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Starting of D-C Motors

Author: E.E. Kimberly

A very small direct-current motor with low armature inertia and relatively high armature-circuit resistance may be started by connecting it without intervening resistance directly to the power supply. Any other type of d-c motor must be started with the armature in series with a rheostat of sufficient size to limit the starting current to a safe value, or the armature must be connected to a variable-voltage power source which will accomplish the same result. Such a device is called a starting rheostat.

Fig. 15-1. Three-Point Starting Rheostat

It is a common practice to limit the starting current for a d-c motor to about 1 1/2 times the full-load rated current. Inasmuch as no back emf is present in the motor at the instant of start, the starting rheostat must contain such resistance that, when it is added to the resistance of the armature circuit, the initial current will be limited to a safe value. These starters are rated according to the size and voltage of the motor which they are intended to start. The resistance of a 10-hp starter would be too small for starting a 5-hp motor, and such a starter might cause distress at the commutator. The resistance of a 10-hp starter would be too large for starting a 20-hp motor, and such a starter might be burned out before the motor reached full speed.

Starting rheostats are intended only for the intermittent duty of starting. No more than 15 seconds should be consumed in moving the control handle from the first point of contact to the final position. When the starting cycle must be repeated many times in rapid succession, as in crane or dredge service, a special resistance unit adapted to continuous service must be used.

Fig. 15-2. Four-Point Starting Rheostat

In Fig. 15-1 is shown a three-point starting rheostat, to which only three connections are required. The solenoid or holding coil, which holds the starting handle in the running position, is connected in series with the motor field. By this connection a break in the shunt-field circuit also opens the holding-coil circuit and permits the starting handle to return to the "off" position; and over-speeding of the motor is thus prevented.

Fig. 15-3. Starting and Speed-Regulating Rheostat

In Fig. 15-2 is shown a, four-point starting rheostat, to which four connections are required. This type of rheostat is required by adjustable-speed motors in which the shunt-field current may be so greatly reduced at high speeds that a series holding coil would release the starter handle. In the four-point starter the holding-coil current is independent of motor operating conditions. The four-point starter is suitable for starting a series motor, whereas the three-point starter obviously is not.

In both the three-point starter and the four-point starter, the handle is returned to the "off" position by a spring if the line voltage becomes too low.

Some starters or controllers, like that illustrated in Fig. 15-3, are intended for adjustable-speed duty and are designed so generously that the starting handle may be left on any control point indefinitely without causing overheating. Such controllers cost more than simple starters and, when such equipment is purchased, it is advisable to specify the type of duty - whether simple starting or combined starting and speed control. By permitting some of the starting resistance to remain in the circuit, it is possible to obtain motor speeds lower than those which may be had by shunt-field control. The efficiency of such control is poor because of the loss of power in the rheostat. Also, the speed regulation of a motor controlled by series resistance in the armature circuit is high (great), and this type of control is thus rendered unsuitable for many purposes.

1 For further study of motor controllers, see Control of Electric Motors, by P. B. Har-wood. John Wiley and Sons.

Last Update: 2011-01-17