# What Type Of Switch?



## Plas62 (Feb 28, 2016)

I am looking at using 1 VFD (KBAC 27D) to control three separate motor setups on grinders. none of the grinders would be used at the same time.

Two of the grinders would have 3ph,  230/460 vac,1.5hp motors with FLA of 4.8 amps. 

One of the grinders would have two 3ph,  230/460 vac,3/4 hp motors with FLA of 2.8 amps each, 5.6 amps total.

What type of switch would I need for this application?

Jim


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## mzayd3 (Feb 28, 2016)

This is a very unorthodox setup. To have 1 drive operate multiple motors, they are typically all the same HP. Using a drive loaded with parameters for one motor will not be setup correctly for a smaller (or larger) sized motor. You will end up with a motor that is inadequately protected. Aside from that, at the very least, you will need interlocked contactors on the load side of the drive. The start / stop circuit should engage the appropriate contactor before the drive is started and disengage the contactor after the drive has stopped. Drives are incredibly capable. They will have a problem with the sudden addition of motor load after starting. Likewise, they have a huge problem with a motor suddenly disconnected. 


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## John Hasler (Feb 28, 2016)

mzayd3 said:


> This is a very unorthodox setup.


I agree.  If you insist on doing this I suggest that you put cords and plugs on the machines and put the VFD in a box with a receptacle.  You will need to reconfigure the VFD each time you change machines.  Put a disconnect ahead of the VFD and never plug or unplug a machine with that disconnect closed.  The alternative would be to spend more on contactors, relays, switches, and enclosures than the price of two more VFDs.

Unless, that is, you already have the contactors.  Did these machines come with controllers?


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## JimDawson (Feb 28, 2016)

As @mzayrd3 said it is a rather unorthodox setup.  But I would use twist lock plugs,  male on the grinders, female coming out of the VFD.  That way, no switches are needed between the VFD and the grinders and you can only plug one in at a time.  Turn on the grinders with the VFD.  Set the VFD for the highest motor load (5.6 amps).  You will not have proper overload protection, but it will work.  This would not be my first choice of setups.


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## uncle harry (Feb 28, 2016)

Plas62 said:


> I am looking at using 1 VFD (KBAC 27D) to control three separate motor setups on grinders. none of the grinders would be used at the same time.
> 
> Two of the grinders would have 3ph,  230/460 vac,1.5hp motors with FLA of 4.8 amps.
> 
> ...



I would lean towards a RPG and not a vfd .  They are very forgiving & not sensitive to open circuit conditions. You could interconnect each machine with it's own power contactor. A rotary selector switch would give power isolation between machines to select the contactor coils.


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## John Hasler (Feb 28, 2016)

uncle harry said:


> A rotary selector switch would give power isolation between machines to select the contactor coils.


No need.  You can just treat the rotary converter as a source of three-phase and hook up the machines as you would if you had three-phase service.


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## John Hasler (Feb 28, 2016)

Plas62 said:


> I am looking at using 1 VFD (KBAC 27D) to control three separate motor setups on grinders. none of the grinders would be used at the same time.
> 
> Two of the grinders would have 3ph,  230/460 vac,1.5hp motors with FLA of 4.8 amps.
> 
> ...


That VFD uses sensorless flux vector control.  It is a very nice one but is likely to be unhappy driving two motors at once.


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## Terrywerm (Feb 28, 2016)

By the time you install the necessary switches and wiring you could pay for a rotary phase converter and go that route. No switches needed other than the standard switches already on the machine. Granted, it would not give you variable speed, but I don't think you will need variable speed for grinders anyway.


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## uncle harry (Feb 28, 2016)

John Hasler said:


> No need.  You can just treat the rotary converter as a source of three-phase and hook up the machines as you would if you had three-phase service.



Agreed. My only concern would be the temptation of starting more than 1 machine at a time. This way the RPG need only be large enough for 1 2/2 the HP of the biggest load.


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## Terrywerm (Feb 28, 2016)

If I remember correctly, an RPG will simply stop working if it is overloaded, correct?


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## John Hasler (Feb 28, 2016)

uncle harry said:


> Agreed. My only concern would be the temptation of starting more than 1 machine at a time. This way the RPG need only be large enough for 1 2/2 the HP of the biggest load.


The RPC would be happy to have all the machines running at once as long as only one is under load and they start one at a time.  Each unloaded motor becomes another RPC.


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## David VanNorman (Feb 28, 2016)

RPC is the way to go cheaper and easier to work with.


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## Plas62 (Feb 29, 2016)

Thank you all for your replies. 

On the two grinders that are 1.5  hp each, one of them is a 2x72 belt grinder and the other is a disc grinder both to be used in knife making. The third grinder is a dual wheel 2x72 grinder with a 3/4 hp motor driving each of the two wheels also to be used for knife making. Variable speed is the main objective of going with a vfd in order to better control the grinding of blades. 

I may have misinterpreted the information I found online about the VFD's, but I thought that if the two 3/4 hp motors running together did not exceed the 1.5 hp setup I had in the VFD that it would be OK.

I understand that I will have to power off the vfd before switching from one grinder to another to prevent damage to the vfd. I had also thought about going with the male / female plug for switching between grinders. I was just wondering if there was a switch that could be used instead of unplugging and plugging in cords. As this would be used in my home shop and I'm the only person using it I didn't have any concern about someone else trying to turn on one of the machines while another one was already in use.

What if I bought a second vfd to use on the grinder with the two 3/4 hp motors would it be more practical to use the the other vfd and switch it between the two grinders each with the same 1.5 hp motor? 

As these vfd's with the NEMA 4 enclosure go for a little over $400 each I was hoping to come up with a solution that did not require a separate vfd for each setup, which results in $800 worth of electronics sitting unused two thirds of the time.

Thanks again for your replies and advice.

Jim


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## Ulma Doctor (Feb 29, 2016)

terrywerm said:


> If I remember correctly, an RPG will simply stop working if it is overloaded, correct?



*dependent on hook up, 
the supply breaker will pop on overload, or the motor overload will pop- if so equipped
after the reset, it should be ready to fire back up in either case*


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## John Hasler (Feb 29, 2016)

Plas62 said:


> I may have misinterpreted the information I found online about the VFD's, but I thought that if the two 3/4 hp motors running together did not exceed the 1.5 hp setup I had in the VFD that it would be OK.


Depends on the type of VFD.  That one has some fancy circuitry that tries to sense the phase of the motor current and adjust itself accordingly.  It's likely to be confused by two motors.  A simpler VFD that just blindly pulse-width modulated wouldn't care.


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## JimDawson (Feb 29, 2016)

John Hasler said:


> Depends on the type of VFD.  That one has some fancy circuitry that tries to sense the phase of the motor current and adjust itself accordingly.  It's likely to be confused by two motors.  A simpler VFD that just blindly pulse-width modulated wouldn't care.



You are correct John.  While it's a waste of VFD functionality, the Sensorless Vector VFDs that I have worked with have a provision for turning that function off and allow them to revert to simple V/Hz operation.


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## John Hasler (Feb 29, 2016)

JimDawson said:


> the Sensorless Vector VFDs that I have worked with have a provision for turning that function off and allow them to revert to simple V/Hz operation.


That would solve the problem.

[Edit] Aren't there cheaper VFDs for small motors, though?  That's a pretty fancy one.


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## fixit (Mar 1, 2016)

Plas62 said:


> Thank you all for your replies.
> 
> On the two grinders that are 1.5  hp each, one of them is a 2x72 belt grinder and the other is a disc grinder both to be used in knife making. The third grinder is a dual wheel 2x72 grinder with a 3/4 hp motor driving each of the two wheels also to be used for knife making. Variable speed is the main objective of going with a vfd in order to better control the grinding of blades.
> 
> ...




In my other life before retirement I designed a machine drive for a multiple motor system with seven different motors on one VFD. The need was to keep all seven drives in sync. All motors could be started & stopped at will without hurting the VFD & all drives would ramp up & down as a single machine. Really cool some said it would not work but we showed them it would. Fact is the VFD was on line and each motor had it's own contactor with overloads for start/stop control. 

fixit


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## JimDawson (Mar 1, 2016)

fixit said:


> In my other life before retirement I designed a machine drive for a multiple motor system with seven different motors on one VFD. The need was to keep all seven drives in sync. All motors could be started & stopped at will without hurting the VFD & all drives would ramp up & down as a single machine. Really cool some said it would not work but we showed them it would. Fact is the VFD was on line and each motor had it's own contactor with overloads for start/stop control.
> 
> fixit



Can't argue with success.  If asked, I would have said it wouldn't work well.  What was the total HP?  Was it possible to start the VFD with all of the motors off, and then switch them in one at a time?  This indicates that a VFD could be used in place of a RPC.  Now I'm going to have to try this.


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## John Hasler (Mar 1, 2016)

JimDawson said:


> This indicates that a VFD could be used in place of a RPC. Now I'm going to have to try this.


There is no reason why you should not be able to do that in principle as long as you can disable all load-sensing interlocks and feedback.  I think that it would be best to hook up a resistive load so that the VFD never sees an open circuit.


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## rdhem2 (Mar 1, 2016)

No offence to anyone trying to help and reason this out, but this whole thread is pretty much why I have given up on trying to help on this site.
One VFD for three grinders--that way the VFD will not be sitting around unused a third of the time.  What about the grinders themselves?  Aren't they just sitting around unused?  Don't mind the folks trying to tell you to do it correctly.  What do they know?

As my Dad used to say, I can bring you 3ph power out with three strands on a barbwire fence.  But is the the thing to do?

Do it correctly and safely or not at all.


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## mksj (Mar 1, 2016)

If a contactor is used between the VFD and the motor, it would probably be wise to wire a spare NC terminal on each contactor/overload relay in series between them and also with the control inputs (i.e. in series with the E-Stop). If any one trips, this would disable the drive signals.  Seems somewhat complicated to have to add contactors/overload relays for each motor, etc. It probably gets tricky with different loads,  acceleration/deceleration, etc. and how this is matched to the VFD drive parameters. You basically give up many of the VFD-motor function/safety features, but no reason it should not work when using a straight volt/Hz control. I have used several of the KB VFD drives (like the KBAC 27D), they have no programming, a few trim pots for minor tweaks, i.e. very simple. It is rated at 6.7 amps. So it may be one reason why it would work, nothing fancy.  Just about any 3 pole motor 2 position changeover switch (like a 12-20A on eBay for $15-20) would work for switching between the drives. This could be hard wired from the VFD to a switch enclosure, then 2 output cords to either grinder (I would use female locking plugs on the wires exiting the enclosure that would connect to the grinder male plugs).
http://www.ebay.com/itm/Plastic-Hou...659693?hash=item4acad6e06d:g:m5UAAOSwQYZWznyX

AC Drives FAQ (part 3): Variable-Frequency Drives and Safety-Interlock Circuits
http://www.industrial-electronics.c...epper_motors_servomotors/AC-drives-FAQ-3.html


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## fixit (Mar 1, 2016)

JimDawson said:


> Can't argue with success.  If asked, I would have said it wouldn't work well.  What was the total HP?  Was it possible to start the VFD with all of the motors off, and then switch them in one at a time?  This indicates that a VFD could be used in place of a RPC.  Now I'm going to have to try this.



The vfd was a 60 hp (over designed) with total connected hp of like 34.5 hp (five 5 hp, 0ne 7.5, & one 2 hp). VFD was on all the time, motors were started one at a time form a PB (start/stop) at operator panel station at machine. The total operation could be ramped up & down to to vary machine output.


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