GB2148611A

GB2148611A - Electric generator

Info

Publication number: GB2148611A
Application number: GB08421344A
Authority: GB
Inventors: Philip Oates
Original assignee: Manufacturing Electrical Design Consultants Ltd
Current assignee: Eaton MEDC Ltd
Priority date: 1983-08-25
Filing date: 1984-08-22
Publication date: 1985-05-30
Also published as: GB2148611B; GB8322835D0; GB8421344D0

Abstract

An electric generator has its field assembly and/or its stator provided by at least one electrical winding and at least one permanent magnet. The magnet (34) may be located adjacent to the field coils (32). The rotor pole pieces (12) are received between the stator windings (22) and the field coils (32). Several magnets (34), eg eighteen, are arranged at spaced locations. The generator has a flame- proof casing and its supply to a load is disconnected until a predetermined output voltage is reached. <IMAGE>

Description

SPECIFICATION Electric generator The present invention relates to electric gener ators, such as dynamos and alternators and is especially, but not exclusively, applicable to brush less alternators particularly those of the flame-proof type for use in potentially explo sive or flammable atmospheres such as, for example, those encountered in coal mines.

Alternators are used widely for the genera tion of electrical energy and in cases where the alternator cannot conveniently be excited by an accumulator as a result of fire risk, for example in coal mines, where the use of accumulators of standard types is severely restricted, the alternator often has a rotor which is composed purely of permanent mag nets with no electrical windings.

Such alternators have a number of disad vantages, at least one of these being that they do not begin to excite until running at about 5000 revolutions per minute, which is a significant proportion of the 6000 revolutions per minute typically used in such alternators at normal operating conditions. This means that to operate successfully they must remain loaded substantially all the time. Damage due to insufficient heat dissipation in the alternator (especially if it is within a flame-proof casing) may result should the load suddenly be re duced for a time before the alternator can be disconnected. The limited speed range also -severely limits the applications of such alter nators.

Electric generators of the kind to which the invention relates generally comprise two electric elements, a stator and a field assembly. In some generators the field assembly forms part of the rotor and rotates with it but in other types of generator such as brushless alternators the field assembly is stationary, the rotor having pole pieces which move between the stator and field assembly. The field assembly and/or stator generally comprise either field coils or magnets, though the latter are not used to a significant degree.

According to the present invention there is provided an electric generator in which at least one of the field assembly and/or the stator comprises at least one electrical wind ing together with at least one magnet.

Preferably the field assembly only com prises at least one electrical winding together with at least one permanent magnet, the stator having no permanent magnet.

Preferably the electric generator is an alter nator.

Most desirably the alternator is of the brush less type, the stationary field assembly comprising a field winding and a plurality of permanent magnets. Desirably the stator is provided with 3 windings in "star" configuration.

Where the alternator is intended for use in potentially explosive or flammable atmospheres, a flame-proof casing is preferably provided.

Preferably there is provided means for disconnecting the electrical supply of the alternator to its load until the output voltage is at least about 50%, preferably at least about 90%, of the nominal rated output voltage.

The disconnection means preferably comprises a semiconductor switching circuit. Advantageously said disconnection means is enclosed within the flame-proof casing where appropriate.

Alternators according to the invention can excite at about 1 300 revolutions per minute and will perform satisfactorily up to about 6000 revolutions per minute.

An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings, in which Figure 1 is a part longitudinal cross-section of an alternator; and Figure 2 is a schematic diagram of components of an electrical control circuit for the alternator shown in Fig. 1, and contained within the casing thereof.

The alternator shown in Fig. 1 generally comprises a rotor assembly stator assembly 20 and a field assembly 30 within a flameproof casing 40.

The alternator is of the brushless type, the rotor having pole pieces 1 2 which are received between stator windings 22 and field coils 32 and a series of eighteen equidistantly spaced permanent magnets 34 of the field assembly 30.

The rotor assembly is mounted in the casing by a needle roller bearing 42 at one end and a double seal ball bearing 44 towards the free end of the rotor shaft 14. Electrical connection 26 from the stator windings 22 pass to a rectifier bridge network supported in a mounting block 28. Positive and negative output wires 29 pass via a fuseholder assembly 46 through a gland 48 into a flameproof terminal chamber 50 (only part of which is shown in Fig. 1 ) adjoining the main chamber 52 containing the moving parts of the apparatus. The terminal chamber 50 has walls (except for a cover plate not shown) formed integrally with the walls of the casing 30 which itself has a removable cover plate 41 so that fuses in the fuseholder assembly 46 may be replaced. The casing 40 is of flame-proof construction, all joints between components of the casing having a long flame path.A mounting plate 56 mounts the various electronics components of the control circuit shown in Fig. 2 (these components not being shown in Fig. 1).

Turning now to Fig. 2, the stator windings 22 are wired in a "star" configuration and are connected to a rectifier bridge network 102, the output of which gives a rectified D.C.

output voltage. Outputs are also taken from the rectifier bridge network via respective diodes D1 in parallel and thence to the field coil 106 and to a control circuit generally indicated as 110. The field coil is also connected to the control circuit 110, which has power rails connected between the positive and negative of the rectified D.C. output. The output from the circuit comprising the stator windings and the field coils, the bridge network, the diodes D1 and the control circuit passes via fuses F1 and F2 to output terminals in the chamber 50 (see Fig. 1).

The control circuit 110 comprises principally a pair of resistors R1 and R2 providing a potential divider with a fine-adjustment variable resistor RV1, powering a zener diode ZD1 connected to a switching network comprising three transistors TR1, TR2 and TR3.

The field coil is connected between the output of the diode trio D1 and the output of TR3, and in parallel with this is connected a clamping diode D10. A further potential divider provided by two resistors R5 and R6 powers a thyristor SCR1 which is connected in series with negative output of the circuit.

Operation of the circuit is as follows: With the alternator running (i.e. shaft 14 being rotated) voltages are generated in the stator winding by virtue of residual magnetism in the rotor and the field due to the bar magnets 26 incorporated in the field assembly. The bridge network 102 converts the stator A.C. output to a D.C. voltage which appears between the rails of the control circuit 110. The stator also supplies D.C. field current through field current diodes D1, the field windings via the transistor TR3 and the bridge network 102.

As the speed of rotation of the alternator increases, and hence the output voltage increases, the voltage between R1 and R2 increases to a value where zener diode ZD1 conducts. Transistor TR1 turns on and transistors TR2 and TR3 turn off. With transistor TR3 turned off the field current and thus the output voltage of the system decreases and zener diode ZD1 stops conducting. Transistor TR1 turns off and transistors TR2 and TR3 turn on again, the field current and output voltages of the system turn back on and the cycle is repeated.

The voltage appearing at the junction of resistors R5 and R6, when the voltage reaches substantially the normal rated output voltage of the alternator, triggers thyristor SCR1 to conduct allowing connection of the output to the load through fuses F1 and F2.

Without the latter facility if the load is connected to the output of the alternator at start up, current (initially low) is immediately drained preventing the alternator from acheiv ing the rated output voltage.

The alternator is preferably rated at 24v D.C. but may be rated at 1 2v D.C. depending upon its construction.

Various modifications may be made within the scope of the invention, for example the casing of the alternator need not be flameproof. The number of permanent magnets used and the relative size of the magnets relative to the field windings may be chosen to suit the application. A control circuit may be used in the alternator which is alternative to that described. The alternator may be made from any suitable components and need not be of the brushless type. One or more permanent magnets may be provided on the rotor as well as or instead of the field assembly. The invention may be applied to a dynamo. One or more magnets may be provided on the stator.

Claims

1. An electric generator in which at least one of the field assembly and/or the stator comprises at least one electrical winding together with at least one magnet.

2. A generator as claimed in claim 1, in which the field assembly only comprises at least one electrical winding together with at least one permanent magnet, the stator having no permanent magnet.

3. A generator as claimed in claim 1 or claim 2, in which the electric generator is an alternator.

4. A generator as claimed in claim 3, of the brushless type, in which the stationary field assembly comprises a field winding and a plurality of permanent magnets.

5. A generator as claimed in claim 3 or claim 4, in which the stator is provided with 3 windings in "star" configuration.

6. A generator as claimed in any one of the preceding claims, in which a flame-proof casing is provided by the generator.

7. A generator as claimed in any one of claims 3 to 6, in which means for disconnecting the electrical supply of the alternator to its load until a predetermined output voltage is obtained is provided.

8. A generator as claimed in claim 7, in which the predetermined output voltage is between 50% and 90% of the nominal rated output voltage.

9. A generator as claimed in claim 7 or claim 8, in which the disconnection means comprises a semi-conductor switching circuit.

10. A generator as claimed in any one of claims 7 to 9, in which said disconnection means is enclosed within the flame-proof cas ing.

11. An electric generator substantially as hereinbefore described with reference to the accompanying drawings.

1 2. Any novel subject matter or combination including novel subject matter herein dis closed, whether or not within the scope of or relating to the same invention as any of the preceding claims.