# Confused about VFD output voltage, amps



## Peewee0413 (Oct 18, 2012)

oodly said:


> I'm curious about the relationship between the input voltage/amps of a vfd and the output. An example is a motor I looked at today. Its nameplate specs are:
> volts - 460
> 3 phase
> hp - 50
> ...


What's the equipment. Blower or pump by chance?

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## paulengr (Oct 8, 2017)

oodly said:


> I'm curious about the relationship between the input voltage/amps of a vfd and the output. An example is a motor I looked at today. Its nameplate specs are:
> 
> volts - 460
> 
> ...




Did you enter a service factor? 57 x 1.15 = 66 A.

More important is your meter true RMS or not? A cheap way to measure AC sinusoidal current or voltage is to rectify it, average it (RC circuit), and scale by 110%. The circuit costs pennies and reuses the DC side of the meter for everything else. Works awesome on sine waves. Drives output something that the motor works on but isn’t a sine wave at all. It’s DC pulses (positive and negative) with some filtering to knock the high frequency edges off so the pulses are rounded off. If you use any meter other than a true RMS one you get false numbers because those only work on real AC.



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## Peewee0413 (Oct 18, 2012)

paulengr said:


> Did you enter a service factor? 57 x 1.15 = 66 A.
> 
> More important is your meter true RMS or not? A cheap way to measure AC sinusoidal current or voltage is to rectify it, average it (RC circuit), and scale by 110%. The circuit costs pennies and reuses the DC side of the meter for everything else. Works awesome on sine waves. Drives output something that the motor works on but isn’t a sine wave at all. It’s DC pulses (positive and negative) with some filtering to knock the high frequency edges off so the pulses are rounded off. If you use any meter other than a true RMS one you get false numbers because those only work on real AC.
> 
> ...


He measured the line side and looked at the load side via the display, meter isn't the issue.

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## frenchelectrican (Mar 15, 2007)

Peewee0413 said:


> He measured the line side and looked at the load side via the display, meter isn't the issue.
> 
> Sent from my VS995 using Tapatalk


The issue if the OP used standard ampmeter it may be ok on reading but with VSD line side I still rather use the true RMS clamp on ampmeter to correctaly read it.

I think paulengr may agree with me but I am not sure what he will reply on that part but I do not know which feedback the OP is using on that part. 

If the OP give us more details maybe we can able look up more closer to get correct info.


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## paulengr (Oct 8, 2017)

I might add most drives can and will put out much more than FLA during acceleration or heavy loads, if you let them. You need this for a lot of loads for starting anyways. You can often set a current or torque limit (almost the same thing) if it’s needed. So if you current limit to FLA then unless the drive has an internal fault or you get a dead short the drive will never trip on overload. It simply slows down. This can become a big problem without stall protection (no cooling so motor burns up), and some loads like compressors will stall out while starting.


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## JRaef (Mar 23, 2009)

The input current to the drive is at a .95 power factor, the output current FROM the drive has to INCLUDE the reactive current going to the motor, so the total output current at the same RMS voltage will be higher by a similar ratio to the difference in PF. Working backward, it would mean the motor PF is at about a .78, which is not at all unusual.

So the motor is running at a .78 PF, but the drive bus caps are "correcting" that motor PF to .95. That is why the current values are different.


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