contactor after vfd and EMF vs USB

[carlcnc]

I learned the hard way that when using a VFD as a phase converter
to NOT put a contactor between the VFD and motor.
I not only blew one VFD ...I blew 2 of them before having a DUH! moment
Hope this saves someone else some $$

on another Weird note
, I have had problems on my USB devices when a contactor coil
"opens". I noticed this when the one for my dust collector would open
[turn off] since it was sporadic,maybe 3 out of 5 times it took days before I caught the relationship.
I made a goss screen out of some mesh and put over the contactor
problem went away

Carl

[dana_swift]

Carl- you did not ask for comment- so with some hesitation I offer the following explanation:

What you are experiencing is the effect of the collapsing magnetic field in the motor (in your first case) and the solenoid coil (in the second case).

What happens is the coils have energy stored in the magnetic fields around them when they are powered. When the current stops flowing the magnetic field collapses and the coils act as generators. The voltage attempts to go to infinity to dissipate the power. Before it gets to infinity, it creates an arc when the air breaks down between the contactor contacts.. and at that same moment destroys the solid state controls on the other side of the contactor. Thats almost certainly what happened to your VFD.

With a contactor coil opens, the voltage through the coil again attempts to go to infinity, and cannot. Two relevant things happen.. one is the collapsing field lines connect with your USB cable which briefly acts like a transformer. You induce a voltage in that cable which sometimes pushes it over its ability to withstand error voltages. Making data transfer unreliable.

When you put a screen over the contactor you created a "Faraday shield" which contains the magnetic field, however it doesn't stop the voltage from attempting to go to infinity and potentially causing other problems.

If the contactor is driven by a DC control signal, a simple diode across the coil can eat the energy in the collapsing field and solve most of the remaining voltage problems. (The shield is still a good idea, because the magnetic field will always be there.)

It the contactor is driven by an AC signal, look into using an MOV (metal oxide varistor) or "surge suppressor". You can find these at any electronics supplier with various ratings. The rated AC voltage of the MOV should be just a little higher than the voltage you are driving the coil with. Again this will eat the energy in the collapsing field and prevent further problems.

MOV's work on DC too, and most of them have a DC "clamp" voltage, it does not hurt the MOV to absorb the surge in a case like what you describe.

Hope that makes sense, and even helps-

D