AC Motors and Their Types

AC Motors

• AC motors are widely available and less expensive than DC motors.
• Most AC motors don't use commutators and brushes, eliminating sparking.
• AC motor speed is determined by the AC voltage frequency applied to the motor.
• Motors are designed for use with either polyphase or single-phase power.

AC Motor Types

• Induction Motors:

  • Most common AC motor type.
  • Single-phase or polyphase.
  • Rotors are energised by induction.

• Synchronous Motors:

  • Typically polyphase.
  • Constant speed.
  • Rotors are energised with DC voltage.

Three-Phase Rotating Fields

• Three phases are connected in a Y-configuration.

• Winding coils are evenly spaced around the stator with 120-degree separation.

• Use the left-hand rule to determine the electromagnetic polarity of the poles at any instant.

• Current flows toward terminals for positive voltages and away for negative voltages.

• Rotating magnetic field is strong, aided by weaker fields.

• A full cycle of the three-phase voltage rotates the magnetic field 360 degrees.

Speeds of AC Motors

• Synchronous speed (Ns) is the rotational speed of the magnetic field within the motor.
• The formula for calculating synchronous speed (Ns) is: Ns = (Frequency x 120) / P, where F = AC power frequency, P = number of poles per phase wound into the motor.
• Some AC motors operate nearly at synchronous speed, called synchronous motors.
• Asynchronous motors operate at less than synchronous speed and are also known as induction motors.
• Slip is the percentage difference between full-load speed and synchronous speed.
• Normal slip is approximately 5%, although it can be higher.

Induction Motor

• Probably the simplest and most rugged electric motor.

• Most commonly used AC motor type.

• Rotor is not connected to an external voltage source.

• AC voltages are induced in the rotor circuit by the rotating magnetic field of the stator.

• Similar to induction between primary and secondary windings of a transformer.

• Typically used to drive loads at a fairly constant speed.

• Two main components: stator and rotor.

• Stator contains a pattern of copper coils arranged in windings.

• As AC is passed through the windings, a moving magnetic field is formed.

• This induces a current in the rotor, creating its own magnetic field.

• The interaction of these fields produces a torque on the rotor.

• Induction rotor is made of a laminated cylinder with slots in its surface.

• Two types of rotors for induction motors use slotted laminated cores.

• Squirrel-cage rotor is the most common type.

• Other type contains actual wound coils placed in rotor slots - wound rotor.

• Regardless of the type of rotor, the basic principle is the same.

• Rotating magnetic field is generated in stator, induces a magnetic field in rotor, and the two interacting fields cause the rotor to turn.

Squirrel-Cage Rotor

• Number of conducting bars running parallel to the motor axis.
• Two conducting end rings.
• Typically, conductors may be copper or aluminum.
• Aluminum is typically used in smaller motors.

Induction Motor – Slanted Rotor

• Slanting conductors in the rotor ensures smooth and steady acceleration during starting.
• Varying physical design features of the rotor can increase their inductance, provide a lower starting current, and produce lower pull out torque.
• No insulation between core and bars as only low voltages induced into rotor bars.
• Very small air gap between rotor and stator – necessary to obtain maximum field strength.

Wound Rotor

• Typically only used when starting requirements are particularly severe.
• Advantages – starting torque can be controlled, and speed of motor can be controlled.
• Wound rotor has three windings and three slip rings.
• Resistances are adjusted simultaneously through three resistors (externally connected).
• Open circuit resistances prevent operation.

Induction Motor – Slip

• Common asynchronous speeds for 50-hertz motors are: 3000, 1500, 1000, and 750 rpm, depending on the poles (2, 4, 6, 8) per phase.

• Common terminology: e.g., a two-pole motor (three phase) has six physical poles.

• Rotor will never reach synchronous speed, resulting in zero torque.

• The formula for slip (S%) is: S% = (Ns - N) / Ns x 100, where Ns = sync speed, N = rotor speed

• Typically the difference between synchronous and rotor speed is not great.

Synchronous Motor

• Characteristic of constant speed between no load and full load.
• May be designed as either single-phase or multi-phase machines.
• Single-phase also spins at synchronous speed.
• Three-phase AC power to stator causes a rotating magnetic field, setting up around the rotor.
• Rotor is energised with DC.
• Strong rotating magnetic field attracts strong rotor field, causing a strong turning force on the rotor shaft.
• Rotor rotates in step with the rotating magnetic field (synchronous speed).
• Rotor cannot be started from a standstill.

Synchronous Motor – Rotor

• Squirrel-cage winding is added to the rotor of a synchronous motor for starting.

• So named because the windings are shaped like a turnable squirrel cage—heavy copper bars shorted together by copper rings.

• A low voltage is induced in the shorted windings by the rotating stator field.

• Relatively high rotor currents flow because of the short circuit.

• This causes a magnetic field to interact with the stator's rotating field, causing the rotor to follow and start rotating.

• Squirrel-cage rotors will be covered in more details shortly.

• AC voltage supplied to the stator

• No DC supplied to the rotor.

• Squirrel cage windings bring rotor near sync speed.

• At that point DC field in the rotor is energised.

• Centrifugal switch typically used to apply DC to the rotor at sufficient speed.

• Rotor is typically built with salient poles.

• When excited with DC it produces alternate N & S magnetic poles.

• These magnetic poles on the rotor outer are attracted in rotating stator field.

• DC may be externally or internally obtained.

Synchronous Motor Principle of Operation

• Principle of magnetic attraction between two opposing magnetic fields.
• One field from the rotating Stator and the other from the Rotor.

Synchronous Motor

• Operates at synchronous speed with no slip.
• Rotor has constant polarity.
• Will run at speed regardless of load variations up to a point called pull-out torque.
• Pull-out torque—maximum value of torque a motor can develop without losing synchronism.
• Higher load than that point will cause the motor to stop.
• Stator is essentially similar to an induction motor stator.

Synchronous Motor – No Load and Load

• No-load—centre lines of stator poles of rotating field and rotor pole coincide.
• With load – backward shift of rotor pole relative to stator pole.
• Angle between rotor and stator poles is known as the torque angle.

Synchronous Motor

• Star wound stator winding will draw less current than a delta wound type

• Common to use a 'star-delta' arrangement for starting of large three Ø induction motors.

• Limits starting current on initial start

• At approx. 75% speed, switches over to 'delta'

• In Star - voltage is reduced to 58%

• In Delta – voltage increases, and current increases

Single-phase Induction Motor

• Probably more One Ø AC induction motors than all other types.

• Unlike polyphase induction motors, stator field in One Ø AC motor does not rotate.

• It simply alternates polarity between poles as AC changes polarity.

• As a result of magnetic induction, magnetic field is produced around the rotor.

• This field will always be in opposition to stator field.

Single-phase Induction Motors

• Several types of single-phase AC induction motors in use today that operate in the same fashion except for starting.
• Once up to operating speed, all single-phase AC induction motors operate the same.
• Various methods for starting single-phase AC induction motors: split-phase, most common; shaded-pole methods.

Split-Phase AC Induction Motor

• One type of induction motor with a starting device.
• Designed to use inductance, capacitance, or resistance to develop starting torque.
• Typically, start winding is disconnected when motor reaches 75% of rated speed.

Capacitor-Start Split Phase

• Stator consists of main winding and a starting winding.
• Starting winding in parallel with main winding.
• A 90-degree electrical phase difference between two windings.
• Obtained by connecting auxiliary winding in series with a capacitor.
• In start winding circuit (with Xc), current leads voltage by about 45°.
• In main winding (with X₁) current lags voltage by about 45°.
• Currents in each winding are therefore 90° out of phase.

Capacitor-start Split Phase

• Effect is two windings act like a two Ø stator to produce a rotating field.
• At 75% speed, a centrifugal device (starting switch) removes start winding
• Motor then runs as a plain single-phase induction motor
• Start (auxiliary) winding is only a light winding
• Split-phase motors therefore come only in small sizes

Permanent-Split Capacitor Motor

• Capacitor of this motor is left in series with start winding.
• Normal operation
• Starting torque is roughly 40% of full load.
• Used on low-inertia loads .

Permanent-Split Capacitor Motor

• Another type of split-phase induction motor that is the resistance-start motor
• Start winding is positioned at right angles to main winding
• Switched in and out of circuit (75%).
• Electrical phase shift is between currents in two windings
• Main winding with high inductance & low resistance
• Start winding low inductance & high resistance
• Starting torque is not as great as in capacitor-start motor.

Shaded-Pole Induction Motor

• First effort in developing a self-starting, single phase motor.
• Field poles extend inward from motor housing.
• A portion of each pole is encircled with a heavy copper ring.
• Copper ring causes magnetic field through ring portion of pole face to lag.
• Results in slight rotation and is strong enough to cause rotation
• Torque created is small, not efficient.

Speed Control

• Induction motors are practically fixed-speed devices.
• Two methods to change rotation speed: frequency converter (VSD), or use a motor with separate windings.

Direction of Rotation—3-Phase

• Direction of rotation depends on direction of the rotating field.
• Reversing any two phases reverses the direction.

Reversing a Split-Phase Motor

• With some split-phase motors, a switch can select direction of rotation.
• Phase b to lag a, or b to lead a—causes motor to follow leading current.
• Only available on some single-phase motors.
• Many motors' start and run windings are physically different.
• Centrifugal switch must also be considered if used.