# Really Dumb Vfd Question?????



## Wheresmywrench? (May 24, 2015)

_Why can you not run your lathe/mill or what have you directly off the VFD??  Why can you not use switches between the VFD and the motor???_


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## mikedoon (May 24, 2015)

Wheresmywrench? said:


> _Why can you not run your lathe/mill or what have you directly off the VFD??  Why can you not use switches between the VFD and the motor???_




A vfd does some tests on the windings of the motor before it starts such as winding resistance tests and magnetising calculations on the motor if you put switches between the motor and vfd and accidentally leave it open the drive will trip all the protection you need is inbuilt within the drive software and design this allows then to be coupled directly to the motor with no extra outgoing side protection hope this helps 

Regards 

Mike


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## Wheresmywrench? (May 24, 2015)

OK but why not design them to work with controls between them and motor? Seems like a more practical method.


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## John Hasler (May 24, 2015)

Wheresmywrench? said:


> OK but why not design them to work with controls between them and motor? Seems like a more practical method.


They are designed to do everything the controls would do.


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## Terrywerm (May 24, 2015)

First, your question is not dumb. It is a question that was on your mind, and you sought the answer, so you asked the question. Excellent!

Maybe you can explain to us why you think placing switches between the VFD and the motor would be more practical. Your thoughts may have some merit, so I for one would like to hear your thoughts in greater detail.

Personally, I do not see how it could be practical at all. Start, stop, forward, reverse, and speed controls are all available on the VFD itself or could all be incorporated in a remote control panel that is separate from, but connected to, the VFD. Just the same, I would still like to hear your thoughts.


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## CluelessNewB (May 24, 2015)

Sensorless vector drives (better VFDs) use feedback from the motor to control the power supplied to the motor.  If you put switches between the VFD and the motor this will screw the feedback up.  Anything you would want to do to control the motor by placing a switch between the motor and the VFD can be done using low voltage and low current controls connected to the VFD control inputs.  Using the low voltage control inputs allows you to use less expensive switches and less expensive wire since then only carry a tiny fraction of the current and much lower voltage.


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## British Steel (May 24, 2015)

As has been said, all the controls can be wired to the VFD - this has major advantages: the controls need only handle a few milliamps at a few volts instead of several amps at a few hundred volts, for one - this means much less expensive switches and control wiring.
A non-VFD lathe or mill needs large, high-current relays (contactors) with multiple sets of contacts to a) switch the curent and voltage reliably and b) create reliable safety interlocks so that (for instance) the forward and reverse relays don't operate at the same time, putting a dead short on two of the mains phases...

Lastly, it's all already there in the VFD - over-current sensing and limiting, under and over-voltage sensing, control of direction and start and stop acceleration, why over-complicate the machine? There are advantages to the VFD's low-voltage, isolated control voltages running around the machine too, much, much safer! My lathe runs 24v dc to its pushbuttons since the VFD conversion, rather than 415v ac! It does, though, have switches between the VFD and the motor - the motor itself is a 3-speed unit, and the big handle selecting speeds is an important part of the "look and feel", so some thought has gone into interlocks that disable the VFD while speed changing!


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## mcostello (May 25, 2015)

Can I put a receptacle between the VFD and motor, to plug motor in? When We get storms I like to unplug the expensive motors.


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## JimDawson (May 25, 2015)

mcostello said:


> Can I put a receptacle between the VFD and motor, to plug motor in? When We get storms I like to unplug the expensive motors.



I would unplug the VFD and the motor from the wall.  There is no sense frying an expensive VFD either.  And it is a lot more susceptible to lightning damage than the motor is.  It's better to have the VFD hard wired to the motor.


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## McRuff (May 25, 2015)

The easiest thing to do is put a disconnect between the vfd and the incoming power. I have a small disconnect box above my vfd, the 220volt single phase comes from my fuse box to the disconnect and then from the disconnect to the vfd. When I'm done using my mill I just pull the lever disconnecting the power to the vfd thus saving it and the mill from power surges during storms and such.


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## wrmiller (May 25, 2015)

McRuff said:


> The easiest thing to do is put a disconnect between the vfd and the incoming power. I have a small disconnect box above my vfd, the 220volt single phase comes from my fuse box to the disconnect and then from the disconnect to the vfd. When I'm done using my mill I just pull the lever disconnecting the power to the vfd thus saving it and the mill from power surges during storms and such.



I use a light switch on both of my VFDs.


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## C&I Harry (May 27, 2015)

Disconnecting both output and input of the VSD should not really harm the VSD if all power is disconnected first. But doing so can introduce a myriad of other problems. One of the main reasons for not putting any device between the VSD and its associated motor, like a switch, circuit breaker or connector is the danger associated with disconnecting under load. When disconnecting a motor under load, the motor generates a back EMF (Voltage) that could be of a magnitude that would destroy the output devices (Transistors, IGBT's) of the VSD. The other reasons were already stated above by other respondents, namely the match required between the VSD and motor. Any items that could cause variations in impedance, capacitive, inductive or resistive, will cause mismatches in the tuning between the VSD and the motor. Typically contactors and switches can introduce resistive changes through contact resistance. changing cables can introduce capacitive and inductive changes. All these changes can cause a mismatch between the previously ideal matched VSD and Motor with resultant loss of ideal control. One would ideally want the connection between the VSD and the motor to be as "stiff" as possible. It is also very important to keep the earthing or grounding between the motor chassis and the dedicated earthing or grounding tab on the VSD as sound as possible. A separate earthing cable between the two is ideal. For lightning and surge protection, the ideal would be to have surge arrestors on the supply side of the VSD, together with well and proper earthing arrangements in the event of not being there when the lightning strikes. A double or triple pole isolator would be the best method to isolate the mains from the VSD during lightning storms. For single phase, use a double pole switch (life and neutral), for three phase, use a triple or quad pole switch (3 phases and neutral). Switching off only the life lines will cause lightning surges to still reach the VSD via the neutral and earth wires.

By the way, in industry some installations require multiple motors to be connected to a single VSD. In such cases it is normal for every motor to have its own motor protection relay. In the event that motor has a fault it would isolate or trip that motor. The other motors still in circuit will protect the VSD from back-EMF damage. The motor protection relay that tripped will then normally switch of the VSD to ensure total system integrity. So yes, one gets disconnecting circuits between VSD's and motors, but in this special case, the remaining motors protect the VSD.

My recommendation is that you do not regularly disconnect your motor from the VSD as it increases your chances of loosing the VSD.


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## Fudooo000 (May 29, 2015)

There is no reason you cannot have a disconnect right at the motor. In fact the code requires it if the VFD or its power disconnect cannot be locked out to service the motor. My last employer had several hundred VFD/motor combinations driving converyors and they ALL had a local disconnect to the motor. The worst that can happen to the VFD when the switch is opened is a fault will be generated requiring a reset (usually by cycling the power to the VFD) after the motor is reconnected. This is true for all makes of VFD I have ever had to work with.


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