FIELD OF THE INVENTION

[001] The present invention relates to a three phase brushless generator and more particularly to a method and a system for starting the three phase brushless generator.

BACKGROUND OF INVENTION

[002] Three phase brushless generator is generally used to provide power supply in locations where utility electric supply from power grid is either temporarily, or permanently, not available. A typical three phase brushless generator is equipped with an Internal Combustion (IC) engine, and an alternator mounted on engine crankshaft. The alternator coupled to the crankshaft is used to generate electric power to provide alternating-current power to substitute utility supply when utility supply is not available.

[003] The alternator comprises two electric machines - an excitation alternator and a main alternator. The main alternator comprises a rotor with a field winding and a stator disposed with three phase shifted windings, typically at 120 degree electrical angle. The three phase windings of the main alternator provide electric power to external electric loads and is the primary output of the generator. The excitation alternator comprises a field winding disposed on the stator and multiple phase shifted windings disposed on the rotor. The purpose of the excitation alternator is to excite the rotor field winding of the main alternator, without using commutator and brush contacts, to maintain electrical contact between a rotating winding and a stationary power source. For this purpose, the electrical power generated in excitation alternator is transferred to rotor field winding of main alternator through a rectifier.

[004] The phase windings of main alternator are also typically connected to input of an automatic voltage regulator (AVR) unit, and the output of the AVR unit is connected to field winding of excitation machine. The AVR unit regulates voltage across phase windings of main alternator when the generator is generating electric power.

[005] For starting the generator described hereinbefore, the IC engine needs to be rotated at sufficiently high speed before self-sustaining combustion process can commence. For this purpose, generator sets are equipped with an electric starter system. The electric starter system typically consists of a brushed DC motor powered by a battery, the motor being connected to crankshaft of IC engine through a suitable power driving mechanism such as a gear train.

[006] Such a starting system results in significant wear and tear in starter motor resulting in reduced reliability of the starting system.

[007] Thus, there is a need in the art for a starting mechanism for a three phase brushless generator which addresses at least the aforementioned problems.

SUMMARY OF THE INVENTION [008] In one aspect, the present invention is related to a method for starting a three phase brushless generator having an IC engine, a main alternator having a field winding disposed on a rotor and three stationary phase windings disposed on a stator, an excitation alternator having a field winding disposed on a stator and a plurality of phase windings disposed on a rotor, and a rectifier connected between the phase windings of the excitation alternator and field winding of the main alternator. The method comprising the steps of receiving a start signal at an Electronic Control Unit (ECU) connected to the phase windings of the main alternator and to the phase windings of the excitation alternator; determining, by the ECU, initial position of the rotor of the main alternator with respect to the phase windings of the main alternator; applying a first voltage, by a voltage source connected to the ECU, to the field winding of the excitation alternator such that a voltage is induced in the phase windings of the excitation alternator resulting in a flow of current through the phase windings of the excitation alternator, whereby the rectifier receives an alternating current from the phase windings of the excitation alternator and supplies a rectified current to the field winding of the main alternator; applying a second voltage, by the voltage source, to the phase windings of the main alternator depending on the initial position of the rotor of the main alternator, such that magnetic field generated by the field winding of the main alternator is out of phase with respect to magnetic field resulting from the phase windings of the main alternator whereby a torque is applied on the rotor of the main alternator causing the rotor of the main alternator to rotate; determining speed of rotation of the rotor of the main alternator by the ECU ; determining whether speed of the rotor of the main alternator is above a threshold value; if speed of the rotor of the main alternator is below the threshold value, determining updated position of the rotor of the main alternator and varying the second voltage depending on the updated position of the rotor of the main alternator such that magnetic field generated by the field winding of the main alternator is out of phase with respect to magnetic field resulting from the phase windings of the main alternator thereby increasing speed of rotation of the rotor of the main alternator; and if speed of the rotor of the main alternator is above the threshold value, removing the application of the second voltage.

[009] In an embodiment of the invention, the method includes the step of varying the first voltage if speed of the rotor of the main alternator is below the threshold value. In another embodiment of the invention, the first voltage is a time varying voltage.

[0010] In a further embodiment of the invention, the second voltage is varied to maintain the magnetic field generated by the field winding of the main alternator 90° out of phase with respect to the magnetic field resulting from the phase windings of the main alternator.

[0011] In another aspect, the present invention is directed to a system for starting a three phase brushless generator having an IC engine, a main alternator having a field winding disposed on a rotor and three stationary phase windings disposed on a stator, an excitation alternator having a field winding disposed on a stator and a plurality of phase windings disposed on a rotor, and a rectifier connected between the phase windings of the excitation alternator and field winding of the main alternator. The system comprising a voltage source; and an Electronic Control Unit (ECU). The ECU is connected to the voltage source and configured to apply a first voltage to the field windings of the excitation alternator and a second voltage to the phase windings of the main alternator. The ECU is further configured to: receive a start signal; determine initial position of the rotor of the main alternator with respect to the phase windings of the main alternator; apply a first voltage, by the voltage source connected to the ECU, to the field winding of the excitation alternator such that a voltage is induced in the phase windings of the excitation alternator resulting in a flow of current through the phase windings of the excitation alternator, whereby the rectifier receives an alternating current from the phase windings of the excitation alternator and supplies a rectified current to the field winding of the main alternator; apply a second voltage, by the voltage source, to the phase windings of the main alternator depending on the initial position of the rotor of the main alternator, such that magnetic field generated by the field winding of the main alternator is out of phase with respect to magnetic field resulting from the phase windings of the main alternator whereby a torque is applied on the rotor of the main alternator causing the rotor of the main alternator to rotate; determine speed of rotation of the rotor of the main alternator by the ECU; determine whether speed of the rotor of the main alternator is above a threshold value; if speed of the rotor of the main alternator is below the threshold value, determine updated position of the rotor of the main alternator and varying the second voltage depending on the updated position of the rotor of the main alternator such that magnetic field generated by the field winding of the main alternator is out of phase with respect to magnetic field resulting from the phase windings of the main alternator thereby increasing speed of rotation of the rotor of the main alternator; and if speed of the rotor of the main alternator is above the threshold value, remove the application of the second voltage.

[0012] In an embodiment of the invention, the ECU has a DC to DC converter for supplying the first voltage and the second voltage predetermined by the ECU.

[0013] In another embodiment of the invention, the ECU has a processor; and a plurality of switches connected with the phase winding of the main alternator and field winding of the excitation alternator, the processor configured to control operation of the switches.

[0014] In a further embodiment of the invention, the voltage source is a DC battery.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Reference will be made to embodiments of the invention, examples of which may be illustrated in accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments.

Figure 1 shows a schematic representation of a main alternator and an excitation alternator of a brushless three phase generator connected to an Electronic Control Unit (ECU) in accordance with an embodiment of the present invention.

Figure 2 shows a block diagram of the ECU and a schematic representation of the main alternator and the excitation alternator connected to the ECU in accordance with an embodiment of the present invention. Figure 3 shows power switches for field winding of the excitation alternator in accordance with an embodiment of the invention.

Figure 4 shows power switches for phase windings of the main alternator in accordance with an embodiment of the invention.

Figure 5 shows power switches connected between phase windings of the main alternator and field winding of the excitation alternator in accordance with an embodiment of the invention.

Figure 6 shows a method for starting a three phase brushless generator in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention relates to a method and a system for starting a three phase brushless generator.

[0017] As shown in Figure 1, the system has a voltage source 500 and an Electronic Control Unit (ECU) 300 connected to the voltage source 500. The ECU 300 is connected to a main alternator 100 and an excitation alternator 200 of a three phase brushless generator (not shown). The main alternator 100 has a field winding 120 disposed on a rotor (not shown) and three phase stationary windings 110a, 110b, 110c disposed on a stator (not shown). The excitation alternator 200 has a field winding 210 disposed on a stator (not shown) and a plurality of phase windings 220a, 220b, 220c disposed on a rotor. Further a rectifier 400 is connected between the phase windings 220a, 220b, 220c of the excitation alternator 200 and field winding 120 of the main alternator 100. In this regard, the rotor of the main alternator 100, the rectifier 400 and the phase windings 220a, 220b, 220c of the excitation alternator 200 are mounted a single shaft and rotatable along an axis of the shaft. The three phase stationary windings 110a, 110b, 110c of the main alternator 100 and field winding 210 of the excitation alternator 200 are connected to the ECU 300.

[0018] As shown in Figure 2, the ECU 200 has a DC to DC converter 310 connected to the voltage source 500, a processor 320 and plurality of power switches 330, 340, 350. In an embodiment of the invention, the voltage source 500 is a DC battery. The first power switch 330 is connected between the processor 300 and the field winding 210 of the excitation alternator 200. The second power switch 340 is connected between the processor 300 and the phase windings 110a, 110b, 110c of the main alternator 100. Further, the processor 320 may be configured to achieve functionalities of an Automatic Voltage Regulator (AVR). Accordingly, the third power switch 350 is connected between the processor 320 and the phase windings 110a, 110b, 110c of the main alternator 100. In this regard, the power switch 350 is also connected to the field winding 210 of the excitation alternator 200. Accordingly, during operation of the generator in a power generation mode, processor 320 monitors the voltage supplied by the generator from the phase windings 110a, 110b, 110c of the main alternator 100 and accordingly adjusts the voltage applied on the field winding 210 of the excitation alternator 200 thereby regulating the output voltage of the generator. In an embodiment of the invention, the third power switch 350 for AVR can be realized by using a Silicon Controlled Rectifier (SCR) and diode bridge as illustrated in Figure 5. [0019] In an embodiment of the invention, power switches 330 for field winding 210 of the excitation alternator 200 can be realized by using MOSFET bridge or IGBT bridge as illustrated in Figure 3. The set of power switches may also include an optional set of switches A, B to disconnect the set of power switches 330 from the field winding 210 of the excitation alternator 200.

[0020] In another embodiment of the invention, power switches 340 for phase windings 110a, 110b, 110c of the main alternator 100 can be realized by using MOSFET bridge or IGBT bridge as illustrated in Figure 4. The set of power switches may also include an optional set of switches A, B, C to disconnect the set of power switches 340 from the field winding 110a, 110b, 110c of the main alternator 100.

[0021] Further, as shown in Figure 1, the ECU 300 receives signal 360 from a set of sensors and switches, such as a position sensor to identify position of the rotor of the main alternator 100 with respect to the phase windings 110a, 110b, 110c of the main alternator 100 and a start switch to start the generator. The ECU 300 also drives a set of electrical loads 370 such as ignition coil and auto-choke solenoid.

[0022] According to an embodiment of the invention, the field winding 210 of the excitation alternator 200 is supplied with a first voltage such that a voltage is induced in the phase windings 220a, 220b, 220c of the excitation alternator 200 resulting in a flow of current in the phase windings 220a, 220b, 220c of the excitation alternator 200. As a result, the rectifier 400 receives an alternating current from the phase windings 220a, 220b, 220c of the excitation alternator 200 and supplies a rectified current to the field winding 120 of the main alternator 100. The phase windings 110a, 110b, 110c of the main alternator is further supplied with a second voltage to rotate the standing rotor of the main alternator 100 when the generator is required to be started. The application of second voltage to the phase windings 110a, 110b, 110c of the main alternator 100 is such that magnetic field generated by the field winding 120 of the main alternator 100 is out of phase with respect to magnetic field resulting from the phase windings 110a, 110b, 110c of the main alternator 100 whereby a torque is applied on the rotor of the main alternator 100 causing the rotor of the main alternator 100 to rotate. In this regard, the ECU 300 is configured to apply a first voltage to the field winding 210 of the excitation alternator 200 through the first switch 330 and a second voltage to the phase windings 110a, 110b, 110c of the main alternator through the second switch 340. The first voltage and second voltage required to start the generator are supplied by the DC to DC converter 310 to step-up or step-down the voltage received from the voltage source 500. In this regard, the DC to DC converter 310 is connected to the power switches 330, 340 and selectively actuated by the processor 320 via DC bus link 310’ to provide a required voltage.

[0023] In operation, as shown in Figure 6, when a start signal 360 is received by the ECU 300 at step 610, the ECU 300 at step 620 determines initial position of the rotor of the main alternator 100 with respect to the phase windings 110a, 110b, 110c of the main alternator 100. Thereafter, at step 630, the ECU 300 actuates the first switch 330 to apply a first voltage by the voltage source 500 to the field winding 210 of the excitation alternator 200 such that a voltage is induced in the phase windings 220a, 220b, 220c of the excitation alternator 200 resulting in a flow of current in the phase windings 220a, 220b, 220c of the excitation alternator 200. As a result, the rectifier 400 receives an alternating current from the phase windings 220a, 220b, 220c of the excitation alternator 200 and supplies a rectified current to the field winding 120 of the main alternator 100. At step 640, the ECU 300 actuates the second switch 340 to apply a second voltage by the voltage source 500 to the phase windings 110a, 110b, 110c of the main alternator 100 depending on the initial position of the rotor of the main alternator 100. In this regard, the second voltage is such that the magnetic field generated by the field winding 120 of the main alternator 100 is out of phase with respect to magnetic field resulting from the phase windings 110a, 110b, 110c of the main alternator 100. As a result, a torque is applied on the rotor of the main alternator 100 causing the rotor of the main alternator 100 to rotate. At step 650, the ECU 300 determines speed of rotation of the rotor of the main alternator 100 based on signals 360 received from a speed sensor. At step 660, the ECU 300 determines whether speed of the rotor of the main alternator 100 is above a threshold value. In an embodiment of the invention, the threshold value is predetermined based on various factors. Typically, the threshold value of the speed of the rotor of the main alternator 100 will be a value at which the generator starts and operation of the IC engine can be self-sustained thereafter. If speed of the rotor of the main alternator 100 is below the threshold value, the ECU at step 670, determines updated position of the rotor and vary the second voltage depending on the updated position of the rotor of the main alternator 100 such that the magnetic field generated by the field winding 120 is out of phase with respect to magnetic field resulting from the phase windings 110a, 110b, 110c increasing the speed of rotation of the rotor of the main alternator 100. In an embodiment of the invention, the ECU also varies first voltage at step 670. Thus, the first voltage may be a time varying voltage. In a further embodiment of the invention, at step 670, the second voltage is varied such that the magnetic field generated by the field winding 120 of the main alternator 100 is 90° out of phase with respect to magnetic field resulting from the phase windings 110a, 110b, 110c of the main alternator 100. If speed of the rotor is above the threshold value, the ECU at step 680, removes the application of the second voltage. At this stage, the generator enters into power generation mode providing power to electrical loads 370, while the AVR regulates output across the phase windings 110a, 110b, 110c of the main alternator 100.

[0024] In an embodiment of the invention, the AVR can use power switches 350 shown in Figure 5. The MOSFET or IGBT bridge shown in Figure 3 of Figure 4 can be used to excite the field winding 210 during starting of the generator and can be continued to be used for field excitation winding 210 during generation mode of the generator for regulating voltage across the phase windings 110a, 110b, 110c of the main alternator 100.

[0025] Advantageously, the present invention eliminates requirement of a starter motor which is typically required for starting a three phase brushless generator. The invention thus results in efficient usage of electrical machines already present in generator and reduces redundancy.

[0026] While the present invention has been described with respect to certain embodiments, it will be apparent to those skilled in the art that various changes and modification may be made without departing from the scope of the invention as defined in the following claims.