Electrical Measurements and Measuring Instruments

Frequency Meters

Frequency MetersIntroduction: Frequency of the local supply can be measured

by many methods simply as from alternator.f=NP/120Where P is no: of poles of alternator and N is the rotor speed in R.P.M.

A.C Bridges are also used for the measurement of frequency.

Frequency can also be measured by portable instruments called frequency meters.

Types of Frequency Meters The different types of frequency meters are :

I. Electrical Resonance TypeII. Mechanical Resonance Type (Vibrating Reed

Frequency Meter)III. Weston TypeIV. Ratiometer TypeV. Saturable Core TypeVI. Digital Frequency Meter

Vibrating Reed Frequency Meter

Vibrating Reed Frequency Meter

Vibrating Reed (contd.)•It is works on the principle of mechanical resonance of thin flat steel reeds arrange alongside and close to an electromagnet, that is why it is known as vibrating reed frequency meter.•It consists of thin metal strips called as REEDS. These reeds are placed in a row closed along an electromagnet.•Electromagnet has a core and a coil connected across the supply whose frequency is to be measured

Vibrating Reed (contd.) All reeds are approx 4mm wide and 0.5mm thick.

Carrying different weights or flags at their top. The reeds are so designed that natural frequency

of one reed differs by one or half a cycle from that of reed next to it.

So if a frequency meter has range from 47-53 Hz then natural frequency of the first reed will be 47 Hz, 47.5 Hz for the second, 48 Hz and so on.

The reeds are fixed at the bottom and free to move at the top (flags).

Reeds on a frequency meter are arranged to be viewed and on they have a portion bent over at free end to serve as a flag.

Vibrating Reed (contd.) When instrument is connected across the supply to

measure the frequency an alternating flux sets up. Due to this field an attractive force is experienced on the

reeds after every half cycles. So all the reeds tend to vibrate. But only the reed whose natural frequency is double to

that of supply frequency will vibrate with maximum amplitude, due mechanical resonance.

The vibration of other reeds so slight as to be unobservable.

If adjacent reeds vibrate with same amplitude then the frequency will be half way between the frequency of the two reeds.

Electrical Resonance Type Frequency Meter

•It consists of iron core of varying cross-section.•A fixed magnetizing coil mounted at one end of the iron core and a moving coil with a pointer attached to it.•It is pivoted so that it can move freely over the iron core.

Electrical Resonance Type Frequency Meter

The moving coil is connected across a capacitor and the fixed coil is connected across the supply circuit whose frequency is to be measured.

When magnetizing coil is connected across the supply circuit magnet field will set up in phase with the current and e.m.f lagging 90o behind the flux is induced in the moving coil.

The current in the moving coil lags or leads the induced e.m.f depending on the circuit of moving coil that whether it is capacitive or inductive and in phase when XL=XC.

Electrical Resonance Type Frequency Meter

The torque on the moving system is given as:T=I i cos (90o+Ɵ); where

I= The Magnetizing Coil CurrentI= The Moving Coil CurrentƟ= The phase angle between current and e.m.f induced in the

moving coil.XL=XC so, Ɵ = 0; hence no torque will be induced on the coil and

coil will become stationary at some point.XC is constant at any particular frequency.XL is dependent on the position of the moving coil from the

magnetizing coil.

Electrical Resonance Type Frequency Meter

Phasor Diagrams

Electrical Resonance Type Frequency Meter

When frequency is higher so XC is reducedso a torque will be produced on the coil to push it away from the magnetizing coil until XL=XC

When frequency is lower so XC is increasedso a torque will be produced on the coil to pull it towards the magnetizing coil until XL=XC

Weston Type The action of this type meter depends on

variation in current in two parallel paths one inductive and other non inductive when frequency changes.

Weston Type

Weston Type It consists two fixed coils A & B placed at 90o to each

other and a thin and soft iron needle is placed in between the two coils.

Coil A is connected in series with inductance LA and across resistance RA.

Coil B is connected in series with resistance RB and across inductance LB .

Inductance L is connected to suppress the harmonics. When frequency is normal currents in both coils A & B

are equal so needle tends to align its axis in between the two coils.

Weston Type If frequency is higher then current in coil B

increases than coil A so its magnetic field becomes stronger and needle tends to align its axis parallel to coil B hence showing higher frequency on the scale.

If frequency is lower then current in coil A increases than coil B so its magnetic field becomes stronger and needle tends to align its axis parallel to coil A hence showing lower frequency on the scale.