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3.2.3 Machine Operation with Starting Delay
The most common cases of starting a machine take place directly after the application of the
START signal in the automation circuit. This START signal can be provided in the automation
circuit by a human, either through the pressing of a button or with the activation of a switch.
Moreover, this command can be provided based on a sensing device, through the closing of an
NO contact, when the sensing requirements are being met. However, there are specific cases where
it is ideal that the machine starts after the elapse of a specific time delay T, from the time instant
of applying the START command. At this point it should be mentioned that the logical question:
“Instead of using the overall operation mentioned, the reason why someone cannot apply the com-
mand and signal START after the same time delay (T)” is justifiable but not correct. To clearly
understand the importance of starting a machine with a time delay after the provided starting
command, the following two examples will be presented.
Example 1: A specific category of pumps, called “hydro-lubricated”, need to be filled with
water through a supporting tank before starting. The water filling of the tank can be completed
in a short time interval of 1–4 minutes. In the case that an automatic operation of the pump is
needed, the following procedure should be followed:
1. In the beginning, the pump water filling mechanism should be set in operation, without the
pump needing to be in operation. Let’s define “T” as the required time interval for complet-
ing this process.
2. After the elapse of time T, the pump should be set in operation.
3. In parallel with (2), the operation of the water filling mechanism is terminated.
An alternative implementation of this operation can also be assumed, where in the beginning a
command can be provided to start the water filling mechanism and afterwards, when we identify
that the pump is full of water and ready to start, to give the second command for starting the
pump and terminating the water filling process. In this implementation, it should be noted that
there is no automatic operation of the pump but a fully manual one.
Example 2: Let’s consider a parts processing station. The parts are being transported to the
station through a conveyor belt. During the part processing, a proper initialization of the station
(before) and the conveyor belt (after) can start. The initialization of the station could include the
heating of specific area, the reception of complementary parts, and specific initialization tasks that
are required to prepare the station for accepting the upcoming parts. This preliminary process of
station preparation lasts a time interval T. It is more than straightforward that the start of the con-
veyor belt should be made automatic and, of course, after the elapse of the T time interval, from
the moment that the station has been put in operation.
Industrial Automation Synthesis
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In both these examples, the problem of starting a machine with a time delay, after the start of
the operation command, is evident. In these cases, where the time is included in the automation
as a parameter of the physical process for implementing the automation circuit, the utilization of
timers (time relays) is needed. A simple automation circuit that achieves the starting of a machine
with a time delay is presented in Figure 3.5. In this case, the timer T is an electronic type time
relay, which can be set at the desired delay time T, while the relay C corresponds to the machine
that we would like to control. For a better understanding of the overall automation circuit and the
utilization of the time relay, the time response of the automation circuit components (T, C) is also
presented in Figure 3.5.
The characteristics of the presented automation circuit are the ability to offer a permanent
operation of the machine with a time delay in starting; a permanent stop through the switch RS;
the ability of repeating the operational sequence as many times as desired; the re-operation; and
the permanent supply of voltage to the time relay during the whole operation. The last character-
istic is of paramount importance, especially if it is considered that the time relay is an electronic
device that should not be under voltage, in case that it is not utilized. Moreover, the time relay
after the elapse of time T is no longer utilized in the automation circuit and thus it is desirable
to have a redesigned automation circuit where, after the elapse of time T, the time relay will no
longer be under voltage. Such a circuit is presented in Figure 3.6, where the time response of the
voltage V
T
at the terminals of the timer is also presented for a better understanding of the overall
50 Hz 230 V
R
N
RS
T
C
T
T
OFF
ON
RS
C
OPEN
CLOSED
t
t
Circuit features:
•Permanent operation or stop of machine C
•Reoperation
•Timer continuously under voltage during operation
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