VFD Input current

[elec_eng]

I asked the VFD manufacturer to provide VFD input current to size the feeder and CB and this is what I have. This is YASKAWA Matrix VFD. As you will notice in the spec, the input current is less than output current. How can this be possible? I thought input current is always higher than output current due the conversion, efficiency loss and such. Is this because this is Matrix VFD?

 

[petersonra]

I asked the VFD manufacturer to provide VFD input current to size the feeder and CB and this is what I have. This is YASKAWA Matrix VFD. As you will notice in the spec, the input current is less than output current. How can this be possible? I thought input current is always higher than output current due the conversion, efficiency loss and such. Is this because this is Matrix VFD?

View attachment 17127

The power factor at the input of the drive is close to unity. the PF at the output of the drive is never unity if it is a motor load so the output current will always exceed the input current.

 

[elec_eng]

Noticed the image is too small to read. Attaching the spec sheet again.

 

[elec_eng]

The power factor at the input of the drive is close to unity. the PF at the output of the drive is never unity if it is a motor load so the output current will always exceed the input current.

You maybe correct on actual current draw from the load but I am asking about the VFD power rating itself.

 

[Besoeker]

You maybe correct on actual current draw from the load but I am asking about the VFD power rating itself.

The VFD input power is greater than the output power because of losses in the VFD. Typically, these losses would be in the region of 3% the drive output rating.
Under a couple of kW for a drive rated at 55kW.

 

[petersonra]

The VFD input power is greater than the output power because of losses in the VFD. Typically, these losses would be in the region of 3% the drive output rating.
Under a couple of kW for a drive rated at 55kW.

he asked about current, not power.

 

[Besoeker]

he asked about current, not power.

He did ask about power in post #4 which is what I answered in post #5.

 

[winnie]

Yes, the key is that output _power_ must be less than input _power_ but output _current_ may be greater.

The combination of the VFD and the inductance of the motor essentially makes a switching power supply. Current can circulate between the VFD and the motor, with the input power supplying losses and the mechanical load. If the VFD is designed to handle it, the output current can be much greater than the input current.

-Jon

 

[ptonsparky]

Yes, the key is that output _power_ must be less than input _power_ but output _current_ may be greater.

The combination of the VFD and the inductance of the motor essentially makes a switching power supply. Current can circulate between the VFD and the motor, with the input power supplying losses and the mechanical load. If the VFD is designed to handle it, the output current can be much greater than the input current.

-Jon

Is this one of the differences in the "better" drives?

 

[Besoeker]

Is this one of the differences in the "better" drives?

The majority of VFDs have the power circuit configured in the same arrangement. Taking the range that the OP mentioned, these are three-phase 400V.
There is a six pulse input diode bridge, a filter, sometimes LC, then to the output inverter which comprises six IGBT. That produces a three phase fairly sinusoidal waveform which feeds the induction motor with a varying and voltage (V) and frequency (f). For many applications, the V/f ratio is made constant.

In short, there are few differences in the power circuit.

 

[Jraef]

The majority of VFDs have the power circuit configured in the same arrangement. Taking the range that the OP mentioned, these are three-phase 400V.
There is a six pulse input diode bridge, a filter, sometimes LC, then to the output inverter which comprises six IGBT. That produces a three phase fairly sinusoidal waveform which feeds the induction motor with a varying and voltage (V) and frequency (f). For many applications, the V/f ratio is made constant.

In short, there are few differences in the power circuit.

Actually no, he said these are MATRIX inverters (actually, converters), it's a totally different power topology from what we all know and love as "VFDs". It's referred to as an AC-AC Converter, as opposed to a traditional VFD being an AC-DC-AC converter. So there is no DC bus in a Matrix Converter (original design), but there are caps on the line side.

There are a couple of different Matrix Converter technologies out there, but Yaskawa now sells what's called an "Indirect Matrix Converter", which solved some of the limitations of the original design (also theirs) and appears to now have a DC bus, although no DC bus caps. One aspect that they have apparently solved from the original design was that they were incapable of supplying the same voltage out as was on the input, so depending on motor design and voltage drop, you often lost motor torque. In the original design, they would then add a boost transformer ahead of the drive, which cost throughput efficiency. Something about this new design is claimed to have fixed that.


The aspect of the drive correcting the power factor of the motor and thus showing a lower CURRENT rating on the input are still the same, it just takes place in a different manner.

As to the POWER, Yaskawa CLAIMS in their advertising that there are "no additional losses compared to across-the-line operation", which I find to be something I would consider an "alternate fact". If that were true, why are there cooling fans? What they ARE good for is that line regeneration capability is inherent and they present ultra low harmonics back to the source. You have to want those capabilities really bad however, because you are going to pay for them and then, you only have one supplier for replacements.

 

[Besoeker]

Actually no, he said these are MATRIX inverters (actually, converters),

Ah, I missed that.
My apologies.

 

[elec_eng]

The aspect of the drive correcting the power factor of the motor and thus showing a lower CURRENT rating on the input are still the same, it just takes place in a different manner.

I am still having a hard time to understand this. This is quite opposite to what I thought before. As all others pointed out, output power is always less than input power. If the voltage is the same for both input and output, I would think, the output current should be less than input.

Of course, the power factor gets into the mix so "I think", it might not be always the case but...I also have seen other VFD manufacturers data sheets show input current is "WAY" higher than output current. I am totally lost here..

 

[Jraef]

I am still having a hard time to understand this. This is quite opposite to what I thought before. As all others pointed out, output power is always less than input power. If the voltage is the same for both input and output, I would think, the output current should be less than input.

Of course, the power factor gets into the mix so "I think", it might not be always the case but...I also have seen other VFD manufacturers data sheets show input current is "WAY" higher than output current. I am totally lost here..

You cannot conflate current and power, specifically BECAUSE of Power Factor.

100kVA at 480V 3 phase = 100,000/1.732 x 480V = 100,000/831.36 = 120A

100kW at 480V, assuming a .8 power factor, = 100,000/1.732 x 480V x 0.8PF = 100,000/665A = 150A

The VFD input is (mostly) in kVA because it is correcting the power factor with caps (either in the DC bus or on the line side in this case).
The VFD output is in kW going to the motor. The VFD output must supply the active PLUS the reactive current to the motor.

 

[elec_eng]

You cannot conflate current and power, specifically BECAUSE of Power Factor.

100kVA at 480V 3 phase = 100,000/1.732 x 480V = 100,000/831.36 = 120A

100kW at 480V, assuming a .8 power factor, = 100,000/1.732 x 480V x 0.8PF = 100,000/665A = 150A

The VFD input is (mostly) in kVA because it is correcting the power factor with caps (either in the DC bus or on the line side in this case).
The VFD output is in kW going to the motor. The VFD output must supply the active PLUS the reactive current to the motor.

Makes sense...one more question though. As I mentioned before, why some of drives have higher input current than output current then? How can you explain that?


Data above is not a Matrix converter. It is typical AC-DC-AC unit. Does it make difference? Matrix vs. rectifier front will make difference on the input current or is there another reason to explain this?

 

[Jraef]

Makes sense...one more question though. As I mentioned before, why some of drives have higher input current than output current then? How can you explain that?
View attachment 17137

Data above is not a Matrix converter. It is typical AC-DC-AC unit. Does it make difference? Matrix vs. rectifier front will make difference on the input current or is there another reason to explain this?

That's a phenomenon that you only see pretty much in Japanese drives like Yaskawa, Mitsubishi etc, and is more of a marketing and UL listing issue believe it or not. Japanese drives are primarily designed for their market, where they have to contend with motors being designed slightly different than the rest of the world, because they have an even worse situation that we in the US do with regard to different power distribution systems. Some parts of the country are 50Hz, some are 60Hz, and the 3 phase voltage levels are odd too. In the 50Hz sections, they use 200 and 400V, in the 60Hz sections they use 220, 400 and 440V. So their motors, to be compatible everywhere in Japan, have to be designed to accept all of the different V/Hz ratios those represent. That then means when they design their VFDs, they are designing them for THEIR motors, and adapting the MARKETING of those designs to be used in the rest of the world.

Then when they send that drive to the US and have to get it UL listed for a particular HP, the HP rating has to correspond to OUR motors, which has to do with the OUTPUT ratings, but the Input ratings have to do with the MAXIMUM input current (per UL rules) without being tied to what is connected to the Output side. So in reality if, picking from the first column on your chart, the input amps are shown as being rated at 44A while the output current is shown as being rated for 38A. That's because it is being sold here as a 25HP 460V drive, which needs to be capable of 34A for that motor. But the drive was DESIGNED by Yaskawa for a different motor in Japan, which would require more output current and take 44A input current. When they took it to UL, the INPUT current is still the same, but because UL insists on this disambiguation of input and output ratings, the 44A input current is still shown. When you actually run that 25HP motor at full load of 34A, the input current will be LESS than 34A.

I know, it's confusing. I had to have a Yaskawa engineer explain it to me once. But you can sum it up as a glitch in how UL requires that VFDs be listed. The 44A is just what UL insists it be listed at, mostly because of the NEC rule on conductor sizing feeding the VFD being based on the VFD maximum input current, not the motor size.

 

[ptonsparky]

That's a phenomenon that you only see pretty much in Japanese drives like Yaskawa, Mitsubishi etc, and is more of a marketing and UL listing issue believe it or not. Japanese drives are primarily designed for their market, where they have to contend with motors being designed slightly different than the rest of the world, because they have an even worse situation that we in the US do with regard to different power distribution systems. Some parts of the country are 50Hz, some are 60Hz, and the 3 phase voltage levels are odd too. In the 50Hz sections, they use 200 and 400V, in the 60Hz sections they use 220, 400 and 440V. So their motors, to be compatible everywhere in Japan, have to be designed to accept all of the different V/Hz ratios those represent. That then means when they design their VFDs, they are designing them for THEIR motors, and adapting the MARKETING of those designs to be used in the rest of the world.

Then when they send that drive to the US and have to get it UL listed for a particular HP, the HP rating has to correspond to OUR motors, which has to do with the OUTPUT ratings, but the Input ratings have to do with the MAXIMUM input current (per UL rules) without being tied to what is connected to the Output side. So in reality if, picking from the first column on your chart, the input amps are shown as being rated at 44A while the output current is shown as being rated for 38A. That's because it is being sold here as a 25HP 460V drive, which needs to be capable of 34A for that motor. But the drive was DESIGNED by Yaskawa for a different motor in Japan, which would require more output current and take 44A input current. When they took it to UL, the INPUT current is still the same, but because UL insists on this disambiguation of input and output ratings, the 44A input current is still shown. When you actually run that 25HP motor at full load of 34A, the input current will be LESS than 34A.

I know, it's confusing. I had to have a Yaskawa engineer explain it to me once. But you can sum it up as a glitch in how UL requires that VFDs be listed. The 44A is just what UL insists it be listed at, mostly because of the NEC rule on conductor sizing feeding the VFD being based on the VFD maximum input current, not the motor size.

Isn't this all common knowledge?



Yea, right.





Thank you again.

 

[Jraef]

Isn't this all common knowledge?



Yea, right.





Thank you again.

Every time someone asks me how I came know all this stuff (which my wife calls "useless"), I answer the same way.

"One mistake at a time, although sometimes the same mistake more than once."

 

[elec_eng]

That's a phenomenon that you only see pretty much in Japanese drives like Yaskawa, Mitsubishi etc, and is more of a marketing and UL listing issue believe it or not. Japanese drives are primarily designed for their market, where they have to contend with motors being designed slightly different than the rest of the world, because they have an even worse situation that we in the US do with regard to different power distribution systems. Some parts of the country are 50Hz, some are 60Hz, and the 3 phase voltage levels are odd too. In the 50Hz sections, they use 200 and 400V, in the 60Hz sections they use 220, 400 and 440V. So their motors, to be compatible everywhere in Japan, have to be designed to accept all of the different V/Hz ratios those represent. That then means when they design their VFDs, they are designing them for THEIR motors, and adapting the MARKETING of those designs to be used in the rest of the world.

Then when they send that drive to the US and have to get it UL listed for a particular HP, the HP rating has to correspond to OUR motors, which has to do with the OUTPUT ratings, but the Input ratings have to do with the MAXIMUM input current (per UL rules) without being tied to what is connected to the Output side. So in reality if, picking from the first column on your chart, the input amps are shown as being rated at 44A while the output current is shown as being rated for 38A. That's because it is being sold here as a 25HP 460V drive, which needs to be capable of 34A for that motor. But the drive was DESIGNED by Yaskawa for a different motor in Japan, which would require more output current and take 44A input current. When they took it to UL, the INPUT current is still the same, but because UL insists on this disambiguation of input and output ratings, the 44A input current is still shown. When you actually run that 25HP motor at full load of 34A, the input current will be LESS than 34A.

I know, it's confusing. I had to have a Yaskawa engineer explain it to me once. But you can sum it up as a glitch in how UL requires that VFDs be listed. The 44A is just what UL insists it be listed at, mostly because of the NEC rule on conductor sizing feeding the VFD being based on the VFD maximum input current, not the motor size.

:thumbsup:

 

[Ingenieur]

I am still having a hard time to understand this. This is quite opposite to what I thought before. As all others pointed out, output power is always less than input power. If the voltage is the same for both input and output, I would think, the output current should be less than input.

Of course, the power factor gets into the mix so "I think", it might not be always the case but...I also have seen other VFD manufacturers data sheets show input current is "WAY" higher than output current. I am totally lost here..

Input 480 x sqrt3 x 100 x 0.95 = 79 kw
Output 480 x sqrt3 x 105 x 0.90 = 78.5 kw

input power > output power
input i < output i

there is reactive i in the output not present in the input