Variable Voltage and Magnetic Clutches

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  • Discuss the types of motors that can be controlled with variable voltage.
  • Discuss requirements for motors that are controlled with variable voltage.
  • Discuss the operation of a magnetic clutch.

The previous section discussed the operation of consequent pole motors that change speed by changing the number of stator poles per phase. Although this is one method of controlling the speed of a motor, it's not the only method. Many small single-phase motors change speed by varying the amount of voltage applied to the motor.

This method does not change the speed of the rotating magnetic field of the motor, but it does cause the field to become weaker. As a result, the rotor slip becomes greater, causing a decrease of motor speed.

Variable voltage control is used with small fractional horsepower motors that operate light loads such as fans and blowers. Motors that are intended to operate with variable voltage are designed with high impedance stators. The high impedance of the stator prevents the current flow from becoming excessive as the rotor slows down. The disadvantage of motors that contain high impedance stator windings is that they are very limited in the amount of torque they can produce.

When load is added to a motor of this type, its speed will decrease rapidly.

Single-phase motors that use a centrifugal switch to disconnect the start windings cannot be used with variable voltage control. This limits the type of induction motors to capacitor start capacitor run and shaded pole motors. Capacitor start capacitor run motors are employed in applications where it's desirable to re verse the direction of rotation of the motor, such as ceiling fans.

Another type of alternating current motor that can use variable voltage for speed control is the universal or AC series motor. These motors are commonly used in devices such as power drills, skill saws, vacuum cleaners, household mixers, and many other appliances.

They can generally be recognized by the fact that they contain a commutator and brushes similar to a direct current motor. Universal motors are so named because they can operate on AC or DC voltage. These motors are used with solid-state speed control devices to operate electric drills, routers, reciprocating saws, and other variable speed tools.

Fgr. 1 Cutaway view of a variable autotransform

Fgr. 2 An autotransformer supplies variable voltage to a motor.

Fgr. 3 Conducting part of a waveform produces pulsating direct current.

Voltage Control Methods

Fgr. 4 Basic triac control circuit.

Fgr. 5 Variable speed control using a triac to control the voltage applied to a motor.

Fgr. 6 Single face magnetic clutch.

There are different methods of obtaining a variable AC voltage. One method is with the use of an auto transformer with a sliding tap (Fgr. 1). The sliding tap causes a change in the turns ratio of the transformer (Fgr. 2). An autotransformer is probably the most efficient and reliable method of supplying variable AC voltage, but they are expensive and require a large amount of space for mounting.

Another method involves the use of a solid-state device called a triac. A triac is a solid-state device similar to a silicon controlled rectifier (SCR), except that it will conduct both the positive and negative portions of a waveform. Triacs are commonly used in dimmers employed to control incandescent lighting. Triac light dimmers have a characteristic of conducting one half of the waveform before the other half begins conducting.

Since only one half of the waveform is conducting, the output voltage is DC, not AC (Fgr. 3). Resistive loads such as incandescent lamps are not harmed when direct current is applied to them, but a great deal of harm can occur when DC voltage is applied to in inductive device such as a motor. Only triac controls that are designed for use with inductive loads should be used to control a motor. A basic triac control circuit's shown in Fgr. 4. A triac variable speed control for small AC motors is shown in Fgr. 5.

Magnetic Clutches

Fgr. 7 Cut-away view of a magnetic clutch.

Magnetic clutches are used in applications where it's desirable to permit a motor to reach full speed before load is applied. Clutches can provide a smooth start for loads that can be damaged by sudden starting, or for high inertia loads such as centrifuges or flywheels.

Magnetic clutches are divided into two primary sections: the field section, which contains the slip rings and coil winding; and the armature section, which contains the clutch disc (Fgr. 6). When power is applied to the field winding through the slip rings and brushes, the armature is attracted to the field, coupling the motor to the load. The force of coupling can be con trolled by adjusting the voltage supplied to the field.

This permits control over the degree of slip between the field section and the armature section. The amount of slip will determine how rapidly the motor can accelerate the driven load and the amount of initial torque de livered to the load. When power is removed from the clutch, a spring separates the field and armature. A magnetic clutch is shown in Fgr. 7.

The clutch illustrated in Fgr. 6 is a single face clutch, which means that it contains only one clutch disc. Clutches intended to connect large horse power motors to heavy loads often contain multiple clutch faces. Double faced clutches have both the armature and field discs mounted on the same hub. A double faced friction lining is sandwiched between them. When the field winding is energized, the field disc and armature disc are drawn together with the double faced friction lining between them. Double faced clutches can be obtained in sizes up to 78 inches in diameter.

Some clutches are intended to provide tension control and are operated with a large amount of slip page between the driving and driven members. These clutches produce an excessive amount of heat because of the friction between clutch discs. Many of these clutches are water cooled to help remove the heat.

Eddy Current Clutches

Fgr. 8 Diagram A shows magnetic armature or rotor and drum. Diagram B shows rotor mounted inside the drum. The rotor is the input shaft of the clutch and the drum is the output shaft.

Fgr. 9 An AC motor is coupled to an eddy current clutch.

Eddy current clutches are so named because they induce eddy currents into a metal cylinder or drum. One part of the clutch contains slip rings and a winding (Fgr. 8A). The armature or rotor is constructed so that when the winding is excited with direct current, magnetic pole pieces are formed. The rotor is mounted inside the metal drum that forms the output shaft of the clutch (Fgr. 8B). The rotor is the input of the clutch and is connected to an AC induction motor. The motor provides the turning force for the clutch (Fgr. 9). When direct current is applied to the rotor, the spinning electromagnets induce eddy currents into the metal drum. The induced eddy currents form magnetic poles inside the drum. The magnetic fields of the rotor and drum are attracted to each other, and the clutch turns in the same direction as the motor.

The main advantage of an eddy current clutch is that there is no mechanical connection between the rotor and drum. Since there is no mechanical connection, there is no friction to produce excessive heat and there is no wear as is the case with mechanical clutches.

The speed of the clutch can be controlled by varying the amount of direct current applied to the armature or rotor. Since the output speed is determined by the amount of slip between the rotor and drum, when load is added, the slip will become greater, causing a de crease in speed. This can be compensated for by in creasing the amount of direct current applied to the rotor. Many eddy current clutch circuits contain a speed sensing device that will automatically increase or decrease the DC excitation current when load is added or removed.


1. Does varying the voltage to an AC induction motor cause a change in synchronous speed?

2. Why do induction motors that are intended to be controlled by variable voltage contain high impedance stator windings?

3. What is the disadvantage of a motor that contains a high impedance stator winding?

4. What type of AC induction motor is used with variable voltage control when it's desirable for the motor to reverse direction?

5. What type of motor that can be controlled with variable voltage is used to operate power drills, vacuum cleaners, routers, etc.?

6. Why are universal motors so named?

7. What type of solid-state component is generally used to control AC voltage?

8. When using a mechanical clutch, what determines how fast a load can be accelerated and the amount of initial torque applied to the load?

9. What is the primary advantage of an eddy current clutch over a mechanical clutch?

10. How is the speed of an eddy current clutch controlled?

Related: Variable Voltage and Magnetic Clutches are often used in the Railroad Industry

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