Circuits
40 Sequence Control: On-Delay
The above circuit shows a standard three-wire circuit for a single motor starter M1. In parallel with M1 is a time-delay relay (TR) whose normally open, timed to close (NOTC) contacts identify it as an on-delay timer. These timed contacts are in series with motor starter M2. The above switching arrangement will allow two motors to be controlled from a single pushbutton station and will guarantee that M2 energizes five seconds after M1 is energized. Once both motor starters are engaged, the motors will continue to run until the stop button is pressed.
If an overload occurs on motor M2, then only that motor will stop, but if motor M1 develops an overload and its OLR contacts open, then the coil will de-energize and its 2-3 holding contact will open, disconnecting the timer coil from the source of supply. Once the on-delay coil is de-energized, its contacts will revert to their original state instantly, so motor M2 will also be de-energized.
If more than two motors are to be sequenced in this fashion, we would simply scale up this switching arrangement by connecting additional timer relays in parallel with each motor starter so that as one motor starts, it also begins a countdown for the next one in sequence.
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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 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.
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 control relay equipped with a delay mechanism that can be used to alter the timing effects of a set of contacts. There are two varieties of timer relays: On-delays which have their delay action when they are energized and revert to their original condition instantly upon de-energization, and off-delays, which change the state of their contacts instantly upon energization, but have a delay before reverting to their normal condition upon de-energization.
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 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 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.
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.
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."
