Showing posts with label DC Motor. Show all posts
Showing posts with label DC Motor. Show all posts

Back EMF | Back EMF Significance in DC Motor

Back EMF | Back EMF Significance in DC Motor

What is Back EMF in DC Motor?


We know whenever conductor cuts the magnetic field,e.m.f will induce in conductor.This also applies for conductors in armature too.When the armature of a d.c. motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field and hence e.m.f. is induced in them as in a generator. The induced e.m.f. acts in opposite direction to the applied voltage  V (Lenz’s law) and in known as back e.m.f or counter e.m.f. denoted with  Eb. 

The back emf  Eb(= PΦZN/60 A) is always less than the applied voltage V, although this difference is small when the motor is running under normal conditions.

Back EMF in DC Motor Circuit Diagram



Significance of Back EMF In DC Motor:


It is seen in the generating action, that when a conductor cuts the lines of flux, emf. gets induced in the conductor. The question is obvious that in a dc. motor, after a motoring action, armature starts rotating and armature conductors cut the min flux.So is there a generating action exiting in a motor ? The answer to this question is 'Yes'

After a motoring action, there exists a generating action.There is an induced e.m.f in the rotating armature conductors according to Faraday's law of electromagnetic induction. This induced e.m.f. in the armature always acts in the opposite direction of the supply voltage. This is according to the Lenz’s law which states that the direction of the induced e.m.f. is always so as to oppose the cause producing it. In a dc. motor, electrical input i.e. the supply voltage is the cause and hence this induced e.m.f. opposes the supply voltage. This e.m.f tries to set up a current through the armature which is in the opposite direction to that, which supply voltage is forcing through the conductor.

So as this e.m.f. always opposes the supply voltage, it is called back e.m.f. and denoted as Eb Though it is denoted as Eb, Basically it gets generated by the generating action which we have seen earlier in case of generators.So its magnitude can be determined by the emf. equation which is derived earlier. So,

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4 Point Starter Working & Construction

4 Point Starter | Working & Construction of Three Point Starter

In the previous post we have seen how does a 3 point starter works. There is only one difference in construction between  3 point starter and  four point starter. Here the no volt coil and field are not connected in series as in 3 point starter but the NVC is connected to the supply independently through a point called N which can be seen clearly in the below figure.
4 Point Starter | Working & Construction of Three Point Starter

Operational difference between 3 point starter and 4 point starter

In 4 point starter NVC gets supply directly from the mains it doesn't depend on field  which can be seen clearly from the diagram. So this NVC will produce force which helps to keep the handle in RUN position all the time and also the current through NVC can be adjusted through R(series with NVC).

What is the disadvatage of this?

As  NVC is independent of  field  NVC will be in RUN position all the time. Now if the field gets opened while motor is in running condition due to the presence of residual flux motor rotates with dangerously high speed (since N is proportional to 1/ᴓ) this happens because still the handle will be in run position only. But in 3 point starter as soon as the field fails handle comes to off position so motor stops. But in 4 point starter as this is not the case the handle will remain in RUN condition all the time it is not suitable to protect the motor from high speeds.

Over load release is similar as in the 3 point starter. When the current increases more than full load current the OLR which is an electromagnet pulls the arm upwards and the triangular piece shorts the two ends of NVC and it demagnetizes the NVC and motor shuts down automatically. But till the full load current the force of attraction gets balanced with the gravitational force.

Tags:difference between 3 point starter and 4 point starter,4 point starter circuit diagram,4 point starter working pdf,ppt,4 point starter wikipedia
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3 Point Starter Working & Construction

3 Point Starter | Working & Construction of Three Point Starter

A starter is a device which helps in smooth  starting and running of a dc motor(mostly dc shunt,compound motors).More technically,an electrical arrangement which limits the starting current supplied to the motor.

Before going to see the working of a 3 point starter let's see why we need a 3 point starter?

We know that in a dc motor to get back emf the armature has to rotate in a magnetic field,due to generator action in the armature core back emf is produced. This back emf opposes the supply voltage which helps in governing the motor. But initially armature doesn't rotate so back emf will be zero so high amount of current will be drawn.Which can damage the motor as we know resistance of armature is low.So by using a 3 point starter we can start dc shunt or dc compound motor without damage. 

Let,
E=Supply voltage
Eb=Back emf
Ia=Armature current 
Ra= Armature resistance
In running conditions of dc motor
Ia=(V-Eb)/Ra
but initially Eb=0 then Ia=V/Ra
So Ia value will be several times higher than rated current as Ra value is small.
To limit the high initial current we use this 3 point starter.

Construction of Three Point Starter

It's well known that a resistor will limit current flow. Exactly what we are using in this is series of resistors which are divided into sections. Here other parts like no volt coil, over load release are the other two main parts which come into picture during the operation of a 3 point starter.
3 Point Starter | Working & Construction of Three Point Starter
In the above diagram of three point starter you can see 3 points.They are,

L- Line terminal and it is connected to the positive terminal of the supply.
F- Connected to the field winding of motor.
A- Connected to the armature of motor.

Here we can clearly see that this L point is further connected to over load release which is nothing but an electromagnet. This other end of over load release is attached to the starter handle which is fixed at one end and at the other end it is free to move and it moves against the force of spring. You can see the other connection from stud 1 to the no volt coil. And the other end of this NVC (no volt coil) is connected to point F. Now last stud is connected in series with armature as shown figure of three point starter.

3 Point Starter:Working of Three Point Starter

Switch on the supply first. Initially the handle will not be in contact with the resistance. Now after switching on the supply move this handle to stud 1 manually. Now field gets supply through the NVC (no volt coil) as this movement of handle on to the stud 1 will form a closed path. As mentioned earlier armature is in series with the resistances the current  through armature will decrease. Now slowly move this handle over the studs  2,3,4 so resistance will be cut down slowly. We need to cut the resistance after initial operation because after the rotation  of armature back emf will be produced so there will not be any necessity with this resistances. The handle will be always maintained in run position during operation.

Handle Operation In 3 Point Starter 

To know this we should know what does a no volt coil do. While supply is on the field is supplied through NVC. NVC is also an electromagnet it magnetizes when handle comes into contact with stud 1 . To this handle a soft iron piece is attached when all the resistances are cut off this handle is attracted to NVC due to soft iron and remains in this position against the spring force during working condition. In three point starter whenever there is failure in supply or damage in field winding this NVC demagnetizes and handle comes to its initial position due to spring force. As the resistances are in series with armature the high current passage will not be there and the motor is protected if we try to start the motor during any damage.

Working of No Voltage Coil of 3 Point Starter ( or )Over Load Release In Three Point Starter 

Motor gets its suppy through OLR. This is also a electromagnet which magnetizes when supply is on. Under this there is an arm and its fixed to fulcrum.In the 3 point starter under over load it magnetizes and attracts the arm due to this arm moves upwards. There is a triangle shaped piece which gets attached to the two ends of NVC when arm is attracted upwards which shorts the NVC. So now voltage across NVC is zero and hence NVC demagnetizes so handle comes to original position in this way motor is protected automatically. During normal condition the force of attraction gets balanced with gravitational force( that is till full load current is reached).So NVC and OLR acts as protecting devices.

Disadvantages Of Three Point Starter

Here the NVC and field are in series.  To get more than rated speed for a motor we need to add extra resistances which makes current through NVC less and there is a chance that NVC doesn't magnetize appropriately and thus soft iron piece doesn't attract as a result handle will move to initial position thus stopping motor. 
To avoid this we use 4 point starter because in this NVC and field are connected in parallel.In the next article we discuss about advantages & disadvantages of Three point starter,Four pointer starter.

Tags:three point starter wikipedia,difference between 3 point starter and 4 point starter
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Retardation Test or Running Down on D.C Machines

Retardation Test or Running Down on D.C Machines 

In the previous articles we have seen brake test,hopkinson's-test which are essential to find out dc machine efficiency.Now in this article we are going to discuss about retardation test on dc machines.Retardation test is also called as running down test.This is very efficient way to find out stray losses in dc shunt motors.In this test we get total stray losses nothing but combination of mechanical (friction & windage) and iron losses of the machine.
The circuit diagram of retardation test on dc machines shown below.A1,A2 are armature terminals.

Retardation Test or Running Down on D.C Machines

Procedure of Retardation Test on D.C Machines 

The main points in the retardation or running down test are discussed below,

1. Now start the dc machine normally,run the machine slightly above the rated speed by adjusting resistance.
2. After achieving above the rated speed just cutoff the power supply to the armature,but keeping field normally excited.
3. Now wait for some time to fall down of speed below rated,then using the tachometer note down the values of speed (in rpm) and time (in sec).
4. The armature consequently slows down and the amount of kinetic energy present in the armature is used to supply the rotational or stray losses which includes iron, friction and winding loss.

If I is the amount of inertia of the armature ans is the angular velocity.
Kinetic energy of armature = 0.5 Iω².
Rotational losses, W = Rate of change of kinetic energy.
Retardation Test or Running Down on D.C Machines
I=Moment of inertia of the armature.

In retardation test of dc machines, the rotational losses are given by

Retardation Test or Running Down on D.C Machines

We have the formula of stray losses in retardation test of machines,but here moment of inertia (I) of the armature is unknown.To find out I we have two different methods.We need to find dN/dt too.


Determination of dN/dt in retardation test of dc machines


The voltmeter (V1) across the armature will give the value of back e.m.f. of the motor. We know that back e.m.f. is proportional to speed so that we calibrate the  voltmeter  to show the speed reading directly.
Retardation Test or Running Down on D.C Machines
When motor is cut off from the supply, the speed decrease in speed is noted with the help of stop watch.You can observe curve drawn between calibrated values of time and speed.



At any point C corresponding to normal speed, a tangent AB is drawn. Then the value obtained from below can be substituted in the expression for W which can give the rotational looses.

dN/dt=OA(in rpm)/OB(in seconds)


Methods of finding moment of inertia (I) in retardation test

(a) Using Flywheel
(b) Without using Flywheel

(a) Using Flywheel in retardation test

In this method we use the fly wheel whose moment of inertia is I1 to find the I value. In first case retardation test is performed with armature alone and dN/dt1 is determined. In next case,flywheel is employed on the shaft,change in speed, dN/dt2 is noted.Addition of fly-wheel will not materially affect the rotational losses.
Retardation Test or Running Down on D.C Machines
Since the values of I1,   t1  and   t2 are known, the moment of inertia I of the
armature can be determined.

(b) Without using Flywheel in retardation test

Without using flywheel, I is eliminated from the expression by an experiment. First, retardation test is performed with armature alone. The rotational losses are given by;
      W = 0.011 IN dN/dt1

Next the motor is loaded with a known amount of power W' with a brake. For
the same change in speed, dN/dt2 is noted. Then,
Retardation Test or Running Down on D.C Machines
Since the values of W', t1 and t2 are known, the value of W can be determined.

The electric loading in retardation test W' (or extra power loss) is given by;
W' = average voltage x average current = V' I'a.
This is the simple article on retardation test on dc machines.In the nest post we try to share a pdf on retardation test lab manual.

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Hopkinson's Test or Regenerative Test On DC Machines

Hopkinson's Test or Regenerative Test On DC Motor

In the previous post we have seen how to determine the efficiency of DC machines using brake testHopkinson's test is also a test of finding  the efficiency of a dc motor. Hopkinson's test or regenerative test is a full load test and it requires two identical machines which are coupled to each other.In this test two identical d.c. machines mechanically coupled to each other and simultaneously tested.One is operated as generator another one as motor,hence we can find efficiency of two dc machines simultaneously.So output power of dc machines are going to be wasted.The mechanical output of motor given to generator through shaft to shaft mechanical coupling.And generator's electrical power supplied to run the motor,where losses will be supplied by external power source.

If there are no losses in the motor-generator set,the electrical power from the generator and mechanical output from motor are enough to run motor,generator respectively.So no need of any external power supply to the motor.But due to losses, the generator output is not sufficient to drive the motor. Thus motor takes current from the supply to account for losses.

Observe circuit diagram of Hopikinson's test. The two shunt dc machines are connected in parallel. In that two machines,one is started as a motor another one operated as generator.Here the only rotor connections are mentioned,stator connections are not shown for simplicity.

Connection Diagram of Hopkinson's Test

First switch S is kept open. The other machine which is coupled to first will act as load on first which is acting as motor. Thus second machine will act as a generator.With the help of field rheostat speed of the motor is adjusted to normal value.Note down the observed voltmeter readings.With the help of generator field rheostat voltage of the generator is adjusted up to voltmeter reading is zero.This is to make sure generator voltage is having same magnitude and polarity of that of supply voltage.By making this we can prevent heavy circulating current flowing in the local loop of armatures on closing the switch.

Now close the switch S. The two machines can be put into any load by adjusting their field rheostats. The generator current I2 can be adjusted to any value by increasing the excitation of generator or by reducing the excitation of motor. The various reading shown by different ammeters are noted for further calculations.
    
The input to the motor is nothing but the output of the generator and small power taken from supply. The mechanical output given by motor after supplying losses will in turn drive the generator.

Calculation of Efficiency by Hopkinson's Test

Let V = Supply voltage
Motor power Input = V(I1 + I2)
Generator power Input = VI1 

We can determine the efficiency of DC machines in two cases.
Case 1:  Assuming that the efficiency of both the machines are same.
Case 2:  Assuming both the machines has same iron loss, friction loss and windage loss.

Case 1:
Assuming that the efficiency of both the machines are same.

Motor output power = η x Motor Input power
      
                           = η V(I1+I2)

i.e., Motor Input power = Generator Input

Now the Generator output = η x generator Input
                
= η x ηV(I1 +I2)
                       
= η2 V(I1+I2)
                           
VI1 = η2V(I1+I2)

∵ Generator output η =  {I1 / (I1+I2)}

Note: The above expression is used to determine the efficiency satisfactorily perfect for a rough test. If case need to find more accuracy then the efficiency of the two machines can be determined separately using the below expressions.
Case 2:

Assuming both the machines has same iron loss, friction loss and windage loss.

However the iron loss, friction loss and windage loss of both the machines will be same due to both the machines are identical. On this notion we can find the efficiency of each machine.
It is not necessary to assume that the efficiency of both the machines are same. It is due to that both the DC machines don’t have the same armature winding and the field winding. 
Let,
Ra = Armature winding resistance of individual machines.
I3 = Shunt field current Generator G
I4 = Shunt fief current of Motor M

Generator armature copper loss = (I1+I32) Ra

Motor armature copper loss = (I1 + I2 – I42) Ra

Shunt field copper loss in G = VI3

Shunt field copper loss in M = VI4

Power drawn from the DC source is VI2 and is equal to the total losses of motor and generator.
VI2 = Motor and Generator total losses

To get the iron loss, friction and windage loss subtract the armature copper loss and shunt copper loss of both the machine from VI2.

Total losses of 2 machines (M & G) 
                              
   = VI2 – [(I1+I3)2Ra + (I1+I2-I42Ra+VI3+VI4)] = W

To find the individual machine losses divide by 2
i.e. Total losses of each machine = W/2

To find the efficiency of Motor in hopkinson's test

Input motor power = V(I1 + I2)

Total Losses = (I1+I2-I42)Ra + VI4 + (W/2) 
                     
= Wm

Efficiency of Motor ηm = (Input – Losses) / Input
                                     
= [V(I1+I2) – Wm] / [V(I1+I2)]

To find the efficiency of Generator in hopkinson's test

Generator output power = VI1

Total Losses = (W/2) + (I1+I32)Ra + VI3
                   
                   = Wg

Efficiency of Generator ηg = VI1 / (VI1+Wg)

Advantages of Hopkinson's Test or Regenerative Test On DC Motor


The various merits of Hopkinson's test are,

1. This test is very economical because it requires only very small power just to compensate the losses which is very small value when compared to full-load power of the motor-generator coupled system.

2. This is performed at full load condition so we can take flux distortion into account.

3. There is no need for arranging any actual load. Similarly by changing the field currents of two machines, the load can be easily changed and a load test over complete range of load can be taken.
4. This test is better suited in case of large machines.

Disadvantages of Hopkinson's Test or Regenerative Test On DC Motor

The various demerits of Hopkinson's test are,
1. It is a big task to check for two identical machines needed for Hopkinson's test.

2. We can not load two machines equally all the time.

3. It is not possible to get separate iron losses for the two machines though they are different because of their excitation.

4. It is difficult to operate the machines at rated speed because field currents vary widely.

5.The machines are not loaded equally in case of small machines which may lead to difficulty in analysis.
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Brake Test On DC Shunt Motor

Brake Test On DC Shunt Motor 

In this article we are going to discuss about brake test on d.c. motor.Brake test is also a method of finding efficiency of dc motors.We took dc shunt motor as running machine.Brake test also called as direct loading test of testing the motor. Because loading will be applied directly on shaft of the motor by means of a belt and pulley arrangement.

How to conduct brake test on dc shunt motors?

Test Requirements:
1. DC shunt motor
2. Water-cooled pulley 
3. Spring balance

Procedure of Brake Test On DC Shunt Motor 

1. By adjusting the handle of the pulley take different readings of the spring balance.
2The tension in the belt can be adjusted using the handle. The tension in kg can be obtained from the spring balance readings.
3. Adjusting the load step by step till full load, number of readings can be obtained.By increasing the load is slowly, adjust to get rated load current.
4. The power developed gets wasted against the friction between belt and shaft. Due to the braking action of belt the test is called brake test.
5. The speed can be measured by tachometer. Thus all the motor characteristics can be plotted.
You can refer figure for the experimental setup for performing brake test on a d.c. shunt motor,belt & pulley arrangement mounted on the shaft of the motor.

Calculation Of  Brake Test On DC Shunt Motor 

Let           R (or) r= Radius of pulley in meters
               N = Speed in r.p.m.
               W1 = Spring balance reading on tight side in kg
               W2 = Spring balance reading on slack side in kg
So net pull on the belt due to friction at the pulley is the difference between the two spring balance readings.

Net pull on the rope = (W - S) kg  = (W - S) X 9.81 newtons......(1)

As radius R and speed N are known, the shaft torque developed can be obtained as,
Tsh = Net pull X R=(W - S) X 9.81 X R   .....(2)
Hence the output power can be obtained as,Say speed of the pulley is N r.p.m., then,The above equation shows the output power of dc shunt motor in brake test.
  Now let, V = Voltage applied in volts
               I = Total line current drawn in amps.
As we know V,I are input parameters of dc motors in brake test.then,
Pin=V.I Watts  .....(3)
We have output and input.Then why late go and find the efficiency of dc shunt motor.

Efficiency (η)=Output/Input [No units]
From equation (2) & (3)

Advantages Of Brake Test On DC Shunt Motor 

1. Actual efficiency of the motor under working conditions can be found out.
2. Brake test is simple and easy to perform.
3. It is not only for dc shunt motor, also can be performed on any type of d.c. motor.

Disadvantages Of Brake Test On DC Shunt Motor 

1. In brake test due the belt friction lot of heat will be generated and hence there is large dissipation of energy. 
2. Cooling arrangement is necessary to minimize the heat. Mostly in our laboratories we use water as cooling liquid.
3. Convenient only for small rated machines due to limitations regarding heat dissipation arrangements.
4.Power developed gets wasted hence brake test method is little expensive.
5. The efficiency observed is on lower side.

This is the complete theory of brake test on dc shunt motor including laboratory manual.Brake test in one of the most simple way of finding efficiency of any kind of dc machine.you can also refer wikipedia for more info or ask here
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