Direct Online Starters
When inspecting and testing on a 3-phase board (or even a single phase), one of the items of equipment that you may have to deal with is a DOL starter. Typically, it will have 3 phases going into a contactor and 3 phases from the other side of the contactor feeding a motor or other machine or socket outlet.
What they do
The design of the DOL starter is such that it encompasses vital safety features in order to comply with the requirement of BS7671 that machines are not capable of unexpected starts that may cause danger. It does this through the contactor which is held in the ‘closed’ position only when there is current passing through a coil that controls the position of the contactor i.e. open or closed. This coil is called the ‘low or no-volt coil’.
What this means is when the supply to the DOL starter fails then the contactor is opened (off) and if the supply is re-instated the only way of re-energising the motor or other equipment is by pressing the green ‘start’ button again (on). Obviously all DOL starters include for safety an emergency stop button.
Whats the problem?
So why do we care about it and what does it mean during inspection and testing? There are two reasons to take DOL starters into account; testing of them is slightly more complex and they must undergo a functional test.