Introduction to Industrial Automation

3.8.4   Operation of Three Air Compressors with Predefined Combinations

Air compressors are devices that consume large amounts of current, especially during their startup. 

The motors of these large air compressors are usually motors with a Y/

Δ

 start, while it is obvious 

that significantly more current is consumed, especially in the case of having two or more compres-

sors starting up at the same time. In such cases, and in order to avoid significant voltage drop (that 

could lead to the generation of faults for electrical devices), a time delay is inserted in the start of 

their operation. If the demand for compressed air is varying, it is generally useful to have the abil-

ity to operate one, two, or more air compressors depending on the general demand. We would like 

to have a semi-automatic operation starting ability for the air compressors in more detail, based 

on the following specifications:

  1. With the help of a rotational selector switch of eight positions, it is desired to pre-select the 

operation of the air compressors, based on Table 3.3.

  2. When more than one air compressors are started, the second one should start with a con-

stant time delay with respect to the first one (positions four, five, six, and seven). The same 

feature of time delay should also exist in the starting of the third motor with respect to the 

second one (position seven).

  3. The motors of the three air compressors should all start in a Y/

Δ

 configuration.

In Figure 3.36, the necessary automation circuit for the described automation is presented. In 

the position of the blocks with the Y/

Δ

 indication, a full automation circuit for the Y/

Δ

 start-

ing of the motors is considered, with two wire commands (without buttons), that have been 

omitted from this design in order to simplify the presentation and the comprehension of this 

automation.