dc machine (Generator / motor)
- External part of the machine used to support the entire assembly of the machine.
- Made up of cast iron or cast steel.
- Field produces uniform magnetic field.
- Field consists of magnetic poles, yoke and field windings.
- Poles are the stationary part and consist of pole body, pole shoes and field windings.
- Main poles are made up of steel laminations of 1 to 2 mm thickness.
- Field windings are placed in the magnetic poles.
- When field windings are excited, magnetic flux is produced. This flux cuts the moving armature conductors to produce voltage in them.
- Air gap b/w stator and armature is 0.5 to 1.5 mm.
- Support the field coils placed on the pole body.
- Get uniform flux distribution in the air gap by spreading out the flux.
- Small poles spaced between the main poles and are connected in series with the armature.
- They ensure sparkles operation of the brushes at the commutator under the loaded conditions.
- Inter poles have number of turns with greater cross-sectional area because they carry armature current.
- Armature is rotating part between magnetic poles of field.
- Consists of several slots, teeth, and armature windings. Armature windings are placed in slots.
- Thickness of sheet steel laminations is 0.4 mm to 0.6 mm.
- Slots of rectangular shape are used in dc machines.
- Each armature winding is connected to each segment of commutator.
Commutator and Brushes
- Commutator converts AC voltage to DC across the brushes.
- Made of hard copper segments insulated from each other by mica sheets and mounted on the shaft of the machine.
- Armature conductors are soldered to the Commutator segments.
- Carbon brushes are used to collect or to deliver dc voltage to dc machine.
- Commutator slides over fixed carbon brushes.
- Brushes are made up of carbon and graphite.
- Bush is accommodated in brush holder where a spring presses it against the commutator.
- Support the rotating part and allow its smooth motion with minimum friction.
- Ball bearings are used for small, Roller bearings for medium & Pedestal bearings are used for large dc machines.
simple loop generator
“Whenever a conductor is moved within a magnetic field in such a way that the conductor cuts the magnetic lines of flux, voltage is generated in the conductor”.
Consider a single turn loop ABCD rotating clockwise in a uniform magnetic field with a constant speed.
(i) When the loop is in position no. 1, the generated e.m.f. is zero because the coil sides (AB and CD) are cutting no flux but are moving parallel to it
(ii) When the loop is in position no. 2, the coil sides are moving at an angle to the flux and, therefore, a low e.m.f. is generated as indicated by point 2 in waveform.
(iii) When the loop is in position no. 3, the coil sides (AB and CD) are at right angle to the flux and are, therefore, cutting the flux at a maximum rate. Hence at this instant, the generated e.m.f. is maximum as indicated by point 3 in waveform.
(iv) At position 4, the generated e.m.f. is less because the coil sides are cutting the flux at an angle.
(v) At position 5, no magnetic lines are cut and hence induced e.m.f. is zero as indicated by point 5 in waveform.
(vi) At position 6, the coil sides move under a pole of opposite polarity and hence the direction of generated e.m.f. is reversed. The maximum e.m.f. in this direction (i.e., reverse direction) will be when the loop is at position 7 and zero when at position 1. This cycle repeats with each revolution of the coil.
Note that e.m.f. generated in the loop is alternating one. It is because any coil side; say AB has e.m.f. in one direction when under the influence of N-pole and in the other direction when under the influence of S-pole. If a load is connected across the ends of the loop, then alternating current will flow through the load. The alternating voltage generated in the loop can be converted into direct voltage by a device called commutator. We then have the D.C. generator. In fact, a commutator is a mechanical rectifier.
How commutator produces dc voltage?
- If, somehow, connection of the coil side to the external load is reversed at the same instant the current in the coil side reverses, the current through the load will be direct current. This is what a commutator does.
- Coil sides AB and CD are under N-pole and S-pole respectively.
- Segment C1 connects the coil side AB to point P of the load resistance R and the segment C2 connects the coil side CD to point Q of the load. Direction of current through load is from Q to P.
- After half a revolution of the loop (i.e., 180° rotation), the coil side AB is under S-pole and the coil side CD under N-pole. The currents in the coil sides now flow in the reverse direction but the segments C1 and C2 have also moved through 180° i.e., segment C1 is now in contact with +Ve brush and segment C2 in contact with – Ve brush.
- Commutator has reversed the coil connections to the load i.e., coil side AB is now connected to point Q of the load and coil side CD to the point P of the load.
- Direction of current through load is again from Q to P.
-Introduction to Transformers