CLIL experienceContent and Language Integrated Learning

4th Fourth lesson

• Another test in order to have motor parameters is blocked rotor test. This test is similar to the short circuit test of transformer.

• Purpose of this test is to determine load dependant losses and stator & rotor reactance and rotor resistance.

• The rotor is blocked to prevent rotation and balanced voltages are applied to the stator terminals at rated frequency. Applied voltage is gradually increased till rated current is achieved.

• Current, voltage and power are measured at the motor input and from this data motor parameters are calculated.

Three-phase induction motorOBJECT: To perform blocked rotor test of 3-phase Induction M.

Theory

Under blocked rotor condition the rotor is at rest and, therefore, the slip is unity. The load resistance R1(1-S)/S is zero. Thus the equivalent circuit of Fig.

IA

Theory

SS

SS IV

Wcos

SSSS WIV cos3

SS

SS IV

W3

cos

In an induction motor, if full voltage is applied across the stator terminals with the rotor not allowed to rotate, about six to eight times its full-load current will flow in the circuit.Blocked-rotor test is conducted at a reduced voltage VS so that current, IS equals to only the full-load current flow through the circuit. The applied voltage VS under blocked rotor conditionis, therefore, about one-sixth to one-eighth of the normal voltage. At this reduced voltage the current I0S flowing through the parallel branch is very small, and can be neglected. Re’ and Xe’ are stationary resistence and reactance in series connected across the applied voltage, VS. If WS is the input power underblocked-rotor condition, IS is the current and cosφS is the power factor, then as a result: SSSS WIV cos3

SS

SS IV

W3

cos

R1 can be measured by ammeter-voltmeter method by applying DC tothe stator winding. To get AC resistance at 50Hz, the value of R1 so obtained is to be multiplied by a factor, say, 1.5. Thus R1 and R2’ ofRe’ can be separated as R2’ = Re’- R1. There is no simple way of separating X1 and X2’ from Xe’ except arbitraly. X1 may be taken equal to X2’ so that X1 = X2’ = Xe’ / 2.Thus the equivalent circuit elements as envisaged in Picture are found out. As mentioned earlier, all calculations regarding the performance of an induction motor can be done using the equivalent circuit with some approximations.

S

Se IVZ

See ZR cos'' See ZX sin''

The impedence

And

Theory

Elaborations

SSSS WIV cos3

WbWaQS 3

S

SS P

Qarctgcoscos

S

Se IVZ

See ZR cos''

See ZX sin''

WbWaWS

SS

SS IV

W3

cos

1''

2 RRR e

2

'

1'2

eXXX

otherwise

Three-phase induction motor

CIRCUIT DIAGRAM

APPARATUS1. Voltmeter2. Ammeter

3. Two wattmeters4. Two Auto transformer

OBJECT: To perform blocked rotor test of 3-phase Induction M.

AIMS•load dependant losses•stator & rotor reactance•rotor resistance

Procedure1. Choose the apparatus and measuring instruments.

2. Choose the connection: star or delta

3. Make the circuit as shown in the picture.

4. Keep rotor of induction motor pressed, so that it cannot rotate even upon energization.

6. start increasing voltage slowly till at the 10% plus of therated current is achieved.

7. Note down the readings of all instruments connected.

8. Repeat the test with decreasing current until zero.

Results

n. IA @ IS VS Wa Wb WS QS cosf S

[A] [V] [watt] [watt] [watt] [var]

1

2

3

4

5

6

78

READINGS ELABORATIONS

ResultsDiagram

VS [volt]

0255075

100125150175200225250275

0 2 4 6 8 10 12 14 16 18 20 22 24

VS [V]

VS

VR

IR Icc

Observation - Results

n. IAz IS VS Wa Wb WS QS cosf S

[A] [V] [watt] [watt] [watt] [var]

1 1,0 7,00 11,0 0,2 11,2 18,7 0,51

2 2,0 15,00 39,0 4,1 43,1 60,4 0,58

3 3,0 25,00 86,0 11,0 97,0 129,9 0,60

4 4,0 36,00 158,0 23,0 181,0 233,8 0,61

5 5,0 44,00 239,0 28,7 267,7 364,3 0,59

7 6,2 56,00 366,0 52,7 418,7 542,7 0,618 7,0 63,00 456,0 65,5 521,5 676,4 0,61

READINGS ELABORATIONS

ResultsWS [watt]

050

100150200250300350400450500550

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0 6,5 7,0 7,5

WS [watt]

Results

cosfS

0,000,100,200,300,400,500,600,700,800,901,00

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0 6,5 7,0 7,5

cosfS

ResultsVS [volt]

0

1020

3040

5060

7080

90

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0 6,5 7,0 7,5

VS [V]

ResultsDiagram result

S

RRCC V

IVI *

VS [volt]

0255075

100125150175200225250

0 1 2 3 4 5 6 7 8 9 1011121314151617181920 21222324252627

VS [V]

VR

VS

IR ICC

36,2456

2,6*220CCI

S

RRCC V

IVI *

Answer to the following questions

1. How will the power factor vary if the voltage supply decreases?

2. To what test is similar the blocked rotor test?3. What are the aims of the blocked rotor test?4. What instrument can the motor speed be measured with?5. How much is the voltage when the current is rated?6. Under blocked rotor condition, how is the slip?7. How much is the percentage on the voltage at blocked rotor

test VS compared with rated current IR?8. Under blocked rotor condition, how is the load resistance

R1(1-S)/S ? 9. What is the procedure in order to carry out blocked rotor test

of 3-phase induction motor?10. How much the current will flow in the circuit when the rotor is

blocked and rated voltage is applied to the stator terminals?

Fill the gaps1) Blocked rotor ……. is similar to short circuit test

of ………..1) The …….. is blocked to prevent ………….2) Applied ………. is gradually increased till rated

…………. is achieved3) From data input of current, voltage and ……… motor

parameters are …………..4) Under ……….. rotor condition the slip is ……. and the

load resistance R1(1-S)/S is ………..

5) The applied ……….. VS under blocked rotor condition is about ………… to one-eight of the normal ………...

6) Under reduced voltage the ………. I0S flowing through the parallel branch is very small, and can be ………….