Moving Iron Type Instrument:
• The most common Ammeters and Voltmeters for laboratory or switch board used at power frequencies are the Moving iron instruments.
• M. I. type instrument may be of attraction type moving iron or repulsion type moving iron.
⇒ Attraction type M. I. instrument is shown below in figure.
⇒ In both type (Attraction or repulsion) of MI instrument, the iron vanes are magnetised in such a way by the current that there is always a force of attraction in the “Attraction type” and repulsion in the “Repulsion type”. Thus we c an say Moving iron type instrument are unpolarised instruments i. e. They are independent of the direction in which the current flows.
So, these “MI instruments” can be used on both AC and DC purposes.
Expression for deflecting torque in MI type:
L = inductance of coil.
I = current through the coil.
θ = deflection of the pointer.
And control torque,
Tc = k θ
At balance condition or at final deflectional
Tc = Td
⇒ The scale is non-linear or non-uniform because θ ∝ I2
⇒ Moving iron type instrument measured RMS value of input quantity.
⇒ MI type instruments are less accurate but cheaper.
Electrodynamometer Type instrument:
(i) To generate spring control torque.
(ii) Provide path for current to the moving coil.
Single-phase Electrodynamometer Power Factor Meter:
⇒Let us consider the case of a lagging power factor of cosø .
Deflecting torque acting on coil A is,
TA = KVI Mmax cosø sinθ
θ = angular deflection from the plane of reference
Mmax = Maximum value of mutual inductance between the two coils.
This towque say acts in th eclock-wise direction.
Direction torque acting on coil B is;
lB = KVI Mmax cos (90º – ø )sin (90º + θ ) = KVI Mmax sinø cosθ
This torque acts in the anti-clockwise direction.
Hence at equilibrium,
TA = TB
or KVI Mmax cosø sinθ = KVI Mmax sinø cosθ
cosø sinθ = sinø cosθ
cosø sinθ - sinø cosθ = 0
sin(ø - θ) = 0
(ø - θ) = 0
ø = θ
Watthour Meter :
• It is not often found in a laboratory but it is widely used for the commercial measurement of electrical energy.
• Figure below shows the elements of a single phase watthour meter.
• In the figure above, both coils (current coils and voltage coils) are would on a metal frame of special design, poviding two magnetic circuits.
• A light Al-disk is suspended in the air gap of current-coil field, which causes eddly currents to flow in the disk.
• The developed torque is proportional to the Fieldstrength of the voltage coils and eddly currents in the disk. The no. of rotations of the disk is therefore proportional to the energy consumed by the load in a curtain time internal, and is measured in terms of kilowatt hours (kWh).
• Damping of the disk is provided by the two small permanent magnets located opposite each other at the rim of disk.
Instrument Transformers :
Instrument transformers are employed for ac measurements (current, voltage, power and energy) in conjunction with the relevant instruments. Such transformers are of two types- Current (or series) transformer and potential (or parallel) transformers.
Current transformer is employed whenever the current of an ac circuit exceeds the sage current of the measuring instrument (e.g. ammeter or wattmeter or energy meter).
Potential transformer is employed whenever and wherever the voltage of an ac circuit exceeds the voltage of 750 volts as it is not easy to provide adequate insulation on measuring instruments for voltage above 750 volts.
(a) Current Transformers (CTs)
The Primary of the CT, which consists of only one of a few turns, is connected in series with the load circuit and so carries the load current to be measured. The secondary winding consists of many more turns than the primary and have the instrument ( an ammeter or ammeter and current coil of wattmeter) directly connected across it. In case of current transformer as the load impedance or 'burden' on the secondary is very small, so the current transformer operates on short-circuit conditions. Also the current in secondary winding is not governed by the load impedance on the secondary but depends upon the current flowing in the primary winding (i.e. main circuit current.) The amount of power which the CT handles is small. The product of voltage and current on the secondary side when the CT is supplying the instrument with its maximum rated value of current, is known as rated burden and is measured in volt-amperes (VA).
There are two types of errors introduced by CT into a circuit These are
(i) ratio error (ii) phase angel error.
CT ratio and phase angle errors are given by equations
Ratio error is taken as + ve when the actual ratio of current transformer is less than nominal ratio. In the above equation of ratio error-ve sign indicates that actual ratio of CT is more than nominal ratio.
The ratio error determines the amount of turns compensation to reduce the error to minimum.
(b) Potential Transformers (PTs)
The potential transformers are used for measurement of high voltages by means of low range voltmeter or for energising the potential coils of wattmeters and energy meters. These are also used for energising relays and other protective equipment. The voltage being measured acts cross the primary winding, which has a large number of turns and connected across the circuit. the secondary winding which has a much smaller number of turns is coupled magnetically through the magnetic circuit to the primary winding. The turn ratio is so adjusted that secondary voltage is applied to the primary.
The errors introduced by PT are less pronounced than those introduced by CTs.
Phase angle error,
The ratio error can be compensated by increasing the turns on secondary winding by the appropriate small amount.
• Instrument transformers (IT) are used to measure AC at generating stations, transformer stations and at transmission lines, in conjunction with AC measuring instruments (Voltmeters, Ammeters, Wattmeters, VARmeters etc.)
• IT performs two important functions and they are :
⇒ They serve to “extend the range” of AC measuring instrument, muxh as the shunt or the multiplier extends the range of DC meter.
⇒ Also, they serve to “isolate” the measuring instrument from the HV power line.