Types of Control
The control system for a single motor may be very simple or it may be reasonably complicated. A process line may consist of a number of motor driven machines. They all have to start and stop as the process demands. There are basically two types of control.
Hand Control
The motor used to drive a basic type pedestal drilling machine is a good example. It is usually controlled as in Figure 41. This is ideal as it provides No Volt Protection. This means that if the supply fails for any period of time, the motor will not restart when supply is restored again. Imagine the result, if the drilling machine started up while the operator was tightening a drill bit in the machine chuck.
The motor used to drive a ventilation fan may be controlled by a one way switch. This switch energises the contactor and the motor starts. If the supply fails for any period of time and the switch is left in the “On“ position, the motor will restart when supply is restored again. In this case No Volt Protection is not provided, as there is no danger to health or safety due to the fan starting unexpectedly.
These are referred to as Hand Control or Manual Control.
Automatic Control
The motor used to drive an air compressor at a petrol filling station is controlled in a slightly different manner. An automatic air pressure switch is attached to the air reservoir. When the pressure in the reservoir reaches the level at which the pressure switch is set, a N/C contact on the pressure switch opens. This disconnects the power from the contactor coil and the motor which drives the compressor, stops. When the pressure falls to a predetermined level the pressure switch contact closes again and applies power to the contactor coil. This starts the motor again. This process continues once the isolator for the air compressor is in the “On” position. As a result there is always an ample supply of compressed air available for any motorist wishing to avail of it.
This is referred to as Automatic Control.