Switch between motor and VFD

So, everyone is saying that it is bad to place a switch between a motor and a VFD (if it is switched when running, of course).

I am sure that it is true, but I am not sure why. Also, is there something that can be done to mitigate a switch between VFD and motor being switched when running.

The question is not entirely idle in nature and pertains to my lathe.

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Yes, there certainly is something that can be done.

Wire your VFD directly to your lathe motor, with five foot leads. Now, wire your switch to control your VFD, using three foot leads. Mount the VFD as far away from your motor as the leads will allow. Now, stretch the leads to the switch in the direction of the motor, and mount the switch there. Your switch will be between your VFD and your motor, and all will be fine.

John Martin

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John Martin

One classic maneuver is to slam the switch from forward to reverse in one fluid motion, thus putting a spinning motor across the power line backwards. The applied power opposes the rotation of the motor, forcing it to deaccelerate to zero and then accelerate in the opposite direction, all very quickly. The motor has no problem with this, but the current surge is substantial.

I'm sure that there are also substantial voltage spikes involved.

I've heard of people getting away with this, if the VFD was sufficiently oversize compared to the motor, but I bet that the VFD manufacturer is agast.

I suppose that another approach is to use the VFD only for generation of bulk 3 phase power, giving up on the variable part. If one has a large

3 phase motor always running, that motor will supply the startup surges, so long as the large motor is large enough to dominate the lathe motor.

Joe Gwinn

Reply to
Joseph Gwinn

I think the inductive spike that occurs on opening the switch would be really hard on the output FETS of the VFD.

The current surge of trying to start a motor at nominal speed would be large. I am not sure it is always the case, but I have my VFD set up to ramp up to 60 Hz over a couple of second period to reduce the startup surge. I am not using the variable speed aspect of the VFD, just as a source of 3 phase 60Hz because my lathe has a good variable speed drive.

As Joe mentioned, "plug" reversing would give really large current surges and I think would put double the output voltage across the FETs for a short time.

About the only thing I can think of that might mitigate the problem is a large idler motor, but that would get rid of my two favorite aspects of the VFD - quiet and low additional power drain. If you really want to switch the outputs, I would probably just build a rotary phase converter. You are 80% there with the idler motor.

Good Luck,


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Bob, thanks. I run the lathe from a phase converter now and it runs great. So the reasons for putting a VFD on it are of minor nature, such as making the lathe quieter, and having a working but cosmeticaly flawed 5 HP VFD that is hard to sell (cracked front cover).

The issue with "switch between the drive and motor" is the two speed forward/reverse lever on the lathe.

The lever lets me switch motor speeds on this reversible two speed motor, REV2/REV1/OFF/FWD1/FWD2.

If I put the lathe on the VFD, everything would work well, I would switch the VFD with the ON/OFF/BRAKE lever connected to its control terminals.

It would work great, except:

This is all problem AS LONG AS I do not try to flip the REV2/REV1/OFF/FWD1/FWD2 switch during running. If I ever do so, the drive may be ruined as I would switch the motor live with VFD powered.

That is my issue. If I could, somehow, keep the drive safe when accidentally flipping that speed selector lever during running, I would be happy.

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Did you mean an on-off switch or a reversing switch?

Pete Stanaitis


Ignoramus19502 wrote:

Reply to


I've got this exact setup on my 10EE. Once/year I forget and go from forward to reverse. It trips the VFD out. OVC alarm code, IIRC. Just reset the VFD and go. No big deal.


Reply to
Karl Townsend

There is a switch that switches from reverse fast, reverse slow, off, forward slow, forward fast. That switch would be a pain to take out or rewire.

That's the switch that I am talking about.

I do not need to switch it during running. I could say that I am fine. But if I do that by accident, it may damage the drive.


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This would be plain awesome if that was the case. A trip is a perfectly fine response. I am delighted at your answer.

I have a VFD that looks bad due to cracked cover, hard to sell (5 HP Toshiba), I think that I will wire it, it is a 1 hour job at most. Worst case, I will lose it. Best case, the lathe would be much more useful and valuable due to not needing the phase converter, the noise, and all of that.

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An additional benefit with the VFD is braking. I really like having my 8" chuck stop in less than a second, even from high speed, when I hit the stop button.

Yes, this is a problem. I'd disconnect all that wiring and just connect the low-speed windings of the motor to the VFD. I assume the 2 speeds are a 2:1 ratio. The VFD can overspeed the motor by putting 120 Hz on the low-speed windings, and you get the high speed, too. You very likely could even wire the switch for the existing lever to the VFD to replicate all these functions, and have smooth switching between all modes handled by the VFD.


Reply to
Jon Elson

Well, my lathe has a mechanical brake. And it generally stops relatively quickly without braking.


I could, I think, but I do not want to, it is too messy.

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Why don't you do the right thing and wire the motor direct to the VFD and wire the control switch to the VFD's control side, including the dual speeds? The VFD's do have the option of control line selected multiple speeds you know...

Reply to
Pete C.

Sorry for starting another thread on this, my news server refuses to show the old thread for some reason.

I re-wired the drum switch on my lathe to supply control signals to the VFD and everything is happy. It was not too difficult to do the wiring. You could probably even get the two speed setup depending on your VFD.

I have a Clausing 1300 which I think is a similar size but different model machine than yours.

If you really, really don't want to re-wire the switch, how about turn the switch on and take the handle off it? That would make it unlikely that you would switch it off while under power. Use the VFD for direction and speed control.

Good Luck, BobH

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Dynamic braking - which most VFDs support, is the way to go IMO.

VFD's are intelligent enough not to fry yer motor, yet they will try to stop the motor in the requested amt of time. Depending on the inertia of moving parts (rotor, gears, spindle, chuck, workpiece) it might do it faster than a mechanical brake. And in case of emergency it can get activated much quicker via a big reg inhibit-abort switch :)

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I have a VFD that currently supplies the three motors on my surface grinder. Within the year, it will be supplying the two motors on my HLV lathe, the three motors on my mill and the motor on my small drill press. All switched after the VFD. There is an idler motor to help damp out spikes and to avoid the possibility of the VFD seeing a leading power factor load. Id copes fine with this treatment. Having said that, it is rated at 23kVA and 650V and running at no more than 7kVA and 300V.

Modern VFD's can cope with overload, they either limit the load or trip. They can't cope with large voltage or current spikes. Current spikes are caused by having a capacitive load (usually overly long cables) and can be eliminated by fitting line reactors sold for the purpose. Voltage spikes can be caused by switching off or reversing a load at the end of the long cables. Mitigate this with an idler motor near the VFD.

Works for me and my VFD anyway.

Mark Rand RTFM

Reply to
Mark Rand

Then you'd want to add a switch on the mechanical brake to a stop line on the VFD. I have that wired on mine.

Wayne D.

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Extract from Electric Motors 2nd edition ISBN-13 978=185486-264-4

3.5.7 Load Limitations

It's perhaps natural to think of a VFD as a sort of portable local three phase supply to which motors can be connected as required. Unfortunately this may not be true. The electronics and protection systems of the VFD are based on the currents and voltages encountered in the normal start and run sequence of the motor load. However at the actual instant connection or disconnection of a motor load the current /voltage relationships are abnormal and transient voltages and currents occur which can overstress the VFD. Because of this, most manufacturers specify that the motor be permanently connected to the VFD output terminals and that motor on/off control is always by the VFD which has built in soft start and soft off functions. A breaker is permitted between the VFD and the input power but it is emphasised that this is for emergency use only. It would normally only be used to isolate the whole system after the VFD has powered down.

In the motor "off" state the power consumption of the VFD is extremely small and it can be left connected to the main power source indefinitely.

The possible overstress occurs when power is suddenly removed or if a second motor is switched in or out of circuit on an already running VFD. In industrial use this limitation is rarely a problem but for a small workshop perhaps powering a small mill which may be fitted with separate drive, traverse and coolant three phase motors it's a real nuisance. Multiple separate VFDs overcomes the difficulty but is expensive. The usual solution is to reserve the VFD for the main drive motor and use the methods of 3.4 or 3.5 to drive the others.

Some leeway is possible for the adventurously minded user. Study the manufacturers literature carefully. Permanent connection of a single motor is the recommendation. Multiple motors and/or direct switching, if forbidden, are not indicated as instant failure hazards but "may increase the failure rate" or "reduce the operating life". These are entirely reasonable reservations on the part of the manufacturer because they have no knowledge of the exact type of motor and load switching to be encountered. The fact that instant failure is not predicted is a pointer that the design safety margins are sufficient to cover normal switching events but may be stretched by unusual motor loads switched at the most unfortunate phase angle and load/speed condition. One manufacturer makes the comment - if it is unavoidable to turn the main power supply on or off to start or stop the motor, it must not exceed once per hour.

It is clearly bad practice but, if there is no reasonable alternative, direct switching can be tried and in many cases the VFD will suffer no more than a small increase in the failure rate (VFD failure rates are normally measured in thousands of hours). This is NOT a recommendation to try this - you are disregarding manufacturers instructions and if something goes wrong you will receive no sympathy from anyone - least of all the manufacturer! The type of failure to be expected is damage to some or all of the output semiconductors - a repairable but expensive fault. Some decrease in the severity of the switching transients can be obtained by permanently connecting a resistive load in delta across the output of the VFD. This provides some damping of the voltage transients and also ensures that the VFD never sees an open circuit load. Three resistors consuming 5 to 10% rated load current is a suitable arrangement.

The load switched must be well within the VFD power rating - if anything like full rated load is switched the transient peak current may activate the VFD overcurrent protection circuits which will power down the VFD..


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here is a quick way to understand why you don't want to disconnect a motor from a VFD under power:

remember the ignition in your car? you put 12V into an inductor, break the circuit and you get HV out the ignition coil's secondary as the magnetic field collapses. Big spark, engine runs. As the breaker points open (remember breaker points), the back EMF from the primary of the coil rises from 12 v to 250 to 400V (I've measured this with a scope on a couple of cars when I was designing CD ignitions)

Now, imagine an electric motor, being driven by the VFD. Some coil in the motor is being energized at near max power, so there is about 250V across it (about 20X what the ignition coil had). Now, you flip a switch and suddenly disconnect the coil from the drive circuitry. The collapsing field will induce a voltage spike just like the ignition coil. And it is not unreasonable to estimate that it will reach about 20X what the ignition coil primary spikes up to - so you might well see 5,000 volts - this is not only bad for the switch (which can arc) but also for the VFD, which will see that voltage reflected through the other coils and through the arching swich contacts.

So, general rule - do not disconnect solid state stuff from a motor when the motor is under power.

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Reply to
William Noble

Bill sez:

..."So, general rule - do not disconnect solid state stuff from a motor when the motor is under power.

As my old boss used to say about such matters: "A word to the wise is superfluous"

Bob Swinney

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Robert Swinney

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