BRAK E TEST ON A

DC SHUNT MOTOR

Circuit Diagram:

BRAKE TEST ON A DC SHUNT MOTOR Aim : To obtain the performance characteristics curves of a D.C. shunt motor by conducting

break test on it. Name Plate Details:

S.NO. Specifications Ratings

01 Voltage 220 V

02 Field Current 0.6A

03 Armature current 12A

04 Speed 1500 rpm

05 Power 3.0 H.P.

06 Ins. Class B

07 Duty SI

Apparatus Required:

S.NO Apparatus Required Rating Type Qty.

01 Voltmeter 0-300 V PMMC 1

02 Ammeter 0-15 A PMMC 1

03 Ammeter 0-2A PMMC 1

04 Tachometer 0-10,000rpm Analog 1

05 Rheostat 300Ω/1.2A Wire Wound 1

06 Fuse 15A TCC --

Theory:

• It is a simple method testing DC machines and consists of applying a brake to a water-cooled drum mounted on the motor shaft.

• A rope is wound round the pulley and its two ends are attached to two spring balances S1 & S2. The tension of the rope can be adjusted with the help of swivels.

• The force acting tangentially on the drum is equal to the difference between the readings of the two spring balances.

Let S1 = Readings on spring balance 1 in Kgf.wt. S2 = Readings on spring balance 2 in Kgf.wt. The net force applied on the brake drum is (S1 – S2) Kgf.Wt. If R = radius of the pulley in meters & N = Motor speed in rpm then, Shaft torque, T, developed by the motor is: T = 9.81 × (S1 – S2) × R Nm Output power = (2×Π ×N×T) / 60 Watts Input power = V (Ia + Ish) Watts Efficiency = Output power / Input power

• This test can be used for small motors only, as, in the case of large motors, it is difficult to dissipate the large amount of heat generated at the brake.

• Where the output power exceeds about 2 H.P., or where the test is of long duration, it’s necessary to use water –cooled pulley.

Procedure: 1. The connections are made as per the circuit diagram. 2. Initially the starter is in off position. 3. The field rheostat is in minimum position. 3. 220 DC supply is applied by closing the DPST Switch. 4. The DC motor is started slowly with the starter and brought to the rated speed. 5. Load is applied on the drum gradually in steps by tightening the belt around it. 6. The readings of the ammeter & voltmeter, two spring balances and the speed at every

step are noted. 7. Drum is cooled through out the loading period by pouring water. 8. The experiment is continued till the full load on the motor is impressed. 9. The machine is switched of by opening the DPST switch.

Graph: 1. η Vs O/P 2. Torque Vs O/P 3. Current Vs O/P 4. Speed Vs O/P Precautions: 1. Loose connections are avoided. 2. Rheostat positions are checked before switching on the motor. 3. Readings should be taken without any parallax error. Result: Viva Questions:

1) what is speed regulation? 2) What are the different types of motor? 3) What are the characteristics of D.C shunt motor? 4) What is the condition for maximum efficiency? 5) What are the different methods to reduce the iron losses? 6) What are the application of D.C Shunt Motor?

MAG NETIZATION CHARACTERISTICS OF A DC SHUNT GENERATOR

Circuit Diagram:

MAGNETIZATION CHARACTERISTICS OF A DC SHUNT GENERAT OR

Aim : To determine experimentally the magnetization characteristics of a DC shunt generator

and to determine the critical field resistance Name Plate Details:

Type DC Shunt Motor DC Shunt Generator

Ratings 3.0 HP 3.0 HP

Volts. 230 VDC 230 V DC

Current 13.6 A 13.6A

Exc. Volts. 220 V DC 220 V DC

Exc. Current 1.0A 1.0A

Duty S1 S1

Ins. Class B B

Speed 1500 rpm 1500 rpm

Apparatus:

S.NO Apparatus Required Rating Type Qty.

01 Voltmeter (0-3--) V M.C 1

02 Ammeter (0-3) A M.C. 1

03 Rheostat 230Ω, 1.7 A WIRE WOUND 1

04 SPST Switch 10 A - 1

05 Tachometer 0-10,000 RPM ANALOG 1

Theory: The magnetization characteristics show the relation between the no load generated emf in armature, E0 and the field (or) exciting current, If at a given fixed speed. These characteristics are known as the no load saturation characteristics or open circuit characteristics. The shape of this characteristics curve is practically same for all generators whether separately excited or self excited. On plotting the relation ship between If and E0 a curve as shown in figure is obtained. Due to the residual magnetism in the poles, some emf is generated even when If = 0. Hence, the curve starts a little way up. The slight curvature at the lower end is due to magnetic inertia. It is seen that the first part of the curve is practically straight. This is due to the fact that at low flux densities, reluctance of iron path being negligible (due to high permeability), total reluctance is given by the air gap reluctance, which is constant. Hence, the flux and consequentially the generated emf is directly proportional to the exciting current. How ever at high flux densities, where µ is small, iron path reluctance becomes appreciable and straight relation between E and If no longer holds good, i.e., saturation of poles start. Critical resistance is that maximum value of the shunt field resistance above, which a machine fails to excite. This resistance corresponds to the straight-line position of the magnetization magnetic circuit do not offer any appreciable reluctance to the magnetic flux. Critical speed is that maximum value of the speed of the machine below which the machine fails to excite. At this speed, the O.C.C. is tangential to the shunt field resistance line Procedure: 1. The connections are made as per the circuit diagram. 2. Initially the starter is in off position. 3. The field rheostat of the motor is in minimum position & the generator is in maximum

position. 4. 220 DC supply is applied by closing the DPST Switch. 5. The DC motor is started slowly with the starter and brought to the rated speed. 6. The Voltmeter readings at the zero field current is noted down. The field current is

increased uniformly in steps by changing the position of the field rheostat. 7. The experiment is continued till the field gets saturated. 8. Now, the experiment is continued on for decreasing values of If, in steps by decreasing

the tapping of the rheostat. 9. The machine is switched of by opening the DPST switch.

Graph: 1. (Eg Vs If) Precautions: 1. Connections should be tight. 2. Check the rheostat positions before switching on the motor. 3. Initially the SPST switch should be kept open. 4. Readings should be taken without any parallax error. Result: Viva Questions:

1) what is O.C.C? 2) What do you mean by critical field resistance? 3) What do you mean by critical speed? 4) How do you obtain the O.C.C at any other speed other than rated speed? 5) What the different types of Generators? 6) What are the applications of D.C Shunt Generators?

SWINBURNE’S TEST

Circuit Diagram:

SWINBURNE’S TEST Aim : To perform Swinburne’s test on a given DC Shunt machine and predetermine its

efficiency at any desired load, both as a motor & as a generator. Name Plate Details:

S.NO. Specifications Ratings

01 Voltage 220 V

02 Field Current 0.6A

03 Armature current 12A

04 Speed 1500 rpm

05 Power 3.0 H.P.

06 Ins. Class B

Apparatus:

S.NO Apparatus Required Rating Type Qty.

01 Voltmeter 0-300 V PMMC 1

02 Ammeter 0-2 A PMMC 2

03 Tachometer 0-10,000rpm Analog 1

04 Rheostat 0-300Ω/1.2A Wire Wound 1

05 Fuse 6A TCC --

Theory: It is a simple method in which losses are measured separately and from this, the efficiency at any desired load can be predetermined in advance. The only running test needed is no load test.

This test is applicable to those machines in which flux is practically constant i.e., shunt and compound-wound-wound machines. The machine is run as a motor on no – load at its rated voltage & its rated speed. The no load current ILO is measured by an ammeter. The no-load armature current is (ILO – Ish ) or Iao. Supply voltage = V No load input = VILO Watts.

∴ Power input to armature = V (ILO – Ish) Power input to shunt = V Ish No load power input to the motor supplies: i) Iron losses in core ii) Friction loss iii) Windage loss iv) Armature & field Copper losses. In calculating armature Cu loss, ‘hot’ resistance of armature should be used. Constant losses Wc = (total input) – (armature Cu loss) ⇒ = VILO - (ILO – Ish )

2 Ra Efficiency as a motor: Let Ia is armature current at which efficiency is required.

Input = V(ILO – Ish ), armature Cu loss = aa RI 2

∴Total losses = aa RI 2 + Wc ∴ % mη = (Input – Losses / Input)* 100

Efficiency as a generator:

Full Load Output = V (ILO – Ish )

Armature Copper losses at full load = aa RI *2

Total Losses = Wc + aa RI *2

% gη = Output / (Output + Losses)* 100

Procedure: 1. The connections are made as per the circuit diagram. 2. Initially the starter is in off position & the field rheostat is kept in minimum position. 3. 220 DC supply is applied by closing the DPST Switch. 4. The DC motor is started slowly with the starter and brought to the rated speed. 5. The no load readings of the motor (input current, field current and input voltage) are

noted. 6. The machine is switched of by opening the DPST switch. PROCEDURE TO FIND Ra: 1. Connections are made as per the circuit diagram. 2. 20V DC supply is applied by closing the DPST Switch. 3. Readings of ammeter & voltmeter are noted by varying the rheostat from maximum

position to minimum position. Graph: 1. η Vs O/P ( For Motor) 2. η Vs O/P ( For Generator) Precautions: 1. Loose connections are avoided. 2. Rheostat positions are checked before switching on the motor. 3. Readings should be taken without any parallax error. Result:

Viva questions:

1) what is the significance of swinburn’s test? 2) what are the advantages & disadvantages of the test? 3) why it is not suitable for D.C series motor? 4) What is the purpose of starter? 5) What happened if field is open in D.C motor? 6) Why we have to keep the field rheostat in minimum position?

LOAD CHARACTERISTICS OF A DC SHUNT GENERATOR

Circuit Diagram:

LOAD CHARACTERISTICS OF A DC SHUNT GENERATOR Aim : To determine the load characteristics of a D.C shunt generator. Name Plate Details: Apparatus: Theory: Procedure: 1. The circuit diagram is connected as per connections shown in the circuit diagram. 2. The supply is switched ON and the motor is started with the help of the starter 3. The generator is made to run at its rated speed by adjusting the field regulator of the

motor.

4. Rated voltage of the generator is ;applied at its terminals by adjusting the field regulator of the generator.

5. The load is applied gradually in steps and the readings of the load current, terminal voltage and field regulator of the generator.

6. The speed is kept constant through out the experiment, i.e., its maintained at its rated value by adjusting the field regulator of the motor.

7. The external characteristics are plotted from the observations. 8. The armature resistance is determined with the help of a multimeter. Graph: 1. The load characteristics (V Vs IL) is plotted. 2. The internal characteristics (V + Ia Ra Vs IL) is plotted. Precautions: 1. The field regulator of the motor must be in minimum position while starting the motor. 2. While loading the generator, at each step, the speed of the generator is maintained at its

rated value. 3. The generator should not be overloaded. Results: The initial and external characteristics of the shunt generator are plotted by conducting load test. Conclusions: From the graphs plotted, we infer that the external characteristics lie below the internal characteristics. And since the terminal voltages of these machines remain almost constant, these machines are used for charging batteries and also for supplying constant power. Viva Questions:

1) what is the difference between internal & external Characteristics? 2) What are the applications of D.C shunt Generator? 3) Why the field rheostat of the Generator should be kept in maximum position? 4) What is voltage regulation ? 5) What is residual voltage? 6) What are the losses in D.C Shunt Generator?

LOAD TEST ON DC SERIES GENERATOR

Circuit Diagram:

LOAD TEST ON DC SERIES GENERATOR Aim : To perform load test on a DC series generator and to draw the initial and external

characteristics. Name Plate Details: Apparatus: Theory: Procedure: 1. We should connect the circuit as per the circuit diagram. 2. The motor fields’ rheostats has been kept in its minimum resistance position and no load

has been connected on the generator. 3. The supply is switched on and the motor is started with the help of a starter.

4. The speed of the motor is to be adjusted by controlling the motor field resistance. 5. The load has to be switched on and the ammeter and voltmeter readings are to be noted

down for different values of load. 6. After the generator has been loaded to its rated value, its load is reduced in steps and

finally, the supply is to be switched OFF. Precautions: 1. Loose connections are to be avoided. 2. The load is to be removed gradually in steps and then the motor is to be switched OFF. Results: The internal and external characteristics of a DC series generator have been plotted. . Conclusions: From the graphs plotted, we can infer that a DC series generator has rising voltage characteristics, i.e., with increase in load , its voltage is also increased. And hence such machines aren’t suitable to be used for power supply as the voltage doesn’t remain constant, but are suitable for being used as boosters. Viva questions:

SEPATARION OF LOSSES IN A D.C.

MACHINE

SEPATARION OF LOSSES IN A D.C. MACHINE Aim : To determine suitable tests on the given D.C. shunt machine and determine from the

experiment, the stray losses and separate these into friction, hysteric and eddy current losses.

Name Plate Details: Apparatus: Theory: Procedure: 1. The connections are made as per the circuit diagram. 2. The motor is started slowly using the starter keeping the field and armature rheostats in

minimum and maximum positions respectively. 3. The field current is adjusted to the rated value at no-lead. 4. The armature ckt resistance is reduced in steps while increasing the speed. 5. The readings of the voltmeter, ammeter and tachometer are taken at constant field

current. 6. The experiment is continued till the maximum speed is obtained by cutting out the

complete resistance in armature circuit. 7. The armature rheostat is brought back to its initial maximum position. 8. The motor is stopped. 9. The armature – resistance is measured using a multimetre. 10. The readings are tabulated.

Graphs: The plot of Ws/N versus N is to plotted to find out the intercept and slopes. Precautions: 1. The rheostats are to be kept in proper positions while starting the motor. 2. Loose connections are to be avoided. 3. The field current is to be maintained constant for a particular excitation though the speeds

are varied, by using the armature rhestat. Results: The stray losses at rated field current and a speed of N = 1230 rpm is 80 watts and the loss has been separated into WH = 29.52 watts, WE = 4.555 Watts, WF = 35.67 Watts, WW = 11.834 Watts. Conclusions: From the observations made, we conclude that the stray losses vary with speed and these stray losses can be separated into various other losses such as heysterisis, eddy current, fiction and windage losses which vary with variation in speed increasing and decreasing characteristics. Viva Questions:

SPEED CONTROL OF DC SHUNT MACHINE

Circuit Diagram:

230 DCSUPPLY

DPSTS1

ZZZ

L A F

A

AA

M

6A

3-Point Starter

230/1.7A

230 DCSUPPLY

DPSTS1

ZZZ

L A F

A

AA

M

6A

3-Point Starter

A (0-2)A

52Ohm/5A

V

+

_

+

_

Field Control Method

Armaturre Control Method

SPEED CONTROL OF DC SHUNT MACHINE

Aim : To determine the speed characteristics of D.C. shunt motor by a) Field control b)

Armature control . Name Plate Details: Apparatus: Theory: Procedure: 1. The connections are made as per the circuit diagram. 2. The field rheostat and the armature resistances are kept at minimum and maximum

positions respectively.

3. The starter is started with the help of the starter. 4. Now, for the field control method, the armature rheostat is adjusted such that the

voltmeter reads 200v. 5. Rf is increased in steps thereby decreasing the field current. The values of If and speed

are noted down. 6. Now, for armature control, the field current is held constant at some selected value. 7. The field current is kept constant and the armature rheostats resistance is varied from

minimum value to maximum value in steps. 8. The speed for each value of Va is noted down. Graph: i) The graph of Ish Vs N is to be plotted. ii) The graph of Va Vs N is to be plotted. Precautions: 1. Keep the rheostats in the proper position while starting the motor. 2. Loose connections are to be avoided. 3. Field current should be maintained constant for armature control and armature voltage

should be maintained constant for field control method. Results: The speed characteristics of the d.c. shunt motor have been determined both in the field and armature control method. Conclusions: From the graphs plotted, we can conclude that during the field control method , the field

current varies inversely with speed as φ N

1α and the curve is a rectangular hyperbola.

And during the armature control method, the armature voltage varies in proportion to the speed. And hence the curve obtained is a straight line. Viva Questions:

1) In how many ways , we can control the sped? 2) Compare the speed control methods? 3) What are the different losses in D.C machines? 4) What is the purpose of starter? 5) How do you reduce the iron losses?

HOPKINSONS’ TEST

Circuit Diagram:

HOPKINSONS’ TEST Aim : To perform Hopkinsons’ test on a given motor generator set and determine the efficiency

of both motor and generator. Name Plate Details: Apparatus: Theory: Procedure: 1. The connections are made as shown in the circuit diagram. 2. The rheostat of the motor and generator fields are kept at minimum and maximum

positions and the switch 3. The motor generator set is started slowly with a starter.

4. The field current is adjusted to rated value by adjusting the field rheostat of the motor. 5. The generator is excited by decreasing the generator field rheostat resistance. 6. When the voltmeter reads zero volts the switch K is closed. 7. The generator is loaded in steps by decreasing the field rheostat resistance of the

generator. 8. The speed and readings of all the metres are noted down for each load. 9. The excitation of the generator is reduced by increasing the field rheostat of the generator

and then the switch K is opened. 10. The supply to the motor generator set is switched off. 11. The observations are tabulated. Graph: The output power versus efficiency curves for both the motor and generated are to be plotted. Precautions: 1. Loose connections are to be avoided. 2. The rheostats are to be kept at proper positions while starting the motor. 3. The readings of all the metres are to be noted down without any error. Results: The Hopkinsons’ test on the given motor generator set has been performed and the maximum efficiency of motor has been determined to be 84.53 % and that of generator is 85.12 %. Conclusions: By performing the Hopkinsons’ test, we can determine the efficiency of the motor or generator by obtaining the stray losses from the test. Form the efficiency curves plotted, we conclude that the efficiency of the generator is more than that of the motor. Viva Questions:

BRAKE TEST ON A DC COMPOUND MOTOR

Circuit Diagram:

BRAKE TEST ON A DC COMPOUND MOTOR Aim : To perform Brake test on a given D.C. Compound motor and obtain the performance

characteristics of the motor from the test observation. Name Plate Details: Apparatus: Theory: Procedure: 1. The circuit diagram is connected as shown. 2. The field regulator is kept at minimum resistance position. 3. The belt on the pulley is checked so that theres’ no load acting on the pulley. 4. The motor is started using the starter.

5. The motor is made to run at its rated speed by adjusting the field current. 6. The load is gradually applied on the pulley by tightening the belt around it. 7. The speed, readings of the ammeter, voltmeter and the spring balance readings are to be

noted down at each step. 8. The pulley is cooled through out the experiment by pouring water on to it. 9. The experiment is continued till the full load of the motor is reached. 10. The observations are tabulated. Graph: i) BHP Vs η , ii) BHP Vs Torque iii) BHP Vs Current iv) BHP Vs speed. Precautions: 1. The pulley is cooled while performing the experiment by pouring water into it. 2. The effective radius of the pulley is considered while measuring the radius of the pulley. 3. The field regulator is to be kept at minimum position while starting the motor. Results: The Brake test on the D.C. compound motor has been performed and the performance characteristics have been plotted. The maximum efficiency of the motor has been found to be 78.63 % and the corresponding torque, output, speed and load current are 7.21 N-m, 1117.23 Watts, 1480 rpm and 6.4 A. Conclusions: On plotting the performance characteristics, we conclude that the torque of the compound motor varies linearly with output power. The load current does not increase linearly with output power as the motor contains both the series and shunt field windings. The speed decreases more pronouncedly than the D.C. shunt motor.