Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
2000-09-01
2002-10-22
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S807000, C318S808000, C318S810000
Reexamination Certificate
active
06469469
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
A variable output induction motor drive system for use with a polyphase variable output alternating current motor.
2. Description of the Prior Art
Induction motors are the most widely used source of electric drive power in industrial and domestic applications. Their popularity is due to the basic simplicity and ruggedness of the basic concept, and relatively low cost manufacture. Induction motors do not use brushes or slip rings, and have the fewest windings and the least insulation requirements compared with other types of DC or AC motors.
However, the induction motor has one major limitation in applications requiring variable speed operation because the number of magnetic poles in the motor and the frequency of the power source determine the speed of rotation. In the United States where the power line frequency is 60 Hz, a two-pole induction motor therefore runs at a nominal 3600 rpm. Unfortunately, such a constant speed motor is not well suited in many industrial processes where variable throughput is required. Until variable speed drives became available for induction motors, one solution was to apply mechanical throttling at the output with the motor running at normal speed. This resulted in an unacceptably wasteful use of power to control throughput. About 25 years ago, the development of low cost semiconductor power devices opened up the possibility of designing cost effective and efficient variable frequency power sources capable of driving variable speed drive systems for three phase induction motors.
However, controlling the speed of an induction motor is more difficult than controlling the speed of a DC motor in which the torque is basically proportional to the product of the flux per pole and the armature current.
In a DC motor, separate connections to the field and the armature windings are generally available so that the field winding may be connected either in series or in parallel with the armature winding, or separately excited. Various well-known techniques have been developed to enable the torque and the speed of a DC motor to be controlled over a wide range. For instance, with separate excitation of the windings, adjustable speed control is obtained by operating with a fixed field flux and varing the armature voltage. The no load speed is determined as the speed where the induced voltage is equal to the applied voltage.
Adjustable torque operation is obtained in a separately excited DC machine by controlling the value of the armature current. With a constant value of field flux, the torque is then directly proportional to the value of the armature current. If the armature current is provided from a current source, the torque can be adjusted as accurately and as rapidly as the armature current can be adjusted and controlled. On the other hand, in a three-phase Y connected AC induction motor, there are only four input connections, one to each stator winding and a neutral connection. The stator winding is connected to the supply and the polyphase currents circulating through the stator winding produce a magnetic field that rotates at synchronous speed. In a three-phase “squrrel-cage” motor, the rotor consists of a number of copper bars with their ends connected to stout copper end rings, causing them to be permanently short circuited on themselves. The lines of force of the stator field cut the rotor conductors to induce current causing the rotor to follow the stator field. The rotor winding is not directly accessible and rotor current is produced by induction rather than by a separately controlled source.
At no load, losses in the motor cause the rotor speed to be slightly less than the synchronous speed of the stator field. When a load is applied, the rotor speed slips behind the synchronous speed developing torque as a function of the difference in speeds. This difference is defined as “absolute slip”. Another useful measure of slip is “fractional slip”, defined as the absolute slip divided by the speed of the stator field. The frequency of the rotor currents is then the synchronous speed of the stator field (the line frequency) multiplied by the fractional slip. Slip may also be measured as a percentage. A motor operating with a slip of 0.02 may therefore be referred to as having 2% slip. In a typical induction motor, full load slip can vary between about 1% in high power motors (10 HP to 100 HP), up to 5% in fractional HP models.
The breakdown torque level represents the maximum torque available from the motor and any further increase in load can not be met by increases in slip. In normal operation with a line frequency voltage source, the full load operating torque of an induction motor is generally limited to about 50% of the breakdown torque to allow for reasonable variations in load. Since the speed of the motor is a function of the line frequency applied to the stator windings, a basic variable speed motor drive system requires a variable frequency power source. In addition, because a constant amplitude air gap flux provides optimum operating conditions for the motor, the amplitude of the input voltages applied to the stator windings should vary linearly with frequency to provide constant V/Hz operation. This technique is widely used in general-purpose applications where fast response time and rapid speed changes are not required. In these simple variable-speed systems, an inverter having an output that is controlled in both frequency and voltage normally provides the variable-frequency drive power required by the motor.
Two types of inverter are widely used in general purpose drives, the six-step inverter and the pulse-width modulated inverter. The six-step inverter typically uses six semiconductor switches in a bridge arrangement. The three-phase line voltage is full wave rectified to produce a DC voltage across a smoothing capacitor. Regulation of the voltage across the smoothing capacitor can be obtained by replacing the input rectifiers with phase controlled SCRS. In this way, the amplitude of the six-step output voltage applied to the motor can be controlled in proportion to the output frequency of the inverter. Gating on IGBT switches in the proper sequence produces the six-step line-to-neutral voltages. This amplitude of the waveforms increases as the frequency is increased. However, the performance of six-step motor drive systems becomes unsatisfactory at slow speeds, e.g. below 5 Hz, due to noticeable torque pulsations that prevent the smooth generation of power.
On the other hand, PWM inverters can simulate sine wave voltages more effectively and produce smooth variations in torque at the slower speeds. These PWM inverters typically employ variable duration high frequency voltage pulses having repetition frequencies between 5 kHz and 20 kHz. The switching command signals for producing the modulated pulses can be generated by comparing a sinusoidal waveform with a high frequency triangular waveform.
In order to improve the motor response performances, many newer PWM designs are using repetition frequencies above 10 kHz. However, these high frequencies have been shown to introduce serious problems in many applications. High frequency pulse currents, generated by the fast rise and fall times of the applied rectangular voltage waveforms, circulate in the motor and can cause break-down in the lubricating oil films, causing seizure of the rotor bearings. Fast switching voltage waveforms produced by pulse-width modulation can also cause corona breakdown in the insulation of the stator windings, and can create unacceptable levels of radiated and conducted EMI. At high power levels, the lengths of the connecting cables between the inverter and the motor are severely limited due to the reflections and distortions produced by the PWM waveforms. These problems do not exist with sinusoidal motor input waveforms.
A major objective of this invention is to demonstrate how true sine wave currents can replace pulse-width modulated sources in variable speed drive applications to improve the performance o
Baker Richard H.
Chambers Derek
Skeist S. Merrill
Fisher III Arthur W.
Ro Bentsu
Spellman High Voltage Electronics Corp.
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