WAZIPOINT Engineering Science & Technology: What is Synchronous Speed?

## Can We Get Synchronous Speed Without a Synchronous Motor?

Synchronous speed refers to the rotational speed at which the magnetic field in the stator of an AC motor rotates. This speed is determined by the frequency of the power supply and the number of poles in the motor.

Synchronous speed (${�}_{�}$) can be calculated using the following formula:

=120×FrequencyNumber of Motor Poles

However, achieving synchronous speed without a synchronous motor can be challenging because synchronous speed is a characteristic of synchronous motors. Other types of motors, such as induction motors, operate at speeds slightly lower than synchronous speed due to slip.

Induction Motors: These motors typically run at speeds slightly below synchronous speed. The difference between synchronous speed and the actual speed of an induction motor is called "slip." The rotor speed is influenced by the slip to generate torque. Variable Frequency Drives (VFDs): VFDs can control the frequency and voltage supplied to the motor, altering its speed. While VFDs allow for speed control, they can't inherently achieve synchronous speed in an induction motor because the motor itself is not designed for synchronous operation. To achieve synchronous speed without a synchronous motor, the motor would need significant modifications, such as altering the rotor design or using specialized controls. Additionally, the power supply frequency would need to match the motor's design frequency precisely, which can be quite challenging and isn't typically done with motors designed for asynchronous operation. In summary, while you can calculate synchronous speed using the formula, achieving that speed without a synchronous motor specifically designed for it is highly complex and impractical in most cases.

### What is Synchronous Speed?

Synchronous speed refers to the theoretical speed at which the magnetic field rotates within an electric motor. It's determined by the frequency of the power supply and the number of poles in the motor.

The formula to calculate synchronous speed (Ns) in revolutions per minute (RPM) is:

Where:

• ${�}_{�}$ = Synchronous speed in RPM
• 120 = Constant
• Frequency = Frequency of the power supply (in Hertz)
• Number of Poles = Total number of magnetic poles in the motor

Synchronous speed is fundamental in understanding the performance of electric motors. In ideal conditions, when the rotor in an AC motor operates at synchronous speed, it moves at the same speed as the rotating magnetic field in the stator. However, due to factors like mechanical losses and slip (the difference between synchronous speed and actual rotor speed), the motor typically operates at a speed slightly lower than synchronous speed when under load.

### Why Synchronous Speed is Required?

Synchronous speed is a fundamental concept in electrical engineering, especially in relation to alternating current (AC) motors. It refers to the speed at which the magnetic field created by the motor's stator rotates. This speed is determined by the frequency of the power supply and the number of poles in the motor.

In an ideal scenario, synchronous speed is necessary for several reasons:

1. Matching Power Supply Frequency: Synchronous speed ensures that the rotating magnetic field within the motor matches the frequency of the power supply. This synchronization is crucial for the motor's efficiency and performance. If the motor operates at a speed significantly different from its synchronous speed, it can experience efficiency losses and overheating.
2. Maintaining Efficiency: AC motors, particularly synchronous motors, are designed to operate at or near their synchronous speed for optimal efficiency. When operating at synchronous speed, these motors minimize energy losses due to friction and internal heating.
3. Maintaining Proper Torque: Synchronous speed is linked to the motor's ability to deliver torque effectively. If a motor operates below synchronous speed, it may not produce the required torque for its intended applications.
4. Frequency Control in Power Generation: In power generation, synchronous speed is critical for generators. It ensures that the generator rotates at the same speed as the grid frequency, maintaining the synchronization needed for the power grid's stability.
5. Industrial Applications: In various industrial processes where precise speeds are required, synchronous motors are used for their ability to operate consistently at their synchronous speed. These motors find applications in industries such as manufacturing, robotics, and precision machinery.

While synchronous speed is essential for synchronous motors, it's worth noting that induction motors, which are more commonly used, typically operate slightly below synchronous speed due to slip—the difference between the synchronous speed and the actual rotor speed. This slip is necessary for induction motors to generate torque and overcome inertia when starting up or operating under load.

In summary, synchronous speed is required in electrical motors, particularly synchronous motors, to ensure efficiency, torque generation, and synchronization with the power supply frequency. Operating at or close to synchronous speed optimizes the motor's performance and efficiency in various applications.