Circuits
32 Three-Wire Circuit with Multiple Push Buttons
This circuit is functionally identical to the three-wire circuit previously discussed, but allows control of the motor from two different locations, often called “local” and “remote” pushbutton stations.
As a rule, a motor can be controlled from any number of locations provided that we follow these simple rules:
- Wire all normally closed stop buttons in series with each other,
- Wire all normally open start buttons in parallel with each other, and
- Connect the holding contact in parallel with the start buttons.
For the above example, we will only consider two push-button stations, but the principle can be scaled up to include any number of pushbutton stations.
With all the control current going through the normally closed contacts of the stop buttons, pushing any one will interrupt the circuit and disconnect the motor starter. Pushing any start pushbutton will provide a path for current to energize the motor starter.
Once energized, the 2-3 holding contact will close, allowing the start button to be released, and the circuit will now run while providing low-voltage protection (LVP) until either an overload occurs or a stop button is pressed.
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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.
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.
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.
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 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.
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.
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.
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.
