Most spindle or feed movements need to run in both forward and reverse directions, so they need to be able to rotate in both directions. In the course, we learned that as long as any two phases of the three-phase winding of the motor stator are switched to the power supply, the phase sequence of the motor stator can be changed, so that the motor can change direction.
If we use two contactors KM1 and KM2 to complete the phase sequence change of the stator winding of the motor, then the combination of forward and reverse start lines becomes a forward and reverse control line.
The working process of the control circuit shown in Figure 1 is: when the motor M stops, if M is to rotate, then press SB2, its normally closed contact opens first, its normally open contact closes later, KM1 coil is energized, its normally closed auxiliary contact is disconnected first, its normally open main contact is closed later, M starts to run in the positive direction, and the normally open auxiliary contact closes for self-protection; When M is rotated, if M is reversed, SB3 is pressed, its normally closed contact is opened first, KM1 coil loses power, M's stator cuts off the positive sequence power supply, and its normally open contact is closed after it, under the premise of KM1 normally closed contact reset, KM2 coil is energized for self-protection, and M is reversed; M reversal to forward rotation is similar to positive transformation to reversal. This control circuit is jointly controlled by composite buttons SB2 and SB3 and contactors KM1 and KM2, which can avoid power short-circuit caused by the sintering of the normally closed auxiliary contacts of KM1 or KM2.
Figure 1 Forward and reverse control lines
Some machine tool tables need automatic round-trip operation, and automatic round-trip operation uses a travel switch to detect the relative position of the round-trip movement.
Figure 2 shows a schematic diagram of the round-trip motion of the machine tool table. The travel switch SQ1 (reverse to forward) and SQ2 (forward to reverse) are respectively fixed on the bed, reflecting the two ends of the cycle, and the bumper block A and B are fixed on the workbench, and the travel switch SQ1 and SQ2 can be depressed with the movement of the moving parts, so that the contact action is made, and the motor is running in the forward and reverse direction.
Fig.2. Schematic diagram of the round-trip movement of the workbench
Fig. 3 shows the control circuit of reciprocating automatic cycle, and its working process is: close Q, press SB2, KM1 coil is energized and self-locking, M is rotated, drive moving parts forward, the current advance is in place, bumper B presses down SQ2, its normally closed contact is disconnected, KM1 coil is powered off, M cuts off positive sequence power supply, but SQ2 normally open contact is closed, KM2 is energized again, M is reversed, moving parts are retreated, when retreating into place, collider A presses down SQ1 to make KM2 power off, KM1 is energized, M is transformed from reverse to forward rotation, and so on automatically reciprocating work。 When SB1 is pressed, M stops. If the reversing travel switches SQ1 and SQ2 fail, the normally closed contacts of the limit switches SQ3 and SQ4 cut off the power supply of M to avoid accidents caused by moving parts exceeding the limit position.
Figure 3 Reciprocating automatic cycle control circuit
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