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Input Frequency Vs. Output Frequency ?


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#1 Matt

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Posted 27 July 2007 - 12:44 PM

Sending a machine down under, Where the Input voltage signal will be 415 VAC @ 50Hz. vs. 460 VAC @ 60Hz Here.
My question is how will the reduced input frequency effect the output frequency. I am using Yaskawa V-7 VFD's in this app.

One VFD is controlling feed speed with a 0-10VDC signal, the others are simply running open loop, And accurate speed control is not needed.
So the one I am concerned with is the feed speed, As I calibrate the speed entry into the HMI, I don't know if it will be accurate once it is installed in the 50Hz enviroment ????


Matt

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#2 jraef

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Posted 27 July 2007 - 03:16 PM

The input frequency has no effect on the VFD. A VFD just uses the line power as a sort of raw material, from which it creates an entirely new power output voltage and frequency. The input can be almost anything and certainly 50 or 60Hz is well within it's range. 400Hz may be a problem, but unless you want to power it up on the jet going over, you shouldn't worry.

Hopefully you have transformers to boost the voltage however, otherwise you will either have to limit your max. speed or oversize your motors.
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#3 mariomaggi

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Posted 27 July 2007 - 03:57 PM

Dear Matt,
no problems with 50 Hz, there is no need to limit speed or oversize your motor if you will select the proper motor winding, i.e. 415 VAC @ 50Hz or a standard EU motor 400 VAC @50 Hz

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#4 marke

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Posted 27 July 2007 - 09:41 PM

Hello Matt

The VFD rectifies the incoming supply, converting it to DC, and then chops it up back into AC again.
Because it is going thtough the DC phase, the output frequency bears no relationship to the input frequency.
The output voltage on a VFD reduces as the frequency is reduced in order to keep the flux in the motor constant. The maximum output voltage is determined by the input voltage, therefore in the 415 volt supply, the output voltage will be limited to 415 volts, so when you are running you VFD with an output voltage higher than 50Hz, the voltage will be low and the flux will be reduced. This will reduce the capacity of the motor at speeds above 50Hz.

One option is to use a motor rated at 415 volts instead of 460 volts.

Best regards,

#5 jraef

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Posted 27 July 2007 - 11:51 PM

QUOTE(marke @ Jul 27 2007, 02:41 PM) View Post

Hello Matt

The VFD rectifies the incoming supply, converting it to DC, and then chops it up back into AC again.
Because it is going thtough the DC phase, the output frequency bears no relationship to the input frequency.
The output voltage on a VFD reduces as the frequency is reduced in order to keep the flux in the motor constant. The maximum output voltage is determined by the input voltage, therefore in the 415 volt supply, the output voltage will be limited to 415 volts, so when you are running you VFD with an output voltage higher than 50Hz, the voltage will be low and the flux will be reduced. This will reduce the capacity of the motor at speeds above 50Hz.

One option is to use a motor rated at 415 volts instead of 460 volts.

Best regards,

Or as I alluded to, if you must have full power at 60Hz from that motor, simply use a transformer in front of the VFD to boost the incoming line to 480V. Even a low cost autotransformer would work for that.
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#6 Matt

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Posted 30 July 2007 - 11:34 AM

QUOTE(jraef @ Jul 27 2007, 07:51 PM) View Post

Or as I alluded to, if you must have full power at 60Hz from that motor, simply use a transformer in front of the VFD to boost the incoming line to 480V. Even a low cost autotransformer would work for that.



Thanks All !!

The Yaskawa V-7 that i'm using, List its input voltage as 380-460VAC, So I should be fine....

Thanks Everyone !!

MATT
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#7 jraef

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Posted 31 July 2007 - 04:30 AM

But your VFD cannot create voltage that is not there!

So if your MOTOR is rated for 460V, you are still either going to have to limit the maximum speed or boost the line voltage to 460V. If you input 415V to the drive, the maximum output voltage will be only 415V. If you put 415V on a motor designed for 460V, and you run the speed up to 60hz, the motor will be under excited and not provide full torque under load, which means increased slip, higher current draw, more heat and could lead to rapid failure of the motor. Here's why:

Motors work on a V/Hz ratio. So a 460V 60 Hz motor needs 460/60 = 7.7V/Hz. But if you make it 415V and 60Hz, the ratio becomes 6.92V/Hz, a decrease of 10% which will represent a 10% loss of torque at full speed. To keep the V/Hz ratio the same as the motor design, you would need to limit your maximum speed to 415V/7.7V/Hz = 54Hz. If that is unacceptable, then the other alternative is to boost the line voltage to 460V


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#8 Matt

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Posted 31 July 2007 - 12:30 PM

QUOTE(jraef @ Jul 31 2007, 12:30 AM) View Post

But your VFD cannot create voltage that is not there!

So if your MOTOR is rated for 460V, you are still either going to have to limit the maximum speed or boost the line voltage to 460V. If you input 415V to the drive, the maximum output voltage will be only 415V. If you put 415V on a motor designed for 460V, and you run the speed up to 60hz, the motor will be under excited and not provide full torque under load, which means increased slip, higher current draw, more heat and could lead to rapid failure of the motor. Here's why:

Motors work on a V/Hz ratio. So a 460V 60 Hz motor needs 460/60 = 7.7V/Hz. But if you make it 415V and 60Hz, the ratio becomes 6.92V/Hz, a decrease of 10% which will represent a 10% loss of torque at full speed. To keep the V/Hz ratio the same as the motor design, you would need to limit your maximum speed to 415V/7.7V/Hz = 54Hz. If that is unacceptable, then the other alternative is to boost the line voltage to 460V



Thanks Jraef !!

The motors are all oversized so the small loss of torque shouldn't be a problem. However you did start me thinking about the V/Hz causing an overheating problem...Thanks !

MATT
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#9 Matt

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Posted 07 August 2007 - 06:29 PM

Set the Max Freq down to 54Hz. and All is well !!

However let me ask this, Since the input Voltage is 415 VAC, And it is going to be rectified into DC, Creating a DC buss voltage of 587 VDC.

Why can the Inverter not chop out 460 VAC ???
Matt

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#10 marke

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Posted 07 August 2007 - 07:49 PM

Because the peak voltage on a 460 volt sinewave is 650 volts.

A DC voltage of 587V will theoretically allow a maximum of 415 volts without distortion. In practice it will be less.

Best regards,

#11 jraef

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Posted 08 August 2007 - 04:07 AM

...now if I could just invent that VFD that would make extra voltage out of thin air, I'd be a rich man!

Must... keep... thinking...

Now where'd I put my hat?

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#12 Matt

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Posted 08 August 2007 - 11:54 AM

QUOTE(jraef @ Aug 8 2007, 12:07 AM) View Post

...now if I could just invent that VFD that would make extra voltage out of thin air, I'd be a rich man!

Must... keep... thinking...

Now where'd I put my hat?

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This should have been Extra Power ! Increasing voltage is easy. At the cost of current...

And it would not be "creating" voltage only manipulating the Voltage / current anyway.


MATT
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#13 Matt

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Posted 08 August 2007 - 12:09 PM

QUOTE(marke @ Aug 7 2007, 03:49 PM) View Post

Because the peak voltage on a 460 volt sinewave is 650 volts.

A DC voltage of 587V will theoretically allow a maximum of 415 volts without distortion. In practice it will be less.

Best regards,


Unless I am really having trouble understanding Inverter operation, How did you calculate this ?

I geuss am having trouble understanding why it is that If I have 587 VDC available with plenty of current, I cannot use an inverter to chop 460 out of that...????
Matt

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#14 Matt

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Posted 08 August 2007 - 05:10 PM

Because:
From Yaskawa,

The conversion of AC to DC results in the DC bus approaching the peak of the RMS AC waveform (460VAC x sqrt 2 = 650VDC). In order to make the same linear sinewave on the output, we get the same conversion back which results in ACin = ACout.
It is true that if you did not care about the waveform (which would sacrifice motor performance), you could make more of a square wave. This would have a higher RMS voltage but would have terrible current distortion. It is the current waveform that determines motor performance.


Matt

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#15 marke

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Posted 08 August 2007 - 07:07 PM

Hello Matt

Yes, you are correct. From a DC bus voltage of 587 V, you can create an output voltage of 460 volts RMS, but it will be non sinusoidal. The result of this is that you will have a 415 volt torque field at 60Hz, plus a smaller counter rotating torque field at 180Hz plus a counter rotating torque field at 300Hz ......
The net result is that the torque produced by the motor at it's synchronous speed will be the same as the 415 volt output. plus some variation due to the lip effects of all the other torque fields which may add, or subtract from the total torque. - No real gain but lots more slip losses in the motor.

Best regards,




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