DC machine is a highly versatile and flexible machine.
It can satisfy the demands of load requiring high starting, accelerating and retarding torques.
If the conversion is from mechanical to electrical energy, the machine is called as Generator.
If the conversion is from electrical to mechanical energy, the machine is called as Motor.
To understand, design and use these machines the following laws must be studied.
Most of the present day machines have one or two electric circuits linking a common magnetic circuit. In subsequent discussions the knowledge of electric and magnetic circuit laws is assumed. The attention is focused on the Faraday’s law and Biot Savart’s law in the present study of the electrical machines.
Application of Faraday's law according to electro mechanical energy conversion results in the generation of both transformer and rotational emf to be present in the coil moving under a changing field. This principle is utilized in the induction machines and a.c. commutator machines. The direction of the induced emf is decided next. This can be obtained by the application of the Lenz’s law and the law of interaction.
Law of induction-Generator action
Law For Motoring Action
Armature Reaction in DC Motor
The effect of magnetic field set up by armature current on the distribution of flux under main poles of a generator. The armature magnetic field has two effects:
(i) It Demagnetizes or weakens the main flux and
(ii) It cross-magnetises or distorts it.
Commutation
Types of DC Machines:
There are two methods of excitation namely, separate excitation and self-excitation.
In separate excitation, the field coils are energised by a separate DC source. The terminals of the winding can be connected across the input voltage terminals or fed from a separate voltage source.
In self-excitation,the current flowing through the field winding is supplied by the machine itself. The field winding can be connected either in series or in parallel with the armature winding
Speed Control of DC Motors
Speed of a DC motor can be varied by varying flux, armature resistance or applied voltage. Different speed control methods for different DC shunt and series methods are there.
Or
Speed Control of Shunt Motors:
Speed Control of Series Motors:
Speed Control Methods
Flux Control Method:
In this method of speed control, Ra and VT remain fixed.
Therefore, from equation
ωm α 1/φ
Assuming magnetic linearity, φ α If
Or,
ωm α 1/If
i.e., Speed can be controlled by varying field current If.
Armature Control Method:
From speed-torque characteristics equation,we know that
For a load of constant torque, if VT and φ are kept constant, as the armature resistance Ra is increased, speed decreases. As the actual resistance of the armature winding is fixed for a given motor, the overall resistance in the armature circuit can be increased by inserting an additional variable resistance in series with the armature.
Voltage control method:
This method is usually applicable to separately excited DC motors. In this method of speed control, Ra and Vt are kept constant.
In normal operation, the drop across the armature resistance is small compared to Eb and therefore: Eb α Vt
Since, Eb = KΦω
ωm is the Angular speed can be expressed as:
ωm = Vt/KΦ
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1. What is a DC machine? |
2. How does a DC machine work? |
3. What are the main components of a DC machine? |
4. What are the advantages of DC machines? |
5. What are the applications of DC machines? |
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