Check my calcs please

[Shaneyj]

My goal is to accurately size a vfd and a phase converter for single phase input (240v) and 3 phase output (480v).
If I understand correctly, the vfd needs to be sized at 1.73 (sq rt 3) times the rated voltage current. Is this correct?
My numbers...
Nameplate v: 480
Nameplate i: 31
480*31*1.73=25.8kVA= rated operating power
25.8kVA*1.73=44.6kVA= operating power on 230v vfd supply
44.6kVA/230= 193.9 A

Would I need a vfd rated at 193A?

Would deriving phase converter current be the same method as vfd?

This is for a plasma table.

Thanks for the input.

Sent from my HTC6545LVW using Tapatalk

 

[kwired]

Are you driving a motor or are you trying to use this as the power supply for the entire machine?

VFD will work fine for directly driving an individual motor, won't work so well for driving other loads.

 

[Shaneyj]

This is to supply a plasma cutter. Nothing else. The numbers in my original post are nameplate voltage and current.

Sent from my HTC6545LVW using Tapatalk

 

[kwired]

I don't think the VFD is going to play well with a plasma cutter as it's output load. They are designed to supply motors

 

[Dennis Alwon]

I don't think the VFD is going to play well with a plasma cutter as it's output load. They are designed to supply motors

He is asking if the calcs are correct. Can we just answer that and then question other things. Something does not look right in the numbers in post 1

 

[kwired]

He is asking if the calcs are correct. Can we just answer that and then question other things. Something does not look right in the numbers in post 1

OK

My numbers...
Nameplate v: 480
Nameplate i: 31
480*31*1.73=25.8kVA= rated operating power
25.8kVA*1.73=44.6kVA= operating power on 230v vfd supply
44.6kVA/230= 193.9 A


Would I need a vfd rated at 193A?

25.8 kVA /240v single phase supply gives you 107.5 amps.

You want an inverter and not a VFD for other then motor load. the so called inverter portion of a VFD is not the same thing, it doesn't put out a true AC wave for one thing, it is a pulsed DC that the motor still sees about the same as an AC wave.

 

[Shaneyj]

OK

My numbers...
Nameplate v: 480
Nameplate i: 31
480*31*1.73=25.8kVA= rated operating power
25.8kVA*1.73=44.6kVA= operating power on 230v vfd supply
44.6kVA/230= 193.9 A


Would I need a vfd rated at 193A?

25.8 kVA /240v single phase supply gives you 107.5 amps.

You want an inverter and not a VFD for other then motor load. the so called inverter portion of a VFD is not the same thing, it doesn't put out a true AC wave for one thing, it is a pulsed DC that the motor still sees about the same as an AC wave.

Not sure what you mean by "inverter".
I've got a single phase source and a 3 phase piece of equipment.
I thought a vfd or a phase converter were the only way to manufacture 3 phase from single phase.
On another note, why would a plasma cutter care about the incoming wave? Does it not condition the input and put out a modified wave?
Maybe a call to hypertherm is in order.

Sent from my HTC6545LVW using Tapatalk

 

[gar]

180625-1403 EDT

Shaneyj:

Look in the manual for the plasma cutter. I have a manual from about 20 years ago. It has specifications and a circuit diagram. My manual is for a MAX80 which is single phase input and good for cutting plate up to 1" thick.

At 12 kW output input single phase is 240 V @ 75 A or 18 kVA. You appear to have a higher power unit.

Do call Hypertherm and ask about modification options, might be cheaper, or other solutions. On the MAX80 they use a bridge rectifier on the output of an input transformer. To go from a 3 phase design to a 1 phase design requires bigger diodes, and more capacitors.

A simulated sine wave generator consisting of rectangular + and - pulses in its simplest form will consist of pulses equal in height to the sine wave peak and width adjusted to produce the required RMS voltage. A good part of the cycle is 0 V. This type of source might satisfy your needs.

A motor generator (alternator) can convert 1 phase to 3 phase.

If this plasma cutter is never to be run at full power, then check with Hypertherm to determine if it could be fed single phase.

.

 

[Besoeker]

He is asking if the calcs are correct. Can we just answer that and then question other things.

Right or wrong, I think what is being questioned is whether it is actually relevant to the application.

 

[Ingenieur]

assuming a suitable vfd load

31 x 480 x sqrt3 = 25.8 kva load
single phase = 25.8 kva / 240 = 108 A
convert to 3 ph = sqrt3 x 240 x 108 = 45 kva
assume 0.8 pf ~ 56 kw or 75 hp
I would use 2/1.732 x 75 ~ 90 hp or next size
1.732 is theoretical, there are losses etc

 

[Shaneyj]

assuming a suitable vfd load
i at 240/1 ~ 200 A or 48 kva
assume a 0.8 pf ~ 60 kw or 80 hp, next size up
I would use 2 vs 1.732 to be safe, so 54 kva or 67 kw or 90 hp, next size up

So my figures make sense?


Sent from my HTC6545LVW using Tapatalk

 

[Besoeker]

assuming a suitable vfd load
i at 240/1 ~ 200 A or 48 kva
assume a 0.8 pf ~ 60 kw or 80 hp,

PF is kW/kVA.

 

[winnie]

Trying to focus solely on the original question:

A VFD takes input AC, rectifies it to DC, and then uses switching electronics to produce variable frequency AC. When used with single phase input the VFD _input rectifier_ needs to be sized to carry the single phase current, and possibly the capacitor bank needs to be larger to handle increased ripple from the lower number of phases. Rather than spending lots of money on a custom built VFD with single phase input and larger capacitor bank, it is cheaper to simply purchase a larger standard VFD.

As a very rough rule of thumb, the input current and output current of a standard VFD are the same. This is not required by physics; the two values need not be the same. It simply is common that this is the case.

For a given voltage and power level, single phase current is 1.732 * three phase current.

Your 3 phase load requires 25.8 kVA (we don't know the power factor, and thus don't know the power required). If we _assume_ this means 25.8 kW, then the input to the VFD will be slightly more than 25.8kW. Ignoring the voltage change, in order to change from single phase to three phase, you would need a VFD _rated_ for about 45 kVA.

Note that the power input remains about 26kW, thus 26000/230 = 113A. This agrees with kwired. People often use the term 'inverter' for VFD or the output section of the VFD. But you need a VFD rated for higher power because that inverter will be rated for 3 phase use, not single phase use.

The output of a standard VFD with 230V single phase input would be roughly 230V three phase output. So you would also need a transformer to step up to 460V.

As others have noted, the VFD would probably not be a good supply for the plasma cutter.

-Jon

 

[Ingenieur]

So my figures make sense?


Sent from my HTC6545LVW using Tapatalk

yes
give yourself some headroom
convert to kw = pf x kva
hp = kw/.0.746
units are typically rated in hp or kw
verify unit is rated for an input current of 200A+
the mfg will have a spec for 1 to 3 ph conversion
I use 2 vs 1.732

 

[Besoeker]

But you need a VFD rated for higher power because that inverter will be rated for 3 phase use, not single phase use.

Does he even need variable frequency for his application?

 

[Ingenieur]

rotary phase converter
http://www.northamericaphaseconverters.com/how-to-size-a-rotary-phase-converter/

you will need a 240:480/3 75 kva xfmr, 45 would be marginal but probably ok

 

[Shaneyj]

180625-1403 EDT

Shaneyj:

Look in the manual for the plasma cutter. I have a manual from about 20 years ago. It has specifications and a circuit diagram. My manual is for a MAX80 which is single phase input and good for cutting plate up to 1" thick.

At 12 kW output input single phase is 240 V @ 75 A or 18 kVA. You appear to have a higher power unit.

Do call Hypertherm and ask about modification options, might be cheaper, or other solutions. On the MAX80 they use a bridge rectifier on the output of an input transformer. To go from a 3 phase design to a 1 phase design requires bigger diodes, and more capacitors.

A simulated sine wave generator consisting of rectangular + and - pulses in its simplest form will consist of pulses equal in height to the sine wave peak and width adjusted to produce the required RMS voltage. A good part of the cycle is 0 V. This type of source might satisfy your needs.

A motor generator (alternator) can convert 1 phase to 3 phase.

If this plasma cutter is never to be run at full power, then check with Hypertherm to determine if it could be fed single phase.

.

Thank you gar.
Per the manual, kW output is 21.9kw.
21.9kw/25.8kva=85
So pf =.85?

Sent from my HTC6545LVW using Tapatalk

 

[Shaneyj]

Does he even need variable frequency for his application?

Only for the purpose of getting 3 phase.
My goal is to give my boss his options and let him decide.
That's why I asked if figuring load for single phase to 3 phase conversion is the same whether vfd or phase converter.

Sent from my HTC6545LVW using Tapatalk

 

[Ingenieur]

Thank you gar.
Per the manual, kW output is 21.9kw.
21.9kw/25.8kva=85
So pf =.85?

Sent from my HTC6545LVW using Tapatalk

pf is correct

here is a static converter 33 kva
http://www.phasetechnologies.com/do...erfect/240VPhasePerfect_Brochure_v2.0_web.pdf
you would still need a 240/480/3 xfmr 30 kva (not the 45 or 75, that was based on vfd 1 ph input)

 

[Shaneyj]

Trying to focus solely on the original question:

A VFD takes input AC, rectifies it to DC, and then uses switching electronics to produce variable frequency AC. When used with single phase input the VFD _input rectifier_ needs to be sized to carry the single phase current, and possibly the capacitor bank needs to be larger to handle increased ripple from the lower number of phases. Rather than spending lots of money on a custom built VFD with single phase input and larger capacitor bank, it is cheaper to simply purchase a larger standard VFD.

As a very rough rule of thumb, the input current and output current of a standard VFD are the same. This is not required by physics; the two values need not be the same. It simply is common that this is the case.

For a given voltage and power level, single phase current is 1.732 * three phase current.

Your 3 phase load requires 25.8 kVA (we don't know the power factor, and thus don't know the power required). If we _assume_ this means 25.8 kW, then the input to the VFD will be slightly more than 25.8kW. Ignoring the voltage change, in order to change from single phase to three phase, you would need a VFD _rated_ for about 45 kVA.

Note that the power input remains about 26kW, thus 26000/230 = 113A. This agrees with kwired. People often use the term 'inverter' for VFD or the output section of the VFD. But you need a VFD rated for higher power because that inverter will be rated for 3 phase use, not single phase use.

The output of a standard VFD with 230V single phase input would be roughly 230V three phase output. So you would also need a transformer to step up to 460V.

As others have noted, the VFD would probably not be a good supply for the plasma cutter.

-Jon

Thanks for the explanation, John.

Sent from my HTC6545LVW using Tapatalk