Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
2001-09-21
2004-03-23
Nappi, Robert (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S254100, C318S434000
Reexamination Certificate
active
06710574
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to motor controllers, and, in particular, to four-quadrant control of series, compound, and shunt wound direct current (DC) motors connected to a DC power source. The invention also relates to four-quadrant control of DC motors connected to a receptive DC power supply.
2. Background Information
Since the early 1900's, current in direct current (DC) motors was controlled by switching resistors in series with the motor's armature and field in order to obtain variable speed or torque as required by a particular application. This method was wasteful of energy, and did not provide very good speed or torque regulation.
During the early 1960's, solid state controllers using SCRs or thyristors were introduced which improved efficiency as well as speed and torque control accuracy. The initial controllers were first used with DC shunt motors and AC power sources, wherein SCRs were employed in AC/DC controlled rectifier configurations with appropriate current and voltage feedback devices.
During the late 1960's, impulse-commutated SCR converters became available which controlled DC motor current and/or voltage when connected to a DC power source. These converters were used mainly for crane and electric vehicle applications powered from DC sources, such as rectified AC sources, or, in the case of some electric vehicles, on-board batteries. In the case of off-board DC power sources, equipment incorporating DC motors was connected by a shoe sliding on a powered collector, rail or overhead wire, or by trailing/festooned cables.
Since it was difficult to obtain good control with such SCR technology when used together with DC series motors during braking operations in crane hoist and railed electric vehicle applications, impulse-commutated SCR converters were mainly used with DC shunt motors in these applications. See, for example, U.S. Pat. Nos. 3,535,605; 3,551,771; 3,553,554; and 3,555,385.
Impulse-commutated SCR converters were relatively complicated low frequency devices, and bulky as a result of the requirement for commutation capacitors and/or reactors. Such converters were prone to failure under high current or fault conditions.
During the late 1970's, reliable high power semiconductor switching devices, such as bipolar junction transistors (BJTs), became available. Such BJTs were employed in DC motor controllers during the 1980's. For example, in 1985, Saftronics Inc., then located in Youngstown, N.Y., produced model 2BC-300 dual DC series motor choppers for electric vehicles, utilizing 400 A/600 V BJTs manufactured by Fuji. The motor's field and armature were in series with the BJT, and a LEM 300 A Hall Effect current transducer was employed to obtain isolated current feedback. This controller made use of the well-known “current amplification effect” to obtain high motor current during stall or low speed “break away” conditions while drawing only a fraction of the motor current from a 320 VDC supply.
In the late 1980's, an improved power switching device, the Insulated Gate Bipolar Transistor (IGBT), became available and was quickly used in many DC motor control applications, instead of BJTs. One such DC/DC controller was the IGBT-based model A 375 for DC series wound motors, as manufactured in 1989 by Saftronics Inc. of Fort Myers, Fla. This controller, rated for 320 VDC, employed an IGBT and current sensor connected in series with both the motor's armature and field as configured in the 2BC-300 dual DC series motor choppers, in order to control the motive effort of a DC series traction motor. The model A 375 was applicable to both crane hoist and travel motion control, as well as motive control for railed and rubber-tired vehicles. However, it had the disadvantage that when applied to hoist control, it was difficult to maintain suitable light hook speed control. Also, braking control during lowering was very load-dependant.
During 1995, Saminco of Fort Myers, Fla. produced the IGBT-based model A812 DC/DC controller with separate control of the DC series wound motor's field as well as armature, providing “field follower” or series motor characteristics during motoring or hoisting, and shunt motor characteristics during regenerative braking conditions. The model A 812 is widely used for railed vehicle applications. However, it is not readily suitable for crane hoist applications without significant alterations to the method of connecting the controller to the industry-standard four-terminal hoist/brake assembly via sliding shoes on collector rails.
U.S. Pat. No. 5,875,281 discloses a microprocessor controlled hoist and travel motion controller, which employs a single IGBT and current transducer in series with the hoist motor's armature and field during hoisting, as employed by the model A 375. However, unlike the model A 375, this controller provides separate field control during a “Lower Fast” mode using a second IGBT to control the motor's field. In both “Lower Slow” and “Lower Fast” modes, resistors are employed to dissipate energy generated during lowering. Although this controller employs the industry-standard four-terminal hoist assembly connections, it is only used with DC series wound motors and cannot readily provide independent field control during hoist “Raise” operations. It also requires a speed feedback device mounted on the hoist motor connected to the controller's microprocessor in order to provide good speed control. Since the hoist motor is usually mounted on a moving structure, it would be very difficult and expensive to achieve this requirement.
For travel motions, the controller of U.S. Pat. No. 5,875,281 utilizes electro-mechanical switches in the series wound motor's field to establish direction of motion of the crane. When it is desired to reverse motion when travelling in a given direction, the series motor's field connections are reversed, and mechanical energy in the moving crane is dissipated in a resistor switched into the circuit by yet another electro-mechanical switch.
Many modern crane controllers for use with DC series motors in crane hoisting and travel applications are still of the “constant potential” contactor/resistor type, with one configuration used for hoisting, and a significantly different configuration used for travel (bridge and trolley) applications. These controllers use contactors which switch under load causing arcing during load break operations. This results in contactor tip burn out which requires frequent maintenance. In addition, much energy is wasted in the resistors during control operations. Furthermore, these controllers can severely stress motor life because of high voltage and current conditions that exist with this technology. Other disadvantages of such controllers include: (1) hook speed during hoisting is highly load dependent and can be relatively very high; (2) field current during low speed dynamic lowering can be as much as 250% of rated current causing possible premature motor damage due to overheating in severe duty applications; (3) armature voltage during high speed dynamic lowering can be as much as 200% of rated voltage giving rise to the possibility of DC motor commutator arc-over; (4) the resistors waste energy and create considerable heat; (5) the load-break contactor tips are a high maintenance item; (6) control can only be achieved in steps, since there are only a finite number of switched resistor stages; (7) it could be possible to overspeed the DC series motor during very light hook duty if the crane operator inadvertently applies full voltage to the hoist motor; (8) there could exist a delay between cessation of motor current at the end of a hoist RAISE motion and the setting of the series brake due to a time delay caused by the current in the series brake windings decaying slowly through a low impedance electrical path—this could cause the load on the hook to sag; and (9) during hoisting, when the operator moves his master switch to “OFF”, decelerati
Davis Anthony J.
Posma Bonne W.
Urbassik Michael A.
Eaton Corporation
Kastelic John A.
Miller Patrick
Moran Martin J.
Nappi Robert
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