AC Motors: An Educational Resource for Electrical Engineering#

Principle of Operation: Slip#

The key characteristic distinguishing an induction motor is its dependence on slip.

Slip (S): The relative difference in speed between the stator’s rotating magnetic field (synchronous speed, N_s) and the rotor’s actual rotational speed (N_r). It is usually expressed as a ratio or percentage:

S = (N_s - N_r) / N_s

Where:

• S is the normalized slip ratio (ranging from 0 to 1)
• N_s is the synchronous speed (in RPM)
• N_r is the rotor speed (in RPM)

The stator’s magnetic field rotates at the synchronous speed (N_s), which is determined by the frequency of the AC supply (F) and the number of magnetic poles per phase winding (p) in the stator:

N_s = (120 * F) / p

Where:

• N_s is in Revolutions Per Minute (RPM)
• F is the AC power frequency (in Hertz, Hz)
• p is the number of poles per phase winding (note: poles always come in pairs, so ‘p’ is the number of poles, e.g., a 4-pole motor has 2 pairs of poles)
• The constant 120 comes from (60 seconds/minute * 2 poles/pair).

For an induction motor to produce torque, the rotor must rotate slower than the synchronous speed. If the rotor were to spin at exactly synchronous speed (S=0), the magnetic field from the stator would appear stationary to the rotor conductors. There would be no change in magnetic flux linking the rotor windings, and therefore no voltage or current would be induced in the rotor (according to Faraday’s Law of induction). With no rotor current, there would be no interaction between the stator and rotor magnetic fields, and thus no torque.

Therefore, the rotor slips behind the rotating magnetic field (N_r < N_s). This relative motion causes the stator’s magnetic field to “cut” across the rotor conductors, inducing a voltage and current in them. These induced currents create the rotor’s magnetic field. The interaction between the stator’s rotating field and the rotor’s induced field generates torque, pulling the rotor along in the direction of the rotating field. The greater the mechanical load on the motor shaft, the more the rotor slows down, increasing the slip. Increased slip results in a larger induced voltage and current in the rotor, generating more torque to meet the load demand.

Under no load, an induction motor runs very close to synchronous speed (slip is very small, typically less than 1%). Under full load, the slip is typically between 2% and 5%.

Types of Polyphase Induction Motors#

Polyphase (e.g., three-phase) induction motors are self-starting and produce a naturally rotating magnetic field. The two main rotor types are:

1. Squirrel-Cage Rotor:

   • Construction: This is the most common type. The rotor “windings” consist of conducting bars (usually aluminum or copper) embedded in skewed slots within a laminated iron core. These bars are short-circuited at both ends by conducting rings, resembling a squirrel’s exercise cage.
   • Operation: The stator’s rotating magnetic field induces currents directly into the rotor bars and end rings. These induced currents create the rotor magnetic field. There are no external connections to the rotor, making it very simple and rugged.
   • Characteristics: High starting torque is possible, but the motor’s speed is relatively fixed by the supply frequency and number of poles (with only slight variation due to slip). Starting current can be high.
   • Analogy: Can be viewed as a transformer where the stator is the primary and the rotating squirrel cage is the secondary. The electrical load on the stator is inherently linked to the mechanical load on the rotor.
   • Applications: Widely used in domestic appliances (washing machines, dryers, fans), pumps, compressors, industrial drives – wherever a constant or near-constant speed is acceptable.

2. Wound Rotor:

   • Construction: The rotor has actual windings (made of wire, like the stator) placed in slots on the laminated core. These windings are connected to external circuits through slip rings and brushes on the motor shaft. The windings are typically wound for the same number of poles as the stator.
   • Operation: Allows for external resistance or impedance to be connected in series with the rotor windings via the slip rings.
   • Characteristics: Adding external resistance to the rotor circuit increases the starting torque and limits the starting current. It also allows for speed control – increasing rotor resistance increases the slip at a given torque, thus reducing the speed.
   • Comparison to Squirrel-Cage: More complex, more expensive, and requires maintenance of brushes and slip rings compared to the squirrel-cage motor.
   • Applications: Historically used for applications requiring high starting torque or variable speed control (e.g., cranes, hoists, traction motors) before the widespread adoption of Variable Frequency Drives (VFDs). Some modern high-power applications use them in doubly fed configurations where power is fed to both stator and rotor for increased power density and precise speed/power control (e.g., large wind turbines, some traction systems).

Starting Polyphase Induction Motors#

Starting a polyphase induction motor, especially large ones, can draw a significant inrush current from the power supply, which can cause voltage dips. Several methods are used to mitigate this:

• Direct-On-Line (DOL) Starting: Full line voltage is applied directly to the motor terminals. Simple but results in high starting current and torque. Suitable for smaller motors or systems with high fault capacity.
• Reduced Voltage Starting:
  • Series Inductors or Resistors: Adding impedance in series with the stator winding reduces the initial voltage and current. Once the motor speeds up, the impedance is shorted out.
  • Autotransformer Starting: Uses an autotransformer to supply reduced voltage to the stator during starting. More efficient than series impedance.
  • Star-Delta (Y-Δ) Starting: Connects the stator windings in a star (Y) configuration for starting (resulting in phase voltage being V_line / √3) and then switches to a delta (Δ) configuration for normal running (full line voltage across each phase). Common in Europe. Reduces starting current but also starting torque.
• Solid-State Starters: Use thyristors or other power electronic devices to control the voltage applied to the motor during starting, providing a smoother acceleration and controlled current.
• Variable Frequency Drives (VFDs): The most sophisticated method. A VFD converts the fixed-frequency AC supply into a variable-frequency and variable-voltage output, allowing smooth speed control from zero up to and sometimes above synchronous speed. VFDs inherently provide soft starting by gradually increasing frequency and voltage, eliminating high inrush currents.

Single-Phase Induction Motors#

Single-phase induction motors are common for lower-power applications in homes and small businesses because single-phase AC power is typically available.

• Challenge: A single-phase AC winding produces a pulsating magnetic field that alternates in polarity but does not rotate. A stationary squirrel-cage rotor placed in this pulsating field will not start on its own. It will simply vibrate. However, if the rotor is given a push in either direction, it will continue to rotate in that direction.
• Solution: Single-phase induction motors require a starting mechanism to create a rotating magnetic field (or a component of one) to initiate rotation. Once the motor is running, the interaction between the main pulsating field and the rotor’s magnetic field (induced by the relative motion) sustains rotation.

Several methods are used to create the necessary starting torque:

1. Shaded-Pole Motor:

   • Mechanism: A simple, low-cost method. A small portion of each stator pole is encircled by a short-circuited copper ring called a “shading coil” or “shading band”. The alternating magnetic flux through the shading coil induces a current in it, which opposes the change in flux. This causes the flux in the shaded part of the pole to lag slightly behind the flux in the unshaded part, effectively making the magnetic field across the pole face appear to move from the unshaded to the shaded portion. This “moving” flux creates a weak rotating magnetic field component sufficient to start the low-inertia rotor.
   • Characteristics: Very low starting torque, low efficiency, speed is roughly proportional to frequency.
   • Applications: Small fans, small pumps, inexpensive household appliances where starting load is minimal.

2. Split-Phase Motor:

   • Mechanism: Uses two stator windings: a main winding and a start winding. The start winding is placed physically 90 electrical degrees apart from the main winding. The start winding is designed with a different impedance (higher resistance, lower inductance) than the main winding. When connected in parallel to the single-phase supply, the current through the start winding is slightly out of phase with the current through the main winding. This phase difference creates a weak rotating magnetic field component, providing starting torque.
   • Operation: The start winding is typically energized only during starting. Once the motor reaches about 70-80% of synchronous speed, a centrifugal switch (or sometimes an electronic relay) disconnects the start winding from the circuit.
   • Characteristics: Higher starting torque than shaded-pole motors. The direction of rotation can be reversed by reversing the connections to the start winding relative to the main winding.
   • Applications: Appliances like washing machines, dryers, larger fans, small pumps, and blowers.

3. Capacitor Motors: These are enhancements of the split-phase design using a capacitor to create a larger phase shift between the currents in the main and start windings, significantly increasing starting torque.

   • Capacitor-Start Motor:

     • Mechanism: Similar to a split-phase motor, but a capacitor is connected in series with the start winding. The capacitor causes the current in the start winding to lead the voltage (or current in the main winding) by a significant angle (closer to 90 degrees), creating a more effective rotating magnetic field for starting.
     • Operation: The capacitor and start winding are disconnected by a centrifugal switch once the motor is up to speed.
     • Characteristics: Provides high starting torque, suitable for hard-to-start loads.
     • Applications: Compressors, pumps, power tools, refrigerators, air conditioners.

   • Capacitor-Start, Capacitor-Run Motor (or Two-Value Capacitor Motor):

     • Mechanism: Uses two capacitors. A larger starting capacitor is in series with the start winding for high starting torque. A smaller running capacitor is also in series with the start winding (or a separate winding) and remains in the circuit during operation.
     • Operation: The starting capacitor is disconnected by a centrifugal switch or relay after starting. The running capacitor remains connected to improve the motor’s power factor and efficiency and provide a more circular rotating magnetic field during normal running.
     • Characteristics: High starting torque and high running efficiency.
     • Applications: Industrial loads requiring high efficiency and starting torque, air conditioning compressors.

   • Permanent-Split Capacitor (PSC) Motor:

     • Mechanism: Uses a single, non-polarized capacitor connected permanently in series with the start (or auxiliary) winding. This winding and capacitor remain in the circuit throughout operation. The capacitor’s value is chosen as a compromise between optimal starting torque and optimal running performance.
     • Characteristics: Moderate starting torque (less than capacitor-start types), simple construction (no switch), relatively good power factor and efficiency compared to shaded-pole or basic split-phase motors. Can be easily reversed and can operate over a wider speed range (especially with voltage control) than split-phase types.
     • Applications: Fans (ceiling fans, blowers in HVAC systems), small pumps, applications requiring variable speed operation.

4. Resistance-Start Motor:

   • Mechanism: A variation of the split-phase motor where the start winding has a high resistance to achieve a phase shift. This is less effective than using a capacitor.
   • Operation: Uses a centrifugal switch to disconnect the start winding after starting.
   • Characteristics: Provides lower starting torque than capacitor-start motors.
   • Applications: Less common than other types, typically used for easily started loads.

Induction Motor Applications Summary#

• Squirrel Cage: Fans, pumps, blowers, conveyors, machine tools, appliances, general industrial drives.
• Wound Rotor: Cranes, hoists, cement mills, traction (historically), large wind turbines (doubly fed).
• Single-Phase (Split-Phase, Capacitor): Residential appliances, smaller pumps, HVAC blowers, fans.
• Single-Phase (Shaded Pole): Very small fans, low-torque devices.

Principle of Operation: Synchronous Speed#

Unlike induction motors, the rotor of a synchronous motor locks in step with the stator’s rotating magnetic field and rotates at the exact synchronous speed (N_r = N_s).

Synchronous Speed (N_s): The speed at which the stator’s magnetic field rotates. This speed is strictly determined by the supply frequency (F) and the number of poles (p) according to the formula: N_s = (120 * F) / p.

The rotor of a synchronous motor has a constant magnetic field. This field can be produced by:

• Permanent Magnets: Used in small motors.
• DC Excitation: Field coils on the rotor are supplied with DC current via slip rings (or brushlessly via a rotating rectifier) to create electromagnets.
• Reluctance: The rotor is shaped with salient poles (protruding magnetic poles) but has no windings or magnets. The rotor aligns itself with the stator’s rotating field due to the principle of magnetic reluctance (the tendency of magnetic flux to follow the path of least resistance).

Once the rotor magnetic poles are locked with the stator’s rotating magnetic poles, they follow it precisely at synchronous speed.

Types of Synchronous Motors#

1. Polyphase Synchronous Motor:

   • Construction: Stator is similar to an induction motor (polyphase windings). The rotor has poles created by DC excitation or permanent magnets. Large motors use DC excitation via slip rings or brushless exciters. Smaller motors may use permanent magnets.
   • Starting: Standard synchronous motors are not self-starting. When AC power is applied, the stator field starts rotating, but the rotor is initially stationary. The alternating pull and push on the stationary rotor results in no net starting torque. Methods to start polyphase synchronous motors include:
     • Using a squirrel-cage winding (damper winding) embedded in the face of the rotor poles. The motor starts as an induction motor. As it approaches synchronous speed, the DC field is applied, pulling the rotor into synchronism.
     • Using a separate pony motor (a small induction motor) to bring the rotor up to near-synchronous speed before applying the AC power to the stator.
     • Using a Variable Frequency Drive (VFD) to start the motor from zero speed and gradually increase the frequency and voltage, allowing the rotor to follow the slowly rotating field until synchronous speed is reached. This is the most common modern method for large motors.
   • Characteristics: Runs at a constant speed regardless of load changes (within its torque limits). Can operate at unity or leading power factor by adjusting the DC field excitation. This is a significant advantage.
   • Applications: Applications requiring precise speed control (e.g., timing devices, synthetic fiber manufacturing), large industrial drives (compressors, pumps, fans), power factor correction (operated without a mechanical load as synchronous condensers). Used in some electric traction systems (e.g., TGV trains). Permanent magnet synchronous motors are increasingly used in electric vehicles due to their high efficiency and power density.

2. Single-Phase Synchronous Motor:

   • Construction: Uses a single-phase stator winding and a rotor that can lock into the alternating magnetic field.
   • Starting: Like polyphase synchronous motors, single-phase types are generally not self-starting based on the main winding alone. They require a starting mechanism.
   • Types:
     • Reluctance Motor: The rotor is a squirrel cage with “flats” ground onto it to create salient poles. It starts as an induction motor using the squirrel cage. As it approaches synchronous speed, the salient poles align with the stator field pulses due to reluctance torque, locking into synchronism. Provides modest torque at synchronous speed.
     • Hysteresis Motor: The rotor is a smooth cylinder of a hard magnetic material. The stator (often similar to a capacitor-run induction motor) creates a rotating or pulsating field. On startup, the field induces eddy currents and magnetizes the rotor material. Due to hysteresis (the lag in magnetization behind the applied field), the induced rotor poles are displaced from the stator poles, creating torque. As the speed increases, the hysteresis effect causes the rotor material to become permanently magnetized with poles that lock onto the stator field once synchronous speed is reached.
   • Characteristics: Runs at constant synchronous speed. Hysteresis motors are known for very smooth rotation (low flutter) and quiet operation. Reluctance motors provide lower torque.
   • Applications: Timing devices (electric clocks), phonographs, tape recorders (capstan drives), small instruments requiring precise speed.

Synchronous Motor Applications Summary#

• Polyphase: Large industrial drives (pumps, compressors, fans), power factor correction (synchronous condensers), some electric vehicles, pumped-storage hydroelectric plants (as motor/generator).
• Single-Phase (Reluctance): Timing applications, record players.
• Single-Phase (Hysteresis): High-quality audio/video equipment capstan drives, timing devices requiring low flutter.