2 measuring instruments(dynamometer type & other)

8/10/2019 2 Measuring Instruments(Dynamometer type & other)

1/24

Dynamometer Type Instruments

Electrodynamic type instruments are similar to the PMMC-type elements exceptthat the magnet is replaced by two serially connected fixed coils that produce

the magnetic field when energized


2/24

i1

i2

PointerMovable

Coil

Scale

Fixed Coil


3/24

Advantages of Dynamometer type Instruments:

Can be used on both DC & AC circuits.

Uniform scale. High degree of accuracy can be obtained.

Disadvantages of Dynamometer type Instruments:

At low power factors, the inductances of the potential coil

causes serious errors.

The reading of the instrument may be affected by stray fields

acting on the moving coil.


4/24

Principle:

When AC current flows through two suitablylocated coils (current coil & potential coil)

They produce rotating magnetic field which is cut by themetallic disc suspended near to the coils, thus an e.m.f.isinduced in the disc which circulates eddy currents in it.

By the interaction of rotating magnetic field& eddy currents, electromagnetic torque is developedwhich causes the disc to rotate.

It is Used on AC system only.

INDUCTION TYPE SINGLE PHASE ENERGY METER


5/24

Fc1 = Friction Compensators

PFC = Power factor compensator

CC = Current coil

VC = Voltage coil


6/24

Construction:

1. Driving System

i) Series Magnet

ii) Shunt Magnet

2. Moving System

3. Braking System

4. Recording Mechanism


7/24

Series Magnet:

The series electromagnet is energized by a coil, knownas current coil which is connected in series with the load so that itcarry the load current.

The flux produced by this magnet is proportional to, and in phase withthe load current.

Shunt Magnet:A coil having large number of turns of fine wire is wound on themiddle limb of the shunt magnet.

This coil is known as pressure or voltage coil and is connected acrossthe supply mains. This voltage coil has many turns and is arranged tobe as highly inductive as possible.

This causes the current, and therefore the flux, to lag the supplyvoltage by nearly090.

DRIVING SYSTEM:


8/24

It consists of a light aluminum disc mounted on a vertical

spindle.

The disc is positioned in the air gap between series & shunt

magnet.

The spindle is supported by a cup shaped jewelled bearing at

the bottom end & has a spring journal bearing at the top end.

Since there is no control spring, the disc makes continuous

rotation under the action of deflecting torque.

MOVING SYSTEM


9/24

A permanent magnet positioned near the edge of the aluminumdisc, forms the braking system.

When the disc moves in the field of the braking magnet, flux is

cut & eddy currents are induced in the disc.

The direction of induced currents is such that it opposes the

rotation, thus braking torque is produced.

Since the induced currents are proportional to the speed of disc

(N), therefore braking torque (Tb) is proportional to the disc

speed. i.e. TbN

BRAKING SYSTEM


10/24

Recording Mechanism:

The function of recording or registering mechanism is to

record continuously a number on the dial which is

proportional to the revolutions made by the moving

system.

The number of revolutions of the disc is a measure of the

electrical energy passing through the meter.


11/24

REED TYPE FREQUENCY METER


12/24


13/24

The vibrating reed type of frequency meter is one of the simplest

devices for indicating the frequency of an ac source.

The current whose frequency is to be measured flows through the coil

and exerts maximum attraction on the soft iron armature twice during

each cycle.

The armature is attached to the bar, which is mounted on aflexible support. Reeds of suitable dimensions to have natural

vibration frequencies of 110, 112, 114, and so forth up to 130 cps are

mounted on the bar.

The reed having a frequency of 110 cps is marked "55"cycles; the one having a frequency of 130 cps is marked "65" cps; the

one having a frequency of 120 cps is marked "60" cps, and so forth.


14/24

When the coil is energized with a current having a frequency

between 55 and 65 cps, all the reeds are vibrated slightly; but the reed

having a natural frequency closest to that of the energizing current(whose frequency is to be measured) vibrates through a larger amplitude.

The frequency is read from the scale value opposite the reed having the

greatest amplitude of vibration.


15/24

Power Factor Meter

INDUCTION TYPE INSTRUMENT


16/24

INDUCTION TYPE INSTRUMENT


17/24

A power factor meter is a type of electrodynamometer movement

when it is made with two movable coils set at right angles to each other.

The method of connection of this type of power Factor meter, in a 3f circuit,is shown in Figure . The two stationary coils, S and S1, are connected

in series in Phase B.

Coils M and M1 are mounted on a common shaft, which is free to

move without restraint or control springs. These coils are connected with their

series resistors from Phase B to Phase A and from Phase B to Phase C.

At a power factor of unity, one potential coil current leads

and one Lags the current in Phase B by 30; thus, the coils are balanced

in the Position shown in Figure .

A change in power factor will cause the current of one potential coil

to become more in phase and the other potential coil to be more out

of phase with the current in Phase B, so that the moving element and pointer

take a new position of balance to show the new power factor.


18/24

Two-wattmeter Method of Power Measurement in a Three-phase Circuit

The connection diagram for the measurement of power in a three-phase circuit

using two watt meters, is given in Figure

This is irrespective of the circuit connection star or delta.


19/24

The circuit may be taken as unbalanced one, balanced type being only a special

case.


20/24

Power measurement using one wattmeter only for a balanced load


21/24


22/24


23/24


24/24

2 measuring instruments(dynamometer type & other)

Documents