Patent GB-1449358-A (GB)
Title: Braking of universal motors
IPC: H02P3/12
Filed: 1974-08-05
Abstract: 1449358 Control of A.C. motors CORDEM CORP 5 Aug 1974 [17 Aug 1973] 34392/74 Heading H2J A universal series motor is braked by disconnecting it from the supply, reversing the connections of the armature winding and connecting a braking resistance across the seriesconnected field and armature windings, generation of voltage in the armature winding to produce the braking effect being ensured by supplying a pulse of current through the field winding and independently of the armature winding, when the motor is disconnected from the supply. The arrangement prevents failure of dynamic braking when the motor is operating in an "overhauling load" condition and the field structure is almost demagnetized, the residual field producing a low voltage that may be unable to energize the field winding due to corrosion on the commutator, which acts as a partial insulator between the commutator and brushes. Such an operating condition occurs when the motor drives a winch, Figures 1 and 2 (not shown), the motor armature 12.1 and field 12.2 being controlled by a reversing switch 18 and push-button switch 19 as shown in Figure 3. With switch 18 in a desired-direction position, the switch 19 is depressed (and held down during motor driving) to energize the members 12.1, 12.2 from an A.C. supply 20, 20.1 through contacts a, 19.1, b and h, 19.3, g, closure of the former set of contacts also charging a capacitor 24 through diode 26 and resistor 27. When the switch 19 is released, contacts a, 19.1, b open to de-energize members 21.1, 12.2, contacts j, 19.4, k close to reverse the connections between members 12.1, 12.2, contacts c, 19.2, d close to connect a brake resistor 25 in series with members 12.1, 12.2 and contacts e, 19.3, f close to discharge the capacitor 24 through the field winding 12.2. In a modification, Fig. 4 (not shown), the switch 19 is replaced by a double pole, double throw, toggle switch (30), operation of which to drive the motor short-circuits the capacitor, diode, resistor circuit, and operation of which to de-energize the motor corrects the latter circuit between the A.C. supply and the field winding, to produce a current pulse through the field winding as the capacitor charges. The circuit of Figure 3 may be modified to short circuit the resistor 25 after a predetermined time delay and so produce an increase in the braking effect after the initial stage of the braking has slowed the motor. This modification, Fig. 5 (not shown), uses a relay (31) whose contacts (a, 31.1) shunt the resistor 25 and whose coil (31.2) is in parallel with a further capacitor (32) forming part of a series circuit of capacitor (32), diode (33) and resistor (34) connected between contact b of switch 19 and the A.C. supply terminal 20.1. The circuit of Fig. 4 (not shown) may also incorporate time-delayed shunting of resistor 25. The motor drives the winch drum (13) through a speed reducing mechanism (15) including two worm/worm gear sets (35, 36; 38, 39) in tandem, Figures 6 and 7 (not shown), the set (38, 39) having the teeth of its gear (39) shaped, Figures 8 and 9 (not shown), to increase the metal to metal contact as the film of lubrication is reduced at low speeds, whereby the set (38, 39) assists in stopping the motor and retaining it so.