AC Motors Overview and Types

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Questions and Answers

What is a primary advantage of AC motors compared to DC motors?

AC motors are typically more complex to manufacture.

AC motors are often less expensive. (correct)

AC motors have a lower reliability.

AC motors usually come with brushes and commutators.

In what type of application are three phase induction motors most commonly used?

Powering hydraulic pumps. (correct)

Controlling oil shutoff valves.

Managing small precision equipment.

Operating electric clocks.

Which equation describes the relationship between the speed of rotation of an AC motor and its operating frequency?

rpm = 120 ÷ Frequency Number of Poles

rpm = 150 × Number of Poles ÷ Frequency

rpm = 60 × Frequency Number of Poles

rpm = 120 × Frequency / Number of Poles (correct)

What is a typical characteristic of single phase synchronous motors?

They require an auxiliary method to start. Signup and view all the answers

How does the speed of a 400-cycle AC motor compare to a 60-cycle commercial motor with the same number of poles?

It operates at about seven times the speed. Signup and view all the answers

Which type of motor is commonly used to operate electric clocks?

Single phase synchronous motor Signup and view all the answers

What is typically avoided in AC motors compared to DC motors?

Mechanical wear from brushes and commutators. Signup and view all the answers

What kind of electrical systems do alternating current motors commonly operate on?

Polyphase or single phase lines. Signup and view all the answers

What is the maximum speed range for a 400-cycle induction type motor?

6,000 to 24,000 rpm Signup and view all the answers

Which component of an induction motor is responsible for producing the rotating magnetic field?

Stator Signup and view all the answers

How is the number of poles per phase in an induction motor calculated?

P = 120 × f / N Signup and view all the answers

What type of motor is also known as a squirrel cage motor?

Three phase AC induction motor Signup and view all the answers

What happens if the rotor of an induction motor turns at the same speed as the rotating field?

No EMF is induced in the rotor. Signup and view all the answers

What is the synchronous speed of a motor calculated based on?

Frequency and number of poles Signup and view all the answers

What is referred to as the stationary part of an induction motor?

Stator Signup and view all the answers

Which element of torque production in an induction motor combines with the revolving magnetic field?

Induced current in the rotor Signup and view all the answers

What type of winding is typically used in the stator of an induction motor?

Distributed windings Signup and view all the answers

Why is there always a difference in speed between the rotor and the rotating field?

To ensure proper torque production. Signup and view all the answers

What is the characteristic feature of a rotor in a squirrel cage motor?

Has longitudinal slots with heavy bars. Signup and view all the answers

What is NOT a factor in the rating of alternating current motors?

Physical dimensions Signup and view all the answers

What dictates the direction of the rotating magnetic field in an induction motor?

Arrangement of wire coils Signup and view all the answers

How can the direction of rotation of a three-phase induction motor be changed?

By reversing two of the leads to the motor. Signup and view all the answers

What happens to a three-phase motor if one connection is broken after starting?

It will deliver only one-third of its rated power. Signup and view all the answers

What is a characteristic of a synchronous motor?

It operates at the same speed as the stator's magnetic field. Signup and view all the answers

Why are synchronous motors not self-starting?

Their rotor is too heavy to reach synchronous speed from a standstill. Signup and view all the answers

Which method is commonly used to start a synchronous motor?

Using a separate AC motor to bring it close to synchronous speed. Signup and view all the answers

What is the consequence of overloading a synchronous motor?

The rotor will pull out of synchronization with the field. Signup and view all the answers

How is the rotor field energized in a synchronous motor?

Through DC supply via slip rings. Signup and view all the answers

In which scenario can a single-phase motor's direction of rotation be reversed?

By reversing connections to the starting winding. Signup and view all the answers

What is a disadvantage of shaded pole motors?

Direction of rotation cannot be reversed. Signup and view all the answers

What is the role of a rheostat in a synchronous motor?

To vary rotor pole strength for load control. Signup and view all the answers

How does the torque of a synchronous motor compare to an induction motor?

Torque is independent and does not rely on rotor currents. Signup and view all the answers

Which of the following is true for AC series motors?

They are a type of single-phase motor. Signup and view all the answers

What is the function of the shading coil in a shaded pole induction motor?

To create a lag in the magnetic field, aiding rotor rotation Signup and view all the answers

What characterizes the starting torque of a shaded pole motor?

Weak, suitable for devices with low initial traction Signup and view all the answers

In a single-phase induction motor, what occurs when the rotor is stationary?

Opposing currents in the rotor create a turning force Signup and view all the answers

How does a split phase motor achieve a rotating magnetic field?

Through a phase shift between the starting and running windings Signup and view all the answers

What is a primary advantage of capacitor start motors over traditional split phase motors?

They offer a higher starting torque, suitable for heavy loads Signup and view all the answers

What happens to the starting winding in a split phase motor once the rotor reaches about 25% of the rated speed?

It is automatically disconnected by a centrifugal switch Signup and view all the answers

How does the rotor of a shaded pole induction motor generate torque while starting?

From the interaction between the waves from the shading coil and stator field Signup and view all the answers

Which of the following types of motors is known for having low starting torque and is suitable for small applications?

Shaded pole induction motor Signup and view all the answers

What is the primary purpose of a centrifugal switch in capacitor start motors?

To disconnect the starting winding at low speeds Signup and view all the answers

Which type of motor uses salient poles with a shading coil for operation?

Shaded pole induction motor Signup and view all the answers

What is slip in the context of electric motors?

The difference in speed between the rotor and synchronous speed Signup and view all the answers

Which factor primarily affects the success of a capacitor start motor in heavy load applications?

The phase shift created by the capacitors Signup and view all the answers

Which benefits the shaded pole motor in applications that require low initial torque?

Simple construction and reliability Signup and view all the answers

What is the primary application area for single-phase induction motors with 1 hp or less?

Small appliances like fans and refrigerators Signup and view all the answers

What causes excessive heating and reduced efficiency in a DC series motor when connected to an AC supply?

Large eddy current and hysteresis losses Signup and view all the answers

Which change is made to reduce eddy current losses in an AC series motor?

Laminate the field poles, frame, and armature Signup and view all the answers

What is the purpose of a compensating winding in an AC series motor?

To neutralize the effect of armature magnetomotive force Signup and view all the answers

In which configuration is the compensating winding connected for operation on both DC and AC circuits?

In series with the armature Signup and view all the answers

What typically indicates that an AC motor is overloaded?

A knocking sound Signup and view all the answers

What is a fractional horsepower AC series motor commonly called?

Universal motor Signup and view all the answers

How is sparking at the commutator reduced in an AC series motor?

By using high resistance leads Signup and view all the answers

What role does the temperature play in the operation of an AC motor?

It is the only limiting operating factor Signup and view all the answers

What is the primary reason for using high-permeability silicon steel laminations in the design of AC motors?

To minimize hysteresis losses Signup and view all the answers

Which type of compensation winding is designed to reduce armature reactance by the action of alternating armature flux?

Inductively compensated winding Signup and view all the answers

What causes a three-phase motor to 'growl' when one lead is disconnected?

Imbalance in electrical supply Signup and view all the answers

What is the effect of the alternating field flux on the armature coils in an AC series motor?

It establishes large currents that lead to short circuits Signup and view all the answers

What type of sound typically indicates that an AC motor is functioning properly?

Humming evenly Signup and view all the answers

Which feature is included to limit induced current and reduce sparking at the brushes?

Preventive leads Signup and view all the answers

To maintain efficiency, which factor should be checked regularly in AC motors?

Coils' cleanliness and dryness Signup and view all the answers

Study Notes

AC Motors

AC motors are often preferred over DC motors due to lower cost and reduced maintenance needs
Reduced maintenance stems from avoidance of sparking at brushes due to the absence of brushes and commutators in many AC motor designs
Well-suited for constant speed applications and some varieties offer variable speed control, within limitations
Operate on polyphase or single-phase lines and various voltage ratings

AC Motor Types

Two primary types: induction and synchronous
Induction motors:
- Three-phase induction motors are used for high-powered applications like starters, flaps, landing gears, and hydraulic pumps
- Single-phase induction motors are suitable for low-power applications, such as surface locks, intercoolers, and oil valves
Synchronous motors:
- Three-phase synchronous motors operate at constant synchronous speeds, commonly used in flux-gate compasses and propeller synchronizers
- Single-phase synchronous motors are used in clocks and other small precision equipment, requiring auxiliary starting mechanisms, usually a separate starting winding

Rotating Magnetic Field

The rotating magnetic field is created by applying three AC voltages (a, b, c) with equal magnitude but differing phases to poles
At different time intervals, the resultant magnetic field has various intensities, producing a revolving field rotation.

AC Motor Speed

Motor speed depends on the number of poles and the power source's frequency, using the formula: rpm = 120 × Frequency / Number of Poles
Aircraft electrical systems (400 cycles) result in significantly faster speeds than commercial motors (60 cycles) with the same number of poles
400-cycle AC motors are suitable for high-speed rotors in lifting and moving loads
AC motors are rated by horsepower, voltage, current at full load, speed, phase number, and frequency; continuous or intermittent operation is also considered

Three-Phase Induction Motor (Squirrel Cage Motor)

Based on the principle of a rotating magnetic field
The stator (stationary part) has distributed windings in slots around its periphery, and the rotor (rotating part) has embedded bars (copper/aluminum) welded to an end ring.
Rotor speed is slightly less than the synchronous speed—the difference is called slip, expressed as a percentage
Slip is essential since a synchronous rotor speed would not induce current or torque

Single-Phase Induction Motor

Single-phase motors have a pulsating field, instead of a rotating one
The rotor's starting direction depends on the initial rotational movement and momentum
The shaded pole induction motor utilizes a copper ring encircling a portion of each pole to generate a rotational component, sufficient for starting

Split-Phase Motor

The split-phase motor has a starting winding 90 degrees apart from the main winding
A centrifugal switch automatically disconnects the starting winding once the rotor reaches 25% of rated speed

Capacitor Start Motor

A variation of the split-phase motor, using capacitors to phase-shift the starting and running windings
Provides higher starting torque and is suitable for applications with heavy initial loads
A centrifugal switch disconnects the starting winding after the rotor reaches 25% of its speed

Direction of Rotation

Reversing connections for two leads (three-phase) or one phase (two-phase) changes the motor's rotational direction
Starting winding connections (single-phase) also reverse the direction.

Synchronous Motors

Based on a reacting pair of magnetic fields (rotating stator and DC-energized rotor) to maintain synchronous speed
Not self-starting and require a starting mechanism (external motor, auxiliary squirrel cage winding)
The rotor is the heavy component limiting self-starting capability

AC Series Motor

Resembles DC series motors with brushes and commutators
Suitable for AC or DC circuits
Limitations when operating on AC, such as eddy current and hysteresis losses, low power factor, and high commutation sparking
Modifications include laminated field poles, frame, armature, and use of compensating windings, that result in lessened sparking and improved power factor for satisfactory AC operation

AC Motor Maintenance

Bearings may need lubrication, depending on type
Keep coils dry and free of contaminants
Use hand-feel to assess (avoid excessive operating temperatures)
Motor sound (humming, groaning, growling) provides trouble indicators.

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Description

Explore the various aspects and types of AC motors, including their advantages over DC motors and specific applications. Learn about induction and synchronous motors, their differences, and typical usage scenarios in different power settings.