# Amp draw increase through VFD



## oodly (Sep 8, 2016)

Hello... at my workplace we have a 3 phase 460v pump which used to start across the line. During some upgrades it was put on a VFD and it seems to me the increase in its amp draw is excessive. Its nameplate full load amps are 25... with the drive in bypass it draws ~21. Through the VFD at 60 Hz it's at 25, almost a 20% increase from directly connected.
Does this sound abnormal? I've noticed VFDs drawing more at full Hz, but never this much more. Any comments on this will be greatly appreciated.


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## Jhellwig (Jun 18, 2014)

How are you measuring current? If you are using an amp clamp are you using it on the line or load side of the vfd? Is the amp clamp true rms? Are you going by what the drive says?

Non true rms meters will not read the output of an inverter correctly.


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## ponyboy (Nov 18, 2012)

Are you reading the current from the drives digital display or other means? You can't just put a clamp on the load side of the drive as you probably know. Is your drive programmed correctly?


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## inetdog (Apr 13, 2016)

In suppose that the motor may be spinning faster with the VFD trying to regulate speed, causing it to draw more power from the increased load. What is the load on the motor?


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## dmxtothemax (Jun 15, 2010)

any reading of a non-sinusuadal waveform is open to errors period.
Most meters are calibrated with pure sine waves
And the output of invertors is usually closer to squarewave
as the area of a square wave is larger than that of a sine wave
slightly more power will be applied and used.


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## Big John (May 23, 2010)

The drive has inefficiencies, that's why bypass exists: You will always use less power running a motor at full speed from a contactor than you will running it at full speed from a drive.

Energy savings only come into play when you use a VFD to run at substantially reduced speed.


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## John Valdes (May 17, 2007)

Biscuits said:


> Are you reading the current from the drives digital display or other means? *You can't just put a clamp on the load side of the drive as you probably know. *Is your drive programmed correctly?


Why not?


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## ponyboy (Nov 18, 2012)

John Valdes said:


> Why not?




I think I got confused and was thinking of load side voltage readings.


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## oodly (Sep 8, 2016)

All readings were from the drive display (ABB ACH550 Eclipse). It's programmed correctly, the only change I made is that I set it to limit at the nameplate amps to avoid future trips (The bypass of the drive had faulted on "MTR OVERLOAD" and I know it was exceeding nameplate FLA when running through the drive). I'm just at a loss to explain that high increase when the drive is selected. I'm going to take some readings at other buildings and see what the norm is.


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## JRaef (Mar 23, 2009)

Any chance you have PFC caps installed on the Bypass starter arrangement? That would explain it.


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## Mike_kilroy (Sep 2, 2016)

Did you verify motor volts L-L is identical in both cases? Maybe u have 245v across line and only 220V from VFD (programmed down)...

Sent from my SM-G900V using Tapatalk


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## John Valdes (May 17, 2007)

Biscuits said:


> I think I got confused and was thinking of load side voltage readings.


Why can't you read voltage on the load side with a true RMS multimeter?
I have seen people read the output with an analog meter.


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## Mike_kilroy (Sep 2, 2016)

John Valdes said:


> Why can't you read voltage on the load side with a true RMS multimeter?
> I have seen people read the output with an analog meter.


Because it will try to read the true rms but that does not work with 2-2.5x voltage spikes caused by the output transistors turning on and ringing at 1-10-100mhz! It will often try to rms those noise spikes right into your answer! 

Now, try it! SOMETIMES some meters are ok and do it. Compare the reading to the drives 'output voltage monitor' reading to see if YOUR meter works on that particular drive, at that particular frequency... Then you know.

And previously you asked about clamp on output - generally that works fine since the current is a failty good sine wave.


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## Big John (May 23, 2010)

Also, analog clamp-ons respond directly to the strength of the magnetic field. They are true-RMS by design.

Digital clamp-ons have a sampling rate that can miss peaks and troughs in a PWM output and just guess at the total based on an averaging calculation. I would be careful trusting a digital ammeter on the loadside of a VFD.


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## inetdog (Apr 13, 2016)

Big John said:


> Also, analog clamp-ons respond directly to the strength of the magnetic field. They are true-RMS by design.



An analog clamp on could just as well be reading average magnetic field by responding to the strength of the magnetic field. 

Old fashioned analog AC voltmeters that measured true RMS were mechanical designs that actually responded to the _square of the voltage_* and averaged that. Then the scale was calibrated to be proportional to the square root of the needle displacement rather than linear. 

* Assume that the needle motion represents the average torque on the coil, working against a linear rate spring. 
The torque is proportional to the rotating field and to the stator field. If both fields are proportional to the voltage/current applied, then the torque will be proportional to the square of the applied voltage. 
By using a coil for both stator and rotor you get that effect. 

Either a clamp meter or a shunt meter or a meter in series with the entire current can be constructed to be either linear or RMS. 
In a digital meter you do need electronics to do the square and square root operations, so an averaging unit is much cheaper.

PS: have you ever looked at old fashioned current or voltage meters and wondered why the DC meters had a linear scale and the AC meters did not? The DC meters just did a mechanical average rather than RMS because before pulsed drives there was no need to explicitly measure DC as RMS. 
PPS: With pulsed DC to produce a desired DC output the (smoothed) current will in fact be proportional to the average DC, not to the RMS DC. But when the high frequency DC is smoothed to produce lower frequency alternating current, that requires the RMS circuitry to be applied to the AC waveform.
PPPS: If the DC pulses are not smoothed but go directly to a resistive load, then the RMS value will correspond to the load heating. 
Bottom line: How a meter should react to pulsed DC depends on how you are using that DC!


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## Big John (May 23, 2010)

inetdog said:


> An analog clamp on could just as well be reading average magnetic field by responding to the strength of the magnetic field.


 I was under the impression that old ammeters were iron vane type. So they all produced a torque directly proprtional to RMS current.


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## Jhellwig (Jun 18, 2014)

Big John said:


> Also, analog clamp-ons respond directly to the strength of the magnetic field. They are true-RMS by design.
> 
> Digital clamp-ons have a sampling rate that can miss peaks and troughs in a PWM output and just guess at the total based on an averaging calculation. I would be careful trusting a digital ammeter on the loadside of a VFD.


True rms digital ones will read right on inverter outputs. That is what they are for.


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## inetdog (Apr 13, 2016)

Big John said:


> I was under the impression that old ammeters were iron vane type. So they all produced a torque directly proprtional to RMS current.


Iron vane type will produce a deflection proportional to the square of the voltage or current. That is, the stator coil induces a field in the iron vane which is proportional to the coil field and the deflection is proportional to the product of the two fields, hence to the square of the applied voltage or current. 
As I noted, the meter scale then converts from MS (mean square) value to the RMS (root mean square) value. 

No disagreement there except between the torque itself being proportional to MS or RMS.


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## John Valdes (May 17, 2007)

Mike_kilroy said:


> Because it will try to read the true rms but that does not work with 2-2.5x voltage spikes caused by the output transistors turning on and ringing at 1-10-100mhz! It will often try to rms those noise spikes right into your answer!
> 
> Now, try it! SOMETIMES some meters are ok and do it. Compare the reading to the drives 'output voltage monitor' reading to see if YOUR meter works on that particular drive, at that particular frequency... Then you know.
> 
> And previously you asked about clamp on output - generally that works fine since the current is a failty good sine wave.


The digital meter cannot react fast enough to see the spikes so you will not read the spikes.
Besides. How important is the output voltage? If you achieve and can maintain speed, that would be all I care about. After all the drive compensates for load (closed loop or torque type control). 
I just want to see the output. Even ifs its not exact. 
I did not ask about a clamp. I said a clamp would work just fine on the VFD output, just as meter will read voltage on the output.



Jhellwig said:


> True rms digital ones will read right on inverter outputs. That is what they are for.


I never trusted the drive readout. I trusted my meter more.


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