VFD switching a relay and kickback voltage

[z28tt]

I'm planning on using my Hitachi SJ200 VFD as a drive for both my lathe and mill. To switch between the motors (1 and 2 hp 3 phase), I was going to use a 120V latching relay (does a 4PDT exist?). To ensure that I don't ever switch when the motor is under load, I was going to have the run signal output terminal switch a normally closed relay to open, preventing the latching relay from getting power and switching motors. When the VFD senses the motor has stopped, the run terminal is no longer high, and the relay is back to NC, feeding 120V to the A/B switch driving the latch relay.

Since I'm only powering relays from the VFD control circuits, rather than switching multi-HP inductive loads, can I get away with not having a diode across the relay coil to prevent voltage kickback (when the relay field collapses)? Or is this really a good idea to do? If so - what diode do you guys suggest? Is there one that is ruggedized (I don't have any circuit boards in the panel, so something that's free standing would be nice).

Andris

[TOTALLYRC]

Originally Posted by z28tt I'm planning on using my Hitachi SJ200 VFD as a drive for both my lathe and mill. To switch between the motors (1 and 2 hp 3 phase), I was going to use a 120V latching relay (does a 4PDT exist?). To ensure that I don't ever switch when the motor is under load, I was going to have the run signal output terminal switch a normally closed relay to open, preventing the latching relay from getting power and switching motors. When the VFD senses the motor has stopped, the run terminal is no longer high, and the relay is back to NC, feeding 120V to the A/B switch driving the latch relay. Since I'm only powering relays from the VFD control circuits, rather than switching multi-HP inductive loads, can I get away with not having a diode across the relay coil to prevent voltage kickback (when the relay field collapses)? Or is this really a good idea to do? If so - what diode do you guys suggest? Is there one that is ruggedized (I don't have any circuit boards in the panel, so something that's free standing would be nice). Andris

Of course you could just use mechanical switches tied together so that when one opens the other one closes.

Three phase, solid state relays would be real nice, but expensive.

Mike

[Al_The_Man]

Reverse EMF diodes should always be used on inductive DC operated devices, especially if the operating device is solid state.
I always keep a quantity of 1N4005 or 1N4007's for this purpose.
All AC inductive devices should have a R/C snubber across the coil, I find this is better than a MOV, which is often used also.
The main problem with using two different motors on the same VFD is that you cannot use the self tuning feature, or tune to one particular motor , due to the different characteristics between them.
You could look into a motor contactor reverser and suitable circuit, this can be wired for two motors and has electrical interlock as well as a mechanical interlock, preventing both from picking up.
Al.

[z28tt]

Three phase, solid state relays would be real nice, but expensive.

Sure, but they're $20ea and SPST. :P Adds up fast when you need six. It's probably easier to buy a second VFD for $150, but I like the challenge of learning something new.

Originally Posted by Al_The_Man I always keep a quantity of 1N4005 or 1N4007's for this purpose.

Thanks! I'll add that to my shopping list.

All AC inductive devices should have a R/C snubber across the coil, I find this is better than a MOV, which is often used also.

Do I need to worry about this when it's just a small relay getting switched by my general house current, or is this more for motors. That being said - I've got a 5 hp single phase air compressor motor, and I've never checked to see if it's got a MOV or R/C. Maybe the contactor has it built in? It (and the rest of my house) has been fine for the last 2 years... What are good ballpark specs for the R/C circuit (both for the 5hp motor and for the 120V relay)? It it dependant on the inductance of the load? Time to buy a better multimeter?

The main problem with using two different motors on the same VFD is that you cannot use the self tuning feature, or tune to one particular motor , due to the different characteristics between them.

Not a big deal w/ the Hitachi SJ200. It's got a motor A and motor B parameters, and I can switch between them using one of the logic input terminal (low runs motor A params, high runs motor B). I'm running one motor at a time.

You could look into a motor contactor reverser and suitable circuit, this can be wired for two motors and has electrical interlock as well as a mechanical interlock, preventing both from picking up.

Thanks! I'll look into that.

Andris

[Al_The_Man]

For small AC devices it is not so neccessary until you get to contactors and solenoids, they can put out a lot of EMI.
A value of around 1uf and 330ohom in series can be used on 120vac.
AC rated capacitor!.
Al.

Originally Posted by z28tt Do I need to worry about this when it's just a small relay getting switched by my general house current, or is this more for motors. That being said - I've got a 5 hp single phase air compressor motor, and I've never checked to see if it's got a MOV or R/C. Maybe the contactor has it built in? It (and the rest of my house) has been fine for the last 2 years... What are good ballpark specs for the R/C circuit (both for the 5hp motor and for the 120V relay)? It it dependant on the inductance of the load? Time to buy a better multimeter?

[TOTALLYRC]

3 phase solid state relays switch all three legs at once, but you would need 2 and they are a couple hundred each. Thats why I said nice but expensive.

Mike