29 Questions
What is the purpose of the shading ring in a shaded pole motor?
To reverse the direction of rotation of the motor
What is the main advantage of a shaded pole motor?
Simple and inexpensive construction
What is the primary application of a shaded pole motor?
Shaft-mounted fans and blowers
What is the main disadvantage of a shaded pole motor?
Low efficiency and low starting torque
How is the direction of rotation reversed in a shaded pole motor?
By shifting the shading ring to the other side of the pole face
What is a disadvantage of a synchronous motor when it comes to starting?
It cannot be started from standstill by applying three-phase AC power to the stator.
What is the purpose of adding a squirrel-cage winding to the rotor of a synchronous motor?
To cause the rotor to start.
Why does a relatively large current flow in the squirrel cage winding of a synchronous motor?
Due to induced low voltage and short circuit in the windings.
What happens to the rotor of a synchronous motor when it is started by the squirrel-cage winding?
It rotates in step with the stator field.
What role does the squirrel cage play in starting a synchronous motor?
Causing the rotor to begin turning.
Why does a synchronous motor require a squirrel-cage winding for starting?
To overcome the lack of starting torque in its purest form.
What is one advantage of using AC motors over DC motors?
AC motors eliminate the problem of dangerous sparking.
Which statement about AC motors is true?
Their speed is determined by the frequency of the AC voltage applied to the motor terminals.
What is a disadvantage of using DC motors compared to AC motors?
DC motors require more maintenance due to brushes and commutators.
Which statement about AC motors is NOT true?
They generally cost more than DC motors.
Which method can be used to control the speed of an AC motor?
Changing the frequency of the AC voltage applied to the motor terminals.
What is one method of producing a rotating field in an AC motor?
Using a capacitor.
What is the primary reason for calling it an induction motor?
AC voltages are induced in the rotor circuit by the rotating magnetic field of the stator
Which of the following is NOT a typical application for induction motors?
High-speed centrifuges
What is the primary advantage of induction motors?
Simplicity and ruggedness
How is the rotating magnetic field in the stator created?
By passing alternating current through the stator windings
What is the purpose of the rotor in an induction motor?
To provide a magnetic field that interacts with the stator's field, producing torque
What is the most common type of winding used in induction motor rotors?
Squirrel-cage winding
What is the main reason why split-phase motors come only in small sizes?
Insufficient torque for heavy loads due to the auxiliary winding.
How is a capacitor start motor recognized externally?
By the bulbous protrusion on the frame where the starting capacitor is located.
What is a common application for Permanent-Split Capacitor Motors due to their low starting torque?
Fans and blowers.
How is the electrical phase shift between the currents in the starting and main windings obtained in a resistance-start motor?
By making the impedance of the windings unequal.
Why is a centrifugal switch not required in Permanent-Split Capacitor Motors?
The capacitor is left in series with the starting winding during normal operation.
Why do resistance-start motors have a starting winding positioned at right angles to the main winding?
To obtain an electrical phase shift between the currents in the two windings.
Study Notes
Shaded Pole Motor
- A shaded pole motor has a slight component of rotation in the field, causing the rotor to rotate.
- The direction of rotation can be reversed by altering the direction of the rotating magnetic field across the pole face.
- Shaded pole motors are simple and inexpensive, but have low efficiency and a very low starting torque.
- Speed regulation is poor, and they must be fan-cooled during normal operation.
- They are used in shaft-mounted fans and blowers, small pumps, toys, and intermittently used household items.
Synchronous Motor
- A synchronous motor works by rotating a magnetic field, but it cannot be started from standstill by applying three-phase AC power to the stator.
- The rotor does not have a chance to get started due to the high-speed rotating magnetic field.
- A synchronous motor has no starting torque, but it has torque only when it is running at synchronous speed.
- A squirrel-cage winding is added to the rotor to cause it to start.
- The squirrel cage is a series of heavy copper bars shorted together by copper rings, which induces a magnetic field that interacts with the rotating field of the stator.
AC Motor Theory
- AC motors have advantages, including wide availability of AC power, lower cost, and fewer maintenance problems.
- Most types of AC motors do not use brushes and commutators, eliminating problems of maintenance and wear.
- AC motors are well suited for constant-speed applications because their speed is determined by the frequency of the AC voltage applied to the motor terminals.
- Industry builds AC motors in different sizes, shapes, and ratings for many different types of jobs.
Induction Motor
- An induction motor derives its name from the fact that AC voltages are induced in the rotor circuit by the rotating magnetic field of the stator.
- Induction motors are probably the simplest and most rugged of all electric motors.
- There are only two main components: the stator and the rotor.
- The stator contains a pattern of copper coils arranged in windings, which form a moving magnetic field that induces a current in the rotor.
- The interaction of the fields produces a torque on the rotor.
Induction Motor Rotor
- The induction rotor is made of a laminated cylinder with slots in its surface.
- The windings in these slots are of two types: squirrel-cage winding and auxiliary winding.
- The squirrel-cage winding is the most common, made up of heavy copper bars connected together at each end by a metal ring.
Other Types of Induction Motors
- Split-phase motors have a starting winding in addition to the main winding and are used for small applications where high starting torque is not required.
- Capacitor start motors have a high starting torque and are used for applications where high starting torque is required.
- Permanent-Split Capacitor Motors have a capacitor left in series with the starting winding during normal operation and are used for low-inertia loads.
- Resistance-Start Motors have a starting winding in addition to the main winding, switched in and out of the circuit, and are used for applications where high starting torque is required.
Test your knowledge on the construction, operation, and characteristics of AC synchronous motors, AC induction motors, and AC motors including single and polyphase. Learn about methods of speed control and controlling the direction of rotation.
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