Electricity: motive power systems – Limitation of motor load – current – torque or force
Reexamination Certificate
2000-02-21
2001-05-22
Masih, Karen (Department: 2837)
Electricity: motive power systems
Limitation of motor load, current, torque or force
C318S254100, C318S132000, C318S376000, C318S375000, C318S599000, C388S812000
Reexamination Certificate
active
06236179
ABSTRACT:
TECHNICAL FIELD
The field of the invention is motor controls for permanent magnet AC synchronous motors and brushless DC motors.
BACKGROUND OF THE INVENTION
AC synchronous motors and brushless DC motors are controlled through commutation of solid state switching devices connected to their stator windings. These motors can be of the permanent magnet (PM) type in which permanent magnets are used on the rotor instead of rotor windings. As the speed of the rotor increases, the voltage developed in the stator (referred to as the “back emf”) increases. This, in turn, requires that higher and higher terminal voltages be applied to produce the desired torques. Base speed is that speed which is at the top of the constant torque range and at the beginning of the constant horsepower range. In many uses, it is desirable to limit terminal voltage at a certain speed and yet maintain a constant horsepower over a certain speed range above base speed. The ratio of the highest speed that can be attained to the base speed at which the limit of terminal voltage is reached is termed the constant power speed ratio. Attaining a desired constant power speed ratio is made more difficult when the motor inductance is in the microhenry range.
PM motors with interior mounted magnets (IPMs) have been shown to have constant power speed ratios of 7.5:1. However, these types of PM motors are not commercially available.
In traction devices such as electric vehicles, the torque-speed specifications call for a constant torque up to some base speed, and then constant horsepower operation up to a higher speed. PM electric motors with rare earth surface-mounted permanent magnets are viable candidates for such applications due to their power density and efficiency. These motors are electrically commutated and are driven by inverters.
Camber et al., U.S. Pat. No. 5,677,605, issued October 14, 1997, discloses a drive system for a brushless DC motor which uses PWM inverter, and phase timing advancement to control operation in the constant power range above base speed. This patent discloses a three-phase brushless DC motor driven by a six-step PWM drive. The commutation switches include IGBT's and MOSFET's for the primary switching devices in parallel with bypass diodes.
As speed increases and commutation takes place at a rapid rate, this arrangement may allow for continuous conduction of the phase current and conduction by the bypass diodes at undesirable times, leading to the loss of power and efficiency. The inverter and the motor may heat up, thereby requiring additional cooling measures.
SUMMARY OF THE INVENTION
The present invention is applied to a surface-mounted PM motor, either the AC-type PM synchronous motor (PMSM) or the DC-type brushless dc motor (BDCM), with operation at speeds well above base speed and with low motor inductances.
The present invention utilizes commutation control switches, which can be pairs of thyristors, or individual triacs or equivalents of these, which are connected to the primary commutation switches to stop conduction of current at the zero crossing and to block currents generated by the induced emf when the motor is operating above base speed. The present invention also utilizes a particular phase advance switching sequence to control the conduction of the primary commutation switches in the inverter, by controlling the firing angles and conduction periods of the primary commutation switches and the commutation control switches added to the inverter.
The present invention allows a smooth transition using standard pulse-width modulation techniques from below base speed to high speed operation above base speed.
The invention is operable up to at least six (6) times base speed (6:1 constant horsepower speed range ratio), but is applicable for attaining even higher speeds.
The present invention provides increased reliability due to the isolation capability of the commutation control switches.
The present invention is applicable to surface mounted PM machines with either trapezoidal or sinusoidal back emfs.
The present invention is applicable when the motor is operated in either a motoring mode or a regenerating mode above base speed.
Other objects and advantages of the invention, besides those discussed above, will be apparent to those of ordinary skill in the art from the description of the preferred embodiments which follow. In the description, reference is made to the accompanying drawings, which form a part hereof, and which illustrate examples of the invention. Such examples, however, are not exhaustive of the various embodiments of the invention, and therefore, reference is made to the claims which follow the description for determining the scope of the invention.
REFERENCES:
patent: 5619109 (1997-04-01), Cameron et al.
patent: 5677605 (1997-10-01), Cambier et al.
patent: 5912522 (1999-06-01), Rivera
Bailey John Milton
Lawler Jack Steward
Lockheed Martin Energy Research Corporation
Masih Karen
Quarles & Brady LLP
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