Watt Meter

MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION � 27

Electrical Measurements (9032) Experiment No. 4

EXPERIMENT NO. 4

(Experiment No. 4 and Experiment No. 15 shall be completed in one practical turn of two hours.)

1.0 TITLE :

To understand the use of Dynamometer type Instrument for

measurement of power by using simple electrical circuit.

2.0 PRIOR CONCEPTS:

• Power in Electrical Circuit.

• Force on a conductor F = B x i x l , Where i is instantaneous current.

• Deflecting torque, controlling torque & damping torque.

3.0 NEW CONCEPTS

Proposition 1 :

In dynamometer type instrument deflecting torque is produced by magnetic effect of electric current.

Control torque is provided by control springs. Damping torque is provided by Air Friction damping.

Proposition 2 :

In a dynamometer type wattmeter the fixed coil (current coil) is connected in series with the load.

This coil is divided in to two parts and they are kept parallel to each other. The coil is thick in cross

section and has less number of turns.

The moving coil (pressure coil) is connected across the load. It is thin in cross - section and has

hundreds of turns. It has a non - inductive high resistance in series with it.

Concept Structure 2 :

Fig. (1) Dynamometer type Instrument

28 � MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION


F, F - fixed coils

M = moving coil

R = High resistance

Fig. 4.2 Wattmeter connections.

Proposition 3 :

Reading of wattmeter is given by multiplication of voltage across pressure coil (V),

current through the current coil (I) and cosine of the angle between V & I.

Concept Structure 3 :

4.0 LEARNING OBJECTIVES:

Intellectual Skills:

• To understand the principle of dynamometer type instruments.

• To understand working of dynamometer type instruments.

• To identify and list the parts of dynamometer type instruments.

Motor Skills

• Ability to draw the diagram of dynamometer type instruments.

• Ability to handle dynamometer type instruments.



5.0 CIRCUIT DIAGRAM :

6.0 EQUIPMENT'S :

• A. C. Ammeter - 10A

• A. C. Voltmeter - 300V

• Wattmeter - 5A/10A, 300Volts, 625 Watts.

• Lamp load

7.0 STEPWISE PROCEDURE :

Extension of range of wattmeter.

When the two parts of fixed coil are connected in series, lower range for current is obtained. When

they are connected in parallel the current range of wattmeter gets extended.

By providing series resistance of different values the voltage range of wattmeter can be extended.

For low current range i.e. (0-5A), join B1 B2 together.

For high current range i.e. (0-10A), join B1 E2 and B2 E1 together.

Power factor rating of wattmeter :

Wattmeter are normally designed for unity p. f. rating. In such case p. f. rating is not marked on the

meter. Some wattmeter's are designed for lower p. f. rating. This improves precision & reduces

error. The p. f. rating is marked on the meter, the rating should be taken into account while finding

out multiplying factor of wattmeter.



Multiplying factor of wattmeter :

As two different ranges of voltage and current are available for wattmeter to calculate actual power

multiplying factor should be used.

Error in wattmeter due to its connection :

a. When M & C are shorted

Wattmeter Reading = (power consumed by load) + (Power consumed by current coil)

This connection is used for low current, high voltage circuit.

b. When L & C are shorted.

Wattmeter Reading = (power consumed by load) + (power consumed by pressure coil)

This connection is used for high current, low voltage circuit, by selecting a proper connection the

connection error can be reduced.



Compensated Wattmeter :

While measuring power in low p.f. circuits, error due to connection is not negligible. So compensating

winding is used. Compensating winding is a winding connected in series with pressure coil. It has

same number of turns as that of current coil, but has small cross - section, such a wattmeter is

called as compensated wattmeter.

8.0 STEPWISE PROCEDURE :

• Open the given wattmeter and observe current coil, pressure coil, controlling system, damping

system, pivot, magnetic shield etc.

• Show the connections of current coils for different ranges of current.

• Connect the wattmeter in simple electrical (resistive) circuit.

• Set the wattmeter to low current range.

• Switch on the supply take three readings by adjusting suitable current.

• Switch off the supply set the wattmeter to high current range.

• Switch on the supply and take three readings for same current as above.

• Switch off the supply.

9.0 OBSERVATIONS :

Readings for measurement of power

10.0 CALCULATIONS :.



11.0 QUESTIONS :

Write answers to Q…Q…Q… (Question numbers to be allotted by the teacher)

1. Open a dynamometer type wattmeter. Draw a neat diagram showing constructional details

to approximate scale of each part you have observed.

2. Label all parts in the diagram.

3. List the materials used for the following parts of wattmeter -

a. Coils, b. control springs, c. spindle, d. pivot, e. vane used in air fiction damping,

f. resistance connected in series with pressure coil.

4. In ……………….. type wattmeter terminals of………….. coil are marked as M - L and terminals

of pressure coil are marked as ………. (Fill in the blanks & complete it.)

5. In a circuit power is measured with a wattmeter with 15A, 400V, 1500Watts F.S.D. The

reading was 750Watts. What is the power consumed by load?

6. What are low p. f. wattmeter? Why are they used?

7. What are the errors that may occur in the dynamometer type instrument?

8. Wattmeter's are not used for measuring power in D. C. circuits, State true or false. Give

reasons.

9. Identify the symbols and give the difference between them.

10. One wattmeter is rated for 10A, 20A, 150V, 300V, and 600V. The full scale deflection is

1500W. Calculate M. F. for these combination.

11. Why does a dynamometer type instrument requires a magnetic shield?

(Space for answers)



(Space for answer)

Signature of Teacher

Documents

Watt Meter