In like manner, TD 2 will prevent the “Forward” pushbutton from energizing M 1 until the prescribed time delay after M 2 (and TD 2) have been de-energized. When TD 1 times out, the contact will close and the circuit will allow M 2 to be energized if the reverse pushbutton is pressed. When the stop button is pressed, contact TD 1 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 M 2 can’t be energized. This being the case, the normally-closed, timed-closed contact of TD 1 between wires 8 and 5 will have immediately opened the moment TD 1 was energized. If the motor has been running in the forward direction, both M 1 and TD 1 will have been energized.
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 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. Let’s begin by adding a couple of time-delay relay coils, one in parallel with each motor contactor coil. 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. 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. This could be problematic if an operator were to try to reverse the motor direction without waiting for the fan to stop turning. 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. Let’s consider another practical aspect of our motor control scheme before we quit adding to it. The “Stop” switch, having normally-closed contacts, will conduct power to either forward or reverse circuits when released. 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.
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