Electricity: motive power systems – Plural – diverse or diversely controlled electric motors – Running-speed control
Patent
1980-05-07
1982-11-30
Dobeck, B.
Electricity: motive power systems
Plural, diverse or diversely controlled electric motors
Running-speed control
318 99, 318112, 318341, H02P 546
Patent
active
043617883
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
Technical Field
This invention relates to electrically operated dual motor drive systems for industrial vehicles and the like and particularly to a control circuit for such systems which enhances the operation thereof under high motor current conditions.
Background Art
U.S. Pat. Nos. 3,243,681 and 3,551,773 to Dannettell disclose overload protection systems for electric motors used in vehicles. The 3,243,681 patent discloses a control system for selectively adding a number of power transistors in parallel to accommodate high current conditions in a single traction motor. The 3,551,773 patent discloses an improvement utilizing low value resistors in the power transistor collector legs to assure even current distribution.
U.S. Pat. Nos. 3,596,154 and 3,646,414 to Gurwicz disclose systems for differentially varying the currents to the left and right wheel traction motors in a dual motor vehicle. Gurwicz, like Dannettell, proposes the use of pulse duration modulation (PDM) for varying mean motor current. In both of the Gurwicz systems the left and right traction motors are energized simultaneously but for differentially varying time periods to accommodate vehicle turning conditions as necessary.
It is highly desirable to achieve the differential variable operation mode of Gurwicz in a dual motor system and to provide overload protection for the high torque low speed operating condition which often occurs in the everyday use of industrial vehicles such as forklift trucks. The Dannettell approach is disadvantageous from a cost and complexity standpoint in the requirement for multiple solid state devices as well as control circuitry for paralleling the devices at various current intervals. Another approach, equally disadvantageous, involves the use of individual power switching transistors having sustained peak current ratings which are sufficient to accommodate the peak current which occurs under a low speed, high torque traction motor operating condition. Current transistors having high power ratings are very expensive and this approach can be as costly or more costly than the Gurwicz approach. From a cost point of view the most attractive overload protection system would utilize, to as great an extent as possible, existing dual motor circuit components and avoid the necessity for high current rating transistors. In addition, the system should enhance efficiency to achieve longer intervals between battery re-charges.
Disclosure of the Invention
In one aspect of the invention, inexpensive and highly effective overload protection means are provided for an electrically operated dual motor drive system, preferably of the type having differentially variable left and right traction motor circuits. In general this is achieved through the use of a simple and inexpensive shunt switch for selectively interconnecting the power switches of the differentially variable left and right traction motor circuits in parallel with one another, and control means for operating the shunt switch in response to a high current condition in at least one of the motors. The result is a division of the motor current between two existing power switches under high current conditions such that each power switch need only be rated at about one-half the sustained peak motor current.
In a second aspect of the invention an improvement in the operation of a differentially variable dual motor drive system is achieved through means for establishing complemental; i.e. time staggered, operating periods for the left and right traction motors, the mean current through each individual motor during its respective energization time being variable to control traction motor speed during straight ahead, reverse and turning conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic circuit diagram of a dual motor system embodying the invention;
FIG. 2 is a schematic circuit diagram showing the controller from the embodiment of FIG. 1 in greater detail;
FIG. 3 is a timing diagram for the various signal quantities which are genera
REFERENCES:
patent: 3243681 (1966-03-01), Dannettell
patent: 3551773 (1970-12-01), Dannettell
patent: 3575648 (1971-04-01), Ridding
patent: 3596154 (1971-07-01), Gurwicz
patent: 3646414 (1972-02-01), Gurwicz
patent: 3870935 (1975-03-01), Abels et al.
patent: 3882365 (1975-05-01), Yemington
patent: 4096423 (1978-06-01), Bailey et al.
patent: 4110668 (1978-08-01), Gurwicz et al.
patent: 4126889 (1978-11-01), Ibamoto et al.
patent: 4168468 (1979-09-01), Mabuchi et al.
patent: 4207478 (1980-06-01), Marumoto et al.
patent: 4247808 (1981-01-01), Hanner
patent: 4275394 (1981-06-01), Mabuchi et al.
Dobeck B.
Towmotor Corporation
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