Conditions- Mach3,parallel port, utilizing two axis ( X and Z) , four axis BOB with one relay , Huanyang VFD. Using mounted relay on BOB to control VFD spindle. Utilizing a stand alone relay (5 volt) to power dust collector from BOB stepper axis output pin 16 . When I write the M7/M8 command in MDI line it powers dust collector motor on just fine. When I write M9 in MDI line, the command powers off the relay but after two-three cycles of M9 command the relay will no longer power off dust collector motor. When I activate the Estop or trigger a  limit switch the motor will not power off  after two-three cycles. Any thoughts on where to start troubleshooting? Thanks for the help.

Hi,
most 5V relays are pretty small, maybe the contacts are 'welding' themselves together as a result of
the current being drawn through them.

Most people use solid state relays for such applications, they are inclined also to offer better isolation from
the control circuit, you really REALLY don't want 230VAC dissapearing into your BoB!

Craig

As Craig says, the relay is likely welding shut.

Always best to use "buffer relays" between the bob and any output - just add a 24v etc relays driven by the BOB which can then have nice heavy contacts for external loads.

It is far easier to replace an external relay if it fails than the one on the BOB, plus if the BOB relay arcs-over for any reason you will get 240v inside the BOB/PC and the magic smoke will come out.

I may have not supplied enough information. The motor load is 120 volt/ 10 amp. The relay I am using is a external relay but not a solid state type. I’m utilizing a single BOB mounted relay to run a VFD which I am NOT having  issues with.
I am now researching a solid state relay with DC input and AC output to control motor. If further supplied information changes opinions, please comment. I appreciate the help. Thanks.

Its external BUT is still only what 5v control?

I would use that relay to operate a bigger relay/contactor and control the load with that.

Solid state relays are practically all DC control AC output  - they are a couple of ££ on eBay, they do fail and mostly fail ON, but failure rates can be pretty low on a good SSR. They mostly have opto-isolated inputs too which is ideal.

I believe this is the way to go. I appreciate the advice. Thank you!