A synchronous motor experiences several types of torques during starting, synchronization, and running. Understanding these torques is essential for selecting the right motor for a given application and for ensuring stable operation under varying loads.
In this article, we will explain the four important torques in a synchronous motor — locked rotor torque, running torque, pull-in torque, and pull-out torque — along with their significance and practical implications.
Table of Contents
Overview of Torques
A synchronous motor goes through different phases during operation — starting, pulling into synchronism, and running at synchronous speed. Each phase involves a different type of torque:
- Locked Rotor Torque — torque at standstill (starting)
- Running Torque — torque during normal operation
- Pull-In Torque — torque needed to pull into synchronism
- Pull-Out Torque — maximum torque before losing synchronism
These torques are always considered when selecting a synchronous motor for any particular application.
Locked Rotor Torque
Locked rotor torque is the minimum torque developed by the motor when the rotor is held stationary (locked) and the motor is supplied with rated voltage and frequency.
Since a synchronous motor is not self-starting, it uses damper windings (amortisseur windings) on the rotor to develop starting torque. These short-circuited bars work on the induction motor principle — the rotating stator field induces currents in the damper bars, producing torque.
Key points about locked rotor torque:
- It is produced by the damper winding only (DC field excitation is not applied during starting)
- Typically 20–50% of rated torque for most synchronous motors
- Must be sufficient to overcome the static friction of the connected load
- Higher locked rotor torque requires heavier damper windings
Running Torque
Running torque is the torque developed by the motor under normal running conditions at synchronous speed. It is determined by the power rating and speed of the driven machine.
Where:
- P = Mechanical power output (watts)
- ωs = Synchronous angular speed (rad/s)
- Ns = Synchronous speed (RPM)
The running torque must always be less than the pull-out torque to maintain stable operation. A safety margin of 25–50% is typically maintained between running torque and pull-out torque.
Pull-In Torque
Pull-in torque is the maximum constant load torque under which a synchronous motor can pull the connected load into synchronism when DC excitation is applied.
How Pull-In Works
The starting sequence of a synchronous motor is:
- Step 1: Motor starts as an induction motor using damper windings (no DC excitation)
- Step 2: Rotor accelerates to about 95–98% of synchronous speed
- Step 3: DC excitation is applied to the field winding
- Step 4: The rotor "pulls in" and locks to the synchronously rotating stator field
The pull-in torque determines the maximum load the motor can carry during this synchronization process. If the load torque exceeds the pull-in torque, the motor will continue running as an induction motor (at sub-synchronous speed) and will never achieve synchronism.
Pull-in torque depends on:
- Moment of inertia of the rotor and connected load
- Strength of the DC field excitation
- Slip at the moment of excitation application
Pull-Out Torque
Pull-out torque (also called breakdown torque or maximum torque) is the maximum torque that a synchronous motor can develop at rated voltage and frequency without losing synchronism.
If the load torque exceeds the pull-out torque, the rotor cannot maintain synchronous speed. The load angle (δ) exceeds 90° (for a cylindrical rotor machine), and the motor "pulls out" of synchronism — it loses lock with the rotating field and stops.
Where:
- V = Supply voltage per phase
- Eb = Back EMF per phase
- Xs = Synchronous reactance per phase
- ωs = Synchronous angular speed
Typical pull-out torque is 1.5 to 3.5 times the rated torque, depending on the motor design and excitation level.
Torque and Load Angle Relationship
The electromagnetic torque of a synchronous motor varies with the load angle (δ) — the angle between the rotor field axis and the stator rotating field axis:
Key observations:
- At no load: δ ≈ 0° (rotor field aligned with stator field)
- As load increases: δ increases, torque increases
- At δ = 90°: Maximum torque (pull-out torque) is reached
- Beyond 90°: Torque decreases — motor loses synchronism
The stable operating region is 0° < δ < 90°. The motor is designed to operate well within this range under rated conditions (typically δ = 20°–30° at full load).
Comparison Table
Practical Significance
- Motor selection: The pull-out torque must be greater than the maximum expected load torque (including transients)
- Starting: The locked rotor torque must exceed the breakaway torque of the driven load
- Load matching: The pull-in torque must be sufficient for the total inertia of the motor-load system
- Protection: If pull-out torque is exceeded, protective relays detect loss of synchronism and trip the motor
- Excitation control: Increasing field excitation increases pull-out torque, providing a larger stability margin. The V curve of synchronous motor shows how excitation affects motor behavior.
FAQs
What happens when pull-out torque is exceeded?
The motor loses synchronism — the rotor falls out of step with the rotating field. The motor either stops or runs at sub-synchronous speed as an induction motor (using damper windings). Protective relays typically disconnect the motor to prevent overheating.
How can pull-out torque be increased?
By increasing the field excitation (higher Eb), increasing the supply voltage, or reducing the synchronous reactance. In practice, increasing field current is the most common method.
Why is pull-in torque important for high-inertia loads?
High-inertia loads (like large compressors or flywheels) are difficult to synchronize because the rotor must accelerate the entire mass to exactly synchronous speed. If pull-in torque is insufficient, the motor will never achieve synchronism and will run inefficiently as an induction motor.
What is the relationship between load angle and stability?
The motor is stable when the load angle is between 0° and 90°. At rated load, the angle is typically 20°–30°. The difference between the current load angle and 90° represents the stability margin. A larger margin means the motor can handle sudden load increases without losing synchronism.
Does a synchronous motor have reluctance torque?
Yes, in salient pole synchronous motors. The reluctance torque arises because the rotor has different reluctance along the direct (d) and quadrature (q) axes. This additional torque component helps the motor maintain synchronism and adds to the total electromagnetic torque.