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Electric Motors andMotor Control Lesson 3

Objectives: Explain what the speed regulation of a

motor is. Describe Starting Current and Torque.

Explain the difference between ShadedPole Motors and Universal Motors.

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Squirrel Cage Rotor Principles

The squirrel cage rotor is made upof bars of conductors that are shortcircuited at each end.

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Squirrel cage rotor

Diagrammaticdrawing of thesquirrel-cage

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Split-Phase Motors and Slip

The speed of the rotating field ofthe rotor is 3600 RPM in this two-

pole (main poles) motor.

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The rotor does not travel as fast. The

rotor must slip behind the synchronousspeed in order to have voltage inducedinto it.

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The rotor bars must be cut by the

magnetic field of the stator to inducevoltage.

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The amount of slip depends on many

factors: the design of the rotor, andtype of rotor conductors.

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There is also air gap between the

rotor and the stator iron, andmechanical load on the shaft.

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Speed Regulation

As the mechanical load on the motorshaft increases, the rotor slows down.

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A perfect motor would have 0 percent

speed regulation. This means therewould be no speed variation betweenno load and full-load speed.

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Connections

Split-phase motors also come in dualvoltage ratings. The motor connectionsare simple and are interchangeable

from one voltage to another.

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Most dual-voltage motors only useone starting winding.

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By placing the running windings in

series with each other and applying230 volts, 115 volts will be presentacross each half of the winding.

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Reversing

To reverse the direction of rotation ofthe rotor, the direction of the rotatingmagnetic field at time of starting must

be reversed.

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Starting Winding Switches

Starting switches and relays for single-

phase motors are used to disconnect thestarting windings of a motor as themotor approaches running speed.

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Starting Current and Torque

When a motor is first connected to the power source, there is a large inrush ofcurrent.

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This large rotor current and statorcurrent produce strong magnetic polesthat produce the torque necessary tostart the rotor spinning.

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As the rotor comes up to speed, the rotor

current decreases because of less inducedvoltage (less slip) and the decrease instator current and line current.

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Capacitor Start, Induction RunMotors

Capacitor start motors are used whensingle-phase power is available, butthere is a need for more starting torque.

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Shaded Pole Motors

Shaded pole motors are one of thesimplest and cheapest motors toconstruct.

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Shading coils

Small shaded-pole motor

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These motors are typically fractionalHP ratings and are used inapplications that do not require a greatdeal of starting torque.

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Shaded pole motors are typically usedfor fans where the blades are directlymounted to the rotor shaft and air passesover the motor.

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3.3-Inch Shaded PoleMotor for

Fan, Pump and

Other General Purposes

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Universal Motors

Universal motors are conduction motors.The name universal indicates that the

motor will operate on AC or DC or

many frequencies of AC.

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Speed control is common in universal

motors. The load may consist of gear boxes.

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There also can be other devices permanently connected so there is aload at all times (for example, drills,saws, vacuum cleaners, and mixers.