Three-Phase Alternator Characteristic Tests

Introduction

For the determination of these three characteristics, test power is very limited; a common denominator in all is that the power output is zero.

The power supplied by a three-phase alternator is given by the expression: P = √3 · U · I · cos φ

Which may be zero if any of the three factors are zero.

The current is zero in the vacuum test.

The voltage is zero in the short circuit test.

A power factor of zero is the condition required in determining the reactive characteristic.

Basis of Practice

A three-phase alternator typically consists of an inductor (field winding) based on salient poles, powered by direct current and generally located on the rotor, and a distributed three-phase winding generally located in the stator, called the armature. The armature is the seat of the balanced system of electromotive forces.

Vacuum Test

The vacuum characteristic is determined by this test. It represents the induced EMF (electromotive force) as a function of the excitation current when the load current is zero and the speed is constant.

The assembly is shown in Fig.

Since the measured voltage is line-to-line, the phase voltage (required for the characteristic) is found by dividing by √3.

The resulting characteristic will be similar to that of a DC machine.

Short Circuit Test

The short circuit characteristic shows the change in armature current (I) with the excitation current (Ie) when the machine terminals are short-circuited.

The scheme is shown in Fig.

With the machine running at synchronous speed, the excitation current is increased to a value sufficient to circulate 1.5 to 2 times the rated current in the armature.

Under these conditions, the short circuit characteristic is practically a straight line.

Reactive Characteristic Test

The reactive characteristic curve represents the terminal voltage in terms of the excitation current (Ie) when the machine, operating as a generator, supplies a constant current at zero power factor (purely reactive current).

To determine the reactive characteristic, the alternator is loaded with adjustable inductive reactance. By adjusting both the excitation current and the load reactance, a constant load current at zero power factor is maintained.

Repeating this process for several current values determines the characteristic, which will have a shape similar to the vacuum characteristic.

Equipment Needed

• A three-phase alternator
• A DC motor
• A DC source
• Two field resistors
• A rheostat for the motor starting current
• A voltmeter
• An AC ammeter
• A small-scale DC ammeter
• Adjustable three-phase reactance
• A tachometer
• A frequency counter