Wednesday, August 15, 2018

August 15, 2018

Sumpner's test or Back-to-Back test on Transformer

In this post we will study about the Sumper's test on a transformer. It is also called Regenerative test or Back-to-Back test.
Please read the whole article carefully for understanding it properly
Main objective of the Sumper's test is to check the transformer efficiency , maximum temperature rise. It is also used to measure the losses in the large transformers.  This method is advantageous for large  transformers because for full load test on large transformers, it would be expensive as a large amount of energy will get wasted during the test. Therefore, large transformers can be tested by Sumper's test.
Sumper's test requires requires two identical transformers. Given figure shows the circuit diagram for back-to-back test (Sumper's test).
Regenerative test of transformer , back to back test of transformer
The primary winding of the  two transformers are connected in parallel and is supplied at rated voltage and frequency. As seen in the figure, a Voltmeter, an ammeter and watt meter are connected to the input.
The secondary of the transformers are connected in series in phase opposition and voltmeter is connected across them. The range of the voltmeter is twice the voltage of secondary winding. Phase opposition of the winding is checked by this voltmeter. If voltmeter reads 0, it means they are in phase opposition , because both windings have equal voltages but in opposite direction due to which overall voltage in the secondary circuit become 0.
Now if the supply is given to the primary circuit, then the total voltage in secondary circuit will be 0, as discussed above.
There will be no current in the secondary winding and it will behave as secondary is open circuited. Only No-load current will flow in the primary circuit which is measured by the ammeter on the primary side.
Hence , watt meter on the primary side will read the total Iron losses of the both transformers.
Now, a small voltage is injected in the secondary circuit with the help of regulating transformer (see the figure). The magnitude of the voltage is adjusted till the ammeter on the secondary side reads the full-load secondary current. This produces the full load copper losses of the both transformers. Thus watt meter on the secondary side reads the total Full-load copper losses.
Thus with the help of this method we have founded the both losses in the transformer. We have loaded the transformer to the full load but the power taken from the supply is just that it will be only sufficient to supply the losses of both transformers.
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Sunday, August 12, 2018

August 12, 2018

The Stepper Motor Basics | Types, Working Operation and Applications

In this article we will study about the various aspects of Stepper motor.
Stepper motor is a special purpose machine , feedback motor in which the rotor of the motor rotates in discrete steps. The angular rotation is determined by the number of pulses fed to the control circuit.
Each input pulse produces the one step of the angular movement. The total rotation of the rotor comprises of various such steps, therefore it is called Stepper Motor.
There are three most popular types of rotor arrangements:

  1. Variable Reluctance (VR) type.
  2. Permanent Magnet (PM) type.
  3. Hybrid type , a combination of VR and PM.
Here we will study only about Variable Reluctance type. Before moving into it , we will focus on a new term called STEP ANGLE.

STEP ANGLE - It is the angle by which the rotor rotates when an input pulse is applied to stator.
Resolution can be defined as number of steps it makes in one revolution of the rotor
Smaller the step angle , higher is the resolution of the positioning of the motor.

VARIABLE RELUCTANCE (VR) STEPPER MOTOR

The principle operation of such motor is based on the property that flux lines always travels/moves through the low reluctance path. The stator and rotor are aligned in such a way that magnetic reluctance is minimum. Variable reluctance motor can be single stack or multi-stack type.
SINGLE STACK VARIABLE RELUCTANCE MOTOR
A variable reluctance motor has a salient pole type stator and winding are placed over it. Stator coils are excited  from a dc source with the help of semiconductor switches , a magnetic field is produced. Due to this the rotor occupies the position which represents the path of minimum reluctance. That is, rotor axis aligns itself to the stator field axis.
Stepper motor , types of stepper motor , variable reluctance stepper motor , single stack reluctance motor
STEPPER MOTOR
As shown in figure above, consider a variable reluctance motor with 4 poles on stator and 2 poles on rotor. Four stator poles A,B,C and D are connected to the dc source with the help of semiconductor switches. These switches are switched in sequence A,B,C,D,A.....
When the winding A is excited , the rotor aligns with the axis of phase A. The rotor gets stable there and cannot move until A is de-energized. Next phase B is excited and A is disconnected . The rotor moves through 90 degree to align with the axis of pole B and in such way this sequence goes on.
The rotor completes one revolution in the 4 steps and rotor moves a step of 90 degree after each pole transition.
Direction of the rotor can be reversed by changing the sequence of the switching i.e. A,D,C,B,A... . Direction does not depend upon the magnitude of the current.
The step angle and resolution of the Stepper motor depends upon the number of poles on the stator and rotor.
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Tuesday, August 7, 2018

August 07, 2018

Construction , Working And Features Of Synchronous Motor

In this post we will study and understand about the various aspects of Synchronous Motor.
Synchronous motor is a type of an AC motor in which the speed of the motor is independent of load and it is synchronized with the frequency of the supply current.
If the frequency of the supply current is kept constant, then it will run at constant speed which is also called synchronous speed.
Synchronous speed is given by:
N = (120*f)/P
where, N = Synchronous speed.
              f = frequency of supply current.
              P = Total number of poles.

CONSTRUCTION OF SYNCHRONOUS MOTOR

Like all motors , it has two main parts called Stator and Rotor.
Stator of the motor carries the 3-phase winding while the rotor is supplied DC.
Stator core is made of thin lamination's to avoid Eddy Current losses.
There are two types of rotor construction
1. Cylindrical type rotor.
2. Salient pole rotor.

WORKING OF SYNCHRONOUS MOTOR

Synchronous motor is doubly excited motor. 3-phase supply is provided to stator while DC is provided to rotor winding.
3-phase supply in stator winding produces a rotating magnetic field while the DC supply produces a constant flux.
Since the rotating magnetic field rotates depending on the frequency. If the frequency is 60 Hz , it means that magnetic field is rotating at 3600 revolutions per minute. At any instant, the rotor poles and the stator poles can achieve same polarity like N-N or S-S , which will result in the repulsion between the poles and at the next instant the poles can achieve same polarity which will result in the attraction between poles.
But due to the inertia of the rotor and rapid attraction and repulsion between the poles, the rotor remains standstill and doesn't moves. That's why synchronous motor are not self-starting.
It is started by some external means and the motor starts to run in the same direction. This causes the magnetic locking between the both fields and the motor continues to rotate even after removing the external means. Since then, due to magnetic locking, motor continues to rotate at constant speed called synchronous speed.

MAIN FEATURES OF SYNCHRONOUS MOTOR

Main features of synchronous motor are ;
1. It runs either at synchronous speed or does not rotate at all.
2. It can work on large range of power factor both lagging and leading, therefore finds its application in power factor improvement.
3. It is not inherently self-starting. It is to be started by some external means.
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Thursday, June 21, 2018

June 21, 2018

Transformer - Introduction & Basic

What is an electrical transformer? 
An electrical transformer is a static device which consists of two stationary electrical circuits connected to each other by a common magnetic circuit for the purpose of transferring electrical energy between them.
This transfer of electrical energy takes place without the change in frequency.
Transformer works on the principle of Electromagnetic Induction. Electromagnetic Induction is one of the basic law of Electrical Engineering. You can read about Electromagnetic Induction.

CONSTRUCTION & WORKING PRINCIPLE  OF TRANSFORMER

A simple transformer consists of two coils wounded on a core as shown in the figure.

There is no electrical connection between these two coils.
When an alternating voltage source V is applied to the coil 1, an alternating current I1 flows in it. This alternating current produces an alternating magnetic flux in the core. This alternating flux links with the turns  T1 of the coil 1 and induces an emf E1 by self-induction.
Thus, all the flux produced by the coil 1 also links with the turns T2 of the coil 2 and thereby inducing an emf E2 by mutual induction.
If load is connected to the coil 2, then an alternating current I2 will flow through it and hence electrical energy will delivered to the load without change in frequency.
Coil 1 which receives the electrical energy from the source of a.c supply is called primary winding.
Coil 2 which is connected to the load and delivers the energy to the load is called secondary winding.
Note that the transformer can only work with the alternating source. It cannot be used for transferring electrical energy in DC form because transformer works on the principle of Electromagnetic Induction and it requires that current should be alternating in nature. So, transformer cannot work on DC.

EMF EQUATION OF THE TRANSFORMER

As we have studied that an emf is induced in both the coils or windings. So the magnitude of the emf can be calculated from the emf equation of the transformer which is ;
E = 4.44* fi * f * T
Where , E = Magnitude of emf induced.
             fi = Maximum flux in Weber.
             f  = Frequency in Hertz.
             T = Number of turns in the coil.
With the help of the emf equation, emf induced in the primary as well as secondary winding can be calculated by providing the suitable number of turns.
It shows that winding having higher number number of turns will have higher voltage and hence it is called high voltage  (hv) winding.
Winding having lower number number of turns will also have lower voltage is called low voltage (lv) winding.

TRANSFORMATION RATIO

The ratio of emf induced in the primary winding to the emf induced in the secondary winding is called transformation ratio. It is denoted by 'k'.
k = E1/E2 = T1/T2
Transformation ratio can also be defined as the ratio of number of turns of primary winding and number of turns of secondary winding. It is also called turn ratio.

STEP-UP & STEP-DOWN TRANSFORMER

Transformer in which the secondary voltage is greater than the primary voltage is called step-up transformer.
Step-up transformer has turn ratio smaller than 1.
Transformer in which the secondary voltage is smaller than the primary voltage is called step-down transformer.
Step-down transformer has turn ratio greater than 1.
Transformer having secondary voltage equal to the primary voltage is called isolation transformer or one-to-one transformer.
Such transformer has turn ratio equal to 1.
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Friday, June 1, 2018

June 01, 2018

Types of damping force in Instruments

A Damping Torque/force is one which acts on the instrument only when it is moving and it always opposes the motion of pointer.
Damping force in any electrical instrument is necessary because it brings the pointer at rest quickly. In the absence of damping force, due to inertia of the moving system, the pointer will oscillate about its final position for some time before coming to rest in steady position. 
The degree of damping is adjusted in such a way that it makes the pointer to deflect to the final position without overshooting the final value. 
If pointer overshoots the final value, then in that case it is called dead beat.


Absence or improper damping in any instrument makes the process of measurement from the instrument slow.
It also decreases the accuracy of the instrument

METHODS OF PRODUCING DAMPING FORCE/TORQUE

Damping torque in an instrument can be produced by using
1. Air friction- It consists of a light aluminium piston attached to the moving system of instrument. This piston is placed in a chamber which is closed at one end. The chamber has a small opening. The cross section of the chamber may be rectangular or circular.
2. Fluid Friction- It is similar to the air friction . Due to greater viscosity of oil, the damping is much effective. However, oil damping is not much used because of several disadvantages such as leaking of oil, keeping instrument in vertical position etc.
3. Eddy Current- This form of damping is the most efficient of the three. It consists of thin disc of conducting but of non-magnetic material like copper or aluminium mounted on a spindle which carries the moving system and pointer of the instrument. Disc is placed between the poles of permanent magnet.
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Thursday, January 14, 2016

How Voltage Drop Affect Electrical Equipment and Installations
January 14, 2016

How Voltage Drop Affect Electrical Equipment and Installations

In an electrical installation, determination of voltage drop from the point of supply to the load is very important. Excessive voltage drop on the supply line impacts most electrical equipment but not to the same degree. The effect of voltage drop on some key electrical equipment is given below.

Effects of Voltage Drop

Electric Motors
In an electric motor, the torque is proportional to the square of the supply voltage. Therefore, if the voltage drops the starting torque will also decrease, making it more difficult to start up motors. The maximum torque will also decrease .
Incandescent Lamps
The more the voltage drops the weaker the beam becomes and the light takes on a reddish tone.
Discharge Lamps
In general, they are not very sensitive to small variations in voltage, but in certain cases, great variation may cause them to switch off.
Electronic Appliances
They are very sensitive to
variations in voltage and that is why they are often fitted with internal stabilizer circuitry.
Electromechanical Devices
Devices such as contactors and auxiliary releases have a minimum voltage below which their performances cannot be guaranteed. For a contactor, for example, the
holding of the contacts becomes unreliable below 85% of the rated voltage.
Voltage Drop Limits for Electrical Installations
To limit the problems of voltage drop on electrical equipment, the IEC 60364-5-52 
Electrical installations of buildings. Selection and erection of electrical equipment - Wiring systems 
Annex G states that the voltage drop between the origin of an installation and any load point should not be greater than certain limits specified in the table below expressed with respect to the value of the nominal voltage of the installation:

Type of Installation Lighting (%) Other Uses (%)
A - low voltage installation supplied directly from a public low voltage distribution system 3 5
B - Low voltage installation supplied from a private low voltage supply. 6 8
Notes:
1. As far as possible, it is recommended that voltage drop within the final circuits do not exceed those recommended for type A installations.
2. When the main wiring systems of the installation are longer than 100 m, these voltage drops may be increased by 0.005% per meter of wiring system beyond 100 m without this supplement being greater than 0.5%
3. Voltage drop is determined from the demand by the current using equipment, applying diversity factors where applicable or from the values of the design current of the circuits

Exceptions to the IEC 60364-5-52 standard
A greater voltage drop may be accepted for

  • Electric motors during starting period
  • For other electrical equipment have high in-rush current
Provided that in both cases it is ensured that the voltage variations remains within the limits specified in the relevant equipment standard
The following temporary conditions are excluded from the voltage drop restrictions in the table above:
  • Voltage transients
  • Voltage variation due to abnormal operation
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Saturday, December 26, 2015

December 26, 2015

Double Cage Induction Motor

Double Cage Induction motor Stator Construction
Double Cage Induction motor Stator Construction

Construction

An induction motor with two cage rotor is used for high starting torque. The slotting arrangement for double cage induction motor is as shown in above figure. As the name indicate the double cage induction motor has two winding in rotor. The outer bars consists of rotor bars having low reactance and high resistance. On the other hand, the inner cage consists of rotor bars having high reactance and low resistance.

Working

At start the rotor frequency is high, the outer cage carries most of the current despite its high resistance. The inner cage has low reactance and is mostly ineffective. This gives high starting torque and low starting current. As the motor picks up the speed, the rotor frequency reduces and the inner cage carries most of the current. Under normal running condition, the outer cage and inner cage are in parallel giving low combined resistance and both the cages are active.
When the speed is normal frequency reduces and it is so small that the reactance of both the cages are practically negligible. Hence it has been made possible to construct a single machine which has high starting torque with reasonable starting current which maintains speed regulation and high efficiency.

Torque-Speed characteristics

Torque Speed Characteristic of Double Cage Induction Motor
Torque Speed Characteristic of Double Cage Induction Motor

Advantages

  1. It has high efficiency and good speed regulation.
  2. It gives higher starting torque.
  3. Lower starting current and are cheaper in cost.
  4. They are more robust and are explosion proof since the risk of sparking is eliminated by the absence of slip ring and brushes.
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