**: Additional equipment (pony motor) is required, which increases the cost and complexity of the system.
### 3. **Using Variable Frequency Drive (VFD)**
- **Description**: A Variable Frequency Drive (VFD) is used to gradually bring the synchronous motor up to speed.
- **How it works**: The VFD starts the motor at a low frequency, and as the motor accelerates, the frequency is gradually increased. Once the motor reaches synchronous speed, the VFD locks the rotor to the synchronous speed and continues operating at that frequency.
- **Advantages**: Smooth start with full control over the motor's speed. The motor can be started under load.
- **Disadvantages**: Expensive compared to other starting methods and requires complex electronics.
### 4. **Using a Rotor DC Motor**
- **Description**: A small DC motor is used to bring the synchronous motor's rotor up to synchronous speed.
- **How it works**: The rotor DC motor is mechanically coupled to the synchronous motor's shaft. It brings the rotor up to synchronous speed, and then the synchronous motor is synchronized by exciting the field winding.
- **Advantages**: Can be used for large motors.
- **Disadvantages**: Requires additional DC motor and mechanical coupling system.
### 5. **Using Direct-on-Line (DOL) Starting with a Prime Mover (for Larger Motors)**
- **Description**: A mechanical prime mover such as a turbine, diesel engine, or gas engine is used to bring the synchronous motor to near-synchronous speed.
- **How it works**: The prime mover accelerates the synchronous motor's rotor to synchronous speed. Once the motor reaches this speed, the field winding is energized, and the motor is connected to the grid.
- **Advantages**: Suitable for very large motors in power plants.
- **Disadvantages**: Complex and requires additional equipment.
### 6. **Using a Reactor Start**
- **Description**: A reactor is inserted into the supply line to limit the initial current, allowing the motor to accelerate slowly.
- **How it works**: The reactor reduces the supply voltage, which decreases the starting torque and current. Once the motor reaches a speed near synchronous, the reactor is bypassed, and the motor is synchronized.
- **Advantages**: Limits starting current and torque.
- **Disadvantages**: Reduced starting torque may not be suitable for loaded starts.
### 7. **Using a Slip Ring Induction Motor Start (Hybrid Method)**
- **Description**: A synchronous motor can be started as a slip-ring induction motor using external resistances connected to the rotor circuit.
- **How it works**: Initially, the synchronous motor behaves like a slip-ring induction motor. External resistance is added to control the starting current and improve the torque. Once the motor reaches synchronous speed, the resistances are cut out, and the rotor is excited with a DC supply for synchronous operation.
- **Advantages**: Smooth start with good control over current and torque.
- **Disadvantages**: More complex, as it requires slip rings and additional resistors.
### Summary of Starting Methods:
| **Method** | **Advantages** | **Disadvantages** |
|----------------------------------|-------------------------------------------------|----------------------------------------------------|
| Damper Winding | Simple, common method | Losses during steady-state operation |
| Pony Motor | Simple, independent of load | Requires additional equipment |
| Variable Frequency Drive (VFD) | Smooth start, full control over speed | Expensive |
| Rotor DC Motor | Suitable for large motors | Requires additional motor and coupling |
| Prime Mover (Mechanical Start) | Good for large motors in power plants | Complex and requires additional mechanical system |
| Reactor Start | Limits starting current and torque | Reduced starting torque |
| Slip Ring Induction Motor Start | Good control over starting current and torque | Complex with slip rings and resistors |
Each method has its advantages and limitations, and the choice depends on factors like motor size, load conditions, and system cost.
### 1. **Using a Damper Winding (Amortisseur Windings)**
- **Description**: Damper windings are copper bars embedded in the rotor pole faces. These act like the squirrel-cage winding of an induction motor.
- **How it works**: When three-phase power is applied, the motor starts as an induction motor because of the induced currents in the damper windings. Once the motor reaches a speed close to synchronous speed, the excitation (DC supply) is applied to the rotor windings, and the motor is synchronized to the mains.
- **Advantages**: Simple and commonly used method in synchronous motors.
- **Disadvantages**: The motor must reach near-synchronous speed before synchronization, and damper windings can cause losses during steady-state operation.
### 2. **Using a Separate Small Motor (Pony Motor)**
- **Description**: A small auxiliary motor, known as a pony motor, is used to drive the synchronous motor up to near-synchronous speed.
- **How it works**: The pony motor is connected to the shaft of the synchronous motor and drives it up to synchronous speed. Once the speed is nearly synchronous, the field winding of the synchronous motor is energized, and the pony motor is disconnected.
- **Advantages**: Simple and independent of load conditions.
- **Disadvantages**: Additional equipment (pony motor) is required, which increases the cost and complexity of the system.
### 3. **Using Variable Frequency Drive (VFD)**
- **Description**: A Variable Frequency Drive (VFD) is used to gradually bring the synchronous motor up to speed.
- **How it works**: The VFD starts the motor at a low frequency, and as the motor accelerates, the frequency is gradually increased. Once the motor reaches synchronous speed, the VFD locks the rotor to the synchronous speed and continues operating at that frequency.
- **Advantages**: Smooth start with full control over the motor's speed. The motor can be started under load.
- **Disadvantages**: Expensive compared to other starting methods and requires complex electronics.
### 4. **Using a Rotor DC Motor**
- **Description**: A small DC motor is used to bring the synchronous motor's rotor up to synchronous speed.
- **How it works**: The rotor DC motor is mechanically coupled to the synchronous motor's shaft. It brings the rotor up to synchronous speed, and then the synchronous motor is synchronized by exciting the field winding.
- **Advantages**: Can be used for large motors.
- **Disadvantages**: Requires additional DC motor and mechanical coupling system.
### 5. **Using Direct-on-Line (DOL) Starting with a Prime Mover (for Larger Motors)**
- **Description**: A mechanical prime mover such as a turbine, diesel engine, or gas engine is used to bring the synchronous motor to near-synchronous speed.
- **How it works**: The prime mover accelerates the synchronous motor's rotor to synchronous speed. Once the motor reaches this speed, the field winding is energized, and the motor is connected to the grid.
- **Advantages**: Suitable for very large motors in power plants.
- **Disadvantages**: Complex and requires additional equipment.
### 6. **Using a Reactor Start**
- **Description**: A reactor is inserted into the supply line to limit the initial current, allowing the motor to accelerate slowly.
- **How it works**: The reactor reduces the supply voltage, which decreases the starting torque and current. Once the motor reaches a speed near synchronous, the reactor is bypassed, and the motor is synchronized.
- **Advantages**: Limits starting current and torque.
- **Disadvantages**: Reduced starting torque may not be suitable for loaded starts.
### 7. **Using a Slip Ring Induction Motor Start (Hybrid Method)**
- **Description**: A synchronous motor can be started as a slip-ring induction motor using external resistances connected to the rotor circuit.
- **How it works**: Initially, the synchronous motor behaves like a slip-ring induction motor. External resistance is added to control the starting current and improve the torque. Once the motor reaches synchronous speed, the resistances are cut out, and the rotor is excited with a DC supply for synchronous operation.
- **Advantages**: Smooth start with good control over current and torque.
- **Disadvantages**: More complex, as it requires slip rings and additional resistors.
### Summary of Starting Methods:
| **Method** | **Advantages** | **Disadvantages** |
|----------------------------------|-------------------------------------------------|----------------------------------------------------|
| Damper Winding | Simple, common method | Losses during steady-state operation |
| Pony Motor | Simple, independent of load | Requires additional equipment |
| Variable Frequency Drive (VFD) | Smooth start, full control over speed | Expensive |
| Rotor DC Motor | Suitable for large motors | Requires additional motor and coupling |
| Prime Mover (Mechanical Start) | Good for large motors in power plants | Complex and requires additional mechanical system |
| Reactor Start | Limits starting current and torque | Reduced starting torque |
| Slip Ring Induction Motor Start | Good control over starting current and torque | Complex with slip rings and resistors |
Each method has its advantages and limitations, and the choice depends on factors like motor size, load conditions, and system cost.