Circuits
41 Sequence Control: Off-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) who’s normally open, timed to open (NOTO) contacts identify it as an off-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. If the start button is pressed both motors M1 and M2 will start instantly. This is because the normally open contacts associated with the off-delay coil will change their state instantly when the coil is energized.
Once both motor starters are engaged, the motors will continue to run until the stop button is pressed. Once pressed, the M1 contactor and the off-delay timer will be de-energized, and their contacts will revert to their original state.
For the motor starter, this will happen instantly, but the timed contacts associated with the timer coil will have a delay of five seconds before they open, during which time motor M2 will continue to run. It is important to note that even though the timer coil has been disconnected from its source of power, it still performs its timing function. It does not need external energy to delay its contacts, that energy is stored in the timer, usually as compressed air or spring tension.
If an overload occurs on motor M2, 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 off-delay coil is de-energized, its contacts will delay for five seconds, then revert to their original state, so motor M2 will continue to run for five seconds after motor M1 has stopped due to an overload.
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 each motor stops one after the other in the prescribed sequence.
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.
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 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.
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.
