Industrial control: Heating of shunt-motor starting resistors

THE USUAL METHOD of starting a shunt motor from a constant-potential source is to insert one or more steps of armature resistance which are short-circuited by contactors operated after a definite time. The required capacity of a resistor for this type of duty is directly related to load conditions and time setting of relays as well as duty-cycle requirements. If time settings are made arbitrarily without some co-ordination with the kind of load, the energy dissipated in the resistor may be unduly high, because the load may be up to speed in a time much less than the setting. On the other hand if the load has a high inertia, the definite time may be too short, causing excessively high peaks on switching points after the first one. Although d-c motors have been used extensively in this way, equations governing such operation do not appear to have been published.