Name any two methods of starting synchronous motors.
2 Answers
Starting synchronous motors can be a bit tricky because they require the rotor to be synchronized with the rotating magnetic field of the stator. Here are two common methods for starting synchronous motors:
### 1. **Direct On-Line (DOL) Starting with Auxiliary Means**
In this method, the synchronous motor is connected directly to the power supply. However, since synchronous motors do not have the ability to start under load without assistance, additional methods are employed:
- **Using an Auxiliary Starter:** Sometimes, an auxiliary starting device, such as a small induction motor or a variable frequency drive (VFD), is used to bring the motor up to near synchronous speed. Once the rotor approaches the synchronous speed, the synchronous motor can be switched to operate in its synchronous mode.
- **Phase-Wound Rotor:** Some synchronous motors have a wound rotor with slip rings and brushes. An external DC source can be connected to the rotor windings to create a magnetic field, allowing the rotor to develop sufficient torque to catch up with the stator's rotating magnetic field once it reaches a certain speed.
### 2. **Using Variable Frequency Drives (VFDs)**
VFDs are increasingly popular for starting synchronous motors. Here's how this method works:
- **Speed Control:** The VFD can start the motor at a low frequency, allowing the rotor to accelerate gradually to synchronous speed. By adjusting the frequency and voltage supplied to the motor, the VFD controls the motor's speed effectively.
- **Soft Start:** This method reduces the initial current draw and mechanical stress on the motor. Once the motor reaches the desired speed, the VFD can switch to the normal operation mode, allowing the motor to run in synchronous mode efficiently.
Both methods have their advantages and are chosen based on specific application requirements, including load characteristics and operational constraints.
### 1. **Direct On-Line (DOL) Starting with Auxiliary Means**
In this method, the synchronous motor is connected directly to the power supply. However, since synchronous motors do not have the ability to start under load without assistance, additional methods are employed:
- **Using an Auxiliary Starter:** Sometimes, an auxiliary starting device, such as a small induction motor or a variable frequency drive (VFD), is used to bring the motor up to near synchronous speed. Once the rotor approaches the synchronous speed, the synchronous motor can be switched to operate in its synchronous mode.
- **Phase-Wound Rotor:** Some synchronous motors have a wound rotor with slip rings and brushes. An external DC source can be connected to the rotor windings to create a magnetic field, allowing the rotor to develop sufficient torque to catch up with the stator's rotating magnetic field once it reaches a certain speed.
### 2. **Using Variable Frequency Drives (VFDs)**
VFDs are increasingly popular for starting synchronous motors. Here's how this method works:
- **Speed Control:** The VFD can start the motor at a low frequency, allowing the rotor to accelerate gradually to synchronous speed. By adjusting the frequency and voltage supplied to the motor, the VFD controls the motor's speed effectively.
- **Soft Start:** This method reduces the initial current draw and mechanical stress on the motor. Once the motor reaches the desired speed, the VFD can switch to the normal operation mode, allowing the motor to run in synchronous mode efficiently.
Both methods have their advantages and are chosen based on specific application requirements, including load characteristics and operational constraints.
Synchronous motors are a type of AC motor that operates at a constant speed, synchronized with the frequency of the supply current. Unlike induction motors, synchronous motors are not self-starting. Therefore, specific methods must be used to bring them up to synchronous speed before they can run on their own. Here are two common methods for starting synchronous motors:
### 1. **Using an Induction Motor (Damper Winding or Amortisseur Winding)**
In this method, the synchronous motor is equipped with additional winding on the rotor, known as damper windings or amortisseur windings. These windings are similar to the squirrel cage winding in induction motors.
**How it works:**
- When the motor is powered on, the stator produces a rotating magnetic field that induces a current in the damper windings.
- This induced current produces a torque, allowing the motor to start and accelerate as an induction motor.
- As the motor approaches synchronous speed, the rotor's magnetic field (produced by DC excitation) locks in with the stator's rotating magnetic field.
- Once the rotor reaches synchronous speed, the damper windings cease to produce significant torque, and the motor continues running synchronously.
**Advantages:**
- Provides a smooth start and acceleration.
- The motor can start under load.
**Disadvantages:**
- Higher starting current.
- Not suitable for very high-power motors without additional assistance.
### 2. **Using a Separate Motor (Pony Motor Starting)**
Another method of starting a synchronous motor is by using a small auxiliary motor, known as a "pony motor," to bring the synchronous motor up to near-synchronous speed.
**How it works:**
- The pony motor is coupled to the shaft of the synchronous motor.
- When power is applied, the pony motor accelerates the synchronous motor to a speed close to its synchronous speed.
- Once the synchronous motor reaches near-synchronous speed, DC excitation is applied to the rotor.
- The rotor then locks into synchronization with the stator's rotating magnetic field.
- The pony motor is then disengaged, and the synchronous motor continues to run at synchronous speed.
**Advantages:**
- Reduces the high inrush current associated with other starting methods.
- Suitable for large motors that cannot tolerate high starting currents.
**Disadvantages:**
- Requires additional equipment (the pony motor).
- More complex setup and higher initial cost.
Both methods are widely used depending on the application, motor size, and the starting conditions. Damper windings are common for motors where the load does not allow for a long starting time, while pony motors are more suited for applications where inrush current must be minimized.
### 1. **Using an Induction Motor (Damper Winding or Amortisseur Winding)**
In this method, the synchronous motor is equipped with additional winding on the rotor, known as damper windings or amortisseur windings. These windings are similar to the squirrel cage winding in induction motors.
**How it works:**
- When the motor is powered on, the stator produces a rotating magnetic field that induces a current in the damper windings.
- This induced current produces a torque, allowing the motor to start and accelerate as an induction motor.
- As the motor approaches synchronous speed, the rotor's magnetic field (produced by DC excitation) locks in with the stator's rotating magnetic field.
- Once the rotor reaches synchronous speed, the damper windings cease to produce significant torque, and the motor continues running synchronously.
**Advantages:**
- Provides a smooth start and acceleration.
- The motor can start under load.
**Disadvantages:**
- Higher starting current.
- Not suitable for very high-power motors without additional assistance.
### 2. **Using a Separate Motor (Pony Motor Starting)**
Another method of starting a synchronous motor is by using a small auxiliary motor, known as a "pony motor," to bring the synchronous motor up to near-synchronous speed.
**How it works:**
- The pony motor is coupled to the shaft of the synchronous motor.
- When power is applied, the pony motor accelerates the synchronous motor to a speed close to its synchronous speed.
- Once the synchronous motor reaches near-synchronous speed, DC excitation is applied to the rotor.
- The rotor then locks into synchronization with the stator's rotating magnetic field.
- The pony motor is then disengaged, and the synchronous motor continues to run at synchronous speed.
**Advantages:**
- Reduces the high inrush current associated with other starting methods.
- Suitable for large motors that cannot tolerate high starting currents.
**Disadvantages:**
- Requires additional equipment (the pony motor).
- More complex setup and higher initial cost.
Both methods are widely used depending on the application, motor size, and the starting conditions. Damper windings are common for motors where the load does not allow for a long starting time, while pony motors are more suited for applications where inrush current must be minimized.