52. Types of Battery Chargers. -The battery charging equipment furnished with RCA
Photophone equipment can be divided into two general groups
1. Motor-Generator sets
2. Vacuum tube rectifiers.
The motor-generator sets are used chiefly in DC installations while the vacuum tube
rectifiers are used in AC installations, but in some cases a motor-generator battery
charger may be used in an AC installation.
53. Motor-Generator Sets. -The battery charging generator is supplied with either an
AC or DC driving motor, depending upon the power supply available at the place of
The motor-generator set consists of a motor, either AC or DC, an eighteen volt DC
generator and a control panel. When a DC motor is used, a starting box is also provided.
Figure 39 shows a motor-generator set with a DC motor, starting box, batteries, and
the "four-pole double-throw" battery change-over switch. The starting box is usually
mounted on the wall near the M-G set, and the battery change-over switch is usually
mounted near the battery box. Figure 40 shows the schematic circuit diagram of the motor
starting box and M-G set. This starting box is the same as that used with the
1/2 kv-a. DC to AC M-G set described in section 34 (see Figure 27).
54. Description of Operation of DC Motor-Generator Charger. -The motor is started by
closing the line switch. This puts the full line voltage across the motor field, and across
the armature in series with a resistor. There is a short-circuiting switch across the
starting resistor which closes when the motor is near full speed. The short-circuiting
switch is actuated by an electro-magnet. The coil of the magnet is connected across the
armature of the motor. When the line switch is closed and the motor is at rest, there is
no voltage generated in the motor to oppose the line voltage. A very large current would
flow through the motor if it were not for the resistor in series with the armature. This
resistor limits the current to a safe starting value. As the motor builds up in speed, the
generated voltage of the motor increases, and soon reaches a value sufficient to close the
magnetic switch, which short-circuits the resistor, thereby connecting the armature
directly across the line.