Split-Phase Motor Operation

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Questions and Answers

What determines the direction of rotation in a reluctance-start motor?

The number of poles

The voltage across the motor

The direction of the exciting current

The width of the air gap (correct)

What causes the starting torque in a reluctance-start motor?

The tapped autotransformer

The single component of pole flux

The varying voltage across the motor

The two out-of-phase components of pole flux (correct)

What happens to the flux when a coil of wire produces flux in a magnetic circuit containing no iron?

The flux lags behind the exciting current

The flux is sinusoidal with the exciting current

The flux leads the exciting current

The flux is in phase with the exciting current (correct)

What happens to the flux when the MMF of a coil produces flux in a magnetic circuit made up entirely of iron?

The flux lags behind the exciting current Signup and view all the answers

What happens to the fluxes when two coils of wire, excited from the same source, produce fluxes that act in magnetic circuits, one of which has less iron?

The flux acting in the circuit with less iron will lead the flux acting in the circuit with more iron Signup and view all the answers

What is a characteristic of reluctance-start motors?

They are applied where starting and overload torque requirements are low Signup and view all the answers

What is used to achieve speed control in reluctance-start motors?

A tapped autotransformer or a tapped reactance coil Signup and view all the answers

Which type of motor is made in larger ratings compared to reluctance-start motors?

Split-phase motor Signup and view all the answers

What is the primary purpose of a supersynchronous motor?

To generate extremely high starting torque Signup and view all the answers

What is unique about the armature structure in a supersynchronous motor?

It revolves freely Signup and view all the answers

What is the purpose of the band brake in a supersynchronous motor?

To lock the armature in place Signup and view all the answers

What is the typical application of synchronous motors?

Constant-duty service Signup and view all the answers

What is the range of motor speeds for 60-cycle service?

1,800 rpm to 120 rpm Signup and view all the answers

What is the typical horsepower rating of a synchronous motor?

Up to 4,000 Signup and view all the answers

What is an economical way to excite a group of synchronous motors?

Use a single motor-generator set to excite all motors Signup and view all the answers

What is the function of the DC machine in starting a synchronous motor?

To function as a generator Signup and view all the answers

What is the purpose of strengthening the field of the DC machine?

To make the DC machine function as a generator Signup and view all the answers

What is the function of the DC exciter in starting a synchronous motor?

To act as a starting motor Signup and view all the answers

What is the purpose of incorporating a squirrel cage in the pole faces of the rotor?

To start the synchronous motor as an induction motor Signup and view all the answers

Why is it necessary to short-circuit the field before closing the main three-phase switch?

To reduce the current drawn from the line Signup and view all the answers

What happens when the revolving field starts in a synchronous motor?

It cuts the field winding at synchronous speed Signup and view all the answers

What is the general procedure for starting a synchronous motor?

Short-circuit the field before closing the main three-phase switch Signup and view all the answers

What is the material of the rotor cylinder in a synchronous motor?

Chrome-steel Signup and view all the answers

Why does the rotor revolve synchronously in a synchronous motor?

Because of the magnetic lock-up with the stator poles Signup and view all the answers

What is the typical speed of a commercial synchronous motor?

3000 rpm Signup and view all the answers

Why do synchronous motors accelerate rapidly?

Because of the lack of teeth or winding on the rotor Signup and view all the answers

What determines the speed of a synchronous motor?

The frequency of the supply source and the number of poles Signup and view all the answers

What happens to a synchronous motor if the counter torque exceeds the maximum torque?

It comes to rest Signup and view all the answers

What is the starting torque of a synchronous motor?

Practically zero Signup and view all the answers

Why are synchronous motors preferred?

Because they can operate at a leading power factor and are less costly Signup and view all the answers

What determines the number of poles for a split-phase motor?

Frequency and desired synchronous speed Signup and view all the answers

How are the two windings connected in a split-phase motor?

In parallel Signup and view all the answers

What is the purpose of the centrifugal device or electromagnetic relay in a split-phase motor?

To disconnect the auxiliary winding at the proper time Signup and view all the answers

What determines the direction of rotation of a split-phase motor?

The connection of the main and auxiliary windings to the source and each other Signup and view all the answers

What is the reason for the high inrush current in split-phase motors?

The motor starts directly from the line Signup and view all the answers

What is a challenge in controlling the speed of split-phase motors?

Special windings are needed for speed variation Signup and view all the answers

What is the typical variation in speed regulation of split-phase motors?

2.5% to 5% Signup and view all the answers

What type of applications are split-phase motors ideal for?

Low starting torque, short starting durations Signup and view all the answers

Study Notes

Split-Phase Motors

Number of poles governed by frequency and desired synchronous speed
Two windings (main and auxiliary) connected in parallel, served by the same single-phase source
Starting torque developed only when both windings are energized
Main winding develops nearly as much torque as both windings at 75% of rated speed
Auxiliary winding cut out by centrifugal device or electromagnetic relay at proper time
Direction of rotation determined by connection of main and auxiliary windings to source and each other
High inrush current (4-6 times rated value) with two parts: larger through main winding and smaller through auxiliary winding
Difficult to control speed; special windings needed for speed variation, with limited adjustments compared to DC motors
Ideal for applications with low starting torque requirements and short starting durations
Speed regulation similar to polyphase motors, with 2.5% to 5% variation between no load and full load

Reluctance-Start Motors

Direction of rotation always from wide air gap to narrow air gap
Starting torque developed due to two out-of-phase components of pole flux
Flux created by single AC exciting winding has two components: one under wide air gap and one under narrow air gap
Important to remember:
- No iron in magnetic circuit: flux in phase with exciting current
- Entirely iron magnetic circuit: flux lags behind exciting current
- Different iron content in magnetic circuits: fluxes out of phase in time
Not as widely used as shaded-pole motors; made in small sizes; applied where starting and overload torque requirements are low; cannot be reversed
Speed control achieved by varying voltage across motor using tapped autotransformer or tapped reactance coil

Synchronous Motors

Operates at an average constant speed regardless of load; momentarily departs from average speed during load changes
Speed determined by two factors: frequency of supply source and number of poles in the machine
Receiving excitation from two sources: AC source through stator winding and DC source through rotor field
Starting torque practically zero; various methods employed to bring machine up to synchronism before assuming load
Preferred for several reasons: can operate at leading power factor, less costly, and economical in installations with a group of synchronous motors

Starting Synchronous Motors

Methods:
- Starting with a DC machine
- Starting with a DC exciter
- Starting with a small induction motor
- Starting as an induction motor
General procedure: short-circuiting the field before closing the main three-phase switch in the starting position, reducing induced voltage in the field winding

Supersynchronous Motor

Generates a starting torque equivalent to the pull-out or stalling value
Unique construction: armature structure mounted in a cradle supported by bearings, allowing it to revolve freely like the rotor
Armature winding supplied with power during startup, and band brake locks armature in place when necessary
Used for constant-duty service, requiring frequent starting, in various applications like fans, blowers, compressors, and pumps, up to 4,000 horsepower

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Description

This quiz covers the operation of split-phase motors, including winding, torque development, and speed. It also explains how the two windings are connected and when the main winding develops maximum torque.