Synchronous Motors Overview

Synchronous Motor

• A synchronous motor is an electric motor that operates at a synchronous speed, converting electrical energy into rotational power.
• Synchronous Motors run at a speed directly proportional to the frequency of the AC power supply.
• Synchronous motors differ from asynchronous motors in their ability to operate at the synchronous speed.
• Synchronous motors are categorized into two types: Non-excited and Excited Synchronous Motors.
• Non-excited synchronous motors have rotors that align with the stator's magnetic field, driving the motor's rotation.
• Excited synchronous motors have an external DC power source that energizes the rotor field, causing rotation.

Construction and Components

• The stator is the stationary part, typically made of silicon-steel laminations with internal slots.
• The stator winding is a 3-phase winding (star or delta-connected) made of copper wire, placed in the stator slots.
• The rotor is the rotating part, made of silicon-steel laminations.
• Rotor winding is an enameled copper winding placed on the rotor poles, energized by a DC exciter.
• The exciter is a small DC generator mounted on the same shaft as the rotor, providing DC current to the rotor winding.
• Slip-rings and brushes facilitate electrical contact between the stationary exciter and the rotating rotor winding.

Working Principle

• When a 3-phase supply is applied to the stator winding, a rotating magnetic field is generated.
• The rotor poles are energized by the DC exciter, creating magnetic poles.
• The interaction of the stator's rotating magnetic field and the rotor's magnetic poles creates a torque that drives the rotor.
• The rotor rotates at the same speed as the stator's rotating magnetic field (synchronous speed).
• Synchronous motors are not self-starting because they don't produce a starting torque.

Advantages

• Power factor control: By adjusting the excitation, the power factor can be controlled (leading, lagging, or unity).
• Power factor correction: Can be used to improve the overall power factor of a system.
• High efficiency: Generally, higher efficiency compared to induction motors, especially in low-speed applications.
• Stable mechanical operation: Wider air gap allows for greater mechanical stability.

Disadvantages

• No self-starting: Requires additional starting methods.
• Requires external DC excitation: Complicates the system and may need maintenance.
• Susceptible to hunting: Sudden load changes can cause instability and oscillation of the rotor (hunting) due to inertia.

Starting Methods

• Direct Online (DOL) Starting: Directly connecting the motor to the power supply. Suitable for small motors with low starting torque requirements.
• Autotransformer Starting: Utilizing an autotransformer to reduce the starting voltage. Enables smooth acceleration.
• Rotor Resistance Starting: Adding resistors in series with the rotor windings. Offers adjustable starting torque.
• Soft Starters and Variable Frequency Drives (VFDs): Electronic devices that gradually increase voltage and frequency, reducing starting current and stress.
• The choice of starting method depends on the motor size, application requirements, and desired performance.

Torque Equation of Synchronous Motor

• The equation calculates the torque developed by a synchronous motor: T = (3/2) * (Es * V / Xs) * sin(?)
• Where:
• T is the developed torque
• Es is the synchronous reactance voltage per phase
• V is the terminal voltage per phase
• Xs is the synchronous reactance per phase
• ? is the power angle

V-curve and Inverted V-curve

• V-curve represents the relationship between field excitation and power factor, keeping torque constant.
• At low excitation, the power factor is low (over-excited, leading power factor).
• As excitation increases, the power factor improves, reaching unity power factor.
• Further increasing excitation beyond the optimum point leads to under-excited (lagging power factor).
• Inverted V-curve shows the inverse relationship between field current and power factor, reflecting the V-curve for power factor.

Power Angle Characteristics

• Power angle (?) is the angle between the stator magnetic field and the rotor magnetic field.
• It determines the rotor's relative position with respect to the rotating stator field.
• Torque is proportional to sin(?), with maximum torque occurring when ? = 90°.
• Synchronous motors operate within a limited power angle range for stable operation (typically ±20° to ±30°).
• Pull-out torque is the maximum torque a synchronous motor can produce before losing synchronism.
• The power angle is important for power system stability, load management, and variable speed drive applications.

Applications

• Power generation plants: Used as synchronous condensers for power factor correction and voltage regulation.
• Industrial applications: Used in pumps, compressors, mills, and heavy machinery.
• Consumer electronics: Found in clocks, turntables, and other appliances.
• Other applications: Used in electric vehicles, wind turbines, and other specialized applications.