VFD for single phase 240 volt to three phase 240 volt 5 HP rated CNC machine

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powerplay

Senior Member
Location
In front of a computer.
Hello,

I was enquiring about connecting a single phase
240 volt 60 amp rated VFD locked at 60 hz to provide three phase 240 volt for a 30 amp 5 hp rated CNC Machine control panel.

I was told only a Rotophase with voltage control will work properly and a VFD needs to be directly connected to the motor and will not work.

If the VFD is locked so as not to vary, is there an issue having it in series with the CNC Machine ?

Thanks for any feedback !!
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Vfds
powerplay said:
Hello,

I was enquiring about connecting a single phase
240 volt 60 amp rated VFD locked at 60 hz to provide three phase 240 volt for a 30 amp 5 hp rated CNC Machine control panel.

I was told only a Rotophase with voltage control will work properly and a VFD needs to be directly connected to the motor and will not work.

If the VFD is locked so as not to vary, is there an issue having it in series with the CNC Machine ?

Thanks for any feedback !!
Click to expand...
VFDs do not have an actual sine wave output so bad idea to use it in the way you want to.
 
P

powerplay

Senior Member
Location
In front of a computer.
petersonra said:
Vfds

VFDs do not have an actual sine wave output so bad idea to use it in the way you want to.
Click to expand...
I had another supplier suggest doubling the HP Rating to 10 HP VFD would work but they are not Engineers nor am I..

May I ask if putting solid state devices in series causes damage, or if it unable to function properly and work but not accurately on the third phase created at 60 HZ ?

Thanks for the feedback !
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
VFDs don't synthesize the third phase like converters. The 3ph output is synthesized symmetrically.

The DC intermediate stage removes all remnants of the original AC input, whether it's 1ph or 3ph.
 
T

topgone

Senior Member
Can't follow you. Let me clear up things here. Are you trying to feed your 3-phase VFD with single-phase power?
If so, that will mean you have to secure a VFD sized such that the usual three-phase amps can be supplied by the single-phase power. Now, the divisor used usually with that change in the usual power supply arrangement is roughly 0.6. This is so because the line amps required will now be divided by a number corresponding to 1/1.732 [1/sqrt(3)] multiplied by the fact that the VFD is not 100% efficient, Say the VFD efficiency is 95%, you'll get 1/1.732 X 0.95 ~ 0.6! To make it a really easy number to remember, sales people say the rule of thumb is use a factor twice the 3-phase rating (1/0.5 = 2).
 
P

powerplay

Senior Member
Location
In front of a computer.
topgone said:
Can't follow you. Let me clear up things here. Are you trying to feed your 3-phase VFD with single-phase power?
If so, that will mean you have to secure a VFD sized such that the usual three-phase amps can be supplied by the single-phase power. Now, the divisor used usually with that change in the usual power supply arrangement is roughly 0.6. This is so because the line amps required will now be divided by a number corresponding to 1/1.732 [1/sqrt(3)] multiplied by the fact that the VFD is not 100% efficient, Say the VFD efficiency is 95%, you'll get 1/1.732 X 0.95 ~ 0.6! To make it a really easy number to remember, sales people say the rule of thumb is use a factor twice the 3-phase rating (1/0.5 = 2).
Click to expand...
Thank you, as you mention sales people tell me to use twice the rating for an VFD supplied single phase for a three phase requirement. The first response was that the VFD doesn’t produce a sine wave, so bad idea to use to turn single phase to three for a CNC Machine.
 
P

powerplay

Senior Member
Location
In front of a computer.
LarryFine said:
VFDs don't synthesize the third phase like converters. The 3ph output is synthesized symmetrically.

The DC intermediate stage removes all remnants of the original AC input, whether it's 1ph or
Click to expand...
3ph output synthesized symmetrically but 120 degrees out of phase ?

Is this possibly the issue noted above about “bad idea” as it isn’t a sine wave but dc pulses ?
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
electrofelon said:
Doesnt a typical CNC just pretty much immediately rectify the AC anyway? I question lots of this concern about non pure sine waves to electronic things when all most electronic things just rectify the incoming power.
Click to expand...
They often have servo motors but the servo drives expect sine wave input power and will fault out if they don't get what they expect.

Vfds would not like a pwm input either.
 
P

powerplay

Senior Member
Location
In front of a computer.
petersonra said:
They often have servo motors but the servo drives expect sine wave input power and will fault out if they don't get what they expect.

Vfds would not like a pwm input either.
Click to expand...
It has been advised by one supplier to go to rotophase or one other expensive alternative, and to only connect directly to the rugged motor with VFD, but a few others use the double the HP “rule of thumb” to make it work for other than strictly a motor.

The “PMW” DC on paper I presume is constant controlled “surges” off an VFD creating heat to simulate the sine wave, but at locked 60 hz sounding like it “works” from some electricians but reducing the lifespan of the solid state in the CNC Equipment ? … the bad idea that may knock off a few years or days on the equipment, or cause the equipment to be inaccurate ?

I certainly don’t want to help them save a few thousand and then have a breakdown on old CNC machine blamed on me !
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
powerplay said:
It has been advised by one supplier to go to rotophase or one other expensive alternative, and to only connect directly to the rugged motor with VFD, but a few others use the double the HP “rule of thumb” to make it work for other than strictly a motor.

The “PMW” DC on paper I presume is constant controlled “surges” off an VFD creating heat to simulate the sine wave, but at locked 60 hz sounding like it “works” from some electricians but reducing the lifespan of the solid state in the CNC Equipment ? … the bad idea that may knock off a few years or days on the equipment, or cause the equipment to be inaccurate ?

I certainly don’t want to help them save a few thousand and then have a breakdown on old CNC machine blamed on me !
Click to expand...
Google PWM VFD waveform.
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
1) VFDs produce output that is designed to run a motor directly. Their output is a bunch of modulated high frequency switching which depends upon the filtering of the motor inductance to create a reasonable sinusoidal current. If you connect any other sort of load you won't get good results. For example, if you connect the output of a VFD to a resistor, that resistor will 'see' the full DC rail voltage switching at high frequency, not the low frequency that the VFD has been set to synthesize.

2) As a rule of thumb, it is pretty reasonable to use a 2X rated VFD to operate a three phase _motor_ from single phase input. But this has to do with properly sizing the VFD input rectifier, and has nothing to do with the capability of the VFD to supply non-motor loads.

3) If you connect the output of a VFD to something like another VFD or servo drive which has an input rectifier, you really don't know what you are going to get. At _best_ the second drive's DC bus will simply get charged do the first drive's DC bus voltage. If you are lucky. The second drive DC rectifier is not really designed to operate at the switching frequency output of the first drive.

-Jonathan
 
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