Safety issue with standard motor starters


Thread Starter


With lots of corrosives and organic solvents in the plant, we have frequent trouble with start/stop buttons sticking in. If a start button sticks in the depressed position, this causes two problems: First, pressing the stop button will only stop the motor while the stop button is depressed. When the button is released, the motor will restart. Second, if the power is turned off to a motor and the start button is stuck, the motor will restart as soon as power is reapplied. Whenever I think about this, I'm really quite shocked that major
manufactures allow such a basic safety flaw out of their doors. I'm sure there are dozens of ways to fix this problem using different types of
electronic push buttons, additional logic etc.. But I would really like to standardize on a starter that doesn't have this problem. Is there a motor starter available that will start only on an off/on transition of the start button?
There are dozens of manufacturers that make varisous safety relays, most are integrated tripple redundant relays with guided contacts. Most require that the start button be pressed, then released, before the relay engages. The have stuck contact sensing and various other features. You can use one of these to drive your motor starter coil. Companies providing
these, among others, are Allen Bradley, Square D, Telemecanique, Schmersal, Guardmaster, Pilz, Banner, Norstat, Tapeswitch, Sti, Schleicher,
and many more. This is not an area that manufacturers have overlooked and the use of these safety relays is often mandated by code in many industries and regions.
There may be motor starters out there like you have mentioned, although I have not seen one. If the system is plc controlled I would
eliminate the problem by using a one shot and a latching contact. If the start button sticks the motor will still stop because the pulse is
over. It would also work the same if the motor was switched on with the start button in. Good luck!
try useing a pull to start / push to stop single button for your aplication. this will add a mechanical override to the sticking problem.
there are buttons available with momentary pull / maintained push. one contact remains closed until pushed - another contact only closes
when pulled. the existing latch circuit will keep the motor from re-srart after power fail. the electrical circuit will remain the same only
the mechanics will be changed.

basic safety flaw out of their doors.

this may not be as much of a safty flaw as much as an aplication flaw. there are nema rateings asigned to electrical push buttons to discribe the enviroment the contacts and operating mechanism are designed to withstand. look at the 800t section of a-b catalog and you will find these rateings.

I'm sure there are dozens of ways to fix this problem

lets not put a $700.00 plc system in a place where a $150.00 push button would work better.

Robert Willis

It sounds like you need to get a pushbutton with a protective boot to prevent the organic material from causing the buttons to stick in the first place. Almost all of the major manufactures have the protective boots and they are recommended for very dirty enviroments or severe hose down.

Robert Willis
Square D Company
The problem is mechanical in nature. Choose a starter that does not expose the moving parts.

Membrane buttons with very good climatic protection (IP-65 or better) with exposed parts made of reisitant materials (PVC, PVDF or whatever is applicable in your case) should resolve your problems.

Also plan to have the starter inside a box with a glass hood so that you get better protection.