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electric driver motor


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The interlock contacts installed in the previous section’s motor control circuit work ¯ne, but the motor will run only as long as each pushbutton switch is held down. If we wanted to keep the motor running even after the operator takes his or her hand of the control switch(es), we could change the circuit in a couple of different ways: we could replace the pushbutton switches with toggle switches, or we could add some more relay logic to “latch” the control circuit with a single, momentary actuation of either switch. Let’s see how the second approach is implemented, since it is commonly used in industry:



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When the “Forward” pushbutton is actuated, M1 will energize, closing the normally-open auxil- iary contact in parallel with that switch. When the pushbutton is released, the closed M1 auxiliary contact will maintain current to the coil of M1, thus latching the “Forward” circuit in the “on” state. The same sort of thing will happen when the “Reverse” pushbutton is pressed. These parallel auxiliary contacts are sometimes referred to as seal-in contacts, the word “seal” meaning essentially the same thing as the word latch.
However, this creates a new problem: how to stop the motor! As the circuit exists right now, the motor will run either forward or backward once the corresponding pushbutton switch is pressed,
and will continue to run as long as there is power. To stop either circuit (forward or backward), we require some means for the operator to interrupt power to the motor contactors. We’ll call this new
switch, Stop:

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Now, if either forward or reverse circuits are latched, they may be “unlatched” by momentarily pressing the “Stop” pushbutton, which will open either forward or revere circuit, de-energizing the energized contactor, and returning the seal-in contact to its normal (open) state. The “Stop” switch, having normally-closed contacts, will conduct power to either forward or reverse circuits when released.


So far, so good. Let’s consider another practical aspect of our motor control scheme before we quit adding to it. If our hypothetical motor turned a mechanical load with a lot of momentum, such as a large air fan, the motor might continue to coast for a substantial amount of time after the stop button had been pressed. This could be problematic if an operator were to try to reverse the motor
direction without waiting for the fan to stop turning. If the fan was still coasting forward and the “Reverse” pushbutton was pressed, the motor would struggle to overcome that inertia of the large
fan as it tried to begin turning in reverse, drawing excessive current and potentially reducing the life of the motor, drive mechanisms, and fan. What we might like to have is some kind of a time-delay
function in this motor control system to prevent such a premature startup from happening. Let’s begin by adding a couple of time-delay relay coils, one in parallel with each motor contactor coil. If we use contacts that delay returning to their normal state, these relays will provide us a “memory” of which direction the motor was last powered to turn. What we want each time-delay contact to do is to open the starting-switch leg of the opposite rotation circuit for several seconds,
while the fan coasts to a halt.

If the motor has been running in the forward direction, bothM1 and TD1 will have been energized. This being the case, the normally-closed, timed-closed contact of TD1 between wires 8 and 5 will
have immediately opened the moment TD1 was energized. When the stop button is pressed, contact TD1 waits for the speci¯ed amount of time before returning to its normally-closed state, thus holding
the reverse pushbutton circuit open for the duration so M2 can’t be energized. When TD1 times out, the contact will close and the circuit will allow M2 to be energized, if the reverse pushbutton is pressed. In like manner, TD2 will prevent the “Forward” pushbutton from energizing M1 until the prescribed time delay after M2 (and TD2) have been de-energized.


لینک به دیدگاه

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