V-curves and inverted V-curves of synchronous motor.

Experiment No.: – 07

Title of the Experiment: –

V-curves and inverted V-curves of synchronous motor.

Objectives: –

To be written by student.

Apparatus Required: –

Sl. No.	Name	Specifications	Quantity
01	Ammeter	(0-2)A, MC	1 no.
02	Ammeter	(0-10)A, MI	1 no.
03	Voltmeter	(0-500) V, MI	1 no.
04	Rheostat	(0-80) Ω, 5A	1 no.
05	Tachometer	Digital type	1 no.
06	Wattmeter	(0-750) W, 10 A, 600V	2 nos.
07	Universal Motor	1440 rpm, 400V, 6A	1 no.
08	Connecting wires	PVC Insulated copper	As per required

Circuit Diagram: –

Theory: –

When the excitation is normal, the power factor is unity and the armature current is minimum for excitation greater than the normal excitation the value of armature current increases and the power factor is lagging for excitation less than the normal excitation, the value of armature current also increases but the power factor is lagging. The curve between power factor and field current is known as inverted V curve.
with constant mechanical load on the synchronous motor, the variation of field current changes the armature current drawn by the motor and also its operating power factor. As such the behaviour of the synchronous motor is described below under three different modes of field excitation.

Normal excitation:
The armature current is minimum at a particular value of field current, which is called the normal field excitation. The Operating power factor of the motor is unity at this excitation and thus the motor is equivalent to a resistive type of load.

Over-excitation:
This occurs when the field current is higher than necessary for the given load in a synchronous motor over-excitation causes the motor noter to draw leading reactive power from the electrical supply. This can lead to an increase in the power factor & can be used to correct a lagging power factor in the system.

Under-excitation
Conversly under-excitation happens when the field current is insufficient for the load in this case, the motor draw’s lagging reactive power from the electrical supply, while some degree of under-excitation is normal during transient condition, prolonged under-excitation can lead to significant decrease in the power factor and can cause instability in the motor operation.

Procedure: –

1. Connect all apparatus as per the Circuit diagram.
2. The TPST switch is closed and the motor is started. by using DOL starter.
3. The DPST switch on the field side is closed.
4. The field current is varied by varying the field rheostat and the corresponding value of line voltage, Line current and wattmeter readings are noted.

Observation Table: –

Sl. No.	Line Voltage (VL) in volts.	Line Current (IL) in amps.	Field Current (If) in amps.	W1 in watts.	W2 in watts.	Power Factor= Cosθ
1
2
3
4
5
6
7
8
9

Calculation:

Precautions: –

1. There shouldn’t be any loose connections.
2. The readings must be taken without any parallel error.
3. The motor field rheostat should be in minimum resistance position.

Conclusion: –

To be written by student.