1. DC Machines Fundamentals
DET 205
Chapter 2
DC Machines Fundamentals

2. There are five major types of DC generators:
Separately excited generator. In a separately excited generator, the field flux is derived from a separately power source independent of the generator itself.
Shunt generator. In a shunt generator, the field flux is derived by connecting the field circuit directly across the terminals of the generator.
Series generator. In a series generator, the field flux is produced by connecting the field circuit in series with the armature of the generator.

3. 4. Cumulatively compounded generator
4. Cumulatively compounded generator. In a cumulatively compounded generator, both a shunt and a series field are present and their effects are additive.
5. Differentially compounded generator. In a differentially compounded generator, both a shunt and a series field are present, but their effects are subtractive.
DC generators are compared by their voltages, power ratings, efficiencies and voltage regulations.
Voltage regulation, VR is defined by

4. Separately Excited Generator

5. Example 2.1
If no load voltage of a separately-excited dc generator is 135V at 850 r/min, what will be the voltage if the speed is increased to 1000 r/min?
Assume constant field excitation.

6. Solution 2.1
Constant field excitation means; if1 = if2 or constant flux; 1 = 2

7. Shunt DC Generator

8. Series DC Generator

9. Total Magnetomotive force
The Cumulatively Compounded DC Generator
Total Magnetomotive force
Cumulatively compounded dc generator with a long shunt connection
Cumulatively compounded dc generator with a short shunt connection

10. Differentially Compounded DC Generator
With a long shunt connection
Equivalent shunt field current,

11. Example 2.2
A short-shunt compound generator delivers 50A at 500V to a resistive load. The armature, series field and shunt field resistance are 0.16, 0.08 and 200, respectively.
Calculate the armature current if the rotational losses are 520W, determine the efficiency of the generator

12. Solution 2.2 Armature Copper Loss Series Field Copper Loss
Shunt Field Copper Loss
Friction + Stray + windage + etc:
Total Losses =

13. Efficiency, η =

14. The equivalent circuit of the separately-excited dc motor
Assignment 2.1
QUESTION 1
The equivalent circuit of the separately-excited dc motor
Figure above shows fixed field voltage VF of 240V and armature voltage VA that can be varied from 120 V to 240 V.
What is the no-load speed of this separately-excited dc motor when Radj = 175 and
a) VA = 120V, b) VA = 180V, c) VA = 240V

15. The equivalent circuit of the shunt dc motor
QUESTION 2
Assignment 2.2
The equivalent circuit of the shunt dc motor
If the resistor Radj is adjusted to 175, what is the rotational speed of the motor at no-load conditions?
Assuming no armature reaction, what is the speed of the motor at full load? What is the speed regulation of the motor?

16. The magnetization curve for the dc motor of Question 1 and Question 2
The magnetization curve for the dc motor of Question 1 and Question 2. This curve was made at a constant speed of 1200 r/min.

17. Power flow and losses in DC machines
DC generators take in mechanical power and produce electric power while DC motors take in electric power and produce mechanical power
Efficiency

18. The losses that occur in DC machine can be divided into 5 categories
Copper losses (I2R)
Brush losses
Core losses
Mechanical losses
Stray load losses
Ia = armature current
If = field current
Ra = armature resistance
Rf = field resistance
VBD = brush voltage drop
- Usually assumed to be 2V

19. Power Losses Core losses – Hysteresis losses and Eddy current losses
Mechanical losses – The losses that associated with mechanical effects.
Two basic types of mechanical losses: Friction & Windage.
Friction losses caused by the friction of the bearings in the machine.
Windage are caused by the friction between the moving parts of the machine and the air inside the motor casing’s
Stray losses (Miscellaneous losses) – Cannot placed in one of the previous categories.

20. The Power Flow Diagram
Pout = VTIL
For generator

21. The Power Flow Diagram
For motor

22. Equivalent circuit of DC generator
Vf = field voltage Rfw = rheostat resistance
If = field current Rf = Rfc + Rfw = field circuit resistance
Ra = armature resistance

23. Ea = KФω
where
Ф = flux generated by field current, If
VT = terminal voltage
Ia = armature current

24. Equivalent circuit of DC motor
Vf = field voltage Rfw = rheostat resistance
If = field current Rf = Rfc + Rfw = field circuit resistance
Ra = armature resistance