Circuits
36 Forward/Reverse Control Circuits
If a three-phase motor is to be driven in only one direction, and upon its initial energization it is found to be rotating opposite to what is desired, all that is needed is to interchange any two of the three line leads feeding the motor. This can be done at the motor starter or at the motor itself.
Once two of the lines have been switched, the direction of the magnetic fields created in the motor will now cause the shaft to spin in the opposite direction. This is known as reversing the phase rotation.
Reversing Magnetic Starters
If a motor is to be driven in two directions, then it will require a Forward / Reverse motor starter, which has two three-pole horsepower-rated contactors rather than just one as in the conventional starter. Each of the two different motor starters powers the motor with a different phase rotation.
When the forward contactor is energized, power contacts connect line L1 to T1, line L2 to T2 and line L3 to T3 at the motor. When the reverse contactor is energized, the power contacts connect line L1 to T3, line L2 to T2 and line L3 to T1 at the motor.
Since the two motor starters control only one motor, only one set of overload relay heaters need be used. The return paths for both starter coils connect in series with the normally closed contacts of the overload relay so that if an overload occurs in either direction, the starter coils will be de-energized and the motor will come to a stop.
Note that the two contactors must be electrically and mechanically interlocked so that they cannot be energized simultaneously. If both starter coils became energized simultaneously, a short circuit will occur with potentially hazardous results.
Forward / reverse starters will come with two sets of normally open auxiliary contacts to act as holding contacts in each direction. They will also come with two sets of normally closed auxiliary contacts to act as electrical interlocks.
Mechanical Interlocks
Forward / reverse starters must never close their power contacts simultaneously. The best way to provide this is through electrical interlocks, which prevent the one coil from being energized if the other is engaged. A failure in electrical interlocking can cause both coils to be energized at the same time.
If both become energized, some form of mechanical interlock is required to prevent both armatures from pulling in. Represented on schematic diagrams as a dotted line between the two coils, a mechanical interlock is a physical barrier that is pushed into the path of one coil’s armature by the movement of the adjacent coil. This means that even if both coils are energized, only one armature will be able to pull in fully. The coil that is prevented from pulling in will make a terrible chattering sound as it tries to complete the magnetic circuit.
Mechanical interlocks should be relied on as a last resort for protection.
Electrical Interlocks
Electrical interlocking is accomplished by installing the normally closed contact of one direction’s coil in series with the opposite direction’s coil, and vice versa. This ensures that when the forward coil is energized, pushing the reverse pushbutton will not energize the reverse coil. The same situation is in effect when the reverse coil is energized. In both situations the stop button will need to be pressed to de-energize the running coil and return all its auxiliary contacts back to their original state. Then the opposite direction coil can be engaged.
Reversing Control Circuit
When designing the control schematic for forward / reverse circuits, we start with the standard three-wire circuit, add a second normally open pushbutton, and add a holding contact branch for the second coil. A single stop button is sufficient to disable the motor in both directions.
The two coils are mechanically interlocked and the normally closed instantaneous contacts provide electrical interlocking.
If the forward pushbutton is pressed, as long as the reverse coil is not engaged, current will find a path through the normally closed reverse contact and energize the forward coil, causing all contacts associated with that coil to change their state. The 2-3 holding contact will close and the normally closed electrical interlock will open. If the reverse pushbutton is pressed while the forward coil is engaged, current will not be able to get past the forward normally closed contact, and nothing will happen.
In order to send the motor in the reverse direction, the forward coil must be de-energized. To do this, the stop button must be pressed, then the reverse pushbutton will be able to energize the reverse coil.
Regardless of the direction the motor is spinning in, this 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.
Pushbutton Interlocks
Pushbutton interlocking requires the use of four-contact momentary push buttons with each pushbutton having a set of normally open and normally closed contacts.
To achieve pushbutton interlocking, simply wire the normally closed contacts of one pushbutton in series with the normally open contacts of the other pushbutton, and the holding contacts will be connected in parallel with the appropriate button’s normally open contacts.
This circuit still requires the installation of electrical interlocks.
Pushbutton interlocking doesn’t require the motor coils to be disengaged before reversing direction because the normally closed forward contacts are in series with the normally open reverse contacts, and vice-versa. Pushing one button simultaneously disengages one coil while starting the other. This sudden reversal (plugging) can be hard on the motor, but if quick reversal of the motor is required, this circuit can be a solution.
A device that controls the flow of electrical power to a motor. It is designed to safely start and stop a motor, and provide overload protection.
The direction that a three-phase motor spins is determined by the phase sequence of the voltage impressed upon it. To reverse the direction of the motor we simple reverse the phase sequence by switching any to line leads.
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.
A contact that under normal conditions has continuity through it. When the contact changes its state it interrupts the flow of current by opening its contacts. Can be associated with pushbuttons, pilot devices or magnetic contactors.
A heater element paired with normally-closed contacts that open once the heater gets too hot. Two types of relays are the bimetallic strip and the melting solder pot.
Normally-closed contacts used in forward/reverse control circuits that prevent both directions coils from being energized at the same time.
A physical barrier that is pushed into the path of one coil's armature by the movement of the adjacent coil in a forward/reversing motor starter.
Contacts on a magnetic starter that are not Horsepower rated. Can come as either normally-open or normally-closed and can be used as maintaining contacts, electrical interlocks or control for pilot lights.
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.
A diagram that shows how a circuit works logically and electrically. It uses symbols to identify components and interconnecting lines to display the electrical continuity of a circuit. It is often used for troubleshooting purposes. Also known as a ladder diagram.
A momentary contact device that has a built in spring to return the button to its normal position once release. Available with either normally-open, normally-closed or both sets of contacts.
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.
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."
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.
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.
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.