Motor Control Circuits for begginner

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The interlock contacts installed in the motor control circuit in the previous section will work fine, but as long as each pushbutton switch is held down, the motor will run.

If we want to keep the motor running after the operator's handle is removed from the control switch, we can change the circuit in a number of ways: we can replace the push button switch with a toggle switch, or we can add more relay logic to complete the control circuit to "lock" one of the switches with a single momentary actuation.

When the "forward" button is pressed, M1 energizes and closes the NO auxiliary contact in parallel with the switch.

When the button is released, the closed M1 auxiliary contact keeps current flowing to the M1's coil, locking the "forward" circuit in the "on" state.

The same happens when the "invert" button is pressed. These parallel auxiliary contacts are sometimes referred to as sealed contacts, where the word "sealed" has essentially the same meaning as the word "latch".

However, this creates a new problem: how to stop the engine! Since the circuit is in place, the motor will run forward or reverse once the corresponding pushbutton switch is pressed, and will continue to run as long as there is power.

In order to stop any circuit (forward or reverse) we need some way for the operator to cut power to the motor contactor. We call this new switch Stop:

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 reverse 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, both M1 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 specified 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. The careful observer will notice that the time-interlocking functions of TD1 and TD2 render the M1 and M2 interlocking contacts redundant.

 We can get rid of auxiliary contacts M1 and M2 for interlocks and just use TD1 and TD2‘s contacts, since they immediately open when their respective relay coils are energized, thus “locking out” one contactor if the other is energized. 

Each time delay relay will serve a dual purpose: preventing the other contactor from energizing while the motor is running and preventing the same contactor from energizing until a prescribed time after motor shutdown. 

The resulting circuit has the advantage of being simpler than the previous example: 

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