EE 106 DC Generators

Part 3

• Separately-Excited

• Self-Excited

– Series

– Shunt

– Compound

• Long shunt

• Short shunt

Types of DC Generators

Separately Excited DC Generator

The Magnetization Curve:

Example 1:

Self-Excited Shunt Generator

Equivalent Circuit:

Compound Generator

• Short shunt: • Long Shunt:

Self-Excited DC Generator: Voltage Build-up

Family of Field-resistance Lines and Corresponding Operating Points

Effect of Speed on Voltage Build-up

Example 2:

a) From the curve, 156 V -> 4.7 A (approx.)

b) From the curve, 140 V -> 3.2 A (approx.)

c)

Establishing arbitrary points:

The arbitrary points: (0 A, 0 V) & (2.1 A, 100 V)

Drawing a straight line, the intersection is 130 V.

d) Draw a tangent line (the critical resistance line) and use arbitrary point on the line:

e)

Plotting the above points as portion of the curve, the intersection with the field-resistance line is approximately

116 V.

f)

***End of the Problem ***

Example 3

The Generated Voltage of Compound DC Generators:

Example 4

Series-Field Diverter

Example 5

Losses in DC Generators:

Total Losses

Copper Losses

Mechanical Losses

Iron Losses

• Armature Cu Loss • Field Cu Loss

• Hysteresis • Eddy Current

• Friction Loss • Windage Loss

Power Flow Diagram of a DC Generator

Power Stages and Efficiency of DC Generators

Paralleling DC Generators

Procedure: 1. Close the generator disconnect switch of the incoming

machine. 2. Start the prime mover and adjust it to rated speed 3. Adjust the voltage of the incoming machine to be a few volts

higher than the bus voltage 4. Close the generator breaker. 5. Turn the shunt field rheostat of the incoming machine in the

“raise voltage” direction, and that of the other machine or machines on the bus in the “lower voltage” direction until the desired load distribution is attained. Load distribution is indicated on the respective ammeters; wattmeters are not used.

One-Line Diagram:

Division of Incoming Bus-Load Between DC Generators in Parallel

• For optimum performance, the voltage regulation should be the same.

• For stability of operation, the voltage regulation should be between 3 and 8 percent.

Characteristic Triangle as a Tool for Solving Load-Distribution Problems:

rated

ratedbus

I

VRV

I

V 100

Example 6

Fig. 12.19:

a)

Example 7

- End of Part 3 -