AC Motors: Universal, Synchronous, Induction

• These Study Notes comply with the syllabus of EASA Regulation (EU) No. 1321/2014 Annex III (Part-66) Appendix I.
• Includes the amendment Regulation (EU) 2023/989.

AC Motors Categories

• Universal
• Synchronous
• Induction

Universal Motor

• Also known as AC-series motor, it operates on either AC or DC power and uses an electromagnet as its stator.
• Similar in construction to a DC series motor but slightly modified for AC power.
• The field coils are connected in series with the rotor windings through a commutator.
• The current in both the field coils and the armature alternates synchronously with the supply.
• The resulting mechanical force occurs in a consistent direction of rotation.
• Has high starting torque, can run at high speed, and is lightweight and compact.
• Commonly used in portable power tools and equipment, as well as many household appliances.
• Relatively easy to control, electromechanically or electronically.
• Universal DC motors run poorly on AC supply if not modified.
• Modifications for proper AC supply operation include a compensating winding, laminated pole pieces, and more coils and plates in the armature.
• Starter motors of piston engines and smaller turbine engines are usually universal motors.
• Smaller motors have around 30% efficiency.
• Larger ones have 70–75% efficiency.
• Speed increases, the counter EMF reduces the voltage across and current through the field windings, weakening the field at high speeds.
• Does not have a theoretical maximum speed for any voltage.
• Can run at 4,000–16,000 RPM, and sometimes over 20,000 RPM.
• Damage may occur from over-speeding.
• Often feature compensating windings in series or inductively coupled, placed at ninety electrical degrees to the main field axis, reducing armature reactance and improving commutation.
• Universal motors have high reactance of the armature and field windings, which limits AC current. AC flux will produce large eddy currents if solid steel is used for the stator frame.

Compensating Windings

• Necessary to ensure satisfactory operation from an AC power source.
• Reduces the reactance of the series field and armature windings as much as practicable.
• Conductively compensation winding is connected in series with the armature winding, opposing the ampere-turns of the armature.
• The compensating winding is displaced by 90 electrical degrees from the field winding.
• Inductively compensated winding may be simply shorted upon itself.
• The induced AC current in the compensating winding again opposes the armature current.

Disadvantages of Universal Motors

• Maintenance and short life problems caused by the commutator and sparks.
• Require mostly clean air at all times to avoid overheating.

Speed Control

• Achieved through a thyristor circuit for continuous speed control on AC.
• Multiple taps on the field coil provide stepped speed control.

Rotating Magnetic Fields

• Utilized in synchronous and induction motors where stators cause rotors to turn.
• A magnetic field in the stator rotates electrically.
• A magnetic field in the rotor can be made to chase it by being attracted and repelled by the stator field, causing the rotor to turn.
• Rotating magnetic fields may be set up in two-phase or three-phase machines.
• To establish a rotating magnetic field in a motor stator, the number of pole pairs must be the same as (or a multiple of) the number of phases in the applied voltage.
• The poles must then be displaced from each other by an angle equal to the phase angle between the individual phases of the applied voltage.

Two-Phase Rotating Magnetic Field

• The stator of a two-phase induction motor is made up of two windings.
• They are placed at right angles to each other around the stator.
• Voltages applied to phases are 90° out of phase, causing respective currents to be displaced by 90°.
• The magnetic fields are also 90° out of phase with each other.
• The two out-of-phase magnetic fields add together, producing a resultant field that rotates one revolution for each cycle of AC.
• Rarely used except in special-purpose equipment.

Three-Phase Rotating Magnetic Fields

• The stator windings are connected to a three-phase AC input.
• The three phases are tied together in a Y-connected stator.
• Individual phase windings are equally spaced around the stator, placing the windings 120° apart.
• The bar magnet, free to move, will follow the stator field.
• A shaft running through the pivot point will rotate at the same speed as the rotating field, known as synchronous speed.

Synchronous Motors

• Construction same as a salient-pole alternator.
• Can be run as an AC motor.
• Constant speed between no load and full load.
• Correct the low power factor of an inductive load under certain conditions.
• May be designed as either single-phase or multiphase machines.
• Application of three-phase AC power can cause a rotating magnetic field to be set up around the rotor.
• The rotor is energized with DC and acts like a bar magnet.
• The strong rotating magnetic field attracts the strong rotor field activated by DC. This results in a strong turning force upon the rotor shaft.
• Can’t be started from a standstill by applying three-phase AC power to the stator, as the high-speed field rushes past the rotor poles too quickly the rotor gets repeled.
• Asynchronous motor in its purest form has no starting torque.
• Practical synchronous motor requires a DC exciter voltage for the rotor.
• Two major types: non-excited and direct-current excited.

Non-Excited Motors

• The rotor is made of steel.
• At synchronous speed, it rotates in step with the rotating magnetic field of the stator.
• Manufactured in permanent magnet, reluctance, and hysteresis designs.

Reluctance Motors

• Rotor consists of a solid steel casting with projecting toothed poles.
• The size of the air gap in the magnetic circuit and thus the reluctance is minimum when the poles are aligned, creating a torque.
• These range from fractional horsepower (few watts) to about 22 kW.
• Small ones have low torque and are used for instrumentation.
• When used with an adjustable frequency power supply.

Hysteresis Motors

• A solid smooth cylindrical rotor, cast of a high coercivity magnetically 'hard' cobalt steel.
• There is a 2-pole low-reluctance bar structure.
• As the rotor approaches synchronous speed and slip goes to zero, this magnetises and aligns with the stator field, causing the rotor to 'lock' to the rotating stator field.

Permanent-Magnet Motors

• Uses permanent magnets embedded in the steel rotor to create a constant magnetic field.
• Requires a variable-frequency power source to start.

DC-Excited Motors

• Require DC supplied to the rotor for excitation.
• DC may be supplied through slip rings.

Synchronous Speed

• Given in RPM by: N = 60 * (f / P)
• Given in radians / second by: ω = 2π * (f / P)
  • f is for the frequency of the AC supply current in Hz,
  • P is the pair number of pole-pairs per phase.

Construction

• The principal components of a synchronous motor are the stator and the rotor, similar to induction motors.
• When the field winding is excited by DC, brushes and slip rings are required.
• Cylindrical, round rotors are used for up to six poles.
• In other cases, a salient pole rotor is used.

Operation

• The stator winding consists of a 3-phase winding is provided with a 3-phase supply, and rotor is with a DC supply. This produces 3-phase rotating magnetic flux.
• One the motor is in operation, the speed is dependent on the supply frequency.

Starting Methods

• Above a certain size, synchronous motors are not self-starting motors because they need a supplemental mechanism.
• Add a squirrel-cage the the rotor.
• Stator is energized, but the DC supply to the rotor field is not energized.

Induction Motors

• An AC motor where the electric current needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding.
• Can be made without electrical connections to the rotor.
• The motor’s rotor can be either wound or squirrel-cage type.
• Derives its name from the fact that AC voltages are induced in the rotor circuit by the rotating magnetic field of the stator.
• Similar to the induction between the primary and secondary windings of a transformer.
• Often induction motors examples are washing machines and bench grinders.
• Stator construction of the three-phase induction motor and the three-phase synchronous motor are almost identical.
• The induction rotor is made of a laminated cylinder with slots in its surface.
• One of two types of windings, squirrel-cage or rotor.

Principle of Operation

• In both induction and synchronous motors, the AC power supplied to the motor's stator creates a magnetic field that rotates in synchronism with the AC oscillations.
• The motor’s rotor rotates at a slower speed than the stator field and is referred to as "asynchronous motors”.
• Wound rotor motors is also referred to as "slip ring motors".

The Wound Rotor and Squirrel Cage Induction Motor Comparison

• Wound rotor:
  • Rotor winding connected to external resistance through slip rings.
  • Starting torque can be adjusted through external resistance.
  • Speed control using rotor resistance control can be used.
  • Frequent maintenance required.
• Squirrel cage:
• Rotor consists of a cylinder that is laminated, having semi-closed slots and is short-circuited at either end.
• Rotor cannot be adjusted.
• Speed control can't be adjusted. Less maintenance needed.

Synchronous Speed

• Can be defined as the ration rate of the stator’s magnetic field (ns).
• ns = 2f / p

Slip

• Slip, s, is defined as the difference between synchronous speed and operating speed (expressed in RPM)
• s = (ns - nr) / ns X 100%

Speed-Torque Characteristics

• Breakdown torque (peak torque), 175-300% of rated torque
• Locked-rotor torque (torque at 100% slip), 75-275% of rated torque Pull-up torque, 65-190% of rated torque

Starting (Competing Small Induction Motors)

• There are single-phase, split-phase and shaded-pole types and small polyphase motors.
• In two-pole single-phase motors, the torque goes to zero.
• Requires alterations to the stator such as shaded.

Shaded Pole Motor

• Starting is done by means of a shaded pole with a copper wire turn around part of the pole.
• Used in desk fans and record players, as the required starting torque energy is low.

Split-Phase Motor

• Have a second stator winding fed with an out-of-phase current.
• currents created by capacitor

Polyphase Motor

• Have rotor bars shaped to give different speed-torque characteristics.
• Rotor bars provide speed torque and the in-rush current and startup.

Single Phase Motors

• The stator field in the singlephase motor does no rotate.
• Simply alternates in polarity with the AC voltage polarity changes.
• All single phase are identical accept for the means of stargint

Capacitor-start

• 90 degree electrical phase occurs.
• Capacitor in serious with auxiliary winding
• Starting torque id not that high.

Resistor start

• The starting winding is positioned at right angels to the main winding.
• Is switched in and out of the circuit just ad it was in the capacitor start motor. The stating torque is not as high a sit is in the capacitor start.

###Rotation Reversal

• Depends on if its either is a three of single phase machine

Power Factor

• Ranges from 0.85 - 0.90

Efficency

• Friction = 5-15%
• Iron or core losses = 15-25%
• Stator losses= 25-40%
• Rotor losses = 15-25%
• Stray load losses = 10-20%