Switching contactors with PLC 1

[JacobGog]

Dear all,

I would like to ask for an advice regarding starting of 4kW motor. We use this motor to power centrifugal pump and we need to start with start - delta starting.

My question is whether it is a good practice to switch between the star and delta contactors with PLC. My understanding was that to make sure that the 2 contactors will never be engaged at the same time there has to be mechanical connection between the contactors that prevents just that and makes sure that only 1 of them is engaged at the same time.

Recently however I found out that competing product just simply uses PLC signal (with delay I imagine) to switch between contactors. For us of course that would be much easier and cheaper. But my question is: Is this a safe practice? Are there any safety precautions we need to take to make sure we never have 2 contactors engaged at the same time?

Thank you
Jakub

 

[BrianPetersen]

Pretty common practice is to include a NC contact of one contactor in the coil circuit of the other one (both directions) thus ensuring by hard-wiring that they cannot be energised simultaneously. These contactors should be the forced-guided ("safety") type to ensure that this NC contact can never be closed if the main contacts (which are NO) are closed.

 

[JacobGog]

Thank you for clarification. That makes sense.

Jakub

 

[jraef]

The mechanical linkage is there to prevent a catastrophic failure should one contact weld and stay closed when the other contactor pulls in. Should that happen, it is a “bolted fault” (phase-to-phase short circuit) that generally results in the fault being cleared a higher level protective device, meaning more than just the one machine will be shut down.

Whether or not that risk of added down time losses is worth the pennies you save in not using the mechanical interlock is your decision, but 99% of the rest of industry uses the mechanical interlock, so you have to imagine that we are not all wrong.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

I agree with Jeff except for one small point. I would take his 99% to 99.99%
A long long time ago I depended only on the NC and NO contacts to prevent both contacts from closing together on a reversing motor.
Bad idea.
After that catastrophe I will never do that again.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Bill,
The elevator industry uses a lot of wye-delta starting schemes for hydraulic elevators, they don't use a mechanical interlock because they are too cheap to do so. In fact they only use two contactors, not three, meaning 3 of the motor leads are permanently connected to the line at all times. I guess their philosophy is that they generally are only going to be serviced by the same people selling the equipment, so once something outlasts the warranty, it's going to generate revenue. Crazy stuff.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

When teaching motor controls, I used a demonstration setup of two contactors with cross connected NC contacts and without mechanical interlocks.
I used a light bulb in place of a direct jumper connection between the phases.
When both contactors were energized simultaneously, the inertia gathered before the NC contacts opened was enough to kiss the main contacts and flash the light bulb.
I haven't had the misfortune of repairing any hydraulic elevators. grin
I guess once they are past the main disconnect on the panel, they are subject to different codes.
I assume that a winding to ground fault will be cleared by the main disconnecting device on the control panel.

Bill
--------------------
"Why not the best?"
Jimmy Carter