How Does Three Phase Slip Ring Induction Motor Is Also Known as Work?

Type of induction 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]

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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 and smooth operation due to variable speed.

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, elevators, and conveyor systems. Also, the travel mechanism of gantry cranes or overhead cranes also used this type of motor, because it has both adjustable speed and high torque. Many gantry cranes and portal cranes used in ship yards used this motor for smooth handling of heavy containers, ship handling, ship part handling , because it is very tolerant with overloads and can precisely handle heavy loads, and also it's speed is not affected by load. 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 Today speed control by use of slip ring motor is mostly superseded by induction motors with variable-frequency drives.

Advantages of this motor are mainly high starting torque with quite low inrush current, high rotational torque, variable speed control with no need of complicated electronics for smooth operation, good speed regulation, good load control, stable rotation under load, high tolerance to overload, meaning it can adapt quick at sudden overload, good efficiency at nominal speed. Owing to its high torque, it was also used in some gearless elevators, with higher number of poles, however, these motors were very big and heavy. Disadvantages are mainly the higher maintenance due to wear out of slip rings, compared to squirrel cage motor it has a more complicated. construction, they are quite noisy (buzzing and humming ) at start-up especially with high loads and have less efficiency compared to squirrel cage rotor motors, and also they are mechanically noisier compared to squirrel cage motor. They are usually bigger than the squirrel-cage motors for the same power. Also, wound-rotor motors are not fully sealed. While this allows a better cooling (however it runs cooler than the squirrel-cage motor), it needs to be protected from excessive moisture (rain ) or excessive dust, and limits somehow it's usage in explosive and damp environments. They are mostly replaced with induction motors with variable frequency drives, or with permanent magnet synchronous motors, depending on applications.

An induction motor is an electrical device that converts electrical energy into mechanical energy. It is most widely used for industrial applications due to its self-starting attribute. Slip ring induction motor is one of the types of 3-phase induction motor and is a wound rotor motor type. Because of various advantages like low initial current, high starting torque, and improved power factor, it is used in applications that require high torque, cranes, and elevators. The rotor windings consist of more number of windings, higher induced voltage, and less current compared to the squirrel-cage rotor. The windings are connected to external resistance through slip rings, which helps to control the torque/speed of a motor.

What is a Slip Ring Induction Motor?

Definition: A slip ring induction motor is referred to as an asynchronous motor as the speed at which it operates is not equal to the synchronous speed of a rotor. The rotor of this type of motor is wound type. It comprises of a cylindrical laminated steel core and a semi-closed groove at the outer boundary to accommodate a 3-phase insulated winding circuit.

As seen in the figure above, the rotor is wound to match the number of poles on the stator. The three terminals of a rotor and three start terminals connecting through slip rings are connected to a shaft. The aim of the shaft is to transmit mechanical power.

Construction

Before we discuss the working principle of slip ring Induction motor, knowing the slip ring induction motor construction is important. So let’s begin with the construction which includes two parts: Stator and Rotor.

• Stator
• Rotor

Stator

The stator of this motor comprises of various slots that are arranged to support the construction of a 3-phase winding circuit connecting to a 3-phase AC source.

Rotor

The rotor of this motor consists of a cylindrical core with steel laminations. Besides this, the rotor has parallel slots to accommodate 3-phase windings. The windings in these slots are arranged at 120 degrees to each other. This arrangement can reduce noise and avoid irregular pausing of a motor.

Working of Slip Ring Induction Motor

This motor runs on the principle of Faraday’s law of electromagnetic induction. When a stator winding is excited with an AC supply, the stator winding produces magnetic flux. Based on faraday’s law of electromagnetic induction, the rotor winding gets induced and generates a current of magnetic flux. This induced EMF develops torque that enables the rotor to rotate.

However, the phase difference between the voltage and current do not meet the requirements to generate high starting torque as torque developed is not unidirectional. The external resistance of high value is connected with the circuit to improve the phase difference of a motor. As a result, inductive reactance and phase difference between I and V is reduced. Consequently, this reduction helps the motor to generate high stating torque. The slip ring induction motor diagram is shown below.

Why Slip Rings are used in an Induction Motor?

Slip is defined as the difference between the flux speed and the rotor speed. For an induction motor to produce torque, at least some difference should be there between stator field speed and rotor speed. This difference is called ‘slip’. The Slip Ring” is an electromechanical device that aids in transmitting power and electrical signals from stationary to a rotating component.

Slip rings are also known as rotary electrical interfaces, electric rotary joints, swivels, or collector rings. Sometimes, based on the application, the slip ring requires higher bandwidth to transmit data. Slip rings improve the efficiency and performance of a motor by improving system operation and eliminating wires that are dangling from motor joints.

Slip Ring Induction Motor Resistance Calculation

The peak torque occurs if

r = Smax. X —— (I)

Where, Smax = Slip at pull-out torque

X = Inductance of a rotor

r = resistance of rotor winding

Adding external resistance R to equation (I),

r+R= (Smax)’. X —— (ii)

From equation (i) and (ii),

R = r(S’ max/Smax – 1) —— (iii)

By definition of Smax, we get Smax = 1 – (Nmax/Ns) —— (iv)

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Putting S’max =1 in equation (iii), we get

R = r. (1/Smax-1) —— (v)

Let’s say, Ns = synchronous speed of rpm and pull-out torque happens at 900 rpm, equation (iv) reduces to Smax = 0.1 (i.e., 10% slip)

Substitute in equation (v),

R = r. (1/0.1 – 1)

R = 9. r

‘r’ is measured using a multimeter. The resistance value of 9 times higher than of a slip ring rotor resistance is connected externally to experience maximum starting torque.

Slip Ring Induction Motor Speed Control

The speed control of this motor can be done using two methods which include the following.

Effect of Adding External Resistance

Generally, the initiation of these motors occurs when it draws full line voltage which is 6 to 7 times higher than the full load current. This high current can be controlled by external resistance connected in series with the rotor circuit. The external resistance acts as a variable rheostat during the motor kick-off and tweaks automatically to high resistance to get required starting current.

The external resistance reduces high resistance as soon as the motor obtains normal speed and increases the starting torque of a motor. The tweaking of external resistance also aids in decreasing of rotor and stator current but improves the power factor of a motor.

Using Thyristor Circuit

Thyristor On/Off circuit is another way to control the speed of a motor. In this method, rotor AC current is connected to a 3-phase bridge rectifier and connected to external resistance through a filter. The thyristor is connected across external resistance and is switched on/off at high frequency. The ratio of on-time to off-time estimates the actual value of rotor circuit resistance that helps in controlling the speed of a motor by controlling speed-torque characteristics.

Difference Between Squirrel Cage and Slip Ring Induction Motor

The difference between these two motors is discussed below.

Slip Ring Motor Squirrel Cage Motor It has a rotor of wound type Its rotor is of squirrel cage type Rotor has cylindrical core has parallel slots, in which each slot has a bar Slots are not parallel to each other Construction is complicated because of slip rings and brushes Construction is simple External resistance circuit is connected with a motor No external resistance circuit as bars of the rotor is completely slotted Starting torque is high Torque is low Efficiency is low Efficiency is high

Advantages and Disadvantages of Slip Ring Induction Motor

The advantages are

• High and excellent starting torque to support high inertia loads.
• It has a low starting current due to external resistance
• Can take full load current that is 6 to 7 times higher

The disadvantages are

• Includes higher maintenance costs due to brushes and slip rings compared to squirrel cage motor
• Intricate construction
• High copper loss
• Low efficiency and low power factor
• Expensive than 3 phase squirrel cage induction motor

Applications

Some of the applications of slip ring induction motor are

• These motors are used where higher torque and low starting current are required.
• Used in applications like elevators, compressors, cranes, conveyors, hoists, and many more

FAQs

1). What is slip in an electric motor?

Slip is defined as the difference between synchronous speed and operating speed, at the same frequency.

2). Where are squirrel cage induction motors used?

They are used in centrifugal pumps, large blowers, and fans, to run conveyor belts, etc.

3). What is a slip ring induction motor?

A motor with a wound-type rotor is known as a slip ring induction motor. Also, rotor windings are connected through slip rings to external resistance.

4). Name one disadvantage of slip ring induction motor and squirrel cage induction motor

The disadvantages are high copper losses and the low torque

5). What is the use of external resistance in slip ring induction motors?

The external resistance acts as a variable rheostat during the motor kick-off and tweaks automatically to high resistance to get required starting current.

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