In order for a dc motor to function properly, it must beprotected from physical damage during the starting period. Atstarting conditions, the motor is not rotating, and armaturegenerated voltage EA = 0 V. Since the internal resistance of anormal DC motor is very low, a very high current flows. It ispossible for a motor to be severely damaged by such currents, evenif they last for only a moment. A solution to the problem of excesscurrent during starting condition is to insert a starting resistorin series with the armature to limit the current flow until EA canbuild up to its rating value. This starting resistor must not be inthe circuit permanently, because it would result in excessivelosses and would cause the motor's torque-speed characteristic todrop off excessively with an increase in load. In modern practice,a starting resistor is made up of a series of pieces, each of whichis removed from the motor circuit in succession as the motor speedsup, in order to limit the current in the motor to a safe value. Inthis design problem, an automatic starter circuit is to be designedfor a 4-point shunt motor rated at 15-hp, 240-V, and 45-A. Thearmature resistance of the motor is (0.5 + 0.XX) Ohm, and the shuntfield resistance is (120 + Y) Ohm. The motor is to start with nomore than (150 + Y) percent of its rated armature current, and assoon as the current falls to its rated value, the starting resistorstage must be cut out. Draw a neat diagram of a 4-point dc shuntmotor starter. Determine the number of steps required in thestarter and the value of the resistors in each step?
