Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1998-11-25
1999-10-19
Shoop, Jr., William M.
Electricity: motive power systems
Induction motor systems
Primary circuit control
B02P 126
Patent
active
059694971
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a starting circuit for an electric induction motor, and is particularly directed to starting a multi-phase motor having a plurality of phase windings to be energised from an electricity supply.
Starting an induction, or asynchronous motor can produce problems because of the current surge drawn by the motor during acceleration of the rotor to rated speed. This is especially a problem for motors connected at sites relatively remote from an electricity supply substation, where the resulting current surge can produce a local reduction in the supply voltage. This arises because of the so called "weakness" of the distribution network providing a significant source impedance. As a result, the size of electric motor that can be connected to a supply for direct on line starting is often restricted by the electricity supplier, even though the supply distribution network would be quite able to support a relatively larger motor during continuous operation.
When the motor is connected to a load requiring a high breakaway torque, the problem is magnified as the duration of the initial surge current drawn by the motor is extended. In fact in extreme cases, the supply distribution network may be unable to supply the full surge current required to generate the required breakaway torque in the machine, so that the motor stalls and fails to start.
There are a number of known solutions to the above problem of high starting currents in electric motors. One is simply to reinforce the distribution network, i.e. provide larger conductors and transformers so that any voltage reduction due to the current surge is minimised. This is obviously extremely expensive and will result in the distribution network having over capacity for steady loads. It is possible to provide electric motors with variable speed drives so that initial starting surges can be reduced by starting the machine at low speed. Power factor correction can also reduce initial current surges. Motors may be arranged to start up on reduced supply voltages but if this is done, the starting torque is reduced. Mechanical clutches may be used to reduce the breakaway torque on start up. Motor modifications such as increased rotor resistance, may be used to alleviate the starting problem.
According to one aspect, the present invention provides a starting circuit for an electric induction motor having a plurality of windings to be energised from an electricity supply, said phase windings having at least three terminals for connection to the supply, the circuit comprising first switching means arranged and controllable to conduct a unidirectional current derived from said supply between a first combination of said terminals to establish a stationary rotor flux in the rotor of the motor, second switching means arranged and controllable to supply a starting current between a second combination of said terminals selected to generate a stator flux at an angle between 0.degree. and 180.degree. to said stationary rotor flux, and switch control means responsive to a motor start signal to control the first switch means to conduct said unidirectional current for a period sufficient to establish said stationary rotor flux, and to control the second switching means to initiate supply of said starting current at the end of said period to provide a starting torque for the motor.
This starting circuit can be arranged to provide a very high starting torque but without drawing a correspondingly high surge current from the electricity supply during start up. During an initial period, the first switching means establishes a stationary rotor flux by directing the unidirectional current between a first combination of the terminals, e.g. through one of the phase windings of a delta connected three phase motor. Only after a substantial stationary rotor flux has been established, is a starting current delivered by the second switching means. This starting current is delivered to a second combination of the terminals to produce a stator flux at an appropriate angle to
REFERENCES:
patent: 3673480 (1972-06-01), Johnstone
patent: 4642545 (1987-02-01), Lewus
patent: 4681191 (1987-07-01), Ikejima
patent: 4792740 (1988-12-01), Smith
patent: 5300870 (1994-04-01), Smith
Baines Geoffrey David
Boys John Talbot
McDonald Stephen Paul
Duda Rina I.
EA Technology Limited
Shoop Jr. William M.
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