Measurement of Field resistance and armature resistance of DC Machine.

Experiment No: – 12

Aim of the Experiment: –

Measurement of Field resistance and armature resistance of DC Machine.

Apparatus Required: –

Sl. No.	Name	Specification	Quantity
1	Ammeter	(0-5)A, PMMC	1 nos.
2	Rheostat	(0-80) ohm, 5A	1 nos.
3	Multimeter	Digital	1 nos.
4	Connecting Wire	PVC insulated copper	As per required.

Circuit Diagram: –

Theory: –

On the basis of armature and field winding connection, there are different types of DC motors.

1. DC Shunt Motor:

The armature and field windings are connected in parallel fashion in a DC shunt motor, as indicated in the below fig.

The field winding is made up of a large number of fine wire turns. The wire used for the field winding in a shunt motor has a lower cross-sectional area than the wire used for armature winding. As a result, field winding has a higher resistance than armature winding.

Total current drawn from the voltage source: I=I_a+I_sh

where I_a= armature current, I_sh= Field current = V/R_sh

Since the applied voltage (V) and the field resistance (R_sh) are almost constant, therefore field current (I_sh) remains constant. As the field current is responsible for flux generation so the flux produced in the shunt motor also remains constant. This is why shunt motor is also known as constant flux motors.

Therefore flux, ɸ α I_sh (constant)

supply voltage: V= E_b+I_aR_a

where E_b is back EMF, R_a is armature resistance.

2. DC Series Motor: –

The armature and field windings in a DC series motor are connected in series with one another as shown in below figure.

A DC series motor’s field winding is made up of a few turns wire. As a result , the series field winding resistance (R_se) is significantly lower than the armature resistance (R_a).

As armature and the field winding are in series, therefore,

I=I_a=I_se

Where I is the total current drawn bythe motor, I_a= armature current, and I_se= series field current.

Total supply voltage V is given by

V= E_b+Ia(Ra+R_se)

The flux produced by DC motors is proportional to the field current. The field current in a series motor, on the other hand, is the same as the armature current. As a result, the armature current (Ia) and by extension the field current both are load-dependent. Hence a variable flux motor is a DC series motor.

3. DC Compound Motor: –

In compound motors both shunt and series field windings are present as shown in below fig.. A portion of the field winding is linked across the armature in these motors, while the remaining is connected in series with the armature. There are two types of compound motors.

a. Cumulative compound motor.

b. Differential compound motor.

Cumulative compound motor

The shunt and series field windings in the cumulative compound motor are coupled in such a way that the current flow direction is the same in both field windings, i. e series field flux strengthens the field due to shunt field winding.

Differential Compound motor:

The shunt and series field windings of the differential compound motor are coupled in such a way that the current flow direction in both field windings is opposite i. e series field flux weakens the field due to shunt field windings.

Procedures: –

1. Connect the circuit as per the circuit diagram.
2. Switch ON the supply to the circuit.
3. For measuring armature resistance, vary the resistance in steps using rheostat and note down the readings of ammeter and voltmeter.
4. For measuring field resistance vary the resistance in steps using rheostat and note down the readings of the armature and voltmeter.
5. From the readings, resistance is calculated by using the formula: R=V/I.

Observation Table: –

For Armature Resistance:

Sl. No.	Voltage across armature (V)	Armature Current (I)	Armature Resistance (ohm)
1
2
3
4

For Shunt Field Resistance:

Sl. No.	Voltage across Shunt Field (V)	Shunt Field Current (I)	Shunt Field Resistance (ohm)
1
2
3
4

For Series Field resistance:

Sl. No.	Voltage across Series Field (V)	Series Field Current (I)	Series Field Resistance (ohm)
1
2
3
4

Precaution: –

1. Connection should be tight.
2. For measuring armature and field resistance initially rheostat should be in the maximum resistance position.
3. take readings from the meter very carefully without parallax error.

Conclusion:

Written by student.