1. Electric Machine DC Generator (2)By
Dr. Shorouk Ossama

2. References
Stephen J. Chapman, “Electric Machinery Fundamentals”, 5th Edition, 2012.

3. Introduction
An Electrical Generator is a machine which converts mechanical energy into electrical energy. The energy conversion is based on the principle of the production of dynamically induced e.m.f.

4. Equivalent Circuit of a DC Machine
The DC machine can be represented by:
- The Armature Winding has a resistance, Ra.
- The Field Winding has a resistance, Rf.

5. In addition, the field voltage is related to the voltage by equation:
Where: VT = terminal voltage
Ia = armature current
Ra = armature resistance
In addition, the field voltage is related to the voltage by equation:
Where: Vf = field voltage
If = field current
Rf = field resistance

6. E.M.F Equation of DC Generator
no. of armature conductors
no. of poles
flux per poles
= 2πN/60
no. of parallel paths
Speed
= 2 for wave winding
= P for Lap winding

7. Example 1:
Find the voltage induced in a DC machine that has flux per pole = 20 m Wb. It has 4 poles and is rotating at 1800 rpm. The machine winding is lap wound with 800 conductors.
Solution
For lap winding the number of parallel paths a = 4, N = 1800 rpm
W = 2πN/60 = 2π x 1800/60 = rad/sec

8. Types of DC generators and Its Characteristic
There is no real difference between a generator and a motor except for the direction of power flow. There are five types of DC generators, classified according to the manner in which their field flux is produced:
Separately Excited Generator
Shunt Wound Generator
Series-Wound Generator
Compound-Wound Generator

9. Separately Excited Generator
A generator whose field current is supplied by
a separate external dc voltage source
The Equivalent Circuit:

10. The terminal voltage of a separately excited dc generator can be controlled by changing the terminal generated voltage Ea of the machine. So if Ea increases, VT will increase. This by two possible ways:
Change the speed of rotation. If W increases then Ea = KϕW increases, so VT increase as well.
Change the field current. If Rf is decreased then the field current increases If = Vf/ Rf . Therefore, the flux ϕ in the machine increases so Ea = KϕW must rise too and the VT increase as well.

11. The Characteristics :
Open-circuit
The Generated Voltage/Field Current Characteristics
Load-circuit
The Terminal Voltage/Load Current Characteristics

12. Applications
This type of generators are used only in special cases, such as:
When a wide variation in terminal voltage, or when exact control of the field current is necessary.
Its disadvantage lies in requiring a separate source of direct current.

13. Example 2: Find the terminal voltage induced of a separately excited Dc generator which modified machine constant is 1.5 its following data: Vf = 240 volts, Rf = 120 ohm, Ia = 80 Amp, Ra = 0.02 ohms, N = 1800 rpm

14. W = 2πN/60 = 2π x 1800/60 = rad/sec Ea = K IfW = 1.5 x 2 x = volt The terminal voltage is: VT = Ea – Ia Ra = – 80 x 0.02 = volt

15. Shunt Wound Generator
A generator that supplies its own field current by having its field connected directly across the terminals of the machine.
The Equivalent Circuit:

16. There are two possible ways to control the voltage of this generator:
Change the shaft speed W of the generator.
Change the field resistor of the generator, thus changing the field current.
This type is more complicated than the analysis of a separately excited generator because the field current in the machine depends directly on the machine’s own output voltage.

17. The Characteristics :
Open-circuit
The Generated Voltage/Field Current Characteristics
Load-circuit
The Terminal Voltage/Load Current Characteristics

18. Applications
The shunt-wound generator is the type most used in practice, but the load current must be limited to a value that is well below the maximum value. This avoid variation of the terminal voltage.
Typical applications are with battery charging and motor car generators.

19. Example 3:
A shunt generator has field resistance of 60 ohms. When the generator delivers 60 kw the terminal voltage is 120 volts. Determine:
- The armature circuit resistance.
- The generated emf when the output is 20 kw and the terminal voltage is 135 v.

20. (a) When delivering 60 kw the load current is:
The field current supplied by the armature is:
Therefore, Ia = IL + Lf = = 502 Amp
The terminal voltage is: VT = Ea – Ia Ra Ra = (Ea - VT)/ Ia = ohm

21. (a) When delivering 20 kw the load current is:
The field current supplied by the armature is:
Ia = IL + Lf = = Amp
The generated voltage is: Ea = VT + Ia Ra
= ( x 0.028) = 139 v

22. Example 4:
A shunt generator supplies a 20kw load at 200v through cables of resistance, R =100m ohm. If the field winding resistance, Rf =50 ohm and the armature resistance, Ra =40m ohm, determine:
The terminal voltage
The emf generated in the armature

23. (a) Load current :
Volt drop in the cables to the load:
I R = 100 x 100 x 10-3 = 10v
Hence the terminal voltage:
VT = = 210 v

24. (b) The field current:
Therefore armature current:
Ia = IL + Lf = = Amp
The generated emf is:
Ea = VT + Ia Ra = (104.2 x 0.04) = v

25. Series-Wound Generator
A generator whose field is connected in series with its armature. Since the armature has a much higher current than a shunt field.
The Equivalent Circuit:

26. The Terminal Voltage/Load Current Characteristics
The Characteristics :
Load-circuit
The Terminal Voltage/Load Current Characteristics
In this type the field winding is in series with the armature and it is not possible to have a value of field current when the terminals are open circuited, thus it is not possible to obtain an open-circuit characteristic.

27. At no load, there is no field current so VT is reduced to a small level given by the residual flux in the machine.
As the load increases, the field current rises so Ea rises rapidly. The values of field resistance and armature resistance in a series wound machine are small; hence the terminal voltage VT is very nearly to Ea

28. Applications
Series wound generators are rarely used in practice, but can be used as a booster on dc transmission line.

29. Thanks