EXPERIMENT 8

8.1Object: No-Load Test on a Separately Excited D.C. Generator (Magnetization Characteristic)

8.2Experimental Setup

A separately excited D.C. generator coupled mechanically with a motor is used to obtain a magnetization characteristic. The generator is run at rated speed, and the open circuit voltage is measured by a voltmeter. Field current is measured by an ammeter in the separately excited winding of the generator.

Figure 8.1

The ranges of the ammeter and voltmeter will depend on the specifications of the generator. The no-load voltage will go on increasing with field current i_f and will reach a maximum point and become constant due to saturation. The field current i_f is increased from zero to some value till e_a becomes constant. The i_f is then decreased and e_a is measured correspondingly. The i_f is brought to zero and e_a is noted. Experimental remits are tabulated.

Figure 8.2

This experiment is made to find the voltage induced in the machine. The induced voltage will be

ea = Kif

(8.1)

The constant K of the machine can be obtained as follows:

EXPERIMENTAL RESULTS (if increasing)

A graph between i_f and e_a is plotted as shown in Figure 8.2. It is called an initial magnetization curve, from point X and Y. The decrease of voltage e_a with i_f is also plotted to obtain curve YZ.

The magnetization curve is shown in Figure 8.2. The curve XY is called the open circuit characteristic of a separately excited generator.

EXPERIMENTAL RESULTS (if decreasing)

8.3Discussion

The experiment is made to find a constant K volts per field ampere for the machine, maximum voltage e_{a max} and the maximum excitation current i_{f max}. The YZ curve encloses an area that will give us the hysteresis loss for a half cycle of field current reversal.

8.4Questions and Answers on the Experiment

Q1. What information do we obtain from the magnetization curve?

Ans. A value of generated EMF is obtained at a given field current and speed. The EMF varies with field current i_f and speed; we have

ea =Kif, K = ea/if

(8.3)

K is a constant of machine volts per field amperes. The determination of constant K is obtained. A voltage due to residual flux is also obtained.

Q2. How does the magnetization curve change when we change the speed?

Ans. The curve as a whole rises with the increase of speed and goes down with decrease of speed.

Q3. What factors affect the shape of the magnetization curve?

Ans. The number of turns on poles, size, shape, and materials of the poles, air gap length, and rotor core dimensions and material affect the shape of the magnetization curve.