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.
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.
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
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.
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.
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
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
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.
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
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.
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.
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.
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...
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.
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
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 :)
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.
Extract from Electric Motors 2nd edition
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
In the motor "off" state the power consumption of the VFD is
extremely small and it can be left connected to the main power
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..
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
So, general rule - do not disconnect solid state stuff from a motor when the
motor is under power. ** Posted from
..."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
Bob Swinney ** Posted from