switching three phase motors - 4 pole double throw switches?

D

Darren Ellis

Joined
Sep 14, 2002
Messages
992
I'm hoping someone has already sourced something like this for switching between three phase motors with a VFD....anyone have a source (and part numbers) for four pole double throw switches that can switch 2 h.p. (7.5A @ 230V) and 1 h.p. (4.5A @ 230V) motors?

Any help is appreciated!

:)

-Darren
 
Hi Darren,

I faced a similar problem in my shop and solved it by using a Hubbell Motor Control Disconnect switch (cat. no. HBL1389D). It is a 3 pole single throw switch in a Nema 1 enclosure and is rated as a motor starting switch.

The way I have things set up in my shop I am using one VFD to run a 9" disc grinder, my KMG, a homemade 2 X 72 grinder, and my surface grinder.

I ran the three phase (plus ground) from the VFD to a 4" square junction box, then from there ran the wires out to each individual switch at each grinder, then on to each grinder motor. The wire type and size used was 14/4 SO cord.

Simply turn on the VFD, select my speed and direction, then turn on the three phase switch at the grinder I want to run. Works great!

My price on these switches was just under forty dollars each.

-Mike-
 
Hi Mike,

Just wanted to say thanks for this great suggestion! I ended up picking up 6 of those three pole switches from a surplus place on ebay for $20 each...they sell for $80 new. I'm in the process of converting everything in my shop to run from three phase motors with the VFDs I have....these switches are just the ticket!

:)

-Darren
 
On my VFD controller you have to change a jumper to switch between 0-1.5 HP, and 2HP+

So, having different power motors wouldn't work.

Just thought I'd bring this up...



That said, I do plan on getting a couple more 2HP motors (to match what I've got already) and run them off a switchbox like what y'all are talking about.

If you've got a link to a similar auction on ebay, let me know Darren. Thanks!
 
Hi Dan, I'm pretty sure it will work...that setting should be your max h.p. rating...or rather, the max current the drive can source to the load. I can let you know soon enough! :) I also emailed the company I've been getting my VFDs through and asked about running two 1 h.p. motors simultaneously through a single 2 h.p. controller. Their response was that I would probably be better off with two 1 h.p. VFDs as I may experience occasional nuisance tripping of the drive. ...for my application, I can live with an occasional drive reset. Again, I'll be able to tell you for sure in the next few weeks once I get everything switched over.

I did a quick search on ebay for those switches and didn't see any currently up for auction like the ones I got a few weeks ago. Here's what you want to look for:

http://www1.mscdirect.com/CGI/NNSRIT?PMPXNO=1941881&PMT4NO=0

I did find these:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7606150773&rd=1&sspagename=STRK:MEWA:IT&rd=1

but the ones I got are like the ones shown above at MSC. I suspect the rotary disconnect switches would work fine too, but I've really not looked into them at all...

:)

-Darren
 
Simply turn on the VFD, select my speed and direction, then turn on the three phase switch at the grinder I want to run. Works great!
Click to expand...


hmmm...this doesn't sound good to me. It is hard to argue with success...but let me just throw my thoughts out there for debate/consideration.

Somwhere, I think I came across a document about VFD...and strong warnings about using them as a Buss source of 3ph power....such that the VFD is always on with muliple motors on standby ready to run by throwing the selected switch.

The way I understood this warning was something like this: VFD's operate with feedback from the motor that they are running by monitoring back-EMF, current and voltage. Based on the feedback from the motor, the VFD will respond accordingly. When the motor is disconnected from the VFD while in operation, the VFD may sense a fault and shut off....or may respond erractically to get the motor to recover. I don't know the technical details but only understood that it is not an ideal practice. I would speculate that it may cause erractic power surges and possible damage to the VFD. After all, the VFD is designed to start and stop the motor with built in canned cycles for accel and decel. The VFD also has motor protection features and some may shut off if they detect a dropped leg.

In my feable mind, I imagine the analogy of driving your car.....then when you want to stop, you turn off the ignition. Now, the brakes and power steering don't work. This is a feature of the car that was designed to operate when the car is running..by turning the car off, you've disabled these important features. Same with the VFD. It has designed features for starting, running and stopping the motor. Use it to do this. So for multiple motors, turn the VFD off, connect the motor of choice, then restart the VFD and use the VFD to operate the motor. Don't leave the VFD on...then switching your selected motor on and off downstream.


Again, you can't argue with success. If you are doing this, and it is working...never mind what I'm saying. I'm just uncomfortable with the idea...but honestly can't provide any technical merit...so, this is just my opinion.

Now I'm 2 cents poorer:D

-Rob
 
Hey Rob, I'll be able to provide some quantitative feedback on my results by the Blade show...I'll let you know how it goes. :)

-Darren
 
Rob,

Whoops, my bad! I didn't realize I had worded my comments the way they came out! You're absolutley right. It is not a good idea to turn the VFD on first.
What I do is to select the motor I want to run by turning the appropriate switch and then turn on the VFD.

Darren,

Your thinking on the hp issue is correct. A 2 hp drive will run a 1 hp motor just fine, however the max current available to the 1 hp motor is more than enopugh to turn it to toast so extra caution on your part is warrented.

As for running two one horse motors at the same time (on a 2 hp drive).........
We've been doing much the same thing at work for over 25 years on several of our drives and there's been no issues at all.
Also, I did the same thing in my shop for quite some time (prior to getting the disconnect switches) and all worked well.
It may be that the drives you have might be more prone to a problem...don't know.

-Mike-
 
Thanks Mike!!! Now I feel better. I normally try to avoid posting notes with an over-protecting Den Mother's tone...( you're gonna shoot your eye out!!) but I couldn't step away from the computer this time. I was just so uncomfortable with concept, I was compelled to say something. At the same time I'm not the sharpest tool in the shed and don't have the expertise to validate my comments on this topic.

Now that that's cleared up.....it's back to work. I'm leaving for the Blade show in exactly 13days, 16hrs, and 22 minutes. Boy-Oh-boy!!! so much work to do....so little time!!!:D :D
 
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