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Questions and Answers
What effect does varying physical design features of a rotor have?
What effect does varying physical design features of a rotor have?
What is a key characteristic of a squirrel-cage rotor?
What is a key characteristic of a squirrel-cage rotor?
What is the primary use of a wound rotor?
What is the primary use of a wound rotor?
According to Lenz's law, what does the induced EMF in an induction motor do?
According to Lenz's law, what does the induced EMF in an induction motor do?
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What does it mean when an induction motor cannot run at synchronous speed?
What does it mean when an induction motor cannot run at synchronous speed?
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What is slip in the context of an induction motor?
What is slip in the context of an induction motor?
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In a wound rotor, what happens if the resistances are set to open circuit?
In a wound rotor, what happens if the resistances are set to open circuit?
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What is a key characteristic of induction motors?
What is a key characteristic of induction motors?
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Which method is NOT used for producing a rotating field in AC motors?
Which method is NOT used for producing a rotating field in AC motors?
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What happens when the rotor speed approaches the stator field speed?
What happens when the rotor speed approaches the stator field speed?
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What determines the speed of an AC motor?
What determines the speed of an AC motor?
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Which of the following is an advantage of AC motors?
Which of the following is an advantage of AC motors?
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Which statement about synchronous motors is true?
Which statement about synchronous motors is true?
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What is a feature of three-phase rotating fields?
What is a feature of three-phase rotating fields?
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How can speed and direction of rotation in AC motors be controlled?
How can speed and direction of rotation in AC motors be controlled?
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What is an important advantage of using AC motors for constant-speed applications?
What is an important advantage of using AC motors for constant-speed applications?
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What is the initial purpose of the squirrel-cage windings in a synchronous motor?
What is the initial purpose of the squirrel-cage windings in a synchronous motor?
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What occurs when the DC field in the rotor is energized?
What occurs when the DC field in the rotor is energized?
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What is the disadvantage of using a rotor with salient poles in a synchronous motor?
What is the disadvantage of using a rotor with salient poles in a synchronous motor?
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How does a synchronous motor operate under varying loads?
How does a synchronous motor operate under varying loads?
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What does pull-out torque refer to in a synchronous motor?
What does pull-out torque refer to in a synchronous motor?
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What principle is the operation of a synchronous motor based on?
What principle is the operation of a synchronous motor based on?
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What device is typically used to apply DC to the rotor at sufficient speed in a synchronous motor?
What device is typically used to apply DC to the rotor at sufficient speed in a synchronous motor?
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What is a potential benefit of using a squirrel cage rotor in a synchronous motor?
What is a potential benefit of using a squirrel cage rotor in a synchronous motor?
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What is the primary use of a frequency converter in induction motors?
What is the primary use of a frequency converter in induction motors?
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How can the direction of rotation in a three-phase motor be changed?
How can the direction of rotation in a three-phase motor be changed?
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What component is necessary for directing current flow and changing the direction of rotation in some split-phase motors?
What component is necessary for directing current flow and changing the direction of rotation in some split-phase motors?
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Why do variable speed drives (VSD) not work with single-phase induction motors?
Why do variable speed drives (VSD) not work with single-phase induction motors?
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What does a shading ring accomplish in a motor?
What does a shading ring accomplish in a motor?
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What purpose does the start (auxiliary) winding serve in a capacitor-start split phase motor?
What purpose does the start (auxiliary) winding serve in a capacitor-start split phase motor?
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What happens to the start winding of a capacitor-start split phase motor at 75% speed?
What happens to the start winding of a capacitor-start split phase motor at 75% speed?
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Which characteristic of a permanent-split capacitor motor is different from a capacitor-start split phase motor?
Which characteristic of a permanent-split capacitor motor is different from a capacitor-start split phase motor?
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What is a common application for a permanent-split capacitor motor due to its low starting torque?
What is a common application for a permanent-split capacitor motor due to its low starting torque?
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In a resistance-start motor, how is the starting torque compared to a capacitor-start motor?
In a resistance-start motor, how is the starting torque compared to a capacitor-start motor?
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What is the main function of the heavy copper ring in a shaded-pole induction motor?
What is the main function of the heavy copper ring in a shaded-pole induction motor?
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What key feature distinguishes shaded-pole induction motors from other types of induction motors?
What key feature distinguishes shaded-pole induction motors from other types of induction motors?
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What is a typical characteristic of the magnetic field in shaded-pole induction motors?
What is a typical characteristic of the magnetic field in shaded-pole induction motors?
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What characterizes an induction motor's rotor?
What characterizes an induction motor's rotor?
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What type of rotor is most commonly used in induction motors?
What type of rotor is most commonly used in induction motors?
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What is the primary mechanism by which an induction motor generates torque?
What is the primary mechanism by which an induction motor generates torque?
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Which of the following accurately describes the construction of a squirrel-cage rotor?
Which of the following accurately describes the construction of a squirrel-cage rotor?
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What occurs when AC voltage is passed through the windings of the stator in an induction motor?
What occurs when AC voltage is passed through the windings of the stator in an induction motor?
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What characteristic of a slanted rotor contributes to its performance during startup?
What characteristic of a slanted rotor contributes to its performance during startup?
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What is the role of the laminations in the induction rotor?
What is the role of the laminations in the induction rotor?
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Which type of rotor includes actual wound coils placed in slots?
Which type of rotor includes actual wound coils placed in slots?
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What best describes the induction motor's rotor regarding its connection to voltage sources?
What best describes the induction motor's rotor regarding its connection to voltage sources?
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Study Notes
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
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Induction Motors:
- Most common AC motor type.
- Single-phase or polyphase.
- Rotors are energised by induction.
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Synchronous Motors:
- Typically polyphase.
- Constant speed.
- Rotors are energised with DC voltage.
Three-Phase Rotating Fields
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Three phases are connected in a Y-configuration.
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Winding coils are evenly spaced around the stator with 120-degree separation.
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Use the left-hand rule to determine the electromagnetic polarity of the poles at any instant.
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Current flows toward terminals for positive voltages and away for negative voltages.
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Rotating magnetic field is strong, aided by weaker fields.
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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
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Probably the simplest and most rugged electric motor.
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Most commonly used AC motor type.
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Rotor is not connected to an external voltage source.
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AC voltages are induced in the rotor circuit by the rotating magnetic field of the stator.
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Similar to induction between primary and secondary windings of a transformer.
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Typically used to drive loads at a fairly constant speed.
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Two main components: stator and rotor.
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Stator contains a pattern of copper coils arranged in windings.
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As AC is passed through the windings, a moving magnetic field is formed.
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This induces a current in the rotor, creating its own magnetic field.
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The interaction of these fields produces a torque on the rotor.
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Induction rotor is made of a laminated cylinder with slots in its surface.
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Two types of rotors for induction motors use slotted laminated cores.
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Squirrel-cage rotor is the most common type.
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Other type contains actual wound coils placed in rotor slots - wound rotor.
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Regardless of the type of rotor, the basic principle is the same.
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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
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Common asynchronous speeds for 50-hertz motors are: 3000, 1500, 1000, and 750 rpm, depending on the poles (2, 4, 6, 8) per phase.
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Common terminology: e.g., a two-pole motor (three phase) has six physical poles.
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Rotor will never reach synchronous speed, resulting in zero torque.
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The formula for slip (S%) is: S% = (Ns - N) / Ns x 100, where Ns = sync speed, N = rotor speed
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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
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Squirrel-cage winding is added to the rotor of a synchronous motor for starting.
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So named because the windings are shaped like a turnable squirrel cage—heavy copper bars shorted together by copper rings.
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A low voltage is induced in the shorted windings by the rotating stator field.
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Relatively high rotor currents flow because of the short circuit.
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This causes a magnetic field to interact with the stator's rotating field, causing the rotor to follow and start rotating.
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Squirrel-cage rotors will be covered in more details shortly.
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AC voltage supplied to the stator
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No DC supplied to the rotor.
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Squirrel cage windings bring rotor near sync speed.
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At that point DC field in the rotor is energised.
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Centrifugal switch typically used to apply DC to the rotor at sufficient speed.
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Rotor is typically built with salient poles.
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When excited with DC it produces alternate N & S magnetic poles.
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These magnetic poles on the rotor outer are attracted in rotating stator field.
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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
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Star wound stator winding will draw less current than a delta wound type
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Common to use a 'star-delta' arrangement for starting of large three Ø induction motors.
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Limits starting current on initial start
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At approx. 75% speed, switches over to 'delta'
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In Star - voltage is reduced to 58%
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In Delta – voltage increases, and current increases
Single-phase Induction Motor
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Probably more One Ø AC induction motors than all other types.
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Unlike polyphase induction motors, stator field in One Ø AC motor does not rotate.
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It simply alternates polarity between poles as AC changes polarity.
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As a result of magnetic induction, magnetic field is produced around the rotor.
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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.
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Description
Explore the fundamentals of AC motors, including their advantages over DC motors and the different types of AC motors like induction and synchronous motors. Understand the principles of three-phase rotating fields and the significance of voltage frequency in motor speed.
