Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1984-12-03
1986-12-09
Smith, Jr., David
Electricity: motive power systems
Induction motor systems
Primary circuit control
318779, 318729, H02P 540
Patent
active
046282416
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and apparatus for controlling the start-up of a polyphase a.c. induction motor.
The start-up of a polyphase induction motor can be most simply effected by connecting the motor directly to the supply phases. However, due to the low impedance of a non-rotating induction motor, a large current will flow initially until motor rotation is established whereupon the current drawn will decrease substantially. Since the large current surges caused by starting motors in this manner produce supply regulation problems, the supply authorities generally specify that these current surges be kept within certain limits (for example, three times rated load current) by some start-up control procedure.
One method of reducing the magnitude of the starting current to a polyphase induction motor is to initially apply only a fraction of the supply voltage to the motor and then progressively increase the applied voltage to the full supply level. This method is relatively easy to implement where voltage regulation merely reduces the magnitude of the voltage waveforms without changing their form (as, for example, in the case of transformer regulation). However, for reasons of economy and ease of control, nowadays the predominant method of effecting voltage regulation for motor control is by the use of controlled switching devices (thyristors, triacs) which interrupt the supply waveforms to produce a regulation of average applied voltage rather than instantaneous applied voltage. One problem of this is that it is difficult to prevent large in-rush currents upon initial motor energisation even when the average voltage applied to the motor is controlled to ramp up from a near zero value.
It is therefore an object of the present invention to provide a method and apparatus for starting a three phase induction motor by the controlled firing of switching devices, which serves to keep down the magnitude of current flowing during the start up period.
According to the present invention, there is provided a method of controlling the start up of a three phase a.c. induction motor by the controlled firing of respective switching means connected in series with each of the supply phases or motor windings, the switching means each being of the type which conduct from the moment of application of a firing pulse until current therethrough ceases, said method being characterised in that it comprises the steps of:
effecting an initial selective firing of the switching means to cause current to flow between two only of the supply phases, this firing being timed relative to a predetermined point on one of the phase voltage waveforms such that the voltage between said two phases is decreasing in value at the moment of firing,
thereafter effecting sequential firing of the switching means such as to progressively increase the angle of conduction of each switching means from an initial minimum value.
The term "angle of conduction" or "conduction angle" as used herein refers, of course, to the conduction period of a switching means and is not to be confused with the angle at which conduction is initiated relative to some reference point of the corresponding supplyphase cycle.
It will be appreciated that by controlling the precise moment of initial firing it is possible to limit the in-rush current to a minimum adequate to magnetise the motor.
The method of invention is applicable both to arrangements where the switching means are in series with respective windings of a delta-connected motor and to arrangements where the switching means are connected in series with the supply phases. In the former case, the initial firing step only involves firing one switching means whereas in the latter case, two switching means must be fired.
Preferably, the said initial firing is effected at a point in time which is in advance of the moment when the voltage between said two phases becomes zero, by an interval corresponding to 45.degree. to 55.degree. of the supply waveform.
Of course, the said predetermined point on one of the supp
REFERENCES:
patent: 3363143 (1968-01-01), Cavanaugh
patent: 4072880 (1978-02-01), Oshima et al.
patent: 4468603 (1984-08-01), Vandermeer et al.
patent: 4482853 (1984-11-01), Bhausar
Bristow Raymond E.
Fitzpatrick Michael
Groom David C.
Fairford Electronics Limited
Smith Jr. David
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