GB2078451A

GB2078451A - Method and apparatus for starting electricity generators

Info

Publication number: GB2078451A
Application number: GB8017481A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-05-28
Filing date: 1980-05-28
Publication date: 1982-01-06

Abstract

A polyphase salient pole electric generator set is started by using the phase windings (A, B, C) of the generator as a motor until ignition of the internal combustion engine or gas turbine driving the set. Pulses are injected into at least one phase winding when another phase is opposite a salient pole thereby applying a torque to the rotor (1) at increasing frequency as the machine speeds up. Injection of the pulses is controlled by position sensors (5) transmitting signals to a direct current inverter (3) when coincidence occurs. The position sensors are preferably apertures in a rotating disc (2), through which light is transmitted to phototransistors when coincidence occurs. <IMAGE>

Description

SPECIFICATION Method and apparatus for starting electricity generators This invention is concerned with the starting of electricity generators driven by gas turbines or engines. It is particularly applicable to high speed generator sets in which the generator is a polyphase salient pole synchronous machine having either electromagnet or permanent magnet excitations.

Conventional starters such as direct current motors supplied from a battery are not suitable for gas-turbine generator sets which have to be brought up to a high speed before the turbine ignites and takes over the drive to the electric generator.

An object of the present invention is to utilize the polyphase windings of the generator as a starting motor, thereby eliminating the need for a separate starting motor with gearing to bring the engine to firing speed.

From one aspect the invention provides a method of starting a polyphase salient pole electric generator set which comprises successively sensing when each pole of the generator is coincident with a phase winding, and injecting at that time a pulse from a direct current fed inverter into at least one other phase winding whereby torque due to the interaction of the magnetic field and the current pulses applied to the set at progressively increasing frequency as the speed is increased until firing occurs.

From another aspect, apparatus for starting a polyphase salient pole generator set according to the invention comprises a plurality of position sensors successively sensing coincidence between each pole of the generator with a phase winding, means for transmitting a signal when coincidence occurs to actuate a direct current inverter to inject a pulse into at least one other phase winding to apply torque to the set whereby pulses are injected at progressively increasing frequency as the speed of the set increases and the engine fires.

The position sensors may be of any suitable type comprising a disc or the equivalent rotating with the machine shaft and operating successive signal transmitting devices when ever coincidence of a pole and a phase winding occurs. Such signal transmitting devices may, for example, be Hall effect semiconductors or pick up coils operated by magnets spaced around the disc, or an oscillator driven coil with a pick up coil, or a proximity switch or switches.

Preferably however, the position sensor devices comprise apertures in a disc corresponding to the phase windings through which light from a light source is transmitted to operate phototransistors to produce a signal to the inverter. These phototransistors may be Photo Darlington amplifiers or separate amplification of the signal may be provided if necessary.

The light may be transmitted from the source to the disc and from the disc to the phototransistors by optical fibres.

The apertures in the disc are preferably elongated and together occupy a sector of the disc corresponding to a pole pitch so that there is little risk of an aperture not being open to transmit light when the machine is at rest whatever the relative position of the poles and windings.

Other parts of the invention are embodied in the preferred forms which will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of one form of the invention; Figure 2 is a schematic diagram of another form of the invention; and Figure 3 is a view in the direction of the arrow Ill of Fig. 2.

Referring to Fig. 1, an alternator AL comprises a three phase winding A, B, C and a salient pole rotor (indicated at 1) which may provide either electromagnetic or permanent magnet excitation. The rotor 1 is driven by a gas turbine or internal combustion engine (not shown).

A disc 2 rotating with the rotor 1 cooperates with signal transmitting devices 5A, 5B, 5C to sense when a salient pole of the machine is opposite one of the phase windings A, B or C respectively. A signal is then transmitted to the correspondinging thyristor TR A, TR B or TR C of an inverter 3 fed by a d.c. source such as a battery 4.

The thyristors are each connected between one pole of the battery 4 and one of the phases A, B or C so that when a thyristor is triggered, a current pulse is injected into that phase. The other pole of the battery is connected to the neutral point of the three phase winding. The arrangement is such that when one phase (say A) is coincident with a salient pole, the position sensor (5A) sends a signal to the thyristor (TR B) corresponding to the reset phase (B) to trigger it and inject a pulse into the next phase winding (B) which will not be opposite a salient pole. A torque proportional to the product of the excitation field and the magnitude of the current pulse is thus applied to the rotor mass to rotate it to a position in which the next phase (B) is opposite a salient pole.This is sensed by the position sensor (5B) to fire the thyristor (TR C) corresponding to the next phase (C) to inject a pulse into this phase (C). When phase (C) comes opposite a salient pole, a signal is transmitted from the sensor (5C) to fire thyristor (TR A) and inject a pulse into phase (A).

The cycle is then repeated, each pulse producing a torque on the rotor and increasing its rotary speed. The frequency at which pulses are injected and torque applied to the rotor is dependent on the speed of the machine so that pulses are injected successively into the phases at increasing frequency as the machine speeds up. Eventually the speed will reach a value at which ignition of the engine or gas turbine will occur. The starter is then disconnected by any suitable means and the machine operates as a generator.

Another form of starter according to the invention is shown in Figs. 2 and 3. In this form, the shaft position signals are derived from the transmission of light through suitably placed apertures in a disc mounted on the shaft. A light source LA, supplied from a battery 3, transmits light through optical fibres OF1 to the apertures 5A, 5B, 5C in disc 2 and optical fibres OF2 transmit light passing through these apertures to phototransistors PD1, PD2 and PD3 respectively. The apertures are each associated with one of the phases A, B, C of the alternator AL and are spaced 1 20 electrical degrees apart.The apertures for each phase are radially spaced so that an aperture is between the ends of the appropriate optical fibres OF1 and OF2 when a salient pole is opposite the corresponding phase.

The output of each phototransistor PD1, PD2, PD3 is connected to the primary winding of a transformer T1, T2, T3 each of which has two secondary windings (S1, S3, S5, S7, S9, S11) each including a diode Dl. These secondary windings are connected to the gates of thyristors TH1, TH2, TH3, TH4, TH5, TH6 of a battery fed inverter 6. Terminals marked 1 to 1 2 in Fig. 2 are connected to the correspondingly marked terminals of the thyristors of the inverter.

When phase A is opposite a salient pole, the corresponding aperture in the disc is between the ends of the optical fibres OF1, OF2 and light is transmitted from source LA, through an optical fibre and through the aperture to the oppositely positioned optical fibre OF2 and thence to the phototransistor PD1.

The output from PD1 is a pulse transmitted to transformer T1 to produce a pulse in each of the secondaries S1, S3. These pulses are transmitted to the inverter 6. The pulse from secondary S1 is applied to the gate of the thyristor TR6 and that from secondary S3 to the gate of thyristor TR 1. Both thyristors are triggered and current flows, from the battery through TR6, through phases B and C and through thyristor TR 1 back to the battery. A pulse is thus injected into both phases B and C and torque applied to the rotor. As successive phases come opposite a salient pole, pulses are injected into the other phases applying torque to the rotor speeding it up until ignition occurs.

The form shown in Fig. 2 has the advantage over that of Fig. 1 that a torque applied to the rotor is greater and that there is no connection to the neutral point of the winding.

In order to reduce the difficulty of starting should the machine be stationary in a position where no phase winding is opposite a pole, the apertures are circumferentially elongated as shown in Fig. 2 so that each occupies substantially one third of a pole pitch.

Instead of an inverter using thyristors, one using transistors may be used.

It will be understood that the invention is applicable to generators having any number of phases greater than one and with any number of pairs of salient poles.

Claims

1. A method of starting a polyphase salient pole electric generator set which comprises successively sensing when each pole of the generator is coincident with a phase winding, and injecting at that time a pulse from a direct current fed inverter into at least one other phase winding whereby torque due to the interaction of the magnetic field and the current pulses is applied to the set at progressively increasing frequency as the speed is increased until firing occurs.

2. A method according to Claim 1, in which coincidence of a pole with a phase winding is sensed by a disc rotating with the rotor of the machine in association with a plurality of signal transmitting device which send a signal when coincidence occurs to operate the inverter to inject a pulse into said other phase winding or windings.

3. A method according to Claim 2, in which said signalling devices are phototransistors controlled by light transmitted through an aperture in the disc when coincidence of a pole and a phase occurs.

4. Apparatus for starting a polyphase salient pole generator set comprising a plurality of position sensors successively sensing coincidence between each pole of the generator with a phase winding, means for transmitting a signal when coincidence occurs to actuate a direct current inverter to inject a pulse into at least one other phase winding to apply torque to the set whereby pulses are injected at progressively increasing frequency as the speed of the set increases and the engine fires.

5. Apparatus according to Claim 4, in which the position sensors comprise a disc driven by the shaft of the machine, and a plurality of signal transmitting devices operated successively by the disc when coincidence of a pole and a phase occurs.

6. Apparatus according to Claim 5, in which said signal transmitting devices are phototransistors, said disc is apertured at positioned corresponding to the phases o the winding and light is transmitted through the apertures in succession when coincidence between poles and phases occurs to operate said photo transistors to operate the inverter and inject pulses into another phase winding.

7. Apparatus according to Claim 6, in which light is transmitted from a light source to the disc and from the disc to the phototransistors by optical fibres.

8. Apparatus according to either Claim 5 or Claim 6, in which the apertures are arranged in concentric circles and are elongated circumferentially of the disc to occupy together a sector of the disc corresponding to one pole pitch.

9. A method of starting a polyphase salient pole electric generator set substantially as described with reference to the accompanying drawings.

1 0. Apparatus for starting a polyphase salient pole electric generator set substantially as described with reference to the accompanying drawings.