Circuits
37 Plugging with a Zero-Speed Switch
When a motor is de-energized, the shaft continues to spin until inertia and friction cause it to slow down and come to a halt. If this condition is undesirable (for example, if we want the motor to stop instantly), we can use a plugging circuit, which requires a forward / reverse motor starter and a zero-speed switch.
Although the motor operates in one direction only, a reversing magnetic starter is needed.
A zero-speed switch is a switch that is activated by centrifugal forces and is attached either directly or via a belt to the shaft of the motor. To provide plugging action, the zero-speed switch must be wired normally open so that any motion of the motor shaft will cause the contacts to change their state and close.
To plug a motor to a stop, the circuit momentarily reverses the motor while it is rotating in the forward direction, but then de-energizes it before it can build up any momentum in the reverse direction.
Under normal operating conditions, the motor is started in the forward direction by pressing the start button. This energizes the forward coil and closes the normally open 2-3 holding contact in parallel with the start button as well as opening the normally open electrical interlocks in series with the reverse coil.
The motor will continue to run and the circuit will operate as a standard three-wire circuit providing low-voltage protection (LVP) until either the stop button is pressed, or an overload occurs.
While the shaft of the motor is spinning, the contacts of the zero-speed switch will change their state, and the forward contact will close. This contact is wired in series with the reverse coil, but cannot energize it so long as the electrical interlock remains open.
When the stop button is pushed, the forward coil is de-energized, dropping out its armature. This causes all the contacts associated with that coil to return to their normal condition, including the normally closed electrical interlock.
Since the shaft of the motor is still spinning in the forward direction due to inertia, the normally open zero-speed switch contacts are being held closed. This means that the instant the stop button is pressed, the reverse coil gets energized, momentarily sending the motor in the reverse direction and cancelling out all of its forward momentum, thus bringing the motor to a halt much quicker.
The instant that the shaft of the motor begins to move in the reverse direction, the held-closed forward contacts of the zero-speed switch open and the reverse coil is de-energized before the motor can build up any momentum in the reverse direction.
The motor has now been plugged to stop. In practice, this all takes place in a fraction of a second.
A circuit that utilizes the momentary reversal of the direction of a three-phase motor to bring it to a sudden stop. Can be very hard on the motor and any driven equipment.
A switch that is activated by centrifugal forces and is attached either directly or via a belt to the shaft of the motor.
When a motor is spinning in one direction and is stopped and suddenly re-energized in the opposite direction before the shaft of the motor has time to come to a complete stop.
A contact that under normal conditions does not have continuity through it. When the contact changes its state it permits the flow of current by closing its contacts. Can be associated with pushbuttons, pilot devices or magnetic contactors.
The conducting part of a switch that makes or breaks a circuit.
Also known as a "maintaining" contact, these are the normally open contacts of a magnetic starter that are connected in parallel with the start button in a three-wire control circuit. When using the conventional NEMA numbering system, they get wire numbers "2" and "3."
In electrical terms, refers to a connection where current has more than one path to flow.
Loads connected in parallel will experience the same potential difference (voltage), but may draw different values of current depending upon their individual resistance.
Normally-closed contacts used in forward/reverse control circuits that prevent both directions coils from being energized at the same time.
In electrical terms, refers to a connection where current has only one path to flow.
Loads connected in series will have the the same value of current flowing through them, and share the total voltage between them. Switches and overcurrent equipment is connected in series with equipment to control and protect it.
In motor control terminology, a three-wire circuit utilizes a magnetic motor starter with a holding contact, along with momentary contact pushbuttons. A three-wire circuit provides low-voltage-protection.
Circuits with low-voltage protection will not automatically turn back on when voltage is restored following a power outage. Examples include the microwave or power tools.
A moderate and gradual rise in the value of current over a relatively long period of time that is caused by excessive amounts of current drawn by a motor due to too much load being put on the motor.
With respect to magnetic contactors, the armature or plunger is the movable part of the magnetic circuit. When a coil is energized the armature is pulled in, opening and/or closing a set or sets of contacts.
