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

[Jraef]

The 50% de-rate is something that is widely touted, but not consistently documented. Some manufacturers state it, some do not, some use higher de-rate values, like 65% (so the max load is 35% of the VFD rating), or require temperature de-rating as well. It all has to do with the DESIGN of a particular VFD. As a gross generalization, drives that use a DC Bus choke in their design can handle a 50% de-rate, because the choke aids in smoothing out the added DC bus ripple from rectifying single phase. But low cost Asian based drive designs tend to use more capacitors rather than a DC bus choke, to reduce the size and cost (for them, because 99% of capacitors come from Asia). Using all capacitors means the added ripple causes the caps to heat up more, requiring additional de-rating.

All of this only speaks to the single phase input aspect. If you want to use a VFD as a phase converter for other than motor loads, you must add a good sine wave filter. Often times, that filter will cost as much as the VFD itself.

It’s often less expensive to just go into the machine power system and separate out the actual 3 phase motors to put VFDs on just those, leaving the rest to be fed by the single phase source. Servos can often be dealt with the same way: just get a larger rectifier unit that can accept single phase input. Many smaller ones are actually already built that way.

 

[wwhitney]

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.

A maybe basic question, just to be clear about this point: If you take the VFD, and hook up a motor as intended, and hook up an oscilloscope across the motor supply to see the voltage waveforms, you'll see something close to 3 phase power. Whereas if you replace the motor by a delta arrangement of resistors, you'll see the spiky high frequency modulated DC.

In other words, in the diagram in post #20, the "simulated AC" waveform shown is not actually what you'd see with the motor hooked up, it's what you'd see if the motor were replaced by resistors.

Is that correct?

Cheers, Wayne

 

[winnie]

A maybe basic question, just to be clear about this point: If you take the VFD, and hook up a motor as intended, and hook up an oscilloscope across the motor supply to see the voltage waveforms, you'll see something close to 3 phase power. Whereas if you replace the motor by a delta arrangement of resistors, you'll see the spiky high frequency modulated DC.

In other words, in the diagram in post #20, the "simulated AC" waveform shown is not actually what you'd see with the motor hooked up, it's what you'd see if the motor were replaced by resistors.

Is that correct?

Cheers, Wayne

If you were to connect an oscilloscope measuring _voltage_, you would see the spiky high frequency modulated DC.

If you were to connect an oscilloscope with a _current_ probe, you would see something more like 3 phase sine waves.

The voltage output of the VFD is the raw switching waveform. The only reason reasonable current flows is because of the filtering effect of the motor.

-Jonathan

 

[LarryFine]

Is the output PWMed, full voltage but varying duty cycle?

Does a motor "average" the pulses like a moving-coil meter?

 

[Jraef]

A maybe basic question, just to be clear about this point: If you take the VFD, and hook up a motor as intended, and hook up an oscilloscope across the motor supply to see the voltage waveforms, you'll see something close to 3 phase power. Whereas if you replace the motor by a delta arrangement of resistors, you'll see the spiky high frequency modulated DC.

In other words, in the diagram in post #20, the "simulated AC" waveform shown is not actually what you'd see with the motor hooked up, it's what you'd see if the motor were replaced by resistors.

Is that correct?

Cheers, Wayne

Sort of, yes. The VFD is INTENDED to be used on a motor, so it takes advantage of the inductive nature of the motor itself. The motor is a form of low pass filter for the pseudo-sine wave PWM output if the drive. Without the motor the output looks more like a square wave and that can have detrimental effects on other equipment.

 

[winnie]

Is the output PWMed, full voltage but varying duty cycle?

Does a motor "average" the pulses like a moving-coil meter?

Exactly.

 

[MD Automation]

To the OP - Your first post leads me to believe you want to connect the output of a VFD to a Control Cabinet, hoping to use the VFD (locked at 60hz) as a single phase to 3 phase converter.

Long story short, you should NOT do this. Anything other than a motor on the load side of a VFD is not going to be happy with the pulse width modulated output of a VFD. Take it from me, I am not ashamed to admit a young and dumb me made this mistake many years ago, thinking it would do exactly what you hoped. The results did not make me happy, and the smoke it generated did not make the facility happy either

A lot of the replies here are (I think) talking about connecting your VFD to a motor. But, again, if you are looking to power control electronics with a VFD - don't.

Winnie and Jreaf touched on this a bit...

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.

Without the motor the output looks more like a square wave and that can have detrimental effects on other equipment.

Jraef's use of "detrimental effects" should be taken quite seriously.

It's possible to do this - typically with a big inductor (choke) on the load side. The choke would act like a motor, a big fat dumb filter (smoothing) of all the fast PWM pulses. But this design would be somewhat unusual and likely not cost effective in a one-of application.

 

[zbang]

It’s often less expensive to just go into the machine power system and separate out the actual 3 phase motors to put VFDs on just those, leaving the rest to be fed by the single phase source. Servos can often be dealt with the same way: just get a larger rectifier unit that can accept single phase input. Many smaller ones are actually already built that way.

Exactly that. Or get a large-enough Rotophase or similar. Either way, it's going to cost and there's no point it trying to cheap-out because that often leads to the smoke coming out.

 

[petersonra]

5 HP is not a lot. You can probably get a rotary convertor for a few grand. Maybe less if used