Electricity: motive power systems – Induction motor systems
Patent
1995-09-19
1997-03-18
Ro, Bentsu
Electricity: motive power systems
Induction motor systems
361 24, H02H 600
Patent
active
056126042
DESCRIPTION:
BRIEF SUMMARY
SUMMARY OF THE INVENTION
The present invention relates to a method of monitoring a temperature rise of a stator winding of a squirrel cage induction motor supplied by a frequency converter, in which method a stator current I of the motor is measured and the temperature rise of the motor at each moment is determined on the basis of this current I, when a rated temperature rise .theta..sub.N of the motor at a rated current I.sub.N, the rated current I.sub.N and a temperature rise time constant .tau. of the motor are known, the method comprising steps of by .DELTA.t long time periods, time period n from the equation ##EQU2## determining a cooling .theta..sub.n(2) of the motor during the same time period n from the equation preceding time period, and .theta..sub.n(2) occurred during the same time period to obtain the temperature rise .theta..sub.N of the motor at the end of the time period n.
BACKGROUND OF THE INVENTION
(1). Description of the Related Art
Similar methods applicable to monitoring temperature rises, though not for frequency converter supply, are known from GB Patent Application 2151862 and EP Patent Application 350 507, for instance.
(2). Field of the Invention
Electromechanical transmission generates power losses in a squirrel cage induction motor and the losses generated appear as temperature rises of the motor. A magnetizing current to be fed to the motor already causes a temperature rise and upon adding the load the motor warms up more. Part of the heat generated can be caused to flow out of the motor by means of cooling. The outflow of the heat depends on the cooling technique of the motor. The net temperature rise of the motor (temperature rise caused by power losses minus heat outflow) determines the maximum allowed loading capacity. The more the motor is cooled, the more power losses may be allowed for it. The motors are provided with temperature limits, which should not be exceeded. The limits are generally informed both as maximum temperature rise of the insulation of the winding and as maximum temperature rise of the whole motor, whereby the endurance of bearings, for instance, is considered. If the motor exceeds the allowed temperature, the result is either a shortened service life of the motor or, in the worst case, a damaged motor. The limits of the temperature rise of the motor thus restrict the loading capacity. This invention relates to an examination of a temperature rise at the generally most sensible point of the motor, the stator winding. Motor identification plates disclose how much the motor can be loaded by sine wave feed at rated frequency, but the situation becomes more complicated when the supply frequency is changed.
Frequency converter supply causes more power losses in a squirrel cage induction motor than sine wave feed, due to which a motor supplied by frequency converter has a lower loading capacity than a motor having sine wave feed. Additionally, the cooling of the motor causes problems with loading capacity, when a frequency converter is used. The cooling capacity of air-cooled rib-cooled motors changes as a function of frequency, which also causes a change in the maximum loading capacity of the motor as a function of frequency. The maximum loading capacity of especially self-ventilated motors decreases considerably at low speeds when the ventilation capacity is weakening.
Loading capacity is generally informed in relation to rated moment. FIG. 1 of the attached drawing shows a typical moment ratio curve of the loading capacity of a self-ventilated motor when a frequency converter is used, i.e. moment/rated moment in relation to the frequency. The first part (0 . . . 45 Hz) of the curve considers the effect caused by weakening ventilation capacity and the latter part (50 . . . 100 Hz) of the curve considers the effect caused by a field weakening on load moment.
The curve of FIG. 1 is a simplification, which does not illustrate the actual loading capacity accurately. A more accurate idea of the effects of one frequency converter on the loading capacity is ob
REFERENCES:
patent: 4547826 (1985-10-01), Premerlani
patent: 5539601 (1996-07-01), Farag
Erkkila Ilkka
Kauhanen Matti
Veijalainen Vesa
ABB Industry OY
Ro Bentsu
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