Operation of Transformer On load

Figure (1) shows a transformer with a load connected across the secondary winding. The load current I₂ flowing through the secondary turns sets up its own m.m.f N₂I₂ which produces the flux ϕ₂.

According to Lenz’s law this flux is in such a direction that it opposes the flux ϕ, produced by the m.m.f N₁I₀ which is set up by the no-load current I₀ flowing throw the primary turns. Consequently the flux is momentarily reduced due to opposing flux ϕ. This in turn causes reduction in induced e.m.f (E₁) in primary according to Faraday’s law E₁ reduces, the difference between applied voltage (V₁) and E₁ increases.

Consequently, the primary will draw more current. Consider \({{{I}’}_{1}}\) to be this additional primary current. It is also known as counter balancing current as it balances between applied voltage and primary e.m.f or it is known as load component of primary current and it is antiphase with secondary current I₂. Now this current \({{{I}’}_{1}}\) sets up its own m.m.f N₁\({{{I}’}_{1}}\) which produces the flux and it is equal in magnitude in such a direction that it opposes the flux ϕ₂. Hence ϕ’₁ and ϕ₂ cancel each other and only flux ϕ flows in the core. Therefore the total flux produced during loaded condition is approximately equal to the flux at no-load.

\[{{\phi }_{2}}=-{{{\phi }’}_{1}}\]

As secondary ampere turns of I₂ are neutralized by primary ampere turns of \({{{I}’}_{1}}\).

\[{{N}_{2}}{{I}_{2}}={{N}_{1}}{{{I}’}_{1}}\]

\[{{{I}’}_{1}}=\frac{{{N}_{2}}}{{{N}_{1}}}{{I}_{2}}\]

The net primary current is the vector sum of primary counter balancing current \({{{I}’}_{1}}\) and the no-load current I₀.

\[{{I}_{1}}={{{I}’}_{1}}+{{I}_{0}}\]

Since the no-load current I₀ is very small compared to the counter balancing current \({{{I}’}_{1}}\), therefore the net primary current is approximately equal to the current \({{{I}’}_{1}}\).

\[{{I}_{1}}={{{I}’}_{1}}\]

\[=\frac{{{N}_{2}}}{{{N}_{1}}}{{I}_{2}}=K{{I}_{2}}\]

‘K’ represents transformation ratio.

\[\frac{{{I}_{1}}}{{{I}_{2}}}=\frac{{{{{I}’}}_{1}}}{{{I}_{2}}}=\frac{{{N}_{2}}}{{{N}_{1}}}=K\]

\[{{I}_{1}}=K{{I}_{2}}={{{I}’}_{1}}\]

Therefore, the primary and secondary currents are inversely proportional to their turns ratio. The total primary current is in anti-phase with I₂ and K times the current I₂.

Phasor Diagram of Transformer with Resistive Load

The phasor diagram for resistive load is drawn as shown in the following figure (2). For purely resistive load, the secondary load current I₂ is in phase with the secondary’ terminal voltage V₂. The counter balancing current \({{{I}’}_{1}}\) is in opposition and equal in magnitude with the secondary load current I₂. The primary current I₁ is the vector sum of \({{{I}’}_{1}}\) and no-load current I₀ respectively. I₀ lags behind V₁ by no-load power factor angle ϕ₀ and I₁ lags behind the voltage V₁ by primary power factor angle ϕ₁.

The post Transformer on Load – Circuit Diagram & Phasor Diagram appeared first on Electrical and Electronics Blog.

When the transformer is on no-load, the secondary winding is opened as in figure (a) and current I₂ is zero. In this condition, the primary winding draws a no-load current ‘I₀‘ which has two components i.e.,

1. Magnetizing component (I_µ), and
2. Working component (I_w).

1. Magnetizing Component (I_µ) :

It lags behind the applied voltage on primary winding ‘V₁‘ by 90º. It is also called as reactive or wattless component of no-load current and is responsible to develop an e.m.f to maintain the flux ‘ϕ’ in the core. It is expressed as \({{I}_{\mu }}=\text{ }{{I}_{0}}\sin {{\phi }_{0}}\).

2. Working Component (I_w) :

It is in phase with the primary applied voltage ‘V₁‘. The component is also called as active component or iron loss component, and is used for describing the core losses such as hysteresis loss and eddy current loss. It is expressed as \({{I}_{w }}=\text{ }{{I}_{0}}\cos {{\phi }_{0}}\).

From the phasor diagram of figure (b),

\[\sin {{\phi }_{0}}=\frac{{{I}_{\mu }}}{{{I}_{0}}}\]

Thus, \({{I}_{\mu }}={{I}_{0}}\sin {{\phi }_{0}}\) is the reactive component of no-load current I₀ and

\[\cos {{\phi }_{0}}=\frac{{{I}_{w }}}{{{I}_{0}}}\]

Thus, \({{I}_{w }}={{I}_{0}}\cos {{\phi }_{0}}\) is the active component of no load current I₀.

Hence,

\[{{I}_{0}}=\sqrt{I_{w}^{2}+I_{\mu }^{2}}\]

cosϕ₀ is the no-load power factor and ϕ₀ is the hysteresis angle of advance.

The post Transformer on No Load – Circuit Diagram & Phasor Diagram appeared first on Electrical and Electronics Blog.

Consider the two winding single-phase transformer shown in figure (1).

\({{{I}}_{1}}\) = Current in the primary

\({{{E}}_{1}}\) = Induced e.m.f in the primary

\({{{V}}_{1}}\) = Voltage applied to the primary

\({{{I}}_{2}}\) = Current in the secondary

\({{{E}}_{2}}\) = Induced e.m.f in the secondary

\({{{V}}_{2}}\) = Terminal voltage of secondary

Here, the primary current \({{{I}}_{1}}\) has two components, one is no-load primary current, \({{{I}}_{0}}\) and the other one is load component of primary current \({{{I}’}_{2}}\). The function of current \({{{I}’}_{2}}\) is to counter balance the secondary current \({{{I}}_{2}}\). The no-load primary current \({{{I}}_{0}}\) leads to the production of losses in the core while magnetizing the core of the transformer. The no- load primary current \({{{I}}_{0}}\) can be resolved into two components i.e., active (or) working component I_w and reactive (or) magnetizing component ‘I_µ‘. The working component ‘I_w’ of no-load current \({{{I}}_{0}}\) leads to the core loss, hence it can be represented by a resistance R₀. The magnetizing current ‘I_µ‘ produces flux which induces e.m.f E₁.

The reactance due to flux is represented by X₀. To account for the core loss and the magnetizing current, an equivalent circuit can be represented by a shunt branch in the primary side as shown in the figure (2).

\[\text{Core loss = }I_{w}^{2}{{R}_{0}}=\frac{E_{1}^{2}}{{{R}_{0}}}\]

To make transformer calculations simpler, transfer voltage, current and impedance either to the primary or secondary

Equivalent Circuit of Transformer as Referred to Primary Side

Secondary parameters transferred to primary side are given as follows,

\[{{{R}’}_{2}}=\frac{{{R}_{2}}}{{{K}^{2}}}\]

\[{{{X}’}_{2}}=\frac{{{X}_{2}}}{{{K}^{2}}}\]

\[{{{Z}’}_{2}}=\frac{{{Z}_{2}}}{{{K}^{2}}}\]

\[{{{I}’}_{2}}=K{{I}_{2}}\]

\[{{{E}’}_{2}}=\frac{{{E}_{2}}}{K}\]

\[{{{V}’}_{2}}=\frac{{{V}_{2}}}{K}\]

Where,

\[K=\frac{{{N}_{2}}}{{{N}_{1}}}\]

We know that,

\[{{R}_{01}}={{R}_{1}}+{{{R}’}_{2}}={{R}_{1}}+\frac{{{R}_{2}}}{{{K}^{2}}}\]

\[{{X}_{01}}={{X}_{1}}+{{{X}’}_{2}}={{X}_{1}}+\frac{{{X}_{2}}}{{{K}^{2}}}\]

\[{{Z}_{01}}=\sqrt{R_{01}^{2}+X_{01}^{2}}=\frac{{{Z}_{02}}}{{{K}^{2}}}\]

The equivalent circuits referred to primary side are as shown in figures (3) and (4).

Equivalent Circuit of Transformer Referred to Secondary Side

Primary parameters transferred to secondary side are given as follows,

\[{{{R}’}_{1}}={{K}^{2}}{{R}_{1}}\]

\[{{{X}’}_{2}}={{K}^{2}}{{X}_{1}}\]

\[{{{Z}’}_{1}}={{K}^{2}}{{Z}_{1}}\]

\[{{{E}’}_{1}}=K{{E}_{1}}\]

\[{{{V}’}_{1}}=K{{V}_{1}}\]

\[{{{I}’}_{1}}=\frac{{{I}_{1}}}{K}\]

\[{{{I}’}_{0}}=\frac{{{I}_{0}}}{K}\]

\[{{{R}’}_{0}}=\frac{{{R}_{0}}}{{{K}^{2}}}\]

\[{{{X}’}_{0}}=\frac{{{X}_{0}}}{{{K}^{2}}}\]

We know that,

\[{{R}_{02}}={{R}_{2}}+{{{R}’}_{2}}={{R}_{2}}+{{K}^{2}}{{R}_{1}}\]

\[{{X}_{02}}={{X}_{2}}+{{{X}’}_{1}}={{X}_{2}}+{{K}^{2}}{{X}_{1}}\]

\[{{Z}_{02}}=\sqrt{R_{02}^{2}+X_{02}^{2}}={{K}^{2}}{{Z}_{01}}\]

The equivalent circuit diagrams referred to secondary side are shown in figure (5) and figure (6).

The post Equivalent Circuit of Transformer – Circuit Diagram & Derivation appeared first on Electrical and Electronics Blog.

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.

1. AC servomotors and
2. D.C servomotors.

AC Servomotor

The principle of operation of A.C servomotor is similar to that of three-phase induction motor. AC servomotors are generally two-phase squirrel cage induction type motors. The stator has two distributed windings. One is the control winding and the other is the reference winding. These two windings are displaced from each other by 90º as shown in above figure. The voltage applied to the control winding will be 90º out of phase with respect to the voltage applied to the reference winding. The current in the control winding will set up a flux and this flux will be 90º out of phase to the flux set up by the current in the reference winding. Thus, a resultant rotating magnetic flux is setup in the air gap, which sweeps over the stationary rotor. Due to this rotating flux, an e.m.f is induced in the rotor, which in turn produces a circulating current in the rotor. This circulating current in the rotor will now set up a flux (rotor flux) which interacts with the resultant flux produced by the stator and thus a torque is developed on the rotor. The effect of this torque is that the rotor starts rotating in the same direction as the rotating magnetic flux.

Applications of AC Servo Motor

1. AC servo meters are used for low power applications.
2. These motors are widely used in radar, process control systems, robotics, servo mechanisms, computers and machine tools etc.
3. These are also used in self balancing recorders, AC position control systems, tracking and guidance systems.

DC Servo Motor

DC motors which are used in servo systems are called DC servo motors. DC servo motor is essentially an ordinary D.C motor except with few variations in its constructional features. These are used when quick response to control signals and high starting torque is required. The figure shows the layout of DC servo motor.

Working Principle of DC Servo Motor

When an electric current flows through the armature winding, the magnetic field is induced in it. This induced field opposes the field, which is set up by the permanent magnets. The difference in magnetic field produces a torque on the rotor. The torque produced by the rotor will be constant throughout the rotation, as the field strength depends on the function of current. The torque of the D.C servo motor is given as,

\[{{T}_{m}}(t)={{k}_{m}}{{I}_{a}}(t)\]

Where,

T_m — Torque produced

I_a – Armature current

k_m – Motor’s torque constant.

Applications of DC Servo Motor

1. D.C servomotors are used for high power applications.
2. These motors are widely used in instruments, tape drives, printers, robot system, air craft control systems etc.
3. These are also used in electromechanical actuators, process controllers and disk drive.

Difference between AC servomotor and DC servomotor

The post What is Servomotor? Working, Diagram, Types (AC & DC) & Applications appeared first on Electrical and Electronics Blog.

Figure 1: Permanent Magnet Stepper Motor.

Construction of Permanent Magnet Stepper Motor

The stator of the Permanent Magnet Stepper Motor is similar to that of the variable reluctance stepper motor, whereas the rotor is replaced by a permanent magnet. In this type of stepper motor the stepping angle would be more due to difficulty in manufacturing the rotor with number of poles. The basic structure of a two pole motor is as shown in figure 1.

Working of Permanent Magnet Stepper Motor

In the equivalent circuit shown with supply source connected (see Figure 2), the current flows in the forward direction, when phase A is energized with A-north and A’-south the position of rotor is as shown in the figure. When phase B is energized with B-north and B’-south the rotor rotates in clockwise direction with an angular shift of β i.e., 60º and again it moves 60º when C phase is energized. After the rotation of 120º, if again phase A is fed supply, then the rotor moves in the anticlockwise direction due to the repulsive force between the like poles.

In order to keep the rotation in same direction, it is required to change the polarity either by interchanging the winding coil terminals or by changing the supply source. The circuit shows the method of changing the source terminals. This change in current should be done after every k angle of rotation.

\[k=\frac{360{}^\circ }{\text{Number of phases}}\]

This motor is also called as variable speed brushless D.C motor.

The post What is Permanent Magnet Stepper Motor? Working, Diagram & Construction 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.

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

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.

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

Open Circuit (O.C.) Test on Transformer

Set-up:

Fig. 1 : Set-up for Open Circuit Test on Transformer.

This test is performed so as to calculate the no load losses (core losses) of a transformer and the values of I₀, R₀ and X₀ of the equivalent circuit. The set up for O.C. test of a transformer is shown in Fig. 1. The primary or secondary winding of the transformer is connected to the rated ac voltage by means of using a variac. The other winding is left open. Generally the high voltage winding is open circuited, and the voltage is applied to the low voltage winding. Assume that this is a step up transformer. A voltmeter is connected across the primary winding to measure the primary voltage. An ammeter is used for measuring the no load primary current (I₀) and the wattmeter is connected to measure the input power. The secondary is open circuited because it is an open circuit (O.C.) test. Sometimes a voltmeter is connected across the secondary to measure V₂ = E₂. Note that the ac supply voltage is applied generally to the low voltage side and the higher voltage side is used as secondary.

Procedure:

• Connect the circuit as shown in Fig. 1.
• Keep the variac at its minimum voltage position.
• Switch on the ac power supply and adjust the variac to get the rated primary voltage as measured by voltmeter V across the primary.
• Now measure the primary current (I₀) and power (W₀) using the ammeter and wattmeter respectively.
• The ammeter reads the no load primary current I₀ whereas the wattmeter measures the no load input power W₀.
• The observation table for the O.C. test is as follows.

• The two components of no load current I₀ are,

\[{{I}_{m}}={{I}_{0}}\sin {{\phi }_{0}}\]

\[{{I}_{c}}={{I}_{0}}\sin {{\phi }_{0}}\]

• The no load power factor is given by cosϕ₀ and the input power at no load is given by,

\[{{W}_{0}}={{V}_{1}}{{I}_{0}}\cos {{\phi }_{0}}\]

• The phasor diagram on no load showing the two components of I₀ is shown in Fig. 2.

Fig. 2 : Phasor Diagram for Open Circuit Test on Transformer.

• The no load current I₀ is very small as compared to the full load primary current. The no load current I₀ is about 3 to 5 % of the full load value.
• As I₂ is zero, the secondary copper loss is zero. The primary copper loss will be negligible because I₀ is small.
• Therefore the total copper loss is very small and can be assumed to be equal to zero. Hence the wattmeter reading W₀ represents the iron losses.

\[{{W}_{0}}={{P}_{i}}=\text{ Iron losses}\]

Calculation of parameters:

The two parameters which can be calculated from the open circuit test are R₀ and X₀. They are calculated as follows.

Step 1: Calculate no load power factor cosϕ_0,

The wattmeter reads the real input power.

\[{{W}_{0}}={{V}_{1}}{{I}_{0}}\cos {{\phi }_{0}}\]

And

\[\cos {{\phi }_{0}}=\frac{{{W}_{0}}}{{{V}_{1}}{{I}_{0}}}\]

Calculate ϕ₀ from this.

Step 2: Calculate I_m and I_c,

\[{{I}_{m}}={{I}_{0}}\sin {{\phi }_{0}}\]

\[{{I}_{c}}={{I}_{0}}\cos {{\phi }_{0}}\]

So calculate I_m and I_c from the above equations.

Step 3: Calculate R₀ and X_0,

\[{{R}_{0}}=\frac{{{V}_{1}}}{{{I}_{c}}}\Omega \]

\[{{X}_{0}}=\frac{{{V}_{1}}}{{{I}_{m}}}\Omega \]

The value of cosϕ₀ is very small. Therefore it is necessary to use the low power factor type wattmeter to avoid any possibility of error in measurements.

Short Circuit (S.C.) Test on Transformer

Set-up:

Fig. 3 : Set-up for short Circuit Test on Transformer.

The set up for carrying out the shown circuit (SC) test on a transformer is shown in Fig. 3. Generally the high voltage side is connected to the ac supply and the low voltage high current side is shorted. Variac is used to adjust the input voltage precisely to the rated voltage. We assume that the transformer used here is a step down transformer. Hence the secondary is shorted and primary is connected to the variac. The voltmeter is connected to measure the primary voltage The ammeter measure the short circuit rated primary current I_sc and the wattmeter measures the short circuit input power. The secondary is short circuited with the help of thick copper

Procedure:

• Connect the circuit as shown in Fig. 3.
• Shown circuit the secondary which is a low voltage high current, low resistance winding.
• Keep the variac at its minimum voltage position and switch on the ac supply voltage.
• Increase the primary voltage very gradually: and adjust it to get the primary current equal to the rated value I_sc. Do not increase the primary voltage further.
• Note down the wattmeter, voltmeter and ammeter readings. The observation table is as shown below in Table.

Parameter calculations:

The primary and secondary currents are the rated currents. Therefore the total copper loss is the full load copper loss. If we adjust the primary current to half the full load current then we get the copper loss at half load. The iron losses are a function of applied voltage. As the applied voltage in S.C. test is small, the iron losses will be negligibly small. Hence the wattmeter reading W_sc corresponds almost entirely to the full load copper loss.

\[{{W}_{SC}}=\text{ Full load copper loss}\]

\[={{P}_{cu(FL)}}\]

We can calculate the parameters R_IT, X_IT and Z_IT of the equivalent circuit from the short circuit (S.C.) test.

We know that

\[{{W}_{SC}}={{\text{V}}_{SC}}{{I}_{SC}}\cos {{\phi }_{SC}}\]

Hence the short circuit power factor is given by,

\[\cos {{\phi }_{SC}}=\frac{{{W}_{SC}}}{{{V}_{SC}}{{I}_{SC}}}\]

But the wattmeter reading W_sc indicates the full load copper loss.

\[{{W}_{SC}}=\text{Copper loss}\] \[=I_{SC}^{2}\times {{R}_{1T}}\]

\[{{R}_{1T}}=\frac{{{W}_{SC}}}{I_{SC}^{2}}\]

Similarly,

\[{{Z}_{1T}}=\frac{{{V}_{SC}}}{I_{SC}^{{}}}=\sqrt{R_{1T}^{2}+X_{1T}^{2}}\]

\[{{X}_{1T}}=\sqrt{Z_{1T}^{2}+R_{1T}^{2}}\]

In this way the parameters R_IT, X_IT and Z_IT can be calculated from the S.C. test. If the transformation ratio K, it is possible to obtain the parameters referred to the secondary side.

Efficiency Calculation from O.C. and S.C. Test:

The expression for full load efficiency IS given by,

\[{{\eta }_{FL}}=\frac{{{V}_{2}}{{I}_{2(FL)}}\cos \phi }{{{V}_{2}}{{I}_{2(FL)}}\cos \phi +{{P}_{i}}+{{P}_{cu(FL)}}}\]

The iron loss P_i can be obtained from the O.C. test because,

\[ {{W}_{0}}={{P}_{i}}=\text{ Total iron loss}\],

And the full load copper loss is obtained from the S.C. test because,

\[{{\text{W}}_{SC}}={{P}_{cu(FL)}}=\text{ Copper loss}\]

Voltage Regulation Calculation from O.C. and S.C. Test:

The percentage regulation (%R) is given by,

\[\%R=\frac{{{I}_{2}}{{R}_{2T}}\cos \phi \pm {{I}_{2}}{{X}_{2T}}\sin \phi }{{{V}_{2}}}\times 100\]

\[\%R=\frac{{{I}_{1}}{{R}_{1T}}\cos \phi \pm {{I}_{1}}{{X}_{1T}}\sin \phi }{{{V}_{1}}}\times 100\]

We can obtain the parameters such as R_IT, X_IT, R_2T, X_2T from the S.C. test of the transformer. Whereas the rated voltages V₁,V₂ and the rated currents I₁ and I₂ in the above expressions are known from the given transformer

The post Open Circuit and Short Circuit Test on Transformer – Experimental Set-up & Procedure appeared first on Electrical and Electronics Blog.