## Single Phase Induction Type Energy Meter Working, Construction & Creeping

### Single Phase Induction Type Energy Meter Construction,Working Principle,Operation,Creeping,Torque Equation

The measurement of energy is the same process as measurement of power except that the instrument not merely indicates the power or rate of supply of energy but must take into account also the length of time for which the rate of energy is continued. There are basically three types of energy meters.
(a) Electrolytic meters
(b) Motor meters
(c) Clock meters.
Out of the above, motor meters are very widely used and among motor meters also induction type watt-hour meters are more commonly used and will be dealt with here. Single phase induction type energy meter working explained including constructional details.
READ HERE : PMMC Working & Operation

### Single Phase Induction Type Energy Meter Construction

The construction of this meter is more or less similar to induction type watt-meter. The main alterations are the provision of only one pressure coil upon the central limb of the shunt magnet and only one copper shading band upon this limb. In addition there are two copper bands placed obliquely on the other two limbs of this magnet, their objective is to provide compensation against friction error in the energy meter (see figure).

In this meter the moving system is allowed to revolve continuously instead of being allowed merely to rotate through a fraction of one revolution as in an indicating instrument. The speed of revolution is proportional to the power in the circuit. It follows, therefore, that the no. of revolutions made by the revolving system in any given time is proportional to the energy supplied. The number of revolutions made by the meter is recorded by a counting mechanism consisting of a train of wheels to which the spindle of the rotating system is geared.

### Single Phase Induction Type Energy Meter Working

The control of speed is brought about by a permanent magnet called the brake magnet.The magnet is placed opposite to the electromagnet used for providing deflecting torque and it induces currents in the disc which produces retarding torque proportional to their magnitude which latter is proportional to the speed of the rotating system. This system attains a steady speed when the retarding torque exactly balances the driving torque produced by the power in the circuit. The braking torque produced by the brake magnet depends upon the strength of the magnet.

Torque Equation:

If  φ is the flux of the brake magnet, i the current induced by the rotation of the moving system in the field of the brake magnet and TB is the braking torque, then
T ∝ i.φ
If n is the speed of the rotating system (disc) and e the voltage induced in the disc of the meter.
∝ n.φ
Let r be the resistance to the path of eddy current i in the disc, hence i = e/r.
and  T φ.(e/r)
∝ φ².(n/r)

This braking torque equals the constant driving torque TD when a steady speed of the disc
is attained. Thus if N be the steady speed of the meter

T' ∝ φ².(N/r)
and at balance T'B = T∝ φ².(N/r)  or N  ∝ (r/φ²).TD

Hence the steady speed attained by the meter for a constant driving torque TD is directly proportional to the resistance of the path of the induced eddy currents and inversely proportional to the square of the flux of the brake magnet. Therefore, it can be seen that it is very important for brake magnet to have constant magnetic strength throughout the use of the energy meter. It is to be noted that the spring control and pointer of the watt-meter have been replaced in the energy meter by a brake magnet which provides braking torque as explained above.

Using the theory of induction watt-meter.
Operating or driving torque ∝ VI cosφ
where V is the supply voltage across the shunt coil and ! the load current through the series coil of the meter and φ the p.f. angle between V and I.
Now the braking torque has been shown to be proportional to the speed N of the disc i.e.
TB  N
Since for a steady speed N, the driving torque TD is equal to TB, we have N  VI cosφ or power is proportional to speed N.Thus the total no. of revolutions which equals N.dt is proportional to VI cosφ dt   i.e. proportional to energy supplied.

The speed of the disc can be adjusted by suitably positioning the brake magnet with respect
to the spindle of the disc. If the brake magnet is moved towards the spindle the braking torque
decreases and if taken away from the spindle the braking torque increases.
An energy meter is rated in terms of the supply voltage, the full load current and number
of revolutions of the disc per kWhr (also known as constant of the energy meter). These are
marked on the dial of the energy meter.

#### What is creeping in energy meter?

Creep:In some meters a slow but continuous rotation of the disc is obtained when the pressure coils are energized and there is no load current passing through the current coil i.e. current coil is not energized. This may be due to incorrect friction compensation to vibration, to stray magnetic field or to the fact that the supply voltage is in excess of the normal voltage.

To prevent such creeping of the meter two holes or slots are cut in the disc on opposite sides of the spindle. The disc tends to remain stationary when one of the holes comes under one of the poles of the shunt magnet. In some cases a small piece of iron wire is attached to the edge of the disc. The force of attraction of the brake magnet upon this iron wire is sufficient to prevent the creeping of the disc under no load condition.

Friction Compensation In Energy Meter:
The two shading bands embrace the flux contained in the two outer limbs of the shunt magnet and thus eddy currents are induced in them which cause a phase displacement between the enclosed flux and the main gap flux. As a result a small driving torque is exerted on the disc, this torque being adjusted by variation of the position of these bands to compensate for friction torque in the instrument.
The friction compensation can be checked whether it is correct or not by connecting light load under which condition it should run at the correct speed.

Energy Meter Phase Angle Error:
It is desired that the phase angle between the applied voltage to the pressure coil and the shunt magnet flux should be at 90° which is adjusted with the help of shading band on the shunt magnet. An error due to incorrect adjustment of the position of this shading band will be evident when the meter is tested on a load whose p.f. is less than unity. An error on the "fast" side under these conditions can he eliminated hy bringing the shading hand further drawn the limb of the shunt magnet i.e. nearer to the disc.

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## Direct Online Starter

In the previous post we have learned about Star Delta Starter. In this post we are going to learn about Direct Online Starter or DOL starter.

### Why We Use Starter?

A starter is generally used to limit the high starting currents. An induction motor has high starting currents which may cause the winding's to damage. In order to prevent this we use starters while starting an induction motor. Direct online starter is one of the simplest motor starter which is used for starting the induction motor. This starter is generally used for starting of cage type induction motors.

Note: Starter action will be only at the start of induction motor i.e, until it gains a required amount of speed (generally until it  reaches 85% of synchronous speed) after that it will be isolated from the circuit automatically by few internal actions.

### Principle Of DOL Starter:

1.We start by closing the contactor and by applying full voltage across the motor winding's. This causes the motor to draw very high amount of staring currents for a short duration.

2. Now the iron coil will get magnetized and the current will be limited to locked rotor current of the motor. Now the motor will develop locked rotor torque and accelerates to achieve full speed.

3. While it is accelerating the current will decrease slowly. Clear drop of current can be seen only after motor reaches 85% of synchronous speed.

4. Note that starting current curve depends only upon terminal voltage and design of motor, it doesn't depend upon the load applied on the motor.

5. Motor load only affects the time taken for the motor to accelerate to full speed but not magnitude of  starting current.

6. Only if the torque developed by the induction motor with DOL starter exceeds the load torque then only induction motor accelerates to full speed if not the induction motor must be replaced with a suitable capacity induction motor which can develop torque more than the load torque.

Key point: DOL starter has maximum starting current and maximum starting torque.

### Main Parts Of DOL starter:

1. Contactors and coil.

let us see use of these parts in detail now.

#### Contactors & Coils:

The contactors that we are using are electromagnetic contactors. With the help of electromagnetic energy they can make or break the circuit.

It consists of a coil wound on a iron core. So when the electrical energy passes through this coil the coil makes the iron gets magnetized as a result it attracts the armature and hence the circuit is made that means a closed circuit is formed. When the current interrupts the iron core gets demagnetized as a result it releases the armature this breaks the circuit. The contactor has three main normally open(NO) contacts and auxiliary contact which has lower power ratings and it can be of normally open(NO) or normally closed(NC) type.

From the name it suggests that it is used for over load protection. Generally over load relays have inverse-trip-time characteristics. It cannot provide protection against short circuit currents it only provides protection when the high currents are drawn for a longer period. This is the main advantage of this over load relay over fuses. As we have high inrush currents at the start of motor for a short period of time if we use a fuse it melts every time when we start a motor but this shouldn't happen, moreover if we use high capacity fuse to prevent this it cannot sense the currents which can damage the motor winding's.

Unlike fuse over load relay is useful in this case since it permits high currents only for short time and high inrush currents at start of motor exists only for shorter duration the over load relay doesn't trip at the start. But later on when the current drawn exceeds the full load current for a longer duration then the over load relay trips the circuit. This means over load relay senses and reacts to the harmful over load currents only. We can classify over load relays based on the characters that are used to detect the over load conditions.

#### Types Of Over Load Relays:

There are 3 types of over load relays. They are as follows:

1. Thermal Relay:

Based on the rising temperature caused by over load currents the thermal relay trips and breaks the circuit. This can be further classified into two types namely melting alloy and bimetallic.

#### 2. Magnetic Relay:

When the current exceeds a certain limit the coil gets magnetized which helps to trip the relay and break the circuit. It will not be affected by temperature.

#### 3. Electronic Relay:

These are very ideal relays and can trip the circuit precisely and in fast manner. They can be installed easily.

### Working Of DOL Starter:

Now let us discuss the working of DOL starter or direct online starter in detail.

We can understand the working of DOL starter clearly by seeing the circuit diagram of DOL starter.

### DOL Starter Circuit Diagram:

1. To switch on the induction motor first press the start push button,S1. Now the contactor gets energized from two lines L1,  L2 .

2. As a result three main contacts and auxiliary contact gets closed and ab is short circuited. So now the motor gets connected across the main supply.

3. Start button goes back due to spring action but the contactor gets energized through ab.

4. If the stop button is pressed supply through contactor is stopped and as a result the coil gets demagnetized and the main contacts and auxiliary contacts are opened as a result induction motor is disconnected from main supply.

5. Now to start the motor again we need to push the start push button.

#### How Under Voltage Protection Is Done?

When the voltage falls  below a certain level or when there is fail in providing supply the coil of contactor gets de-energized as a result supply to induction motor stops. In this way under voltage protection is done.

#### How Over Load Protection Is Done?

When there is over load the over load coils in the circuit gets energized and the normally closed coil D will change to open position, contactor coil gets de-energized which causes the supply to  induction motor to stop. In this way induction motor winding's are saved from burning or over heating due to over loads.

### Relation Between Starting Torque And Full Load Torque Of Induction Motor:

Ist be the starting current drawn from the main supply per phase.
Ifl is the full load current drawn from the main supply per phase.
Ʈest is the starting torque.
Sfl is the slip at full load.

we know,
rotor copper loss = S x rotor input  (S means slip)

At start, S = 1, I2 = I2st, Ʈe = Ʈest.

So we get,
At full loads = sfl, I2 = I2fl, Ʈe = Ʈefl

If we neglect no load current, we have

Equating 6 and 7 equations we get,

Equating 5 and 8 equations we get,
If V1 is the stator voltage per phase equivalent

Ze10 is the standstill impedance per phase of the motor referred to the stator.

Then the current at the starting is given by the equation as shown.

Starting current is equal to the short circuit current.
From equation 9 and 10 we get,
This the relation between start torque and full load torque of induction motor.

1. It is the simplest form of starter.

2. It is more economical starter.

3. It gives high starting torque.

4. Control circuit is simple and can be easily troubleshooted.

5.Occupies less space.

6. Easy to find fault.

1. High starting currents.

2. High starting currents of motor causes the large voltage dip or drop of electrical supply which affects the other appliances connected to the supply.

3. High starting torque required by the load may cause increasing mechanical stresses on motor mechanical parts as well as the loads.

4. Not suitable for motors having higher ratings( above  10 KW).

In this post we have learnt about the working of DOL starter or direct online starter.

In our previous posts we have discussed  synchronous motor working principle ,why synchronous motor is not self starting? etc.The most important thing is why mostly we use synchronous motors over induction motors.Here we have the reasons..!

Synchronous motor always runs at constant synchronous speed irrespective of the load and variable power factor operation. As seen earlier its power factor can be controlled by controlling its excitation. For over-excitation its power factor is leading in nature, which is very important from the power factor correction point of view.

### Applications of Synchronous Motors

We use synchronous motors where we need almost constant speed in the operation.So applications of synchronous motors list goes like this, we use them in,

• Machine tools,
• Motor generator sets,
• Synchronous clocks,
• Stroboscopic devices,
• Timing devices,
• Belt driven reciprocating compressors,
• Fans and blowers,
• Centrifugal pumps,
• Vacuum pumps,
• Pulp grinders,
• Textile mills,
• Paper mills line shafts,
• Rolling mills,
• Cement mills etc.

### Some more uses of  synchronous motors:

Not only for the above purposes we also use synchronous motors for power factor correction.You might have known about synchronous condenser[If not read here].Also used as phase advancers and phase modifiers for voltage regulation of the transmission lines. This is possible because the excitation of the synchronous motor can be adjusted as per the requirement.

### Disadvantages of synchronous motor :

The disadvantages of synchronous motor are their higher cost, necessity of frequent maintenance and a need of d.c. excitation source, auxiliary device or additional winding provision to make it self starting. Overall their initial cost is very high.

## Principles of Electronics by V.K. Mehta and Rohit Mehta Pdf-Free Download

### Principles of Electronics by V.K. Mehta and Rohit Mehta Pdf-Free Download

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## Two Watt Meter Method For Power Measurement

Two watt meter method is used for measuring power in 3 phase 3 - wire star or delta connected systems either with balanced or unbalanced load. Let us discuss two watt meter method for 3 phase power measurement  with star and delta connection in detail.

### Measurement of power by two watt meter method in star connection:

In this method we use two watt meters and the current coils of these watt meters are connected in series with two lines and potential coils are connected across these two lines  and load is connected in star as shown in the following figure.

From figure

The instantaneous current flowing through the watt meter W1 is Ir

The voltage across the potential coil of watt meter W1 is

Instantaneous power measured by the watt meter, W1 is given by,

The instantaneous current flowing through the watt meter W2 is Iy

The voltage across the potential coil of watt meter W2 is
Instantaneous power measured by the watt meter, W2 is given by
The total 3 phase power will be sum of power measured by watt meters W1 and W2

Adding equations 1 and 2 we get following results.

Where P is the total 3 phase power consumed by load.

### Measurement of power by two watt meter method in delta connection:

In this method also we use two watt meters and the current coils of these watt meters are connected in series with two lines and potential coils are connected across these two lines and load is connected in delta as shown in the following figure.

The instantaneous current flowing through the watt meter W1 is,

The voltage across the potential coil of watt meter W1 is Erb

Instantaneous power measured by the watt meter, W1 is given by,

The instantaneous current flowing through the watt meter W2 is,
The voltage across the potential coil of watt meter W2 is Eyb

Instantaneous power measured by the watt meter, W2 is given by,
The total 3 phase power will be sum of power measured by watt meters W1 and W2

Adding equations 3 and 4 we get following results.

Where P is the total 3 phase power consumed by load.

Note : The power measured by watt meter is average power because of the inertia of their moving system.

In this post we have discussed about measurement of three phase power using two watt meter method.

## Electrical Bus - Bar And Types Of Bus - Bar Arrangements And Its Advantages And Disadvantages

### What is a bus bar?

A bus - bar is a conductor or group of conductors and it collects electric energy from incoming feeders and distributes them to outgoing feeders. So bus - bar is a junction where all incoming and out going currents meet. Bus - bar is generally made up of aluminium but not with copper because aluminium has special characters like higher conductivity, lower cost, excellent corrosion resistance, etc.

### Criteria For Choosing Type Of Bus - Bar Arrangement:

We have different types of bus - bar arrangements we need to choose the required arrangement. This depends on various factors such as

1. System voltage.
2. Position of a substation in the system.
3. Reliability of supply.
4. Flexibility.
5. cost.
6. Availability of alternative arrangements if outage of any of the apparatus happens.
7. Bus - bar arrangement should be simple and easy to maintain.
8. In case of load - growth there must be possibility to extend the system to meet the load requirements.
9. The installation should be as economical as possible, keeping in view about the needs and continuity of supply.

### Types Of Bus - Bar Arrangements:

1. Single bus-bar arrangement.

2. Single bus-bar arrangement with bus sectionalized.

3.Double bus arrangement.

4. Double bus double breaker arrangement.

5. One and a half Breaker arrangement.

6. Main and transfer bus arrangement.

7. Double bus system with bypass isolators.

8. Ring main arrangement.

9. Mesh arrangement.

Let us see each of bus - bar arrangements in detail now.

Note: Through outgoing feeders power is supplied to loads.

### Single bus - bar arrangement:

This single bus - bar arrangement consists of only one bus - bar and all the incoming feeders and outgoing distributors are connected to this bus - bar only. All the fuses, circuit breakers, generators and transformers are connected to this as shown in the following figure.

### Advantages of single bus - bar arrangement:

1. It is easy in operation.

2. Initial cost s less.

3. Requires less maintenance.

### Disadvantages of single bus - bar arrangement:

1. When damage occurs then there will be whole interruption of power supply.

2. Flexibility and immunity are very less.

### Single bus - bar arrangement with bus sectionalized:

In  single bus-bar arrangement with bus sectionalized we divide a single bus - bar into two sections with the help of a circuit breaker and isolator switches and load is distributed equally among both sections as shown in the following figure:

### Advantages of sectionalized single bus - bar arrangement:

1. As we are using circuit breaker to divide a bus - bar into two sections fault on one section will not interrupt power supply on the other section only few loads will have lac of power supply.

2. The fault level can be reduced by adding a current limiting reactor.

### Disadvantages of sectionalized single bus - bar arrangement:

1. We are using extra isolators and circuit breakers so that the cost will be high.

### Double bus arrangement:

Double bus arrangement has two bus bars and the incoming feeders and outgoing feeders are connected in parallel to both buses with the help of isolators. By closing the isolator switch we can connect the feeders either to bus - bar 1 or to bus - bar 2 . We can divide the load among two buses with the help of isolator switches by closing the isolator switch that is connected to bus - bar 1 and feeder the load can be connected to bus - bar 1 and by closing the isolator switch connected to bus - bar 2 and feeder the load gets connected to bus - bar 2.  This can be shown in following figure:

We have a bus coupler breaker which is used for bus transfer operation. When we need to transfer load from one bus to other bus we need to close the bus coupler first and then close isolators of the associated bus to which load is to be connected and open the isolator switch coupled to fault bus and then open the bus coupler breaker.

### Advantages of double bus system:

1. It has greater flexibility.

2. During fault conditions there is no interruption of power supply to load.

### Disadvantages of double bus system:

1. We cannot transfer load from one bus to other bus without interruption of power supply for few minutes.

### Double bus double breaker arrangement:

In double bus double  breaker arrangement we connect a feeder in parallel to both buses with the help of two circuit breakers and isolator switches instead of bus coupler as shown in the following figure:
Here we energize both the feeders and divide feeders among both the buses but we can connect desired feeder to desired bus at any time for this purpose we need to close the isolator and then circuit breaker associated with the required bus - bar and later open the circuit breaker and then isolator from which is has to be disconnected.

### Advantages of double bus double breaker system:

1. During fault conditions load can be transferred to one bus so there will not be interruption in power supply.

2. Here we are not using a bus coupler so there will not be much delay in power supply while closing circuit breaker to transfer load from one bus to other bus.

3. High flexibility.

### Disadvantages of double bus double breaker system:

1. The number of circuit breakers used are high so cost is very high.

2. Maintenance cost will also be high.

So this type of arrangement is used very rare.

### One and a half breaker arrangement:

Because of high cost of double bus double breaker arrangement we use one and a half breaker arrangement. Here the two feeders are fed through their corresponding bus - bars and these two feeders are coupled by a third circuit breaker called tie - breaker as shown in the following figure:
During normal conditions all the three circuit breakers are closed and the both circuits operate in parallel and power is fed to feeders from the two bus - bars. If fault occurs on one bus bar then  with the help of second  bus - bar feeder circuit breaker and tie breaker power is fed to feeders. This means each feeder breaker has to be rated to feed both feeders which are coupled by tie breaker.

### Advantages of one and a half breaker system:

1. There will be no interruption of power in case of fault because all the feeders can be transferred to other healthy bus immediately.

3. Cost is less compared to double bus double breaker arrangement.

### Disadvantages of one and a half breaker system:

1. This arrangement is complicated because during fault two circuit breakers are to be opened.

2. Maintenance cost is high.

### Main and transfer bus arrangement:

In main and transfer bus arrangement we have two buses one is main bus and the other is transfer bus. With the help of isolator switches it is connected to the transfer bus which are called bypass isolators and with the help of circuit breakers and isolator switches it is connected to the main bus. There is also bus coupler as shown in the following figure:

In normal conditions the feeders are fed through main bus but during fault conditions load is transferred to the transfer bus. In order to transfer load from main bus to transfer bus we need to close the bus coupler first and then close by pass isolators of feeder to be connected to transfer bus and open the isolator switch of feeder coupled to main bus and then open the bus coupler breaker.

### Advantages of main bus and transfer bus system:

1. No interruption of  power supply because in case of fault load can be shifted to transfer bus.

2. Load can be divided into two groups since they can be feed from either of the buses.

### Disadvantages of main bus and transfer bus system:

1. Two bus - bars are used which increases the cost.

### Double bus arrangement with by pass isolators:

Double bus system with bypass isolators is combination of double bus system and main and transfer bus system. Here the feeders are connected to both the buses with the help of isolators during fault conditions loads can be transferred to healthy bus by closing isolators of feeders associated to healthy bus and opening isolators of feeders associated to faulty bus. The connection diagram is given below:
A bus coupler is also provided so while transferring load to healthy bus close the bus coupler breaker first and close the isolator of feeder to which it has to be transferred and open the isolator switch of feeder coupled to fault bus and then open the bus coupler breaker.

### Advantages of double breaker system with by pass isolators:

1. No interruption of  power supply because in case of fault load can be shifted to transfer bus.

2. Load can be divided into two groups since they can be feed from either of the buses.

### Disadvantages of double breaker system with by pass isolators:

1. cost is high as we are using two bus bars and extra isolator switches.

2. Complex in nature.

### Ring main arrangement:

Ring main arrangement provides double feed to each feeder circuit. Here the end of the bus bars is returned upon themselves to form a ring. Hence it is called main ring arrangement. This arrangement is shown in the following figure:
Here if one circuit beaker is damaged it is opened and the feeder can be supplied from the other circuit breaker which is near to it.

### Advantages of ring main system:

1. Since each feeder is fed from two circuit breakers even if fault occurs in one system the feeder can be fed from other system so there will be no interruption of power supply under fault conditions.

2.The effect of fault is localized to that section alone and the rest of the section continues to operate normally.

### Disadvantages of ring main system:

1. There will be difficulty to add  any new circuit in the ring.

### Mesh arrangement:

In mesh arrangement between the mesh formed by bus bars circuit breakers are installed as shown in the following figure:
From the node point of mesh circuit is tapped. We need to open two circuit breakers when the fault occurs so that protection can be obtained but switching is not possible.

1. Provides protection against fault.

2. For substations having large number of circuits this arrangement is suitable.

1. It doesn't provide switching facility.

2. Not suitable for all type of substations.

These are the different sub station layouts or bus bar arrangements.

In this post we have learnt about electrical bus -bar and types of bus - bar arrangements and its advantages and disadvantages.

To download this post on  electrical bus -bar and types of bus - bar arrangements and its advantages and disadvantages as PDF click cntrl+p and save as PDF.

## Star - Delta Starter

To start a large induction motor we use star - delta starter because large induction motors(cage type) with delta connected stator when started directly on line it produces large starting current surges which causes fluctuations in voltage on supply line. To prevent this we run induction motor at reduced voltage at the start of induction motor and later on after getting require speed we operate induction motor at full supply voltage for this purpose we use star delta starter.

### Working Principle Of Star - Delta Starter:

As discussed above to reduce fluctuations in voltage on supply line we need to reduce starting surge currents so we need to reduce the voltage at the start of induction motor for this purpose we firstly connect the stator windings of induction motor in star connection and later on we connect the stator windings in delta to operate at full supply voltage. See the below circuit diagram how the star - delta starter is connected to induction motor.

#### Connection diagram of star - delta starter:

 star delta starter control circuit diagram
During start position of switch the stator windings are connected in star as shown in the following figure.

Now the induction motor gradually picks up it's speed. When the speed becomes 80 percent of its rated speed then the switch moves to run position as a result stator windings get connected in delta as shown in the below figure.

### Theory And Calculations Of Star - Delta Starter:

To understand how voltage is reduced by star connection of stator windings see the below calculations.

In star connection the phase voltage is 1/√3 times of line voltage as the torque is directly proportional to square of voltage applied the torque is reduced to 1/3 times than the torque produced by starting with direct delta connection.

Let,

VL be the line voltage.

V1 is the phase voltage.

Istyp be  starting current per phase when the stator windings are connected in star.

Istyl is the starting line current when the stator windings are connected in star.

IstΔp is the starting current per phase by direct switching with the stator windings connected in delta

IstΔl is the starting line current by direct switching with the stator windings in the delta.

IscΔp is the short circuit phase current by direct switching with the stator windings in the delta.

Ze10 is the standstill equivalent impedance per phase of the motor, referred to the stator

In star connection,  line current will be equal to phase current.
In delta connection, the line current is equal to the √3 times of the phase current. so we get,

This shows that with star delta starter, the starting current from the main supply is one-third of that with direct switching in the delta.
This shows with star delta starter, the starting torque is reduced to one-third of the starting torque obtained with the direct switching in the delta.
Here,
IflΔp is the full load phase current with the winding connected in delta. But
So the above equations show how voltage and torques are reduced with star delta starter than direct delta starting.

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