Fig. 748.--Allen-Bradley type Z automatic motor starter. The operation of this machine is as follows: When the main switch is closed, the motor circuit is made through the fuses, resistance unit, current relay, and the motor armature. At the same time, the solenoid circuit is closed (this is connected directly across the line, and takes a current which is a small fraction of an ampere), and the plunger of the solenoid is drawn up, which produces a pressure on the resistance unit, and increases the current in the motor circuit to the predetermined value at which the current relay is set. When this value is reached the current relay operates and opens the solenoid circuit, which reduces the magnetic pull and allows the solenoid plunger to drop back slightly. This action increases the resistance in the motor circuit, which decreases the current sufficiently to allow the relay to close again. Similar cycles of operation are repeated as the motor accelerates, and each time the plunger is drawn a little farther into the solenoid, until the short circuiting contacts on the top are pushed together, which short circuits the current relay and resistance unit, making them inoperative, and completing the operation of starting the motor. It will be noted that in starting a motor with this device the current is always held down to a certain predetermined value, and it is impossible to overload the motor by too rapid starting. The current relay is calibrated in amperes, and may be set to suit existing conditions. The action of the starter being controlled by a current relay and not by an oil or air dash pot, the motor will start rapidly when under a light load, and slowly when more heavily loaded. The fuses or circuit breakers may be set at a value that will furnish protection to the motor under running conditions.
Precautions with Shunt Motors.--With motors of this type, because of the large amount of self-induction in the shunt windings, it is important to note: 1, that in switching on the field magnet, the current may take an appreciable time to grow to its normal value, and 2, that in switching off, especially with quick break switches, high voltages are induced in the windings, which may break down the insulation.
Fig. 749.--Monitor starter giving automatic start with knife switch control; designed for use with printing presses. It consists of a set of solenoids connected in series and so interoperating as to cut resistance out of the armature circuit of the motor as the apparatus it is driving comes up to speed. This type is for small motors or where no need arises for speed regulation; there is, therefore, no adjustment of speed possible aside from an actual change in motor conditions. At full speed the motor is directly across the main supply line.
Fig. 750.--Monitor automatic starter, equipped with relay for push button control.
Ques. What provision is made so that the magnetizing current will have time to reach its normal value?
Ans. The field connections are generally separated from the actual starter, and taken to the main switch, so that wherever the main switch is closed, the current flows through the field coils, before the starting lever is moved.
Ques. How are the connections arranged to avoid excessive voltage in the windings due to self-induction?
Ans. Generally the armature and field magnet circuits are placed in a closed circuit that is never opened.
In other cases, in order that the rise of voltage may not injure the insulation when the shunt is opened, a special form of main switch is sometimes used which, before breaking from the supply, puts a non-inductive resistance across the shunt of the motor. This is known as a flashing resistance.