Rotary cam switch

Stromberg Knives

Stromberg Knives

strombergknives.com
Joined
Jan 3, 2015
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856
Hey guys!

In my current setup I use rotary cam switches to switch between belt grinders/buffers/disc grinders connected to my VFDs.

I'm about to get myself a third belt grinder and want to utilize an existing VFD. The VFD in question is already connected to two devices with a three position 3 pole rotary cam switch.

The current setup uses the middle position (of the three) as a neutral position when I switch between devices. I always thought it was a good safety measure since you don't want to have two devices sharing the VFD at once.

But how tight are these cam switches?

Would it be a bad idea to connect my third device to the same cam switch (the middle position) and then lose the "neutral position"?
 
Are there 3 poles connected in the middle position? On my drum switches there are no contactors being connected in the center.
 
Good point! I assumed - and we all know that those are the mother of all f-ups. :D

Thanks for the input, but let's say that I procure a 4 position switch (usually 0-1-2-3), my question remain. Is it too big of a risk to not have a neutral position in between the devices?
 
Is your concern, a machine is unintentionally 'on' when VFD power is applied via the rotary switch ?
An easy albeit easily ignore to actually use solution is add a 'master' toggleswitch ahead of the rotary. Switch off the master, Then select whatever machine on the rotary.
 
Thanks for your input! Well, my concern could very well be completely unwarranted. ;) I'm just double checking my thinking before I do anything stupid.

But you're right my concern mostly covers an accidental switch between machines with the motor running. And I've never done this by accident on any of my VFDs.

I guess I'm also worried that the cam switch would "leak current" between the poles when you switch, so that two motors (for a very short period of time) are connected to the VFD at the same time. But if it's in Idle mode, this may not even be a problem for it to handle?
 
I think you are overthinking this.

I have never seen any cam switch that did not disconnect one set of contacts before making contact with the next. They are made that way for obvious reasons. If they "Leaked", people could get killed. The below is the standard series of functions to switch any power.

All you need to do is:
1) turn the VFD off
2) turn the machines off
3) switch the cam switch to the selected machine
4) turn the VFD back on.
5) turn the selected machine on
 
Yeah, I understand my questions may come across as a bit overcautious. Haha, but as I said, just wanted to get additional eyes on this before I start implementing.

But it seems pretty watertight, thanks for your help!
 
Keep in mind that all the motors must be the same horse power or you will also have to change the jumper connection inside ..
 
using plugs and receptacles costs less, is less work, and you never worries about what is connected to what. most industrial VFD makers advise against any downstream switches, contactors or relays between VFD and motor.
 
razor-edge-knives said:
Keep in mind that all the motors must be the same horse power or you will also have to change the jumper connection inside ..
Click to expand...

That is true in a sense as far as safety to the motor, all it does is limit the available current to the motor (see quote below). I use a KBAC-29 1P. My grinder is 3hp and my drill press I converted to a 1.5hp 3ph motor I had laying around and works fine with the jumper on 3hp. I don't suspect it would draw more current than the label unless I was drilling a very large diameter hole. I probably wouldn't do it on a grinder. I have drilled some 1" holes in 1/2" plate and it didn't even bog down, it cut through it like butter.

Additional Product Information
The KB Electronics KBAC Adjustable Frequency Drive is a variable speed control housed in a rugged die cast NEMA-4X / IP-65 washdown and watertight enclosure. They are designed to operate 208-230 and 460 Volt 3-phase AC induction motors through 5 HP. The main features of the KBAC include adjustable Current Limit and I2t Motor Overload Protection. In addition, Adjustable Slip Compensation with auto-tune provides excellent load regulation over a wide speed range. Power Start delivers over 200% motor torque to ensure startup of high frictional loads. Electronic Inrush Current Limit (EICL) eliminates harmful AC line inrush current and Adjustable Linear Acceleration and Deceleration make the drive suitable for soft-start applications. Also provided is a Horsepower Selection Jumper which enables the KBAC to be used on motors below its maximum rating without recalibrating Current Limit and Slip Comp. Standard front panel features of the KBAC include diagnostic LEDs, a Start/Stop Switch, and Main Speed Potentiometer. Run / Fault Relay output contacts are also provided. Optional accessories for the KBAC include a Power On/Off Switch, Forward-Brake-Reverse Switch, Signal Isolator, Multi-Speed Board and RFI Filters. The drive can be manually readjusted with trimpots and jumpers or digitally programmed with optional Drive-Link or DownLoad Module (DLM).
Click to expand...


scott.livesey said:
using plugs and receptacles costs less, is less work, and you never worries about what is connected to what. most industrial VFD makers advise against any downstream switches, contactors or relays between VFD and motor.
Click to expand...

I've never seen one of these advisories. I think the rotary switches I bought were about $20 and the extra female twist lock receptacle was $30, you have to have the male ends anyway. The added cost to me of not having to swap out plugs is well worth the cost. I purchases a large enough box to mount 7 receptacles in and 2 rotary switches and still have room if I ever need them and have the switch locations labeled. Never needing to unplug them eliminates the worry of whats being powered by the VFD. Once I get to the point of 4 motors the 4th position of the first switch will send power to the second switch and allow me up to 4 more motors. If the HP jumper ever becomes that big of a deal I could simply wire my own multi position switch and label each position of the motor selection with the motor HP and change the HP selection via a switch. All in all if I ever get to running 7 motors I'll have much less money tied up than having multiple VFD's. YMMV, but this works for me.
 
KnuckleDownKnives said:
That is true in a sense as far as safety to the motor, all it does is limit the available current to the motor (see quote below). I use a KBAC-29 1P. My grinder is 3hp and my drill press I converted to a 1.5hp 3ph motor I had laying around and works fine with the jumper on 3hp. I don't suspect it would draw more current than the label unless I was drilling a very large diameter hole. I probably wouldn't do it on a grinder. I have drilled some 1" holes in 1/2" plate and it didn't even bog down, it cut through it like butter.






I've never seen one of these advisories. I think the rotary switches I bought were about $20 and the extra female twist lock receptacle was $30, you have to have the male ends anyway. The added cost to me of not having to swap out plugs is well worth the cost. I purchases a large enough box to mount 7 receptacles in and 2 rotary switches and still have room if I ever need them and have the switch locations labeled. Never needing to unplug them eliminates the worry of whats being powered by the VFD. Once I get to the point of 4 motors the 4th position of the first switch will send power to the second switch and allow me up to 4 more motors. If the HP jumper ever becomes that big of a deal I could simply wire my own multi position switch and label each position of the motor selection with the motor HP and change the HP selection via a switch. All in all if I ever get to running 7 motors I'll have much less money tied up than having multiple VFD's. YMMV, but this works for me.
Click to expand...

Yes good point... It WILL work even left on the higher jumper you are correct. But when I posed the question "what would happen if I run my .75hp motor w/ the jimper set to 1.5hp?" to a KB Electronics tech support rep, he responded that it would burn my motor out and cause it to quit working prematurely if I did this on a regular basis.
 
Josh, I think the K&B tech was providing the lawyer's response, not a tech response. I'm not at all familiar with K&B drives, I use the cheap Chinese drives. I have used Allen Bradley and Hitachi drives in industry. It's my understanding JT is correct with his statements and I wouldn't even consider it to be a problem.

The tech's response is based on this: the drive will protect your motor if it gets overloaded and is overloaded down the drive will stop providing power at the programmed (jumper in K&B's case) amp load. If you leave the jumper in the 3 hp position, and put a 1 hp motor and the motor gets overloaded, the drive will keep providing power and burn the motor out.

Scott is correct, VFD drive manuf folks do say not to have contactors, switches, etc between drive and motor. Opening the circuit downstream of drive while drive is producing power is VERY hazardous to the drive, possible cause damage to drive. For our use, you're not planning to switch while motor is running and that's all that's of concern.
 
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Although sometimes saving money takes priority over everything else, there are at least 2 reasons why running all your machines off of 1 VFD may not be the greatest of ideas.

First, if the VFD goes down, all your machines go down....

Second, you lose the ability to easily optimize the VFD to the particular machine with the jumpers and trim-pots. So all your machines will be running with the same acceleration, frequency, braking, etc. values. For some machines this won't matter, others may have special needs... I recently had to replace the VFD on one of the wood lathes, I discovered that the OEM had set the original VFD to output 120 hz and was over-speeding the 60 hz motor, but insuring that the TEFC motor had adequate airflow anytime the lathe was at half speed or above. But some other motors in the shop would not perform well at more than 60 hz.
 
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