Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
1999-04-09
2001-11-20
Martin, David S. (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S717000, C318S724000, C318S587000, C318S114000, C318S148000
Reexamination Certificate
active
06320350
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a modulation control type of AC machine, and more particularly, to a modulation control type of AC machine which can remove drawbacks of a conventional AC machine to improve the characteristics of industrial facilities and has many application fields.
2. Description of Related Art
Conventional AC machines can be classified into AC motors and AC generators and the structures of AC machines are different from each other depending upon the types. Generally, the AC motor is employed for driving and controlling various types of industrial facilities and the AC generator is used for generation of power, using a prime mover. However, the AC machines differ from each other in the characteristics and purposes depending upon the types. Therefore, the type of the AC machine is to be selected based on a usage purpose. For instance, there are a drawback of rotation speed control in a synchronous motor and a drawback of change of rotation speed due to the load of a rotor in an induction motor. The problems of the conventional various types of AC machines will be described below.
The conventional various types of AC machines such as a synchronous AC machine, an induction AC machine and a selsyn motor have different structures, operation principle and characteristics from each other and have the respective inherent problems. The modulation control type of AC machine according to an aspect of the present invention has an indirect relation to the conventional various types of AC machines. The basic principle of the modulation control type of AC machine according to an aspect of the present invention can be explained by using the relation. However, in order to make clear the difference between the present invention and the conventional AC machines, the problems of the conventional AC machines will be described below.
A synchronous motor uses a rotary magnetic field which is constituted by a DC main magnetic flux which is proportional to the frequency of an AC power source and rotates with a synchronous rotation speed. Therefore, DC electromagnetic force generated by field windings through which DC field current is flown acts as synchronous torque for rotating a rotor with a synchronous rotation speed. In this case, however, the rotor has a delay angle in relation to the DC main magnetic flux. The synchronous motor requires an AC power source and a DC power source and this is a drawback on the structure. The rotation speed and synchronous torque are controlled by changing the AC power source frequency and the DC field current, respectively. For purpose of increasing the rotation speed, if the AC power source frequency is increased, the magnetizing current, DC main magnetic flux and synchronous torque are decreased. Accordingly, the voltage of AC power source needs to be increased for the purpose. Further, if the AC power source frequency is decreased for purpose of decreasing the rotation speed, the magnetizing current is increased to cause magnetic saturation, as contrary to the above case. Accordingly, the AC power voltage needs to be decreased. As a result, the size of synchronous motor becomes larger and larger for response to the change of AC power source voltage, resulting in increased weight and reduced efficiency.
In the synchronous motor, alternate magnetomotive force is generated which is proportional to the product of the number of turns of the stator winding for each value of phase and load current. However, the synchronous motor has no secondary circuit for canceling the AC magnetomotive force. Therefore, there is a drawback in armature reaction which generates reactance to prevent the load current. The armature reaction is the causes of distortion influencing to the DC main magnetic flux generated by the magnetizing current and demagnetizing action. If the alternate magnetomotive forces of respective phases are synthesized, DC magnetomotive force can be obtained. Further, if the load torque of the rotor is greater than the synchronous torque, the synchronous motor causes the step-out from the synchronization to stop rotation, resulting in loss of the synchronous torque. If the DC field current is increased in order to prevent the step-out, the polarization by the DC excitation causes magnetic saturation. Therefore, the AC power source increases the exciting current so that the power factor is decreased and the temperature is increased.
A synchronous generator is necessary to be equilibrium between the AC power source voltage and the generated voltage, power is supplied through the generated voltage and its phase. Accordingly, in order to keep the synchronous rotation speed, a speed control unit with precise control capability is applied to a prime mover. Since the power is generated based on a lead angle of the rotor, a voltage difference is necessary between the AC power source and the synchronous generator. In the power generation, the load current and magnetizing current are supplied from the synchronous generator to the AC power source. If the direction of voltage difference is inverted, the synchronous generator acts as a synchronous motor. If the generated power is changed, the load current and armature reaction change the generated voltage and the phase. There is a drawback that circulating current flows between synchronous generators operated in parallel so that turbulence of generated power is readily caused due to the synchronizing function.
In the induction motor, since primary winding corresponds to stator winding of the synchronous motor, the same rotary magnetic field as in the synchronous motor is generated in the stator. The relation of the primary winding and the secondary winding is similar to the electromagnetic coupling in a transformer and a load is connected to the secondary winding. Since the load current is supplied from the AC power source, the induction motor has a simple structure, compared to that of the synchronous motor. Since the load current flowing through the primary winding in each of the phases is proportional to that flowing through the secondary winding of the corresponding phase, the alternate magnetomotive force representative of the product of the number of turns in the primary winding and the load current flowing in the primary winding and that of representative of the product of the number of turns of the primary or secondary winding and the load current are equal to each other and cancelled. Therefore, there is no armature reaction and any reactance preventing the load current is removed.
Exciting current flows in each phase of the primary winding. If the components of reactive current as the magnetizing current are synthesized over all the phases, an equivalent DC magnetizing current is obtained which generates a DC main magnetic flux. When secondary reactive currents having the same phase relation as the magnetizing currents are synthesized over all the phases, an equivalent DC current is obtained similarly which corresponds to a DC field current of a synchronous motor. Thus, in the induction motor, the rotor is rotated based on the synchronous torque which is generated with the same principle as in the synchronous motor. Further, it indicates that the secondary reactive current is necessary for the rotation. Since the secondary active current in each phase has a phase difference of &pgr;/2 in relation to the magnetic flux generated by the primary winding in the phase, an average of AC electromagnetic forces which is proportional to the product of the secondary active current and the number of turns of the secondary winding is zero. Therefore, the secondary active current has no relation to the synchronous torque.
In the induction motor, the rotation speed changes due to slip of the rotor so that the secondary voltage and the frequency decrease. Equivalent DC current reduces because of the reduction of secondary voltage and the rotation speed is decreased from the synchronous rotation speed because of the slip. Actually, since the rotation of the roto
Allen Kenneth R.
Martin David S.
Townsend and Townsend / and Crew LLP
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