Thermal co-ordination of motors, control, and their branch circuits on rower supplies of 600 volts and less

THE protection of a motor against an overload or the protection of a motor and its branch circuit against a short circuit are basically the same problem. The entire circuit is heated at a rate which is a function of the amount of current flowing, and this current must be interrupted before the winding or any part of the circuit exceeds a prescribed temperature. The means used to protect against these two magnitudes of power may be the same, or it may be entirely different as will be brought out later. It is the purpose of this article to show what the heating characteristics are of the motors, the control, and the branch circuit wiring, and what the characteristics of the protective devices should be in order that the power will be disconnected from the main feeders under all conditions of excess current before excess temperatures are attained.