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Monday, April 4, 2011

HOPKINSON’S TEST


HOPKINSON’S TEST

AIM:
            To conduct Hopkinson’s test on a pair of identical DC machines to pre-determine the efficiency of the machine as generator and as motor.

APPARATUS REQUIRED:
S.No.
Apparatus
Range
Type
Quantity
1
Ammeter
(0-1)A
(0-20) A
MC
MC
1
2
2
Voltmeter
(0-300) V
(0-600)V
MC
MC
1
1
3
Rheostats
1250W, 0.8A
Wire Wound
2
4
Tachometer
(0-3000) rpm
Digital
1
5
Resistive Load
5KW,230V
-
1
6
Connecting Wires
2.5sq.mm.
Copper
Few




PRECATUIONS:
1.    The field rheostat of the motor should be in the minimum position at the time of starting and stopping the machine.
2.    The field rheostat of the generator should be in the maximum position at the time of starting and stopping the machine.
3.    SPST switch should be kept open at the time of starting and stopping the machine.

PROCEDURE:
1.    Connections are made as per the circuit diagram.
2.    After checking the minimum position of field rheostat of motor, maximum position of field rheostat of generator, opening of SPST switch, DPST switch is closed and starting resistance is gradually removed.
3.    The motor is brought to its rated speed by adjusting the field rheostat of the motor.
4.    The voltmeter V1 is made to read zero by adjusting field rheostat of generator and SPST switch is closed.
5.    By adjusting field rheostats of motor and generator, various Ammeter readings, voltmeter readings are noted.
6.    The rheostats and SPST switch are brought to their original positions and DPST switch is opened.














TABULAR COLUMN:
S.No.
Supply
Voltage
V(Volts)

I1
(Amps)

I2
(Amps)

I3
(Amps)

I4
(Amps)

I1 + I2
(Amps)
Motor
Armature
Cu Loss
W (watts)
Generator
Armature
Cu Loss
W(watts)
Total
Stray
losses
W (watts)
Stray
Loss
Per M/c w/2 (watts)




























AS MOTOR:

S.No.

V
(Volts)

I1
(Amps)

I2
(Amps)

I3
(Amps)

Motor Armature
Cu Loss
W (Watts)


Field Loss
(Watts)

stray losses
/2(Watts)

Total Losses
W
(Watts)

Output Power
(Watts)

Input Power
(Watts)

Efficiency
h%

















AS GENERATOR:

S.No.

V
(Volts)

I1
(Amps)

I2
(Amps)
Motor Armature
Cu Loss
W (Watts)
Field Loss
(Watts)
Stray losses
/2(Watts)
Total Losses
W
(Watts)
Output Power
(Watts)
Input Power
(Watts)
Efficiency
h%

























PROCEDURE:
1. Connections are made as per the circuit diagram.     
2. Supply is given by closing the DPST switch.
3. Readings of Ammeter and Voltmeter are noted.
4. Armature resistance in Ohms is calculated as Ra = (Vx1.5) /I


TABULAR COLUMN:

S.No.
Voltage
V (Volts)
Current
I (Amps)
Armature Resistance
Ra (Ohms)



















FORMULAE:

Input Power                                                  =  VI1 watts
Motor armature cu loss                   =  (I1+ I2)2 Ra watts
Generator armature cu loss           =  I22 Ra watts
Total Stray losses W                        =  V I1 - (I1+I2)2 Ra + I22 Ra  watts.
Stray loss per machine                   =  W/2 watts.

AS MOTOR:
Input Power                                      =  Armature input + Shunt field input
                                                            =  (I1+ I2) V + I3V = (I1+I2+I3) V
Total Losses                                      =  Armature Cu loss + Field loss + stray loss
                                                            =  (I1 + I2)2 Ra + VI3 + W/2  watts
    Input power – Total Losses        
Efficiency h%                                   =  ------------------------------------- x  100%
      Input Power
AS GENERATOR:
Output Power                                   =  VI2 watts
Total Losses                                      = Armature Cu loss+ Field Loss + Stray loss
                                                            = I22 Ra + VI4 + W/2 watts
                Output power                               
Efficiency h%                                   =  -------------------------------------- x 100%
                                                                Output Power+ Total Losses
MODEL GRAPH:




 













RESULT:
Thus Hopkinson’s test is conducted on a pair of identical DC machines the efficiency of the machine as generator and as motor are pre-determined




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