Characteristics of Single Phase Induction Motor

The following are the inherent characteristics of single phase induction motor.

1. There is no starting torque in this motor.
2. If the motor is made to rotate by any means, the motor picks up the speed and continues to rotate in the same direction developing the operating torque.

Construction of Single Phase Induction Motor

Figure 1.

The single-phase induction motor mainly consists of two parts. They are as follows,

1. Stator
2. Rotor.

1. Stator

The stator is a stationary hollow cylindrical structure and it is the outer covering of the motor. The stator core is usually made up of cast iron or cast steel. A large number of axial slots are cut around the inner periphery of the core and these slots shelter the stator conductors. The stator winding of a single-phase induction motor is provided with concentric coils as shown in figure (1). The most widely used number of poles in the induction motor are 2, 4, 6, so that the induction motor can be wound for even number of poles. Practically, each coil has a number of turns but for convenience, only one turn of the coil is shown in figure (l). The stator core is made up of laminations which are usually 0.036 to 0.06 cm thick. Generally, the motors consisting of squirrel cage type are provided with 2 stator windings (except for the shaded pole motor). Both the windings are identical to that as shown in figure (l). Among these, one of the stator winding is provided with the heaviest wire, and these 2 stator windings are arranged in such a way that they are in space quadrature with respect to each other. In the motor which works with both the windings energized, the winding with much thin wire is known as auxiliary winding and the other is called as the main winding.

2. Rotor

It is the part of the motor that develops the driving torque and rotates. In practice there are two types of rotors, and the choice of the rotor is made on the basis of the application for which the motor is employed. The two types of rotor are,

1. Squirrel cage rotor
2. Slip ring rotor.

(i) Squirrel Cage Rotor: The rotor core is cylindrical and is usually made of cast iron or cast steel. All along the periphery of the core, longitudinal slots are made and these slots are embedded rotor conductors. The rotor conductors are usually thick bars of copper or aluminium. They are permanently welded to two copper end lings as shown in figure (2).

By this arrangement, the rotor always forms a closed-circuit. This type of construction is termed as squirrel cage construction.

(ii) Slip Ring Rotor: The rotor core is cylindrical. Slots are cut around the periphery of the core, and these slots house the rotor windings. The rotor conductors are in the form of copper wire. The slip rings of the rotor are shown in figure (3).

Applications of single phase induction motor

Ans: Single phase induction motors find their applications in,

1. Fans
2. Refrigerators
3. Vacuum cleaners
4. Centrifugal pump
5. Machine tools
6. Blowers
7. Washing machines
8. Grinders
9. Compressors
10. Conveyors.

Q1. What are the disadvantages of a single phase induction motor when compared with a 3-phase induction motor?

Ans: The following are the disadvantages of single phase induction motor when compared with 3-phase induction motor.

1. Single phase induction motors are not self-starting, whereas 3-phase induction motors are self starting.
2. The power factor of single phase induction motors is lower than 3 -phase induction motors.
3. Single phase induction motors have lower efficiency than 3-phase motors.
4. For the same rating, the output of single phase induction motor is half that of 3-phase induction motors.
5. For the same output, single phase motors are costlier than 3-phase motors.

Q2. Why a single phase induction motor needs an auxiliary winding?

Ans:

Single phase induction motor needs an auxiliary winding because of the following reasons.

1. To establish a rotation in magnetic field of stator because the main winding alone cannot establish a rotating magnetic field.
2. To predetermine the direction of rotation of motor.

Q3. How is the direction of rotation of a single phase induction motor reversed?

Ans: The direction of rotation of a single phase induction motor can be reversed by reversing the connections of start winding without disturbing the connections of run winding and vice-versa. Hence, the direction of a single phase induction motor can be reversed by reversing the connections of either start or run winding but not both at the same time.

Q4. Why single phase induction motor is not self starting? Mention any one method of starting.

Ans: Whenever the stator windings of a single phase induction motor are excited by a single phase A.C supply, an alternating flux is produced in the rotor which has the same axis to that of the stator. The rotor flux tries to oppose the main field flux.

Due to the lack of relative motion between the stator and rotor fluxes, the rotor fails to rotate resulting in no torque. Hence the single phase induction motor is not self starting. There are different methods to start induction motor, split phase method is one among them.

Q5. What are the various methods available for making a single-phase motor self-starting?

Ans: The various methods used for starting of single phase induction motor are,

1. Split phase method
2. Capacitor start method
3. Capacitor run method
4. Shaded pole method.

Q6. Name the motor being used in ceiling fans.

Ans: Single phase induction motor with split phase is used in ceiling fans due to its smooth torque-speed characteristics and ability to run very efficiently at constant speed.

The post What is Single Phase Induction Motor? Construction, Parts, Diagram & Applications appeared first on Electrical and Electronics Blog.

A capacitor start and run motor is also known as a two value capacitor run motor. The capacitor run induction motor is same as the capacitor start induction motor, where the capacitor is connected in series with the starting winding throughout its operation.

Under this condition, the motor runs as if it is a two-phase motor but with unbalanced currents. As the capacitor is connected all the time, it is selected in such a way to have longer duty cycles, generally the capacitors connected are paper or oil capacitors. They have the torques less than the capacitor start motor but higher than split phase motor. It does not require any centrifugal switch since starting winding is continuously kept in operation.

Circuit Diagram & Working of Capacitor Run Induction Motor

Figure (1) shows the circuit diagram of a two-value capacitor run motor supplied by single-phase supply. It consists of main winding, auxiliary winding, two capacitors C₁, C₂ and switch ‘S’. It is similar to the single value capacitor run motor. But the main difference here is the auxiliary winding and a capacitor C₁, are always connected in the circuit. The main function of capacitor C₂ is to start the motor. For this purpose, it is called the start capacitor and capacitor C₁ is called the run capacitor. It improves the power factor of the motor. In general, the starting capacitor C₂ is about 10 to 15 times as large as running capacitor C₁. At the time of starting, the centrifugal switch ‘S’ is closed, both the capacitors C₁ and C₂ are in parallel and the total capacitance is the sum of their individual capacitances. After the motor reaches to 75% of the full-load speed the switch is opened and the only capacitor C₁ is present in the auxiliary winding circuit. In this way, best starting performance with high capacitance and best running performance (best torque condition) with low capacitance is achieved. Such motors produce continuous torque thereby reducing the pulsating torques. By means of the two-value capacitor run motor, it is possible to obtain phase shift (β) (i.e. the angle between the currents in main winding and auxiliary winding) equal to 90º. Run capacitor C₁ and auxiliary winding can be designed in such a way that they provide balanced two-phase field. The balanced two-phase field avoids the backward rotating field and improves the power factor and efficiency of the motor.

The torque-speed characteristics of two-value capacitor run motor are shown in figure (2). From the characteristics it can be observed that, when auxiliary winding is used with the main winding, improved torque is obtained.

Applications of Capacitor Run Induction Motor

These are also used in applications like pumps, compressors, refrigerators, air-conditioners, conveyors, machine tools etc.

Q 1. How can the direction of a capacitor run motor be reversed?

Ans: The direction of a capacitor run motor can be reversed by reversing the connections of start winding without disturbing the connections of run winding and vice versa. Hence, the direction of a capacitor run motor can be reversed by reversing the connections of either start or run winding but not both at the same time.

The post What is Capacitor Run Induction Motor? Working Principle, Diagram & Applications appeared first on Electrical and Electronics Blog.

Fig. 1. Capacitor Start Induction Motor.

This capacitor is designed for short duty service. The phase displacement between the two phase currents is 90º, so the starting torque developed is more (twice that of split-phase motor). A centrifugal switch is connected in series with the capacitor and the starting winding so that they could be isolated at speeds near the full-load speeds. Because of the presence of capacitor, the starting current could be in phase with the operating supply voltage. The circuit is as shown in figure (1).

Construction of Capacitor Start Induction Motor

The construction of capacitor start induction motor is almost same as that of a split phase induction motor. In this motor capacitor is connected in series with auxiliary or starting winding and are mounted on top of the motor in any convenient external position by means of metal casing, in some cases it may be mounted inside the motor. The capacitor used in this motor provide higher starting torque and limits the starting surge of current to a lower value than developed by the split phase motor.

Working of Capacitor Start Induction Motor

The schematic diagram of capacitor start induction motor is shown in figure 2(a). In this motor an inexpensive and small A.C electrolytic type of capacitor is connected in series with the starting winding or the auxiliary winding. So that the current through the main winding, I_m lags behind the current of starting winding, I_S by an θ ( approximately equal to 90º)  as shown as figure 2(b). This results in high starting torque. The starting torque of a capacitor start induction motor, ranges between 3 to 4.5 times the full-load torque which is twice that of split phase induction motor. A centrifugal switch is connected in series with auxiliary winding and capacitor. The purpose of this switch is to disconnect the capacitor when motor attains 75% of full-load speed. At rated speed motor operates with main winding.

Figure 3.

The speed-torque characteristics of capacitor start induction motor are shown in figure (3). These motors are quite expensive than split phase induction motor because of the addition of capacitor.

Applications of Capacitor Start Induction Motor

1. Due to high starting torque, capacitor start induction motors are used for high inertia loads and also where regular starts are needed.
2. These are also used in applications like pumps, compressors, refrigerators, air-conditioners, conveyors, machine tools etc.

Q1. List out the characteristic features of single-phase capacitor start motor.

Ans: The characteristic features of single-phase capacitor start motors are as follows.

1. Capacitor start motors can be used for dual voltage ratings.
2. They can also be used in applications where starting torque requirement is high.
3. They have two windings i.e., start and run winding. When motor is started both the windings are energized but when motor attains 75% of full load speed, start winding and capacitor are disconnected from the circuit by a centrifugal switch.

Q2. How can the direction of rotation of the capacitor-start motor be reversed?

Ans: The direction of rotation of capacitor start motor can be reversed by interchanging the connections of starting winding without disturbing the capacitor connections. The direction of motor can be reversed only before the starting operation of the motor i.e., when the motor is at rest and the centrifugal switch is in closed position. Because once the motor comes in normal operating condition it will be running as single phase induction motor and at this moment the reversal of motor is attempted, then there will be no effect on the direction of rotation as centrifugal switch will be in open position and the developing torque will be in the direction of rotation.

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Fig. 1. Shaded Pole Induction Motor.

Shaded pole induction motor is the simplest and inexpensive type of motor similar to single-phase induction motor. The stator poles of this motor are wound only with main winding, so as to make them electromagnets. The pole has a slot cut at one third of one end for housing high reactance but low resistance copper bars enclosing a part of the pole. This part is called the shaded part and the other is called the unshaded part. The rotor of such motor is similar to squirrel cage rotor.

Working Principle of Shaded Pole Induction Motor

The required phase-split for making self-start is obtained through induction principle (transformer principle) when a shaded pole motor is supplied with a single-phase A.C supply. This single-phase current produces two pole alternating flux which is as explained follows.

Working of Shaded Pole Induction Motor

Consider a positive half cycle with different time instants i.e t₁, t₂, t₃ as shown in figure (2). Consider one electromagnet in which the current flowing is positive half.

Along Time 0t₁ :

During this time, as the exciting current is increasing, there will be an e.m.f generated in the shaded coil and hence large current is set-up. The current flows in a direction so as to oppose its cause and thus reduces the flux under the shaded pole. So most of the flux is concentrated under the unshaded part, thus moving the magnetic axis as shown in figure 3(1).

Along Time t₁t₂ :

During this time, there is no change in the exciting current and hence the flux will be uniform throughout the pole. This moves the magnetic axis to center of pole as shown in figure 3(ii).

Along Time t₂t₃ :

During this time, the exciting current is decreasing thus, inducing e.m.f in the shaded coil. The currents flowing in the shading coils are in a direction so as to oppose the exciting current and the flux produced will be adding with the main field flux and thus the magnetic axis moves under the shaded pole as shown in figure 3(iii). This motion of magnetic axis from unshaded part to the shaded part (this effect) is similar to that as if the real poles are actually sweeping in space. So, the rotor starts rotating in the direction of magnetic axis (unshaded part to shaded part). As the torque is very small, this type of motors are generally used in toys, hair driers, desk fans, electric clocks etc.

Torque-Speed Characteristics of Shaded Pole Induction Motor

A shaded pole motor is a self start single-phase induction motor. This property of self start is achieved by splitting the phase by induction principle using shaded ring. So, this motor is thereby named as shaded pole motor. From torque-speed characteristics, it is known that with the increase in speed initially the torque increases and then decrease.

Disadvantages of Shaded Pole Induction Motor

The following are the limitations of shaded pole motors.

1. Compact size
2. Less power rating
3. Poor starting torque
4. Less power factor
5. Less efficiency due to I²R and copper losses in the shading ring
6. Complexity in speed reversal.

Applications of Shaded Pole Induction Motor

Shaded pole motors have the characteristics of low starting torque, low power factor, high losses and hence low efficiency. These are preferred for applications requiring low power ratings in the order of 40 W. Some of the applications of shaded pole motor are, relays, table fans, exhaust fans, hair driers, fans for refrigerators, air conditioning equipments, toys, film projectors, photo copying machines, record players, tape recorders, single-phase synchronous timing motors etc.

Q1. What will be the direction of rotation of a shaded pole single phase induction motor?

Whenever the exciting winding of a single phase induction motor is excited by a single phase A.C supply, the magnetic axis will shift from the unshaded part of the pole to the shaded part of the pole. This shifting is similar to the actual rotation of poles. So, the rotor starts rotating in the direction of magnetic axis. Hence, the direction of rotation of a shaded pole single phase induction motor will be from unshaded part to the shaded part.

Q2. State the method of reversal of rotation of shaded pole motor.

There are two common methods of reversing the rotation of shaded pole motor. They are,

1. By using two shaded poles and one main winding
2. By using one shaded pole and two main windings.

Figure shows reversing the direction of rotation of a shaded pole meter.  In the first method, the switch is moved to the other position, connecting the alternate winding and the previous winding which was being used is disconnected. Due to this other side of the main winding is connected to the shaded pole which causes the rotor to move in opposite direction.

In the second method the two main windings are wounded in the slots such that shaded pole is positioned on the opposite side of each winding. As rotor will move towards the shaded pole, this will cause the rotor to rotate in reverse direction.

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No-load Test is conducted on the 3-phase induction motor to determine no-load current I₀, no-load power factor windage and friction losses, no-load copper loss (I₀²R), no-load power input P₀, no-load resistance R₀ and magnetizing reactance X₀ etc.

The test is carried out with different values of applied voltage below and above the rated voltage when the motor is running on light load or without load. The input power is measured by two wattmeters, current I₀ by an ammeter and voltage V by a voltmeter, which are connected in the circuit as shown in figure (1 As the motor is running on light load the power factor is very low i.e., less than 0.5. Hence, the total power input will be the difference of both the wattmeter readings.

Using,

\[{{W}_{0}}=\sqrt{3}{{V}_{L}}{{I}_{0}}\cos {{\phi }_{0}}\]

\[p.f.=\cos {{\phi }_{0}}\frac{{{W}_{0}}}{\sqrt{3}{{V}_{L}}I}\]

I₀ has two components,

\[{{I}_{w}}={{I}_{0}}\cos {{\phi }_{0}}\text{ (Working components)}\]

\[{{I}_{\mu }}={{I}_{0}}\sin {{\phi }_{0}}\text{ (Reactive components)}\]

\[{{R}_{0}}=\frac{{{V}_{L}}/\sqrt{3}}{{{I}_{w}}}\text{ (Iron loss resistance)}\]

\[{{X}_{0}}=\frac{{{V}_{L}}/\sqrt{3}}{{{I}_{\mu }}}\text{ (Magnetizing resistance)}\]

Thus, the equivalent circuit of induction motor on no-load is shown in figure (2).

Blocked Rotor Test

This test is conducted to determine the short circuit current (I_sc) with rated current drawn by the stator, power factor on short circuit and equivalent resistance (R₀₁), equivalent reactance (X₀₁) referred to stator side. To obtain rated current reduced voltage is applied to stator. The determined values can be used in the construction of circle diagram of induction motor.

In this test the circuit diagram is same as in no-load test but meters are replaced by higher values. The rotor is held firmly and stator is connected across the variable supply voltage. The readings of voltmeter, wattmeters and ammeter are noted.

As in this case, power factor is greater than 0.5 both the wattmeter readings are additive for total power. The equivalent circuit diagram under blocked rotor test is shown in figure (3).

Using the relation,

\[{{W}_{sc}}=\sqrt{3}{{V}_{s}}{{I}_{s}}\cos {{\phi }_{sc}}\text{    (Short circuit power)}\]

\[\cos {{\phi }_{sc}}\text{ }=\frac{{{W}_{sc}}}{\sqrt{3}{{V}_{s}}{{I}_{s}}}\text{     (Power factor on short circuit)}\]

\[{{I}_{sN}}={{\left( \frac{V}{{{V}_{s}}} \right)}^{2}}\times {{I}_{s}}\text{      (Current at normal voltage)}\]

\[{{W}_{sN}}=\frac{V}{{{V}_{s}}}\times {{W}_{s}}\text{      (Power at normal voltage)}\]

The post No-load and Blocked Rotor Tests on Induction Motor appeared first on Electrical and Electronics Blog.

Figure 1: Torque-speed characteristics of an induction motor.

The torque developed in the 3-ϕ induction motor depends on its speed. The relationship between torque and speed can be represented by the curve as shown in figure (1), which represents the speed-torque characteristics. The starting torque is called the locked rotor torque while the maximum torque is called the breakdown torque. Under full-load conditions, the motor runs at a speed of ‘N’. When the mechanical load increases, the speed of the motor keeps decreasing until the torque becomes equal to load torque. When the two torques are balanced, the motor runs at a constant speed. But, if the torque exceeds maximum torque or breakdown torque then the motor stops abruptly.

Effect of Rotor Circuit Resistance on Torque Speed Characteristics of Induction Motor

The speed of an induction motor can be controlled by adding resistances in the rotor circuit. The torque-speed characteristics of an induction motor are modified due to a variation in the rotor resistance as shown in figure (2). These characteristics show that the speed of the motor decreases with an increase of resistance value in the rotor circuit. Thus, this method controls speed from rated speed to lower speed and smoother speed control can be achieved by increasing external resistance in steps.

Effect of Rotor Circuit Reactance on Torque Speed Characteristics of Induction Motor

The torque of a 3-ϕ induction motor can be controlled by changing stator reactance or rotor reactance or both. Stator reactance can be varied for both squirrel cage and wound rotor motor, but the rotor reactance can be varied only for a wound rotor motor by placing an external reactor in the rotor circuit through slip rings. The torque-speed characteristics of an induction motor with changing equivalent reactance are shown in figure (3). These characteristics show that the speed can be controlled from rated speed to lower speed in downward direction. The range of speed control in this method is limited. The number of steps of external reactors determine the smoothness of the speed control which is stepped. Stepless and smooth reactance variation can be achieved by using saturable reactors.

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Figure 1: Torque-slip characteristics of an induction motor.

The torque-slip characteristics (see Figure 1) of an induction motor, is the curve drawn using the equation relating torque and slip is given as,

\[T=\frac{K\varphi s{{E}_{2}}{{R}_{2}}}{R_{2}^{2}+{{(s{{X}_{2}})}^{2}}}\]

Case (i): No-load Condition

When there is no load on the induction motor, the actual speed of the rotor is equal to zero, i.e.,

\[s=\frac{{{N}_{s}}-0}{{{N}_{s}}}=\frac{{{N}_{s}}}{{{N}_{s}}}=1\]

Therefore at no-load condition, slip is maximum this is the staffing condition and the torque at s = 1 is called as the starting torque.

Case (ii): Increasing Load Condition

As the load is increased, slip decreases, but it has got a significant value. Hence at this condition R₂ is negligible when compared to sX₂. Torque is inversely proportional to slip,

\[T\propto \frac{1}{s}\]

Therefore the torque continues to increase.

Case (iii): Maximum Torque Condition

As the load is increased, the torque continues to increase. Now if the load is increased in such a way that s = R₂/X₂ then the torque reaches a point called as maximum torque or breakdown torque or stalling torque.

Case (iv): Further Increasing the Load

If the load is further increased after reaching the maximum torque condition, slip reduces to low values and the torque (T) is directly proportional to slip (s) i.e., the torque also staffs decreasing. The torque-slip continues to decrease till they become zero.

The graph shows the different regions of operation, they are,

1. PQ, the unstable region
2. ST, the stable region
3. OP, the staffing point.

In the stable region, the torque varies linearly with slip i.e., it is the region between the points s = 0 and s = T_max. In the unstable region, the load torque decreases as the slip is increased. This is the region after the maximum torque occurs.  OP represents the starting torque i.e., the torque at s = 1. If the load torque is less than OP, the matrix will accelerate till the torque developed by the motor is equal to load torque.

MR is the maximum value of torque called as break down torque. The maximum value of torque occurs at a slip value of s = R₂ / X₂. From the load torque characteristics, it can be seen that motor with different values of R₂ will have different regions of operation i.e., as the value of R₂ is increased, the slip at which maximum torque occurs increases, which in turn increases the stable regions of operation. Hence in order to improve the operating regions of induction motor the higher values of R₂ must be considered.

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Working Principle of 3 Phase Induction Motor

A 3 Phase Induction Motor works on the principle of electromagnetic induction. When the three-phase winding of the stator is connected to the three-phase supply, the three phase current in stator winding produces a rotating magnetic field, which rotates round the stator at synchronous speed (N_S). This rotating field or flux passes through the air gap and cuts the rotor conductors, since rotor conductors are stationary and flux is moving. As a result of relative speed between the rotating field and stationary rotor conductors, e.m.f is induced in rotor conductors. Since the rotor conductor circuit is closed, current flows in that as a result of induced e.m.f. Thus, whenever a current carrying conductor is placed in rotating magnetic field it produces its own flux. Due to this, a magnetic flux called rotor flux is produced in rotor.  Thus, a torque is produced due to the interaction between the air gap flux and the rotor flux. The effect of this torque is that the rotor starts rotating in the same direction as the rotating magnetic field (according to Lenz law) and the speed of the rotor starts increasing from 0 to N.

Since the rotor speed is increased it rotates in the same direction as the rotation of magnetic field and the relative speed between the two decreases. As a result. the induced e.m.f in rotor current and rotor frequency gets reduced and also the torque gets reduced. But the rotor continues to accelerate and gains the speed. Under no-load conditions equilibrium is reached when the developed torque equals the torque due to losses. At equilibrium condition, the motor runs almost at synchronous speed but not equal to synchronous speed. As soon as the motor is loaded, the load torque plus the torque due to losses become greater than the developed torque. Therefore. the motor slows down. However, a reduction in rotor speed means an increase in relative speed as well as rotor current and hence an increase in the developed torque. As a result of this, the developed torque equals load torque plus torque due to losses. Hence equilibrium is reached in loaded condition. The motor then runs at a new steady speed, which is less compared to no-load speed. Thus, the speed ofmthree-phase induction motor varies with the load.

Construction (or Parts) of 3 Phase Induction Motor

The main constructional features of a three phase induction motor are stator and rotor. Usually stator receives three phase supply and working currents are developed in rotor.

Stator

Figure 1.

Stator consists of cast iron stator frame and laminated steel stampings of cylindrical stator core as shown in figure (1). These stator core steel laminations have minimum hysteresis and eddy current losses compared to other materials when alternating current passes through them. Stator core have the slots for inserting the polyphase winding.Slots provided on the stator core are of three types. They are open type, semi-closed type and closed type slots. Closed slots facilitate uniform air gap thereby flux distribution is also uniform. They have a limitation due to their high initial labour cost for winding and their increased winding inductance. Open slots have the advantage of easy removal and replacement of defective coils. But non-uniform air gaps are formed by open slots. Among all the three, semi-closed slots are advantageous because of less ripple content in their e.m.f waveform and less initial labour cost for designing.

Rotor

Induction motors are of two types depending upon the type of rotor used. The two types of rotors are squirrel-cage type and slip ring type.

Squirrel cage type rotor has slots on its outer periphery which are circular in shape. The rotor winding is placed in the rotor slots and its ends are shorted by same winding material known as end rings as shown in figure (2). This facilitates the rotor suitability for any number of stator poles and insertion of external resistance to the rotor winding is not possible. Hence the starting torque of squirrel cage type induction motor is double to that of full load torque only for the application of rated voltage. Squirrel cage induction motors (SCIM) are simple and economical in construction and require less maintenance.

Slip ring type rotor also have slots on its circular outer periphery. The rotor winding which is to be placed in the rotor slots is similar to that of its three phase stator winding so that number of rotor poles are equal to stator poles. Slip ring type of rotors have a facility to insert external resistance at their output terminals as shown in figure (3). Therefore its starting torque can be changed as per the requirement, by placing suitable resistance in the rotor circuit.

Advantages of 3 Phase Induction Motor

The following are some of the advantages of three phase induction motor.

1. Simple design
2. Low initial cost
3. Simple maintenance
4. Rugged in construction
5. Easy and reliable operation
6. High efficiency
7. Simple speed control.

Applications of 3 Phase Induction Motor

The following are the various fields of application of three phase induction motors.

1. Fans
2. Centrifugal pumps
3. Lift irrigation
4. Rice mills
5. Pumps
6. Cranes.

Frequently Asked Questions on 3 Phase Induction Motor

Q1. Why an induction motor will never run at its synchronous speed?

Ans1: At starting of induction motor, the rotor rotates and tries to achieve the speed of the rotating magnetic field. But the rotor runs at a speed which is always less than the synchronous speed of the stator field. If there is no difference between the two speeds, then there would be no relative motion, hence no induced E.M.F and no current would flow and thus no torque would be present to maintain rotation. By this action the motor stops and again there exists a relative motion between the rotor and rotating magnetic field and thus the motor starts. Due to inertia of rotor, the rotor rotates at a speed slightly less than the synchronous speed of a rotating magnetic field. Thus an induction motor will never run at its synchronous speed.

Q2. Explain why an induction motor, at no-load, operates at very low power factor.

Ans2: At no-load operation, the magnetizing component, I_m of no-load current is much greater than the loss component, I_c of no-load current as shown in figure. Hence, due to this reason an induction motor operates at very low power factor less than 0.4 lagging at no-load condition.

Q3. How can the direction of rotation of 3-phase induction motor be reversed?

Ans3: The direction of rotation of three phase induction motor can be reversed by reversing the connection i.e., if the motor is connected to three phase as RYB then by connecting motor as RBY or YBR, the direction of rotation of induction motor can be reversed.

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Working of Induction Generator

When a 3-ϕ induction motor is given A.C supply and is mechanically coupled to a prime mover then it acts as a synchronous generator if it is driven at a speed greater than its synchronous speed. The function of induction generator is to convert mechanical energy from the prime mover to electrical energy which is received through stator.

The operation of induction motor as induction generator can be better explained using figures (1), (2) and (3). Figure (1) represents the motoring action of an induction motor and when same motor is driven by a prime mover with its speed higher than it synchronous speed then its generating action i.e., of induction generator is shown in figure (2). Initially, in figure (1), the direction of torque and rotation are for motoring operation where the running speed is less than the synchronous speed and hence the slip (N_s – N) is positive but when the same machine is operated as induction generator, the machine is now made to run at super synchronous speed i.e., above synchronous speed and the slip for this action becomes negative. Also, the direction of torque will now be in the opposite direction i.e., in induction motor when the speed is increased above its synchronous speed with the help of prime mover. Now, the rotor rotates faster than the rotating magnetic field and the rotor conductor cuts the main magnetic field in opposite direction to that of motor rotation due to which the current and e.m.fs in the rotor will be in reverse direction and the machine acts as an induction generator.

Induction generator does not contain a field circuit and hence it is not self-exciting. But a magnetizing current is always required to be supplied to the stator winding even for motor action and also for generator action for the production of rotating magnetic field. Therefore, it becomes necessary to operate induction generator along with any other generator in parallel which can supply the exciting current of fixed frequency to it for the production of rotating magnetic field.

In induction motor at the instant when the motor speed becomes greater than the synchronous speed, the motor starts delivering active power to the 3-phase line. The power developed will be negative. In order to develop its own rotor magnetic field, it starts absorbing reactive power from the line. Hence, it requires some source of energy to supply reactive power. This reactive power is supplied by means of static capacitor connected in parallel as shown in figure (3).

For both the motoring and generating actions, torque slip characteristics are as shown in figure (4). From the construction point of view, both the machines are similar. The main difference is in the direction of rotation of rotor i.e., both are opposite to each other and the equivalent circuit for induction motor also holds good for induction generator for the slip s introduced with negative sign.

Advantages of Double Cage Induction Motor

1. The generator need not be synchronized.
2. It is suitable for higher speeds.
3. When an induction generator is short circuited, a small amount of power will be delivered to load since the excitation is quickly reduced to zero.
4. The construction of an induction generator is robust, requiring less maintenance.
5. There is no problem of hunting or drop out of synchronism.

Disadvantages of Double Cage Induction Motor

1. The major disadvantage of an induction generator is that it is difficult to operate alone as a standby or emergency generator.
2. It has very low power factor.
3. Since, it can generate only leading currents, an additional synchronous apparatus with huge KVA capacity is required to deliver the total quadrature lagging currents.
4. The efficiency of an induction generator is poor.

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Construction of Double Cage Induction Motor

In double cage rotor induction motor, the stator is similar to that of ordinary induction motor. The only difference is in the rotor construction. The rotor of double cage induction motor is constructed by using two sets of squirrel cage windings which are provided on the rotor separated by narrow slit or construction. The upper cage or the starting cage which is very close to the air gap is made of high resistivity material such as aluminium, bronze etc., and the lower cage is made up of copper. The cross- section of upper cage is smaller than the lower cage and hence the resistance of upper cage is higher than that of lower cage. Common short circuiting end lings can be provided for both the cages or separate short circuit end ring can be provided for each of the cage. The schematic of a double cage rotor is shown in figure (1). Double cage induction motor has two cages on the same rotor i.e., a high resistance and low reactance bars on outer cage and low resistance and high reactance bars on inner cage respectively.

Working of Double Cage Induction Motor

The cross-sectional view of double cage induction rotor is shown in figure (2). Initially, the rotor current frequency is high due to large slip, which is nearly equal to supply frequency. The current supplied to inner and outer cages of rotor will be inversely proportional to their impedance. However, the inner cage have a high impedance compared to outer cage due to its high leakage reactance. Thus, the current in inner cage is small but the outer cage current is enclosed which has a high resistance. This gives a very good starting torque.

When the speed of the motor is increased, the frequency of the rotor current gets decreased. Thus, the leakage reactance of inner cage is reduced when the motor is running at full speed. Therefore, the reactances of both the cages become too small and the current is divided in the ratio of resistances (i.e., the resistance of outer cage is 5 to 6 times of inner cage). Further, maximum current flows in inner cage and the motor runs at low slip.

Torque-speed Characteristics of Double Cage Induction Motor

Figure (3) shows the torque-speed characteristics of double cage induction motor. From figure (3), it can be seen that, the outer cage provides a high starting torque whereas the inner cage helps in obtaining the sufficient speed at the normal running condition. Hence, both characteristics combined provide an excellent torque-speed characteristic curve. Due to these characteristics, double cage induction motor is also known as low starting current, high starting torque, low slip motor.

Equivalent Circuit of Double Cage Induction Motor

Figure 4.

The equivalent circuit diagram referred to stator of a double cage induction motor is shown in figure (4).

Where,

R₁ – the per phase stator resistance

X₁ — the per phase stator reactance

R_i — the inner cage resistance/phase referred to stator

X_i — the inner cage reactance/phase referred to stator

R₀ — Outer cage resistance/phase

X₀ — Outer cage reactance/phase

I_w — Working component of current

I_m – Magnetizing component of current

K — Transformation ratio and

s — The considered slip.

Figure 5.

In figure (5), if the magnetizing current is not considered and neglected, then the equivalent circuit of double cage induction motor changes to as shown in figure (5).

Where,

\({{{{R}’}}_{0}}\) is the outer cage resistance/ph referred to stator

\({{{{X}’}}_{0}}\) is the outer cage reactance/ph referred to stator.

The total impedance referred to stator is given by,

\[{{Z}_{01}}=({{R}_{1}}+j{{X}_{1}})+\frac{1}{\frac{1}{{{{{Z}’}}_{i}}}+\frac{1}{{{{{Z}’}}_{0}}}}\]

\[={{R}_{1}}+j{{X}_{1}}+\frac{1}{\frac{{{{{Z}’}}_{0}}+{{{{Z}’}}_{i}}}{{{{{Z}’}}_{i}}.{{{{Z}’}}_{0}}}}\]

\[{{Z}_{01}}={{R}_{1}}+j{{X}_{1}}+\frac{{{{{Z}’}}_{0}}.{{{{Z}’}}_{i}}}{{{{{Z}’}}_{0}}+{{{{Z}’}}_{i}}}\]

Advantages of Double Cage Induction Motor

1. It reduces the starting current.
2. It increases the starting torque.

Disadvantages of Double Cage Induction Motor

1. Maximum torque is less.
2. Power factor is low.
3. Efficiency is less.
4. Rotor copper losses are high.
5. Cost is high.

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