WO1990000833A1

WO1990000833A1 - Rotary induction machine having control of secondary winding impedance

Info

Publication number: WO1990000833A1
Application number: PCT/US1989/002928
Authority: WO
Inventors: Samuel Heller
Original assignee: Heller-Dejulio Corp.
Priority date: 1988-07-12
Filing date: 1989-07-10
Publication date: 1990-01-25
Also published as: AU3970189A

Abstract

A rotary induction machine having rotor and stator with primary and secondary windings (11, 12, 13 and 17, 18, 19) and impedances (31, 41, 46; 32, 42, 47 and 33, 43, 48) connected in the secondary windings to correct the power factor and increase efficiency.

Description

ROTARY INDUCTION MACHINE HAVING CONTROL OF SECONDARY WINDING IMPEDANCE

FIELD OF THE INVENTION This invention relates generally to rotary induction machines of the type having a wound rotor and more partic¬ ularly to wound rotor induction motors and generators in which the impedance of the secondary is controlled by connecting impedance components to the secondary windings.

BACKGROUND OF THE INVENTION In induction machines the current in the secondary winding (usually the rotor) are created solely by induction. These currents result from voltages induced in the secondary windings by rotating magnetic fields in the primary winding which arise from the application of line voltages thereto.

When the machines are operated as a motor, the rotor rotates at speeds below the synchronous speed. The difference in speed is referred to as the slip speed, usually expressed as a decimal of the synchronous speed.

The rotor windings are generally connected to slip rings and adjustable resistances are connected in series with the windings. The resistances limit the secondary currents during "start." As the motor picks up speed the secondary resistance is gradually reduced whereby the efficiency increases. The resistance can also be used to control the speed; however, this method of speed control is very inefficient. When the rotor is driven at above the synchronous speed, the machine acts as a generator. With resistance in th secondary windings, the output power can be maintaine somewhat constant over a narrow range of rotor speeds.

For the past thirty years or more when driving a generato with various types of prime movers, the speed of th electrical generator was kept nearly constant. Various mechanical methods for controlling speed have been use depending on the prime mover. When using an alternato with DC excited fields, the rpm must be kept constant to very close tolerance; less than one revolution of 1800 o 3600 revolutions per minute. When using a squirrel-cag induction generator, the most common cogeneration gener¬ ator, a few percent above base rpm is necessary. I inadvertently, a higher speed is supplied by the prim mover, the generator completely releases its load and "runaway condition" exists. Under such circumstances, th prime mover, a wind or steam turbine, or a diesel, may rac to destruction in a very few minutes or seconds.

In equipment for wind, water, wave power, thermal, etc., mechanical means are used to maintain constant speed. Spoilers, blade pitch control and mechanical brakes ar used to limit the speed. All these methods have a ver short life and are costly to maintain._*

In U. S. Patent 2,648,808 there is described a motor havin a wound primary winding (stator) in which the effectiv impedance of the primary windings is varied to improve th torque-speed characteristics of the motor. -More particu larly, the power factor of the motor is improved by con trolling the impedance of the primary windings by addin thereto external series impedances.

OBJECTS AND SUMMARY OF THE INVENTION It is a general object of this invention to provide a improved rotary induction machine. It is a further object of this invention to provide a wound rotor induction machine in which the impedance of the secondary winding is controlled to improve the efficiency.

It is a further object of this invention to provide a wound rotor induction generator in which impedance is added in series and/or in parallel with the secondary winding to control the current flowing therethrough to increase the power output and efficiency.

It is another object of this invention to provide an efficient variable speed motor.

It is a further object of this invention to provide an induction generator which can be operated efficiently over a wide range of rotor speeds.

The induction generator described herein has a number of advantages over existing machines. No exact rpm is neces¬ sary and two times base rpm can easily be accommodated. Since a generator in accordance with the invention generates more power as it speeds up, it is always under electrical control, and thus can eliminate runaway entire¬ ly. The generator can be made to produce its rated output at any rpm above its base speed within its range, and so can be matched to the prime mover at its location using external impedance.

An objective for power generation is to get maximum power output. The generator of the present invention can deliver power over a much wider range and a larger capacity than known generators. The generator maintains its frequency over a variable speed range. The generator is ideally suited for "peaking" requirements with diesel, steam or gasoline type prime movers.

The generator of this invention has the impedance components added to the secondary. The primary voltage is of no concern. On other attempted variable speed drives, the primary has always been used. This limits the gener¬ ator primary to operation under 600 volts because higher voltage requires more costly impedance components.

In many areas, the induction type squirrel-cage generator is tied to a grid of very limited capacity. When starting the generator, it is desirable to reach near base speed as soon as possible before applying the prime mover. Unfor¬ tunately, the squirrel-cage induction generator takes from six to ten times rated amperes at starting. On a limited capacity grid, this voltage dip will lower the voltage noticeably and may even cause contactors and relays to drop out. The power lost with each start may be more annoying than the results.

The wound-rotor type generator, when used as a motor, has the highest torque-per-ampere of any AC motor type. The inrush amperes can be easily cut to half or less of the squirrel-cage type. This effect not only increases the KW output but prevents the voltage dip problem almost com¬ pletely.

The frequency in the secondary of the generator is in direct proportion to the rpm above the base speed. Thus, in a 900 rpm, 8 pole, 60 Hz machine, the frequency in the secondary at 900 rpm is zero. The rise in frequency is 60 Hz for each 900 rpm above base speed. So it is 60 Hz at 1800 rpm in the secondary of this machine.

This is important. It means that the generator is being driven in the same direction that it would run as a motor. Thus, there is no need to reverse any connections to use the generator as a motor for assisting the generator to reach base speed.

It should be noted that unlike the capacitors found in the primary circuits of almost all machines, the capacitors in this generator see changing values of both voltage and current as well as frequency depending on the rpm of the generator. It is this effect in conjunction with the other external components that make this generator operate so well over so wide a range of speeds. The inductors and the capacitors complement each other in controlling the effici¬ ency and rpm of the generator as well as its range.

These and other objects of this invention are accomplished by inserting in the secondary windings an impedance which modifies the impedance of the secondary windings to control the current flowing therethrough.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a schematic diagram of an induction motor in accordance with the prior art;

FIGURE 2 is a graph showing power output as a function of slip for the prior art device shown in FIGURE 1;

FIGURE 3 is a schematic diagram of a wound rotor induction machine in accordance with one embodiment of this invention;

FIGURE 4 is a schematic diagram of a wound rotor induction machine in accordance with another embodiment of this invention;

FIGURE 5 is a graph showing the operating character¬ istics of an induction device in accordance with the embodiments of FIGURES 3 and 4;

FIGURE 6 is a schematic diagram of another embodiment of this invention;

FIGURE 7 is a bar chart comparing the operation of a generator in accordance with this invention with a prior art generator; and

FIGURE 8 is a schematic diagram of a single phase induction machine in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The prior art induction machine shown in FIGURE 1 includes primary windings 11, 12, 13 shown connected in a "Y" con¬ figuration to the voltage supply 1 . The windings are wound in the stator of the machine in accordance with well known winding practices. The voltage applied generates currents in the windings which generate a primary field "F" which is coupled to the wound rotor 16 including windings 17, 18, 19. The rotor is connected to a shaft, not shown. Each of the secondary windings is connected to a slip ring 21, 22, 23 respectively and in turn to a resistive network 24 which includes shorting switches 26 and resistors 27.

As described above, all switches are open as voltage is first applied to start the machine. The resistance in series with the rotor windings is maximum, thereby limiting the starting current. As the rotor speed increases the switches are sequentially closed to thereby remove more and more resistance, thereby allowing the speed to increase and provide maximum torque at higher speeds. The current re¬ duces because the relative speed between the primary magnetic fields and the rotor windings is reduced, thereby reducing the induced current.

Referring particularly to FIGURE 2, the curves show the torque as a function of slip rate. It is seen that with the resistance R4, the maximum power is achieved at the slip rate or rotor speed and that as the resistance is decreased, higher torque is achieved at lower slip rates. The efficiency is substantially higher when the resistance is low because the resistive losses in the rotor secondary circuit are minimized.

In accordance with this invention, the operating character¬ istics of "the induction motor are substantially improved by adding in the secondary windings a reactive impedance. Referring to FIGURE 3, potentiometers 31, 32 and 33 are connected in series, one with each winding. The winding resistance is shown at 36, 37 and 38. The potentiometer wiper is connected to a parallel combination of a bridging capacitor 41, 42 and 43 and an inductor 46, 47 and 48. In FIGURE 4 the capacitors 41a, 42a and 43a are shown con¬ nected in parallel to the secondary windings with the inductors and resistors in series. As a result, the reactive impedance of each winding is increased. The phase lag of the secondary current with respect to the secondary voltage is decreased, thereby increasing the power output torque for the lower slip rates. This is shown in the family of curves of FIGURE 5 which are shown for various impedance values Z^, Z₂, Z3 and Z4. It is noted that for the lowest impedance values Z^, the maximum power is delivered over a broad range of slip rates, thereby im¬ proving the speed torque characteristics of the motor.

The rotor and stator primary and secondary may be reversed in accordance with well known principles. Therefore, re¬ ferring to FIGURE 6, the rotor windings are shown connected to the input power source as the primary while the stator, or secondary windings, are shown connected with series or parallel impedances comprising the parallel combination of potentiometers 51, 52 and 53 and capacitors 56, 57 and 58. Other elements like inductors may be inserted in the secon¬ dary winding in the same manner as in FIGURES 3 and 4.

In the embodiment of FIGURE -6, the potentiometers are connected for common adjustment by drive shaft 59 which is driven by a servo motor and gear 61 and 62. A potentio¬ meter 63 is connected to the shaft and its output is applied to summing amplifier 66. A phase detector 67 senses the power factor angle in one of the windings and provides its output to the summing amplifier 66.

The amplifier moreover may receive a bias input from an adjustable resistor 68 tied to a reference voltage. The system shown controls the phase angle and therefore the power output of the motor. The response of the machine shown in FIGURE 6 will be substantially identical to that shown in FIGURE 4. When operated as a generator, the generator power is controlled over a wide range of shaft speeds.

In one example, a machine was operated as a generator with resistors only in the secondary in accordance with the prior art, and also in accordance with this invention, with impedance in the secondary. The output was compared for identical slip speeds. The results are shown in the bar chart of FIGURE 7. In each case, the resistance in each phase of the secondary of the ordinary generator was varied. The results were noted when the full rated current (11 amperes) was reached in the machine for various speeds. Then an impedance was connected to the secondary and the same current maintained. The results for various imped¬ ances and speeds were recorded. The bar chart shows the results for various speeds which are indicated as decimal numbers. The decimal numbers^"are the quotient of the speed for maximum current divided by the speed for zero watt output, i.e. the point where the motor becomes a generator. The top of each bar graph shows the percent of rated load output of the generator operated at various speeds hatched for normal generator with R only and vertically lined for a generator in accordance with this invention with R,C and L.

When operated in accordance with the prior art, the generator had difficulty obtaining two kilowatt output, that is, 75% of rated load output. As seen by the other bars in all instances, the generator maintained 100% load over the wide range of speeds shown. In one instance it reached 140%, or about 4.2 kilowatt output.

FIGURE 8 shows an induction machine connected for single phase operation. The single phase primary may be used to start the generator in accordance with known starting methods. This may include starting winding and centrifugal switches, not shown. The secondary is shown including secondary impedances in accordance with this invention. Tne generator can be operated over a wide range of speeds with increasing output for increasing speed.

Therefore, it is seen that an improved motor having a relatively constant torque over the wide range of slip speeds is obtained when the secondary winding is modified in accordance with this invention. It is also seen that in a generator, the output power is maintained substantially constant and high over a wide range of speeds by variation of the static components in the secondary circuit.

Claims

WHAT IS CLAIMED:

1. A rotary induction machine comprising a stator having wound thereon stator windings; a rotor mounted for rotation in said stator and having wound thereon rotor windings; said stator or rotor windings adapted to be connected to a source of power and serving as primary windings where¬ by the applied power causes current to flow in said wind¬ ings and provide a rotating flux; said other windings serving as a secondary windin coupled to said flux whereby currents are induced in sai secondary windings; and impedance means connected in said secondary windings to correct the power factor in said winding to thereby increase the efficiency of said machine.

2. A rotary induction machine as in Claim 1 in which said rotor is connected to drive a load.

3. A rotary induction machine as in Claim 1 in which sai stator windings are connected to the source of power and said reactive impedance is connected to said rotor windings.

4. A rotary induction machine as in Claim 3 in which sai rotor has a rotating speed less than that of the rotatin flux and delivers mechanical power to a load connecte thereto.

5. A rotary induction machine as in Claim 3 in which th rotor is driven at a rotary speed greater than that of th rotating flux to delivery electrical power to said stato windings and connected power source.

6. A rotary induction machine as in Claim 1 in which sai impedance includes capacitive and inductive means.