VFD driving 2 motors, 1 switched. OK?

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Doozer
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VFD driving 2 motors, 1 switched. OK?

Post by Doozer » Fri Jun 13, 2003 11:55 am

I know you are not supposed to interupt the output of a VFD because the inductive spike will be higher than the threashold of the IGBT's can handle. But can you run 1 motor and switch another on and off while in run mode? I know the switch contacts will still induce a spike, but mabe not as bad? Could a solid state device be wired across the switch contacts, simmilar condenser to reduce the spike? I was thinking along the lines of a MOV, but they do degrade with use. What about putting a bridge rectifier AC terminals across the contacts, and have the DC output go to an electrolytic cap? Not sure is this would work at all, just a brain storm. I could borrow my buddy's scope and experament, just wondered if any electronics junkies have run into this before? The application is I am trying to power a Hardinge lathe with a 5hp VFD and it has a small 1/10th hp 3 phase motor that runs a leadscrew that changes the pulley pitch, and thus the drive ratio. I thought of using the single phase 220v and using a start cap and current sensing startup relay on the third leg to run it, cause it will be used very intermitantly. I share my VFD with a few machines, so finding a way to switch the output on this little motor while the main motor is running would make wiring simpler (perhaps?) Any ideas appreciated. Thanks, --Doozer in Buffalo

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itspcb
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Re: VFD driving 2 motors, 1 switched. OK?

Post by itspcb » Fri Jun 13, 2003 1:55 pm

Doozer: Which Hardinge Lathe do you have?

Peter B
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Re: VFD driving 2 motors, 1 switched. OK?

Post by mbensema » Fri Jun 13, 2003 3:34 pm

Never put a capacitor on the output of a VFD, it can explode! The same holds true to any other device that is sensitive to high frequency signals. There are load reactors for VFDs that may help reduce the spike from switching the motor, but anything you use must be rated for the high carrier frequency. Switching a motor while another is running can be done, but it would be best to check with your VFD manufacturer to make sure yours can handle it. You are relying on the other motor to absorb the spike, so the small motor may not have much of an effect if the larger motor is still running

Keep in mind that all motors will run at the same speed on the VFD output, you cannot individually vary one motor's speed. If you are running your main motor at half speed, you will only get half speed of the second motor, and about half HP. The other danger is no overload protection on the smaller motor. The VFD settings will have to be set for both motors so the small motor can easily fry before the VFD even notices the problem. You will have to check with your VFD manufacturer to see if overloads for the small motor will cause problems with the control electronics, most do not recommend it.

Mike

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Re: VFD driving 2 motors, 1 switched. OK?

Post by AAFRadio » Sat Jun 14, 2003 7:18 am

With a 5HP VFD? Don't even worry about it. I have a relatively small GE 1HP VFD on my 460v HLV-H and it runs the speed control motor just fine without a whimper. A 5HP won't even know it's there. In general, the VFDs have quite a bit of self protection and will normally simply shut down if you put too big a load on them too suddenly, or switch off the load using a breaker on the load side. They have to be able to take that in an industrial environment. One of my neighbors (DoN Nichols) has a 5HP VFD of the early type, and he shunts all sorts of loads on and off of it without turning it off at all. The later ones, as I said, won't usually permit that because of their self protection features. It won't damage them, they just require a reset.

While we're at it, there are some other comments that have been mentioned about reactors and capacitors, so some discussion of that subject might be useful. Most VFD manufacturers have a section in their manuals about adding these components to their VFDs. The VFDs I'm familiar with have two major, separate sections - an input rectifier to obtain high voltage DC, and an inverter section to create the three phase AC. It is in the interface between these two sections that the reactors and capacitors are sometimes added. The reactor or choke, normally a large iron cored inductor, allows the inverter section to pull short term, sharp current peaks out of the DC supply without exceeding rectifier design ratings. They also extend the life of the smoothing capacitor on the DC buss in the VFD. Its position is in series with the rectifier diodes, so sometimes you will see them installed on the input phase(s) rather than in the DC buss section - they do the same job either place. The additional capacitors are intended to be added across the DC buss to reduce longer term sag of the DC voltage. Both of these are useful when the load isn't a quiet, steady load. Both have limits for somewhat complex reasons, and the VFD manufacturer will state maximum values of inductance and capacitance that can be added. For example, the GE manual for the AV300i suggests a 1.67mH inductance for 5HP. You only need one if you're feeding the unit with single phase, but will need three for 3 phase input. They don't recommend any capacitance be added because of rectifier diode limitations in this particular model. There are normally some screw terminals provided on the VFD for this purpose. If there aren't, the VFD manufacturer didn't design the innards with enough spare capacity to accommodate them.

Best wishes,
Mike

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Re: VFD driving 2 motors, 1 switched. OK?

Post by mbensema » Sat Jun 14, 2003 1:42 pm

My comments had nothing to do with the internals of the VFD, it was for the output. The problems with switching motors on running VFDs come from the IGBTs not able to handle the current spike. From his post, it sounded like he wanted to add a capacitor to limit the spike from switching a motor. The high frequency carrier frequency will destroy a capacitor if added to the output of a VFD.
The load reactor is designed to filter out some of the carrier frequency and will also buffer some voltage and current transients from switching motors on and off on the output with the VFD running. This is different then a line reactor which is on the input side of the VFD and is meant to reduce harmonics in the incoming lines.

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Re: VFD driving 2 motors, 1 switched. OK?

Post by AAFRadio » Sun Jun 15, 2003 6:36 pm

Completely agree. I wasn't attempting to criticise your comments, simply contribute to an excellent analysis. This sort of discussion can easily turns into a "tastes great / less filling" kind of conversation and that wasn't my purpose - too much of that goes on over in RCM. :-) The placement (and manufacturer's stated intent) of the reactor/choke/inductor is largely a function of the their design parameters for the unit involved, and I have seen it specified on the input side, the DC buss prior to the filter cap (usually on the older systems), and the load side, as you said. If we get into those design parameters and the conventions necessary to understand each other I fear we will lose most of the readership in technical minutia, so it's probably best left at that. The main point I was trying to make was that Doozer shouldn't be too worried about it. Turning a heavy feed interrupted cut at slow speeds will likely produce higher current transients than he will get with that tiny speed control motor popping in and out.

Happy Father's Day,
Mike

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mbensema
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Re: VFD driving 2 motors, 1 switched. OK?

Post by mbensema » Sun Jun 15, 2003 7:45 pm

I completely agree with you, just wanted to make sure there was no confusion.



Mike

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Re: VFD driving 2 motors, 1 switched. OK?

Post by Doozer » Mon Jun 16, 2003 11:21 am

Thank you all for your input. I have a Tsugami lathe which is a copy of a Hardinge HC (no tailstock, 8 position ratcheting toolpost). This has that speed change motor (1/10hp) that I was worried about. This will be a shared VFD, and I am also using it on my Gorton mill (very proud to own it) which has seperate motors for the power quill feed and power X and Y feeds. I think the quill motor is 1/8 hp and table motor is 1/4hp. I would like to also switch these feed motors in and out of the circut, so that combination is a 3hp main motor and switching on and off the 1/4 table motor. It also has a 1/10hp 3phase coolant pump that I might be switching on and off. Of corse I can shut off the VFD when switching these motors, but the lathe has to be running to change the belt speeds, hence the reason for my post. It sounds like I would inflict little hurt an a 5hp size VFD, so that is good to hear. I have yet to purchase the VFD. I have a hitachi SR100 1hp and I like it a lot. Was gonna order a 5hp hitachi from automation direct. I figured on getting the plain medel, not the flux-vector one, as the multiple motors would probably screw up the feedback sensing anyhow. A little cheaper too. So one last thing.. If I wanted to be paranoid and install a choke or line reactor, where should I install it. It was sugested I install it on the power input or as a load reactor, between the motor? If I can fund the proper Henrey rating and find one cheap enough, I would like to use one for extra protection, as I think the VFD price from automation direct is like $380 or their abouts. Again, thanks for your help. --Doozer

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Re: VFD driving 2 motors, 1 switched. OK?

Post by itspcb » Mon Jun 16, 2003 12:16 pm

Doozer, don't want to appear negative but there is another effect that caused me to add other modifications to my HLV-H when it would not work properly with either a Rotary convertor or a VFD. Whilst I agree the 5HP model probably won't blink at the motor issues, in my experience when the main motor is switched on the load is high and vfd's can, sensing this, back off on frequency and voltage to get a soft start. In fact on my Hardinge the start up time is said to be 3 secs. With my vfd device it actually starts it up faster than this, BUT to do so it drops the volts and frequency way down low and then rapidly ramps them up. Why is this important, because all the relays in the control cabinet drop out causing the motor to immediately stop. Good news is that there is a solution. Take a 220v supply from your shop single phase and power the relays separately no drops no fall outs.You may need a transformer and or a rectifier to power the control circuits depending on the relay power supply type and voltage.
I personally would not go near a capacitor or inductor between the vfd and the load.
If you need more come back with your circuit diagram and I'll make specific comments.

Peter B
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Re: VFD driving 2 motors, 1 switched. OK?

Post by Doozer » Mon Jun 16, 2003 3:28 pm

I was going to hook the main spindle motor direct to the VFD output, no switch, as recommended. The small belt changing motor I was gunna hook to the VFD output also, but switched thru the factory fwd-off-rev switch. I will not be using any of the origional relay contactors. I know better than to mess with high freq and capacitors so thanks for the words of caution. My hitachi manual talks about line reactors on the input side and the need if you are supplying dirty voltage and such but does not say too much about load side reactors. It does include both of them in an example hook up diagram, though. And it does list an equasion for finding the size of an input reactor, I just haven't sat down and tried to decipher the values for the equasion yet. I would still like to know if I need input side or load side reactors if I am gunna be switching a second small motor in and out of the main motor feed while this thing is in run mode. A few of you have said I will probably be OK, but I would rather spend a little extra money as insurance that I am not gunna blow up my new VFD. Thanks a bunch, --Doozer

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Re: VFD driving 2 motors, 1 switched. OK?

Post by itspcb » Mon Jun 16, 2003 4:43 pm

Doozer, keep in mind these vfd's are for use in very tough industrial ciircumstances. Tough equals lots of line variations due to other machinery coming on and off plus fast switching spikes etc. Inductors (and capacitors ) on the input side help here by holding up the line for a short time and filtering out the high frequency noise in to that all important capacitor in the vfd. Unless you live right amongst such an industrial environment I would not worry about it. Vfd's are tough too and protected against all sorts of faults.
Inductors are sometimes shown in the output lines to act as rfi filters to keep interference down.
Just my 2cents
Has anyone on this forum blown a vfd? If so how?


Peter B
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Re: VFD driving 2 motors, 1 switched. OK?

Post by AAFRadio » Tue Jun 17, 2003 8:52 pm

That sounds like it will work fine. I would agree with Peter that the control circuitry is best powered from the 220/240v single phase service if you can arrange that. The real Hardinges have a single phase transformer that can be strapped for several different voltages and I followed Peter's approach with mine, but I'm unfamiliar with your lathe. I'm a little surprised at the forward / off / reverse switch for the speed change motor, as the Hardinges use raise / lower momentary pushbuttons to run the motor forward and backward. Either will work, of course, just seems odd. Also probably not worth hooking up a line side reactor, though the GE literature is big on this. They have a capacitor life check function as a part of the extensive internal diagnostics, and they claim a significant increase in capacitor life with a series inductor. Oddly enough, they don't discuss the other benefits that Mike and Peter mentioned, though they are real enough. I still don't think you need a load side reactor unless the manufacturer requires it, and the three supplier manuals I checked quickly here don't even mention them except for much larger (>100hp) units with special loads.

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