Fig. 1: Autotransformer (Construction & Symbol).

The normal transformer has separate primary and secondary windings. But the autotransformer is a special transformer in which a part of winding is common for the primary and secondary windings. The construction of an autotransformer is as shown in Fig. 1. It consists of only one winding wound on a laminated magnetic core, with a rotary movable contact. Thus from the autotransformer three terminals are brought out for connection. The autotransformer can operate as a step down or a step up transformer.

Autotransformer as Step Down Transformer

Connection diagram

Fig. 2: Step-down autotransformer

The connection of autotransformer as a step down transformer is shown in Fig. 2. It shows that the two fixed terminals A and B are connected to the single-phase AC supply V₁. Thus winding AB acts as the primary winding. A part of the complete winding i.e. CB acts as the secondary winding across which the load is connected. The operating principle of an autotransformer is same as that of the normal transformer. Therefore the load voltage for this configuration is given by,

\[{{V}_{2}}=\frac{{{N}_{2}}}{{{N}_{1}}}\times {{V}_{1}}\]

Where,

N₂ = Number of turns corresponding to secondary i.e. CB

N₁ = Number of turns corresponding to primary i.e. AB.

As the number of turns corresponding to winding CB i.e. N₂ is less than that of winding AB i.e. N₂, this configuration operates as step down transformer.  If we neglect the losses, the magnetizing current and the leakage reactances, then the transformation ratio is defined as,

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

Where,

I₂ = Primary current

I₁ = Load current.

Autotransformer as a Step Up Transformer

Connection diagram

Fig. 3: Step-up autotransformer

Fig. 3 shows the connection of an autotransformer for operating it as the step up transformer. Note that the part C₃ of the complete winding acts as the primary winding. The ac input voltage V_I is applied between terminals C and B. The full winding AB acts as a secondary winding and the load is connected between these terminals. As the number of turns of winding AB (now N₂) is higher than the number of turns of winding CB (now N₂) the autotransformer now acts as a step up transformer. Neglecting the losses, the magnetizing current and the leakage reactances, the load voltage V₂ is given by,

\[{{V}_{2}}=\frac{{{N}_{2}}}{{{N}_{1}}}\times {{V}_{1}}\]

Copper Saving in Autotransformer

The cross sectional area of a wire is decided by the value of current flowing through it. Larger the area, smaller is the resistance and higher is the current carrying capacity.

Area ∝ I

The length of the wire is proportional to the number of turns.

Length     l ∝ N

The weight of copper required is proportional to area and length.

\[\text{Weight of copper }\propto \text{ Area }\times \text{ Length}\]

But Area ∝ I and l ∝ N

Thus,

\[\text{Weight of copper }\propto \text{ NI}\]

Weight of copper in a two winding transformer

Fig. 4: Two winding transformer

Consider a two winding transformer of Fig. 4. Let the total weight of copper be W_c.

\[{{W}_{C}}={{W}_{1}}+{{W}_{2}}\]

Where W₁ is the weight of copper for primary and ‘W₂’ is the weight of copper for secondary.

But W₁ ∝ N₁ I₁ and W₂ ∝ N₂I₂.

Total weight of copper W_C ∝ (N₁I₁ + N₂I₂)…(1)

Weight of copper in a step down autotransformer

Fig. 5: Step down autotransformer

Consider the step down autotransformer of Fig. 5.

Number of turns

\[XZ={{N}_{1}}\]

\[YZ={{N}_{2}}\]

\[XY={{N}_{1}}+{{N}_{2}}\]

Weight of copper of section XY ∝ (N₁ – N₂)I₂

\[\text{ }\propto \text{ (}{{\text{N}}_{1}}\text{-}{{\text{N}}_{2}}\text{)}{{\text{I}}_{2}}\]

Weight of copper of section YZ ∝ N₂(I₂ – I₁)

\[\text{ }\propto \text{ }{{\text{N}}_{2}}\text{(}{{\text{N}}_{2}}\text{-}{{\text{N}}_{1}}\text{)}\]

Hence the total weight of copper is given by

\[\text{ }{{W}_{A}}\propto \left[ \left( {{N}_{1}}-{{N}_{2}} \right){{I}_{2}}+{{N}_{2}}\left( {{I}_{2}}-{{I}_{1}} \right) \right]…(2)\]

Saving of copper

Take the ratio of Equations (1) and (2) to get

\[\frac{{{W}_{c}}}{{{W}_{A}}}=\frac{{{N}_{1}}{{I}_{1}}+{{N}_{2}}{{I}_{2}}}{\left( {{N}_{1}}-{{N}_{2}} \right){{I}_{1}}+{{N}_{2}}({{I}_{2}}-{{I}_{1}})}…(3)\]

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

I₂ = I₂/K and N₂ = KN₂. Substituting in Equation (3) we get,

\[\frac{{{W}_{c}}}{{{W}_{A}}}=\frac{{{N}_{1}}{{I}_{1}}+K{{N}_{1}}({{I}_{2}}/K)}{({{N}_{1}}-K{{N}_{1}}){{I}_{1}}+K{{N}_{1}}({{I}_{2}}/K-{{I}_{1}})}\]

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

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

\[=\frac{2{{N}_{1}}{{I}_{1}}}{2{{N}_{1}}{{I}_{1}}\left( 1-K \right)}\]

\[\frac{{{W}_{c}}}{{{W}_{A}}}=\frac{1}{\left( 1-K \right)}\] \[{{W}_{A}}=\left( 1-K \right){{W}_{c}}\]

This shows that the copper required for the autotransformer is less than that required for a two winding transformer.

\[\text{Saving of copper = }{{W}_{C}}-{{W}_{A}}\] \[={{W}_{C}}-\left( 1-K \right){{W}_{C}}\]

\[=K{{W}_{C}}\]

So the saving of copper for a step down autotransformer is K times the total copper weight of the two winding transformer. For a step up transformer we can prove that,

\[\text{Copper saving = }\frac{{{W}_{C}}}{K}\]

Advantages of an Autotransformer

1. As only one winding is used, the copper required for the transformer is very less.
2. The size and hence the cost is reduced as compared to the conventional transformer.
3. The losses taking place in the winding are reduced hence the efficiency is higher than the conventional transformer.
4. Due to reduced resistance, the voltage regulation is better than the conventional transformer.

Disadvantages of an Autotransformer

1. There is no electrical isolation between the primary and secondary windings. This can prove to be dangerous for high voltage applications.
2. If the common part of the winding (winding CB) breaks (open circuited) then the transformer action is lost and full primary voltage appears across the secondary.
3. It possess a low impedance, hence if the secondary circuit is short circuited, then a large current will flow on the secondary side.

Applications of an Autotransformer

1. It can be used as a variac, i.e. variable ac supply to vary the ac voltage applied to the load smoothly from 0 V to about 270 V.
2. In order to start the ac machines such as induction motors or synchronous motors.
3. To vary the supply voltage (as per requirements) of a furnace.
4. When the variac autotransformer is used to control the intensity of lamps in the cinema halls etc., it is called as the dimmerstat.

Autotransformer as Dimmerstat

Fig. 6: Autotransformer as Dimmerstat.

When an autotransformer is used for the application of light dimming in the cinema halls or on the stage of play, it is called as dimmerstat. By varying the position of the variable contact, we can adjust the ac voltage applied to the lamps as shown in Fig. 6. The secondary voltage,

\[\text{}{{V}_{2}}=\frac{{{N}_{2}}}{{{N}_{1}}}\times {{V}_{1}}\]

With change in position of the variable contact the value of N₂ changes. This will change the value of V₂ and hence the intensity of lamps which are acting as load.

The post What is an Autotransformer? Connection Diagram, Symbol, Construction, Working Principle & Applications appeared first on ElectricalWorkbook.

Connection diagram of DC Series Motor

Fig. 1: DC Series Motor.

In the DC series motor, the armature and field windings are connected in series with each other as shown in Fig. 1. The resistance of the series field winding (R_s) is much smaller as compared to that of the armature resistance (R_a).

Construction of DC Series Motor

A DC series motor is a type of DC motor where the field windings are connected in series with the armature winding. Its main components include:

1. Armature Winding: A rotating part where current flows and interacts with the magnetic field to produce torque.
2. Series Field Winding: A coil made of thick wire with fewer turns to carry the same current as the armature.
3. Commutator: A device that reverses the current direction in the armature to maintain unidirectional torque.
4. Brushes: Carbon blocks that provide electrical contact to the rotating commutator.
5. Yoke and Poles: Provide structural support and house the magnetic field.

Voltage and Current relations in DC Series Motor

The total current I supplied by the supply voltage (see Figure 1) is same as the field current I_s and armature current I_a.

\[I={{I}_{s}}={{I}_{a}}\]

The total supply voltage V is given by,

V = Volatge across feild + Volatge across field

\[={{I}_{a}}{{R}_{a}}+({{E}_{b}}+{{I}_{a}}{{R}_{a}}+{{V}_{brush}})\]

But,

\[{{I}_{s}}={{I}_{a}}\]

\[V={{E}_{b}}+{{I}_{a}}{{R}_{a}}+{{I}_{a}}{{R}_{s}}+\text{ Brush drop}\]

Neglecting the brush drop we get,

\[V={{E}_{b}}+{{I}_{a}}({{R}_{a}}+{{R}_{s}})\]

The flux produced is proportional to the field current. But in series motor, the field current is same as armature, the armature current.

\[\phi \propto {{I}_{a}}\]

Since the armature current flows through the series field winding, the flux is proportional to the field current as well.

\[\phi \propto {{I}_{s}}\]

The armature current I_a and hence the field current I_s will be dependent on the load. With increase in load I_a and I_s will Increase. Hence in a DC series motor the flux ϕ does not remain constant. So the DC series motor is not a constant flux motor.

Characteristics of DC Series Motor

Torque – Armature Current Characteristics of DC series motor

Fig. 2: DC Series Motor Torque – Armature Current Characteristics.

For the DC series motor, the torque produced is given by,

\[{{T}_{a}}\propto \phi {{I}_{a}}\]

But,

\[\phi \propto {{I}_{a}}\]

∴

\[{{T}_{a}}\propto I_{a}^{2}\]

Where, T_a represents the gross torque produced by the motor.

As the torque is proportional to square of armature current, the starting torque of DC series motor is much higher than that of DC shunt motor. The torque armature current characteristics of a DC series motor is as shown in Fig. 2. The characteristics of T_a versus I_a can be divided into two parts :

1. In part-I, the torque is proportional to the square of armature current. (upto point S in Fig. 2). Hence torque increases exponentially with increase in armature current.
2. But at point S, the saturation of the field electromagnet may take place. It means that there is no change in flux even when there is a change in field current. Hence in part II of the characteristics in Fig. 2, the torque produced is directly proportional to the armature current and not proportion to its square. Hence the torque increases linearly. The dotted characteristics of Fig. 2 shows the variation of shaft torque T_sh with the armature current. Note that,

\[{{T}_{sh}}={{T}_{a}}-{{I}_{f}}\]

T_sh = 0 at no load i.e. when I_a = I_a0 and the curve of T_sh can be obtained by subtracting the constant lost torque from the gross torque T_a.

Speed – Armature Current Characteristics of DC series motor

Fig. 3: DC Series Motor Speed – Armature Current Characteristics.

The speed-armature current characteristics of a DC series motor is shown in Fig. 3. We know that,

\[N\propto \frac{{{E}_{b}}}{\phi }\]

Substituting the value of E_b as E_b = V – I_a (R_a + R_s) and ϕ ∝ I_a we get,

\[N\propto \frac{V-{{I}_{a}}\left( {{R}_{a}}+{{R}_{s}} \right)}{K{{I}_{a}}}\]

Due to the small values of R_a and R_s E_b ≈ V which is constant,

\[N\propto \frac{1}{{{T}_{a}}}\]

Hence the speed decreases with increase in I_a. as shown in Fig. 1.

Speed – Torque Characteristic of DC series motor

Fig. 4: DC Series Motor Speed – Torque Characteristics.

The speed-torque characteristics of a DC series motor is as shown in Fig. 4.

We know that,

\[T\propto I_{a}^{2}\text{    and    }N\propto \frac{1}{{{I}_{a}}}\]

\[{{I}_{a}}\propto \sqrt{T}\text{   and   }N\propto \frac{1}{\sqrt{T}}\]

This shows that the speed decreases increase in the value of torque (i.e. with increase in load). Note that the nature of speed-torque characteristics is same as that of speed-armature current characteristics. DC series motors are preferred for the traction applications because of their capability to generate a high starting torque.

Advantages of DC Series Motor 

1. Provides high starting torque, suitable for heavy loads.
2. Compact and lightweight compared to other motors of similar power ratings.
3. Simple construction and low initial cost.
4. High efficiency at full load.

Disadvantages of DC Series Motor

• Cannot be used without a load (can overspeed dangerously).
• Speed regulation is poor under varying loads.
• Maintenance is required due to brushes and commutator wear.
• Not suitable for precise speed control.

Why is DC Series Motor Never Started on No Load?

For a DC series motor ϕ ∝ I_a. At no load I_a is small, hence also is small. But motor speed is inversely proportional to ϕ.

\[\text{N}\propto \frac{1}{\phi }\]

Hence at no load when is small and the motor speed can be excessively high. This can damage the motor mechanically. In order to avoid this, we should never start the DC series motor on no load.

Uses of DC Series Motor

Transportation Systems

• Electric Trains and Trams: DC series motors provide the high starting torque required to move heavy train cars.
• Trolleys and Locomotives: Used for propulsion systems in traction applications.

Lifting Equipment

• Cranes and Hoists: Ideal for lifting and lowering heavy loads due to their ability to generate strong torque at low speeds.
• Elevators: Used in industrial and commercial settings for efficient and reliable operation.

Automotive Industry

• Starter Motors for Vehicles: DC series motors are used in car starters because they can generate the high torque needed to crank the engine.

Industrial Applications

• Conveyor Belts: For heavy material handling in industries.
• Rolling Mills and Presses: To handle large mechanical loads in manufacturing processes.

Applications of DC Series Motor 

Series motor characteristics indicate that it develops very high starting torque and it has adjustable varying speed. Therefore, for any load which needs high starting torque, series motor is the only suitable DC motor. Series motors are very widely used for the following applications :

1. Electric trains,
2. Diesel electric locomotives,
3. Cranes,
4. Hoists,
5. Trolley cars and trolley buses,
6. Rapid-transit systems,
7. Conveyers etc.

Characteristics of DC Series Motor 

1. High Starting Torque: Torque is directly proportional to the square of the current.
2. Variable Speed: Speed decreases with an increase in load due to higher current draw.
3. Non-linear Torque-Speed Relationship: Torque increases significantly with current, but speed decreases.

Summary of DC Series Motor

The post What is DC Series Motor? Working, Diagram, Characteristics & Applications appeared first on ElectricalWorkbook.

Connection diagram of DC Shunt Motor

In the DC shunt motor, the armature and field winding are connected in parallel as shown in Fig. 1. The parallel combination of the two windings is connected across a common dc power supply.

Fig. 1: DC Shunt Motor.

The resistance of the shunt field winding (R_sh) is always much higher than that of the armature winding (R_a).

Voltage and Current relations in DC Shunt Motor

From Fig. 1 we can say that,

\[\text{I }=\text{ }{{\text{I}}_{\text{a}}}+\text{ }{{\text{I}}_{\text{sh}}}\]

Where,

I = Total current drawn from the voltage source

I_a = Armature current

I_sh = Field current.

Therefore the field current will be,

\[{{\text{I}}_{\text{sh}}}=\frac{\text{V}}{{{\text{R}}_{\text{sh}}}}\]

The supply voltage is given by,

V = Voltage across armature = Voltage across field.

\[=\text{ }{{\text{E}}_{\text{b}}}+\text{ }{{\text{I}}_{\text{a}}}{{\text{R}}_{\text{a}}}+\text{ }{{\text{V}}_{\text{brush}}}=\text{ }{{\text{I}}_{\text{sh}}}{{\text{R}}_{\text{sh}}}\]

Thus,

\[\text{V }=\text{ }{{\text{E}}_{\text{b}}}+\text{ }{{\text{I}}_{\text{a}}}{{\text{R}}_{\text{a}}}+\text{ }{{\text{V}}_{\text{brush}}}=\text{ }{{\text{I}}_{\text{sh}}}{{\text{R}}_{\text{sh}}}\]

Neglecting the drop across brushes we get

\[\text{V }=\text{ }{{\text{E}}_{\text{b}}}+\text{ }{{\text{I}}_{\text{a}}}{{\text{R}}_{\text{a}}}\]

The field current I_sh always remains essentially constant. Since V and R_sh both are constant. Hence the flux produced also remains constant, because field current is responsible for the generation of flux.

\[\Phi \propto {{\text{I}}_{\text{sh}}}\]

This is why the DC shunt motor is also called as the constant flux motors.

Speed–torque characteristic curves for a DC shunt motor

Selection of DC Shunt Motors for a Particular Application

D.C. shunt motor is used to drive pumps, lathes machine tools, prime mover for generators due to constant speed characteristic.

DC Shunt Motor Applications

Characteristics of DC Shunt motor indicate that it is a nearly constant speed motor having its starting torque (torque at N = 0) 30

1. Various machine tools such as lathe machines, Drilling machines, milling machines etc.
2. Printing machinery
3. Paper machines
4. Centrifugal and reciprocating pumps
5. Blowers and fans etc.

Summary of DC Shunt Motor

The post What is DC Shunt Motor? Working, Diagram & Applications appeared first on ElectricalWorkbook.

Construction of Commutator

The commutator is a split ring of larger size with large number of splits (commutator segments). It is called a mechanical rectifier in generator and an inverter in motor. The connections to the commutator depends upon the type of armature windings. These are made of hard copper so as to withstand the brush forces which are placed upon the commutator segments. The position of the brushes is generally based on the winding. Commutator consists of number of segments or bars insulated from each other and are combined together tightly to form a cylinder as shown in figure (1), and fitted on the insulated shaft of the armature. This is known as commutator.

Figure (1): Commutator.

Figure (2): Sectional View of Commutator

To insulate the segments of commutator thin layers of (0.5 to 1 mm) mica is used. The ends of the coils wound on the armature are soldered on the segment of commutator. The insulating mica sheet is usually M or V shaped so as to prevent the segments from flying out due to centrifugal forces as shown in figure (2).

Working of Commutator

(a) DC generator with simplified commutator

(b) Simplified construction

(c) Initial equivalent circuit

(d) Equivalent circuit after half rotation

Figure (3): Commutator Working.

We know that the emf induced into the rotating conductor is always sinusoidal. The commutator is used to convert it into a unidirectional (dc) emf. Thus “commutation” is the process of converting ac voltage into dc. It is similar to rectification.

The action of commutator is as follows, Refer Fig. 3(a) which shows a single turn dc generator, with the commutator in its simplest form. We assume that the commutator has been divided only into two segments namely segments A and B. The simplified construction is shown in Fig. 3(b). The commutator segments A and B are connected to the brushes P and Q respectively. Commutator segments, A and B are connected to conductors “ab” and “cd” respectively and rotate together.

According to Fleming’s light hand rule, the induced current in coil “ab” and “cd” as shown in Fig. 3(b). Hence brush P becomes negative and Q becomes positive as shown in Fig. 3(b). The current I_load through external load resistance R_L flows from bottom to top as shown in Fig. 3(b).

After half the rotation, the segments A and B of the commutator will change their positions as shown in Fig. 3(d). So brush P is in contact with segment B and Q is in contact with segment A.  The directions of currents induced in conductors “ab’ and “cd” are reversed as shown in Fig. 3(d). So brush P continues to be negative and brush Q continues to be positive. Hence the current through the external load continues to flow from bottom to top as shown in Fig. 3(c). Thus the load current and load voltage has become unidirectional. Also the commutator converts the alternating voltage produced by the single turn alternator into a DC voltage.  Thus a commutator operates as a rectifier which converts the ac voltage to DC voltage.

Function of Commutator in a DC Machine

1. As the voltage build up in the armature conductors is A.C voltage, to convert it into D.C voltage commutator is used in the external circuit in generator operation. whereas in D.C motor it produces unidirectional torque.
2. It facilitates the collection of current from armature.
3. It helps in connecting the armature with the external circuit.
4. It converts alternating quantity into a direct quantity (i.e., voltage or current) and vice-versa.
5. It keeps rotor or armature m.m.f stationary in space even though the armature is rotating.

The post What is Commutator? Working, Diagram & Function appeared first on ElectricalWorkbook.

A double cage rotor motor gives excellent starting and running characteristics than a deep-bar cage rotor motor. It also provides a high starting torque with a low starting current. Therefore it is also known as low-starting current, high starting torque, low slip-motor. The rotor is well designed in order to have high resistance rotor circuit during starting and low resistance rotor circuit when operating at running conditions. In case of double squirrel cage motor, the starting torque ranges between 200 to 250 percent of full-load torque whereas starting current ranges in between 400 to 600 percent of full-load current.

Double Cage squirrel cage induction motor with a specialty of different types of rotor having not only one cage but two different independent cages of different resistances and cross-sectional areas (Fig. 1).

Fig. 1: Double Cage Induction Motor.

Construction of Double Cage Induction Motor

The squirrel cage motor has two main parts the stationary and rotory parts. The stationary parts are those which remain stationary during the operation of motor viz, body or yoke, side covers, bed sheet, flench covers, terminal box etc. The rotory parts are those which rotates during the working of motor viz, rotor, fan, shaft, bearings, etc. The stator of the motor has slots and in them a three phase winding whether in star or in delta is done according to the designing of the motor winding.

The rotor of this type of motor is specially designed. There are two distinct rotor windings say two different cages (See Fig. 1). The stampings are respectively designed having provision of two separate windings, one having rotor bars close to the surface and other having some distance i.e., deep into the rotor stampings. Both the cages are short circuited independently at both the ends, i.e., in the same fashion as that of single squirrel cage rotor winding.

The resistances of both inner cage and outer cage are different. The  outer cage have high resistance compared to inner cage. It is generally made of iron or brass bars. Since this cage is near the periphery, the reactance of the windings due to magnetic leakage will be less. The inner cage is made of low resistance material wire such as copper deeply embedded, as a result more flux as compared to upper cage will be linking and resulting more reactance. The resistance of both cages (apart from the selection of material) is made different by choosing different cross sectional area of the conductors. The outer cage is made of less cross sectional area resulting more resistance. The inner cage is made of larger cross sectional area resulting less resistance.

Working of Double Cage Induction Motor

When the three phase supply is given to the stator a rotating magnetic field is produced. The flux links the rotor conductors and according to the Faradays Laws of electromagnetic induction an e.m.f. is induced in the rotor bars. Initially, when the rotor is stand-still, the magnetic flux linking the bars, will have the same frequency as that of supply voltage. The flux links both the windings i.e., cages inducing an emf. As both the squirrel cages are short circuited, by the independent end rings, the circulating current starts flowing. The quantity of current will be different in both the cages. The inner cage have maximum reactance resulting more impedance and so less current in this cage bars. The upper cage being on top results less reactance and hence the impedance, as compared to inner cage will be considerability less as a result more current will flow through the upper cage. Thus the starting torque produced by the outer cage will be more (the outer cage has more resistance). There will be less current flowing through innercage (which has less resistance) thus the torque produced will also be less.

As the motor speeds up, the slip speed comes into existence and this the flux linking the rotor conductors reduces its speed and hence frequency. The reactance of the inner cage becomes less resulting less impedance and more current. The outercage has more resistance as compared to inner cage causes more impedance and as a result less current and less torque.

Thus in starting the motor is started mainly because of outer cage (having more resistance) and in running condition the torque is produced by the inner cage (having less resistance). Thus we get high starting torque and less starting current with high running efficiency.

Torque-speed characteristics of Double Cage Induction Motor

The torque-speed characteristics of the outer cage, inner cage, and double cage induction motor (Resultant torque) are shown in Fig. 2 separately.

Fig. 2: Torque-speed characteristics.

Advantages of Double Squirrel Cage Induction Motor

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

Disadvantages of Double Squirrel Cage Induction Motor

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

The post What is Double Cage Induction Motor? Working, Construction & Characteristics appeared first on ElectricalWorkbook.

If in such case the motor does not stall and if it is carrying more than half the full load, both stator and rotor will be seriously over heated. Because the load is shared by two phases of winding, instead of three phases and stator and rotor current increases increasing temperature. The temperature rise in the stator will be most marked in two phases if it is star connected and in one phase if stator is delta connected. The protections used for single phasing fault are:

1. Thermal overload relays
2. Single-phase preventer

Thermal Overload Relays

Thermal over load relays operate due to increase in current of two phases due to single phasing. These relays can operate trip circuit and disconnect the motor from supply.

Single Phasing Preventer

Main parts of the circuit.:

1. Control coil,
2. Negative sequence filter,
3. Contactors,
4. Normally closed NC contact,
5. Normally open NO contact,
6. C.T.S.,
7. Thermal relay.

Fig.1: Single Phasing Preventer.

In Fig. 1 the parts are shown connected to the protective circuits and motor. The preventive circuit is connected in secondary of C.T. and R.Y.B. lines act as primaries of the CT. The negative sequence filter is shown connected. The output of this lifter is fed to a level detector. This sends the tripping command to the starter and NC contact gets opened and contactors get opened, stopping the motor as it is disconnected from the supply R.Y.B. This type of preventer is generally used for small/medium capacity motors.

The post What is Single Phasing Preventer? Explanation & Circuit Diagram appeared first on ElectricalWorkbook.

Fig. 1: Blocked rotor test of Induction motor

The rotor is held stationary or rotor is blocked i.e. it is not allowed to rotate. The stator supply voltage is gradually increased till motor carries rated or full load current. The corresponding readings are noted down. The voltage required to circulate rated current is very less (merely 5 to 8

Observation Table of Blocked Rotor Test

V_sc = Voltage required to circulate rated current at short circuit condition/ blocked rotor condition

I_sc = rated current at short circuiting condition

W_sc = Copper loss at full load

Calculations of Blocked Rotor Test

W_sc = total copper loss

\[{{W}_{SC}}=3I_{SC}^{2}{{R}_{01}}\]

Where R₀₁ is equivalent resistance of motor referred to stator or primary side

From which

\[{{R}_{01}}=\frac{{{W}_{SC}}}{3I_{SC}^{2}}\]

Equivalent impedance,

\[{{Z}_{01}}=\frac{{{V}_{SC}}}{{{I}_{SC}}}\]

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

\[{{X}_{01}}=\sqrt{Z_{01}^{2}+R_{01}^{2}}\]

Thus parameters of equivalent circuit referred to stator side are calculated.

Using no load an blocked rotor test efficiency of motor can be calculated as follows

\[\eta =\frac{\text{Output}}{\text{Output + Iron loss + Copper loss}}\times 100\]

\[\eta =\frac{\text{Output}}{\text{Output + }{{\text{W}}_{0}}\text{ + }{{\text{W}}_{\text{SC}}}}\times 100\]

The post Blocked Rotor Test of Induction Motor appeared first on ElectricalWorkbook.

Construction of Permanent Magnet Synchronous Motor

Fig. 1 shows the circumferential cross-section of a typical two-pole permanent magnet synchronous motor. The stationary member of the motor, called stator can be of a wide variety of constructions and designs and can be wound for single-phase or polyphase excitation. The permanent magnets made up of high permeability and high coercivity materials like rare earth or samarium-cobalt form the poles of the rotor, equivalent to the wound field poles of conventional synchronous motors. Permanent magnet poles are inherently salient. Many permanent magnet synchronous motors may have physically smooth cylindrical rotors, but electrically each permanent magnet pole have a salient pole structure.

Fig. 1: Circumferential cross section of a two-pole permanent magnet synchronous motor

Apart from permanent magnet poles, rotor structure of the motor may include a number of other structural and magnetic components. Damper winding, forming the cage arrangement of conducting bars is provided in most of the motors (of power ratings above a few hundred watts) used in power applications. The principal purpose of damper winding is to dampen oscillations about the synchronous speed. It is also used to start the synchronous motor and bring it up to the synchronous speed.

Principle of Operation of Permanent Magnet Synchronous Motor

The principle of working of permanent magnet synchronous motor is same as that of conventional synchronous motor. Initially, the motor starts as a squirrel-cage induction motor with the help of damper winding. When a three-phase winding (if provided) of a stator is energized from a three-phase supply, rotating magnetic field is set up in the air gap. At synchronous speed, the rotor field poles get magnetically locked with the stator poles to produce torque and hence, rotor continues to rotate.

Characteristics of Permanent Magnet Synchronous Motor

Permanent magnet synchronous motors have same operating and performance characteristics as conventional synchronous motors.

Applications of Permanent Magnet Synchronous Motor

They are widely used in various industrial applications such as automobiles, compressors, blowers, conveyors, fans, pumps, machine tools, elevators, traction, aerospace, printed circuit motors, computers, robotics, textile and glass industries, steel rolling mills, fibre spinning mills, etc.

The post What is Permanent Magnet Synchronous Motor? Working, Construction, Diagram & Applications appeared first on ElectricalWorkbook.

Constructional of Switched Reluctance Motor

Fig. 1 shows the cross-section of a typical 4-phase switched reluctance motor having eight poles in the stator and six poles on the rotor. Both the stator and rotor have laminated construction to reduce the core losses and have salient poles. While the rotor has no windings or permanent magnets, each stator pole has a concentrated winding around it. Each pair of diametrically opposite coils are connected in series and form one phase of the motor. In the figure, only one phase winding is shown for simplicity.

Fig. 1: Schematic diagram of a 4-phase switched reluctance motor.

Switched Reluctance Motor drive Block diagram

The complete drive system comprises of a switched reluctance motor coupled with a load, a power converter, a control system involving rotor position transducer and current sensor as shown in Fig. 2.

Fig. 2: Block diagram of a switched reluctance motor.

Principle of Operation Switched Reluctance Motor

Torque is produced by exciting any phase of the stator winding by means of unidirectional current. This results in the magnetic attraction of an adjacent rotor pole as it tends to align into a position of minimum reluctance. When the number of stator and rotor poles differ, the sequential switching of the excitation from one set of stator poles to the next, in synchronism with the rotor position, results in an almost constant torque causing uniform rotation. The synchronization of the switching on of the excitation with rotor position can be accomplished with simple rotor position feedback.

Torque-Speed Characteristic of Switched Reluctance Motor

Fig. 3 shows the torque-speed characteristic of a switched reluctance motor with the simplest form of a control. From the figure, it will be observed that the starting torque of such a motor is high. By choosing appropriate switching angles and current levels together with an appropriate electromagnetic design, torque-speed characteristics of the switched reluctance motor can be tailored to suit the application.

Fig. 3: Torque-speed characteristic of a switched reluctance motor with the simplest form of control.

Reversal of Rotation of Switched Reluctance Motor

The direction of rotation of a switched reluctance motor can be reversed by reversing the sequence of switching the current into the stator windings.

Applications of Switched Reluctance Motor

The applications of these motors include compressors, fans, centrifuges, food processors, washing machines, vacuum cleaners, fluid pumps, process control industries, hybrid/electric vehicles, electromechanical brake systems, electric power steering, starter generator systems, aircraft applications, servo drives, etc.

The post What is Switched Reluctance Motor? Working, Diagram, Construction & Applications appeared first on ElectricalWorkbook.

Constructional of Synchronous Reluctance Motor

The stator of this type of single-phase synchronous motor is similar to that of a single-phase induction motor (either resistance split-phase, capacitor split-phase or shaded-pole type). The rotor has salient poles without any winding for d.c. excitation. The salient-pole structure is given to the rotor by removing some of the teeth of a squirrel-cage rotor as shown in Fig. 1 (a). The remaining teeth which serve as salient poles carry short-circuited copper bars and form squirrel-cage type damper winding.

Fig. 1: Synchronous reluctance motor constructional features of 4-pole.,

Synchronous Reluctance Motor Principle of Operation

When the stator winding is energised, the motor starts as an induction motor and reaches near about synchronous speed. When the magnetic axis of the revolving magnetic field of the stator aligns with the centres of cutout slots of the rotor, the reluctance of the magnetic circuit is greatest and it is least when the magnetic axis of the revolving field aligns with the centres of the groups of rotor teeth serving as salient poles. This is because of the dissimilarity in the air-gaps in two conditions. The reluctance of the magnetic circuit is thus a function of the relative position of the rotor with respect to the rotating magnetic field. This variation in reluctance produces a tendency for the salient portions of the rotor iron to lock in with the rotating stator field in a position of minimum reluctance, just as any piece of iron located in a magnetic field tends to line itself up with the densest portion of a magnetic field. The torque so produced is called reluctance torque. It is this torque which pulls the rotor into step with the revolving field. The rotor then continues to run at the speed of the revolving field i.e. at synchronous speed.

Torque-Speed Characteristic of Synchronous Reluctance Motor

Torque-speed characteristic of a typical induction start synchronous reluctance motor is shown in Fig. 2. From the figure, it will be observed that the starting torque of such a motor is highly dependent upon the rotor position. This is because of the projecting nature of its rotor poles.

Fig. 1: Synchronous reluctance motor torque-speed characteristic.

Reversal of Rotation of Synchronous Reluctance Motor

The direction of rotation of such motors can be reversed in the same manner as in a single-phase induction motor.

Applications of Synchronous Reluctance Motor

Even though the efficiency and power factor of these motors are poor, their constant speed characteristic are accompanied by other advantages like rugged construction, non-requirement of d.c. supply and the minimum maintenance has made such motors very suitable for varieties of applications such as signaling devices, recording instruments, clocks and all kinds of timing devices, teleprinters, sound recording and sound producing apparatus (e.g. gramophones, tape recorders, etc.), control apparatus, automatic regulators, etc.

The post What is Synchronous Reluctance Motor? Working, Construction, Diagram & Applications appeared first on ElectricalWorkbook.