Type of induction motor

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Schematic symbol of a slip ring motor

A wound-rotor motor, also known as slip ring-rotor motor, is a type of induction motor where the rotor windings are connected through slip rings to external resistance. Adjusting the resistance allows control of the speed/torque characteristic of the motor. Wound-rotor motors can be started with low inrush current, by inserting high resistance into the rotor circuit; as the motor accelerates, the resistance can be decreased.[1]

Compared to a squirrel-cage rotor, the rotor of the slip ring motor has more winding turns; the induced voltage is then higher, and the current lower, than for a squirrel-cage rotor. During the start-up a typical rotor has 3 poles connected to the slip ring. Each pole is wired in series with a variable power resistor. When the motor reaches full speed the rotor poles are switched to short circuit. During start-up the resistors reduce the field strength at the stator. As a result, the inrush current is reduced. Another important advantage over squirrel-cage motors is higher starting torque.

The speed and torque characteristics of a wound-rotor motor can be adjusted by changing the external resistance, unlike a squirrel cage motor which has a fixed characteristic. This is useful for speed control of the motor.[1]

A wound-rotor motor can be used in several forms of adjustable-speed drive. Common applications include hoists and conveyor systems. Certain types of variable-speed drives recover slip-frequency power from the rotor circuit and feed it back to the supply, allowing wide speed range with high energy efficiency. Doubly-fed electric machines use the slip rings to supply external power to the rotor circuit, allowing wide-range speed control. Today speed control by use of slip ring motor is mostly superseded by induction motors with variable-frequency drives.

References

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1. a b

   Harold J. Herbein Rotating Machinery, Rinehart Press, , SBN 03--7, pages 215-218

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Difference between Slip Ring & Squirrel Cage Induction Motor

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There are two types of 3-phase induction motors viz. squirrel-cage induction motor and slip-ring or wound-rotor induction motor.

• Squirrel cage induction motors are widely used in industrial applications because they are very cheap, robust, efficient and reliable.
• On the other hand, slip-ring or wound-rotor induction motors are rarely used in industries since these motors have several disadvantages such as high copper loss and frequent maintenance.

Following are the key differences between slip ring and squirrel cage induction motors &#;

• The construction of a slip-ring induction motor is complicated because it consists of slip-rings and brushes while the squirrel cage induction motor has simple construction.
• The slip ring induction motors have high starting torque with low starting current whereas squirrel cage motors have low starting torque and high starting current.
• Slip-ring motor has smooth acceleration under heavy loads as compare to the squirrel cage motor.
• There is no abnormal heating during starting of the slip ring motor whereas adequate heat is generated in squirrel cage motor due to high starting current.
• The speed control of slip-ring motor is very easy as compared to squirrel cage motor.
• For starting the slip ring induction motor rotor resistance starter is required whereas the squirrel cage motor may not require any starter.
• Slip ring motors have good running characteristics as compared to squirrel cage motors.
• For the slip ring induction motors, the initial and maintenance costs are higher than those of squirrel cage induction motors.
• Slip ring induction motors have lower efficiency than the squirrel cage induction motors.
• The power factor of slip ring induction motors is low as compared to squirrel cage motors.
• The operation of slip ring induction motor is more sensitive to the fluctuations in supply voltage as compared to the squirrel cage motor.
• The copper loss is greater in a slip ring induction motor as compared to squirrel cage motor.
• Slip ring induction motors are mainly used in applications where high starting torque is required such as cranes, hoists, conveyors, etc. while the squirrel cage induction motors are used in general application like lath machine, drill machine, pumps and fans, etc.

Summary of Differences between Squirrel Cage & Slip Ring Induction Motors

Parameter of Comparison
Squirrel-Cage Induction Motor
Slip-Ring Induction Motor
Rotor TypeSquirrel cage type rotor with permanently short circuited rotor bars.
Wound type rotor with 3-phase windings similar to stator winding.
Construction
Simple
Complicated
External Rotor Resistance
It is not possible to add external resistance because the rotor bars are permanently shorted with the help of end rings.
External resistance is to be added during starting.
Starting Torque
Low
High
Brushes
Not used
Used
Maintenance
Less maintenance is required.
Frequent maintenance is required.
Copper Loss
Low
High
Efficiency
High
Low
Power Factor
High
Low
Speed Control
Not possible
Possible
Starting Current
High
Low
Cost
Cheap
Expensive
Application
In lath machine, fans, pumps, blowers, etc.
In cranes, hoists, elevators, conveyors and loads which require high starting torque, etc.

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