Really Dumb Vfd Question?????

[Wheresmywrench?]

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???

 

[mikedoon]

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

 

[Wheresmywrench?]

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

 

[John Hasler]

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.

 

[Terrywerm]

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.

 

[CluelessNewB]

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.

 

[British Steel]

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!

 

[mcostello]

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.

 

[JimDawson]

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.

 

[McRuff]

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.