Separately Excited Generator

When the field winding is supplied from external, separate d.c. supply i.e. excitation of field winding is separate then the generator is called separately excited generator. Schematic representation of this type is shown in the Fig.1.

Fig. 1  Separately excited generator

       The field winding of this type of generator has large number of turns of thin wire. So length of such winding is more with less cross-sectional area. So resistance of this field winding is high in order to limit the field current.

 

  Voltage and Current Relations :

       The field winding is excited separately, so the field current depends on supply voltage and resistance of the field winding. 

       For armature side, we can see that it is supplying a load, demanding a load current of I_L at a voltage of V_t which is called terminal voltage.

       Now   I_a = I_L

       The internally induced e.m.f. E is supplying the voltage of the load hence terminal voltage V_t is a part of E. But E is not equal to V_t while supplying a load. This is because when armature current I_a flows through armature winding, due to armature winding resistance R_a ohms, there is a voltage drop across armature winding equal to I_a R_a volts. The induced e.m.f. has to supply this drop, along with the terminal voltage V_t. To keep I_a R_a drop to minimum, the resistance  R_a is designed to be very very small. In addition to this drop, there is some voltage drop at the contacts of the brush called brush contact drop. But this drop is negligible and hence generally neglected. So in all, induced e.m.f. E has three components namely,

 

i) Terminal voltage V_t

ii) Armature resistance drop  I_a R_a

iii) Brush contact drop V_brush 

 

       So voltage equation for separately excited generator can be written as,

       E = V_t + I_a R_a + Vbrush

       Where E = (ΦPNZ)/(60A)

       Generally V_brush  is neglected as is negligible compared to other voltages.

Types of DC Generator

1. Separately Excited Generator :

 In a separately excited DC generator, the field winding is excited by an external independent source.

2. Self Excited DC Generator. These are generators in which the field winding is excited by the output of the generator itself. As described before – there are three types of self excited dc generators – they are 1) Series 2) Shunt and 3) Compound.

Q. What is Residual Magnesium?

Q. What is armature reaction in DC Generator ?

Q. What is the reason of the generated emf reduce while increase the load current ?

Brushes Used in DC Generator

Q.Why is carbon used in the brushes in a DC generator?

The inductances of the energized windings, and the mechanics of a brush type motor or generator make arcing and sparking an unavoidable characteristic of the machine at the brush interface. Electrical arcs produce very high temperatures. High enough to melt metals. 

Voltage drop across brushes is 1-2 volts

 

Carbon has a very high melting point (~3500degC) compared with other conductors. Brushes made with copper or steel would wear out faster because of factors including:

1) The melted metal will break away from, or even be vaporized, leaving the brush with less material to do its job.
2) If the melted material doesn't vaporize or otherwise move away from the interface, it creates a mess for the brush. It can weld together bristles of the brush. It can build up material that interferes with the motion of the rotor.
3) As the metal melts in the presence of an arc, conductive material can be liberated, therby prolonging the arc, exasperating the wearing action.

Carbon is less prone to those high temperature effects and associated impacts to the generator operation.

Also, Graphite is conductive and auto-lubricant (low friction coefficient) so, it provides a good dynamic contact to rings with minimum erosion. 


Carbon Graphite - Carbon Graphite brushes have been used in motor applications since the use of actual copper wire brushes that wore away the commutator. This material is mainly used as a base for impregnating metal graphite brushes today. Carbon graphite can be used in lower current density and lower speed applications because of its high resistivity.

Electrographitic – Electrographitic brushes are baked in excess of 2000°C to increase the strength of the material. These can stand up to very high current densities and high speed applications. With a lower resistance, electrographitic brush grades can also tolerate higher temperatures. There are many different grades in this material and it is the most widely used brush on the market today.

Graphite – Graphite brushes are available in several grades with different resistances and strengths needed for specific applications, such as high speed turbo generators and slip ring motors. Generally low to medium current density and lower speed applications use this grade as a cheaper alternative to electrographitic or metal graphite brushes.

Metal Graphite – Metal graphite brushes offer very low resistance and high current density which means a much lower voltage drop across the brush. These are used commonly in low voltage machines that require these specifications such as DC motors, low voltage generators, and slip ring inductions motors. The low resistance also offers less EMI (electromagnetic interference) if that may be a problem for a particular industrial application around sensitive equipment. 

Basic Working Principle of DC Generator

• A DC generator operate on the principle of Dynamic induced emf explain by fleming right hand rule

• Fleming's right-hand rule :  (for generators) shows the direction of induced current when a conductor moves in a magnetic field. It can be used to determine the direction of current in a generator's windings.

• Induced emf in generator : e=B*l*v*sinθ, Here the induced emf is in form of Sine, so generated emf in DC generator is AC instead of DC. To convert in unidirectional DC signal use rectifying as Commutator.

DC Generator

IMP_Question Bank_3rd EE_DCMT-2130904

GTU Question Paper_DCMT-2130904 (Old and New Syllabus)
Basic Working Principle of DC Generator 

Brushes Used in DC Generator 

 

Types of DC Generator