Newbie- VFD for 3 phase supply

[ErnieD]

Hi,

I am wondering if I can use an appropriately size VFD to take 240 VAC single phase input and output pure (?) 240 VAC 3 phase to a 3 phase motor
which would become the RPC. If this would work then I would not have to
mess with starting or running capacitors to balance the legs. I would then hook this up as the power supply, to a Bridgeport EZTRAK-SX. I would want the ability to plug (instantly reverse the spindle motor) the spindle motor switch for tapping.

Thanks,

Ernie

[Al_The_Man]

I assume you want to do floating tapping? as rigid tapping requires an encoder on the spindle and a controller to gear the Z axis to it.
You cannot plug a motor the way you would do it with contactors, the VFD must have control at all time.
Depending on the HP and the size of inertia of the spindle, you may find you need brake resistors to dissipate the reverse energy of rapid decel.
Output of a VFD is not 'pure' sine wave as it is composed of a PWM signal.
Also you did not mention if the machine is manual or under CNC control?
Al.

[mike_Kilroy]

ernie,

yes you can.

as al said u may need db resistor for quick reversal.

pm me if u want quote on hitachi units that will do this.

[ErnieD]

Mike,

PM sent.

Ernie

[mactec54]

ErnieD

You can not do what you want to do, Mike is incorrect or miss read what you want to do, The VFD can only power the spindle motor, you need another source of power for the rest of the machine which 120/240V should be all you need, you will have to check the transformer to see what you need to power the rest of the machine, But the right size VFD will take care of your spindle

[mike_Kilroy]

Originally Posted by mactec54 ErnieD You can not do what you want to do, Mike is incorrect or miss read what you want to do, The VFD can only power the spindle motor, you need another source of power for the rest of the machine which 120/240V should be all you need, you will have to check the transformer to see what you need to power the rest of the machine, But the right size VFD will take care of your spindle

Ernie,
I am not sure what is going on. your question on practical machinist was about running the whole machine: I said unequivocally NO you cannot.

ur pm said:
Hi Mike,
Over on the Practical Machinist site you gently chastised me for wanting to use a VFD to provide 3 phase to my Bridgeport EZTRAK-SX machine. Are you now saying that I can use the VFD to start an idler, let the idler make the third leg and then connect to the input on the mill?

NO.

mactec54 is correct; i must have mis understood ur rephrased question here.

I am sorry for miss understanding you Ernie. it is still true you cannot run a WHOLE machine on the vfd - just the motor. If I misunderstood ur request here I apologize. You can run the machine MOTOR ONLY on the vfd, NOT the machine.

[Al_The_Man]

If the rest of the machine runs on 1 phase, just use the VFD for the spindle and feed the rest, control, servo's etc with 240 1 phase, if this is what it takes at present..
Al.

[neilw20]

Regardless of all of that you CANNOT put contactors or any switching on the output of a VFD (Variable Frequency Drive) and it needs to be used closely following the VFD user manual.
The (3 phase) motor wires go DIRECTLY to the VFD.
If there are any 2 speed/reversing switches in the path to the motor they must NEVER be changed until the power to, and in the VFD has discharged, which maybe even MINUTES.
Generally one VFD, one motor, and you control the low voltage input logic to the VFD to control the motor. Also see the application information in the manual.

Breaking the rules often means the smoke gets out (of the VFD) and can't be put back. It mightn't even smoke, but still might die silently.

Trying to run a single phase motor with a centrifugal switch breaks the rules of NOT switching motor wires, and the single phase motor won't like running below it's design frequency in most cases.

[Al_The_Man]

Originally Posted by neilw20 If there are any 2 speed/reversing switches in the path to the motor they must NEVER be changed until the power to, and in the VFD has discharged, which maybe even MINUTES.

That is not quite true, it is not necessary to wait until the Power Supply has discharged only that the output is at zero freq.
It is done all the time by usually setting one of the outputs as 'At Zero speed' when this bit is detected, the output can be contactor switched, I did this in one instance on a dual motor set up, when swapping motors was desired.
As mentioned in post two, contactor plugging Cannot be done.
Al.

[neilw20]

Sorry Al, but I can show you some VFD manuals that specify this.
Depends on internal biasing etc whether it is safe.
Just because it works doesn't mean it wont fail.
Not really the disconnection, unless it bounces, but is when the connection is made, and it probably will have some bounce in it too,
which upsets internal biasing and very much depends on the high side drive method used. This can cause transients.
If the high side drive is a true isolated drive, then most likely OK, but if charge pump method is used for high side drive then beware.
If internal inspection makes it obvious each high side drive has an isolated floating supply then that is a good sign.
Always refer to the application details before doing this.
Some OK, others not.

!!!Safety Precautions
★ Live dip setting or measurement for the driver is not permitted.
★ Connection, installation and dip setting must be carried out after three-minute
power-off.
★ Three-minute interval must be carried out between the first and second
power-on and power-off to avoid fault alarming.
★ Input voltage of the driver shall meet the technical requirement.
★ Make sure that power supply cable, motor cable and signal connection cable are
connected correctly and solidly before plugging in.
★ After complete connection of cables and before plugging in, the resistance
measured between terminals A, B, C of the driver and grounding terminal, using
the millimeter’s resistance gear, shall be infinite. Every two-phase resistance
value measured at terminals A, B, C of the driver shall be the same, avoiding
shorted circuit among phases of the motor or any damage caused by insufficient
phase of the motor.

Pasted straight out a manual.
That's one I won't use.

[Al_The_Man]

Well I guess the bottom line is if faced with an application such as this then just select a VFD that will do it.
Of the failed VFD's that I have cannibalized for parts consists of a mains fed 3 phase bridge, which feeds directly to the switching devices.
The only isolation is usually the switching supply for the logic, allowing earth ground reference or common gnd if needed.
Not all VFD's have configurable logic outputs, I prefer the type that do, I use mainly Mitsubishi, WEG and Telemecanique Altivar, all high end VFD's.
I would add, the failed ones I stripped were not from any of my installations..
Al.