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PMMC Dynamometer

The main types of instruments used as ammeters and voltmeters are permanent magnet moving coil (PMMC), moving iron, electrodynamometer, electrostatic, induction, rectifier, and thermocouple. Permanent magnet moving coil instruments have high accuracy but higher cost, can only be used for DC, and are subject to friction and heating errors. Electrostatic instruments can be used for both AC and DC, have low power consumption, and are free from hysteresis and eddy current losses but are only suitable for low voltages. Rectifier type instruments use a PMMC instrument along with a rectifier.

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130 views16 pages

PMMC Dynamometer

The main types of instruments used as ammeters and voltmeters are permanent magnet moving coil (PMMC), moving iron, electrodynamometer, electrostatic, induction, rectifier, and thermocouple. Permanent magnet moving coil instruments have high accuracy but higher cost, can only be used for DC, and are subject to friction and heating errors. Electrostatic instruments can be used for both AC and DC, have low power consumption, and are free from hysteresis and eddy current losses but are only suitable for low voltages. Rectifier type instruments use a PMMC instrument along with a rectifier.

Uploaded by

Hasibur
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PPT, PDF, TXT or read online on Scribd
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Types of Instruments

The main types of instruments used as


ammeters and voltmeters are:
(i) Permanent magnet moving coil (PMMC)
(ii) Moving iron
(iii) Electrodynamometer
(iv) Electrostatic
(v) Induction
(vi) Rectifier
(vii)Thermocouple
Permanent magnet moving coil (PMMC)
Permanent magnet moving coil
(PMMC)
Deflection torque:
When current, I passes through the
Con
coil then the force in each side of the
coil is:
TC 
F  BIl At b
For N numbers of turns- TC 
F  NBIl Kθ
Advantages:
(i) The accuracy is high.
(ii) Power consumption is very low.
(iii)The scale is linearly divided.
(iv)One instrument may be used for different
ranges of voltage and current.
Disadvantages:
(i) It is only for DC.
(ii) The cost of PMMC is higher than that of
other instrument.
(iii)The friction and heating errors are
present.
Electrostatic
When applied voltage isInstrument
increased the
capacitance current is,
If C is the capacitance between the plates
dq
A d(CV)
B dV dC
i  then,
 store energy
 C 1 CV  V2
dt dt dt
2 dt
F dVP dC
Input energy  Vidt Spring
 V(C V )dt T
dt dt
 CVdV  V dC 2


Electrostatic Instrument

ciple of the convertion of enregy


 Change in store energy 
Mechanical work done
1 2
dC  V dC  CVdV  F.dx
2
C
Advantages:
(i) It can be used for both AC and DC.
(ii) Power consumption is low.
(iii) It is free from hysteresis and eddy
current loss.

Disadvantages:
(i) They are suitable only for low
voltages.
Rectifier Type Instrument

Rs
Supply
PMMC
Instrument
Types of Wattmeter

(i) Electrodynamometer type wattmeter


(ii) Induction type wattmeter
(iii) Electrostatic wattmeter
Electrodynamometer type
wattmeter

C.C
I

P.C
Load
R
) Electrodynamometer type wattmeter
RP
Operation:
An instantaneous torque is given by-
dM
Ti  i1i2
d
Let V and I beFthe BIl
r.m.s values of the
voltage and current being measured.
So the instantaneous voltage across the
pressure coils, v  2 V sin wt
age by an
Measurement of power by using three
Voltmeter

V2

R
Supply V1 V3 Load
V1
V  V  V  2V2V3 cos 
1
2
2
2 2
3 V3
2
 V2  V  2 IRV3 cos 
3
2

2
 V2  V  2 RP (where, P  V3 I cos  )
3
2

2 2
V1  V2 φV 2
P 3
2R V2 =IR
Measurement of power by using three
Ammeter

I1 A1 I3
I2 A3

Supply R
Load
A2
Types of Frequency Meter:
• Mechanical resonance type
• Electrical resonance type
• Electrodynamometer type
• Weston type
• Ratiometer type
• Saturable core type
Electrodynamometer type Frequency Meter
Supply

L1 L2

C2
C1
Fixed
i1 Coil 2 i2
Fixed
Coil 1 i1 +i2
Moving
Coil

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