Delayed Automatic Forward Reverse Motion of Overhead Crane Driven by a Motor

This is another situation that requires the overhead crane to rest for a while upon reaching both of the rail. After a pre-determine time the overhead crane will re-start in the opposite direction.

The control circuit of this system is shown in Figure 8. Two times TR1 and TR2 are connected to the limit switches Y and X respectively at terminal 3.

The terminal no. 1 of the instantaneous contacts TR1(1-3) and TR2(1-3) are connected to the terminals no. 3 of the limit switch Y and X respectively. The delay contact TR1(6-8) and TR2(6-8) are connected to the contacts R(13-14) and F(13-14) respectively.

Supposing the overhead crane is moving in the forward (right) direction that means, contactor F(A-B) is at energized and contactor R(A-b) is at de-energize condition. When the overhead crane reaches the right end limit and touches the limit switch Y, the contact Y(1-2) of limit switch Y will open to de-energize contactor F(A-B). This will bring the overhead crane to stop moving in the forward right direction and it will remain at stand still for a pre-determine time set by timer TR1. Also the contact Y(1-3) of the Y limit switch will close to de-energize timer TR1. The instantaneous contact TR1(1-3) will close instantly to maintain TR1 continously energized.

After the pre-determine time set by TR1, the delay contact TR1(6-8) will close to energized contactor R(A-B). This will cause the overhead to start moving in the reverse (left) direction. At this point the connection of limit switch Y contacts Y(1-2) will open and contact Y(1-3) will close.

When the overhead crane reaches the left and it touches the limit switch X, the contact X(1-2) of limit switch X will open to de-energized contactor R and timer TR1. Instantaneous contact TR1(1-3) will open. At this point the overhead crane will stop moving in the reverse (left) direction and it will remain at stand still for pre-determine time set by timer TR2. Also the contact X(1-3) of the X limit will close to energized timer TR2. The instantaneous contact TR2(1-3) will close instantly to maintain TR2 continuosly energized. After the pre-determine time set by timer TR2, the delay contact TR2(6-8) will close to energize contactor F(A-B).

When the contactor F(A-B) is energized, it will cause the overhead crane to start moving in the forward (right) direction. Immediately after, the connection of contact X(1-3) will open and contact X(1-2) will close again. Timer TR2 will remain energize because its instantaneous contact TR2(1-3) is maintain close. Timer TR2 will be de-energized when the overhead crane reach the right end limit and touches limit switch Y. 

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Automatic Forward and Reverse Motion of Overhead Crane Driven by a Motor

Notice that when contact (1-2) of limit switch X opens to de-energized contactor F, its contact (1-3) will almost simultaneously closed to energized the contactor R. This will bring about the automatic reversal of the overhead crane from the left to the right. 

On the other hand where contact (1-2) of limit switch Y opens to de-energized contactor R, its contact (1-3) will almost simultaneously closed to energize the contactor F. This will bring about the automatic reversal of the overhead crane from the left to the right.

Next Article: Delayed Automatic Forward Reverse Motion of Overhead Crane Driven by a Motor  

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Limiting the Forward and Reverse Motion of an overhead crane driven by a motor

The motion of an overhead crane moving forward and reverse can be automatically stop at both end by placing a limit switches X and Y in the control circuit of the Forward – Reverse Motor Control shown in Figure below (d).

Refer to Figure below (d). When contractor F is energized, the motor runs clockwise. The motor drives the overhead crane to move the right (forward) direction as shown in Forward Reverse Motor Control . Before the crane could reach the right end wall, it will hit the Limit switch X. Limit switch X will open its normally closed contact X(1-2) to de-energized contactor F. With contractor F de-energized. The overhead crane will stop and it will never have a chance to hit the right end wall. Pressing the forward push button will never make the motor run and the overhead crane remains immobilized.

However, pressing the reversed push button will energized contractor R and will enable the motor to run in counterclockwise direction. The motor will drive the overhead crane to move to the left (reverse) direction. Before the crane could reach the left end wall, it will hit the limit switch Y. Limit switch Y will open its normally closed contact (1-2) and de-energized contactor R. With contractor R de-energized, the overhead crane will stop and prevented from hitting the wall at the left end. Pressing the reverse push button will never make the motor run and will render the overhead crane immobilized.

                The motion of an overhead crane shown in Figure above (d) can be automatically change to opposite direction when it reaches both end, that is hitting the limit switch X and Y without pressing the forward and reverse push button switches any longer. This can be done by connecting terminal 3 of limit switch X to terminal 11 of contact F(11-12). And connecting the terminal 3 of limit switch Y to terminal 11 of contact R(11-12), see Next Article: Automatic Forward and Reverse Motion

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Forward Reverse Motor Control

The forward reverse motor control is used i a system where forward and backward or upward and downward movement in the operation are needed.

An example of which are shown in figures below (a) and (b). Figure below (a) shows forward and backward lateral movement of an overhead crane driven by motor M. Figure below (b) shows a downward and upward movement of a load is driven by motor lifter M.

(a) Lateral movement of an overhead crane driven by a motor.

   

  (b) Vertical movement of a lift by a motor.

Control Operation

      The clockwise (forward) and counterclockwise (reverse) rotation of a motor can be caused by interchanging the connection of any of the two of its three terminals. Figures (c) and (d) below will show how this can be done.

Contractors F and R are interlocked. It means that if the contacts (1-2, 3-4, 5-6) of contractor R can not be closed. On the other hand if  the contacts (1-2, 3-4, 5-6) of contractor R are closed, contacts (1-2, 3-4, 5-6) of contractor F can not be closed. 

        Note that when the contacts of contractor F in Figure (a) are closed, L1 is connected to T1 through contact F (1-2), L2 is connected to T2 through contact F(3-4), and L3 is connected to T3 through contact R(5-6).The motor M will run forward. 

        When the contacts of contractor R in the Figure (a) are closed, L1 is connected to T3 through contact R (1-2), L2 is connected to T2 through contact R(3-4) and L3 is connected to T1 through contact R(5-6). The motor M will run reverse.


        Referring to the control circuit on Figure (a), pressing the forward push button F will energize contactor F. Maintaining contact F (13 14) will close to maintain contact or F continually energized even if the forward push button is release. Contactor F (11-12) will open to prevent contactor R to be energized. Contacts F(1-2, 3-4, 5-6) will close to run motor M on forward direction.


       Pressing the stop push button will de-energized contactor F. This will cause the contacts R(1-2, 3-4, 5-6) to open and stops the motor M from running forward. Contacts F(11-12) will close again. Maintaining contact F(13-14) will open.  


       Pressing the reverse push button R will energize contactor R. Maintaining contact R(13-14) will close to maintain contactor R continually energize evenif the reverse push R button  is release. Contacts R(11-12) will open to prevent contactor F to be energized. Contacts R (1-2, 3-4, 5-6) will close to run motor M on reverse  direction.

     Pressing the stop push button again will de-energized contactor R. This will cause the contacts R(1-2, 3-4, 5-6) to open and stops the motor from running reverse. Contact R(11-12) will close again. Maintaining contact R(13-14) will open. This brings the condition of the circuit the same as the one shown in Figures (a) and (b) below.  

(a) Control circuit of Forward – Reverse Motor Control with limit switches X and Y.

(b) Power circuit of Forward Reverse. 

(c) Connection of Motor to power lines at forward condition.

 (d) Connection of Motor to power lines at reverse condition.

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WYE-DELTA REDUCE VOLTAGE STARTER

Some motors starters at high current more than several times its current at full load condition. The effect of this to other is necessary tripping of circuit breakers.

          One way of reducing the high starting current of a motor is the start the motor with the low voltage at its winding. After a few seconds, when the motor is already running at its stable condition, the rated full voltage is applied to its winding. This system is called reduced voltage starting.

          One method of reducing the starting current of a motor is to connect the winding of the motor into WYE connection during the starting process and when the motor is running, the winding are connected into Delta, see figures 9(c) and (d). Note that at WYE connection the voltage across the motor windings is 220 volts, Figure below (d).

CONTROL OPERATION

Figure below(a) and (b) shows the power and control circuit of a “WYE-DELTA Reduced Voltage Starter”. A time delay relay (Timer) is in the control circuit. The timer TR will delay the opening and closing of its normally closed contact TR (8-5) and normally open TR (8-6) respectively when its coil terminals TR (2-7) is energized, the duration of delaying I set by adjusting a knob in the timer TR.

          Referring to figures below (a) and (b), pressing the start push button will energized M1 contactor, timer TR, delay relay, and S contactor. Contactor M1 (13-14) will closed to maintain control circuit. Contact S (11-12) will open to prevent unnecessary energizing of contact motor M2. Contacts M1 (1-2,3-4,5-6) and S (1-2,3-4,5-6) will closed simultaneously to connect the motor windings (T1-T4,T2-T5,T3-T6) into WYE connection see figure below (c).

After a few seconds the normally closed the normally closed contact TR (8-5) will open and normally open contact TR (8-6) will close. Contactor S will be de-energized to open its contacts S (1-2,3-4,5-6) and closed its contact S (11-12). Also contactor M2 is energized when TR (8-6) closes. Contacts M1 (1-2,3-4,5-6) will closed. At this contacts M1 (1-2,3-4,5-6) remains closed. At this condition the motor winding (T1-T4,T2-T5,T3-T6) is change into delta connection as shown in figure below (d).

          Pressing the Stop push button will de- energized contactor M1,timer TR, and contactor M2. This will bring the motor into halt.

(a) Control Circuit of the WYE-DELTA starter.

(b) Power Circuit of the WYE-DELTA starter.

(c) WYE connection of the motor windings.

(d) Delta connection of the motor winding.

 

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TWO STATION MOTOR CONTROL

Most often a motor is controlled from two locations as shown in figure below. The stations are station 1 and station 2.

The motor M can be started and stopped from the push buttons located in the circuits of the “Two Station Motor Control” is shown in figures below.

 

Control Operation

The start and stop operation of the “Two Station Motor Control” is the

same as the start and stop operation of the “Across The Line Magnetic

Starter” except the two stop and two start push button are used in the two

station motor. The two stop push button are connected in series while the

two start push button are connected in parallel.

Pressing any one of the two start push button will complete the current

flow L1, O.L. (95-96), stop push button station 1. Stop push button station 2,

contacts (3-4) of either any one of the start push buttons at station 1 or 2,

contactor C to line L2. This will energize contactor C  and cause the motor M

to run. Pressing the stop push button either at station 1 or station 2 will de-

energized contactor C and will cause the motor M to stop.

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