Amp draw on 50hp VFD

[M_A_A]

Could someone please help me with this?
Last week at the plant we were testing out a used mixer powered by a 50HP 480v 3p motor and a VFD.
When it was running at full speed under no load the amp draw on all phases was less than 3 amps on the input of the VFD and 20+ on the output side phases.
I presume this is because the mixer was not under load. As the load increases the input amp draw should increase as well, correct?
It has been a while since I worked on such a large unit and was wondering if the readings sound normal or maybe there is a problem with the VFD.
The unit is older and documentation is somewhat unclear as it only states maximum specs. All of the parameters have been doublchecked to be correct.

 

[GeorgeB]

powered by a 50HP 480v 3p motor and a VFD.
When it was running at full speed under no load the amp draw on all phases was 20+ on the output side phases.

That is reasonable; I usually assume the no load current will be between 25% and 50% of full load current. NOTE the power drawn is NOT 25-50% however.

When it was running at full speed under no load the amp draw on all phases was less than 3 amps on the input of the VFD.

I've never thought about it, but the huge lagging power factor of the motor would not exist on the input; a true 3A at unity PF would be around 3.3HP or 7% loss ... a little high, but you did say "less than 3" and there is probably still a non-unity PF.

 

[jim dungar]

FYI,
Remember the output of a VFD is not a sine wave. You need to be using a true RMS meter to have a number that can be related to the input.

 

[kwired]

FYI,
Remember the output of a VFD is not a sine wave. You need to be using a true RMS meter to have a number that can be related to the input.

He may want true RMS for measuring input also. I'm sure the input current is not a true sine wave either.


3 amps for a no load full speed 50 hp motor?? The rotor alone will probably require more power than that just to turn it.

Most drives will tell you what output current is if you find the correct display setting. I can't recall one that tells you input current, it would probably require additional components to be installed to be able to do this, otherwise it probably could give you a calculated amount, that may not always be accurate however.

 

[Besoeker]

3 amps for a no load full speed 50 hp motor?? The rotor alone will probably require more power than that just to turn it.

That's VSD input current, not motor current.
A 50 hp motor is 37kW. At say, 92% efficiency, that would equate to about 3kW losses when running at rated power. On no load, the losses would be significantly lower.
As GeorgeB noted the 3A at unity pf would be about 3.3 HP or 2.5kW - when will you guys get with the programme ad ditch HP then I wouldn't have do do all these conversions....
And he is right in pointing out that there might be a non-unity PF.
Power factor is W/VA and it comes in two flavours. Displacement and distortion.
If the VSD has a good sized reactor after the input rectifier you might get reasonably level current on the DC link. At that, distortion PF is about 0.95. And quite a bit worse without one.

In short, I entirely agree with George. The currents being measured are quite within expectations.

 

[Jraef]

He may want true RMS for measuring input also. I'm sure the input current is not a true sine wave either.


3 amps for a no load full speed 50 hp motor?? The rotor alone will probably require more power than that just to turn it.

Most drives will tell you what output current is if you find the correct display setting. I can't recall one that tells you input current, it would probably require additional components to be installed to be able to do this, otherwise it probably could give you a calculated amount, that may not always be accurate however.

That's right. It takes a fairly sophisticated digital meter to properly read and interpret the input current of a VFD. I would not trust your 3A reading. But before you get too involved, why do you need to know the input current?

 

[M_A_A]

Thank you everyone for the replies.

The reason for my concern is the fact that the mixer and vfd are both used and it would be best to catch any problems before they were put into service. Also as I suspected the input draw is so low (as opposed to the output) due to the fact that the motor is not loaded.
I just wanted to run it by the experts on this forum to be sure.

Also we now have somewhat of a baseline to check the second motor against when we test run it (we have 2 used mixers)

I recall this scenario on much smaller equipment some years back and I haven't worked with vfds for a while so I'm a little foggy on theory.

Jim : I was pretty sure RMS played into it as far as accuracy goes but it was the difference between input and output that threw me. If using a true RMS meter would the output read lower? Our department is too cheap to provide a decent meter.

Kwired : The drive is capable of displaying output freq,volts,or amperes. (calculated by drive I believe)

Besoeker: Sorry for all the math!!

Jraef : Not so concerned with total accuracy,just the spread sounded high to me and the senior electrician. Also as mentioned we are trying to establish a baseline of sorts for future reference.

Great knowledge and info here, Thanks Again!!

Mark A.

 

[chris kennedy]

Our department is too cheap to provide a decent meter.

I would be very concerned testing 480 with anything other than a 600V/IV-1000V/III. In fact I wouldn't do it.

 

[kwired]

That's VSD input current, not motor current.
A 50 hp motor is 37kW. At say, 92% efficiency, that would equate to about 3kW losses when running at rated power. On no load, the losses would be significantly lower.
As GeorgeB noted the 3A at unity pf would be about 3.3 HP or 2.5kW - when will you guys get with the programme ad ditch HP then I wouldn't have do do all these conversions....
And he is right in pointing out that there might be a non-unity PF.
Power factor is W/VA and it comes in two flavours. Displacement and distortion.
If the VSD has a good sized reactor after the input rectifier you might get reasonably level current on the DC link. At that, distortion PF is about 0.95. And quite a bit worse without one.

In short, I entirely agree with George. The currents being measured are quite within expectations.

I understand what you are saying, just thought a 50 hp motor running at no load/full speed would use more than 3 hp just to turn its own rotor. 5-8 amps on the input and I may not have even questioned it. But again OP may not have been using true RMS meter and 3 amps may mean nothing.

I don't know if we will ever get rid of our units of measurement and fully go metric. I do know metric and know that just moving decimal place is all that is needed to convert from one unit to another, but been using feet, inches, cup, pint, quart, ounces, pounds, farenheit, for so long, it would have other difficulties. I can get good guesstimate of length in meters simply because a yard is close enough for guesstimate, but tell me something is 100 kilometers away and I have a harder time thinking about what that is than if you told me about 60 miles. (Didn't look up the conversion but off the top of my head, if I am right it should be 62.5 miles)

 

[Jraef]

I understand what you are saying, just thought a 50 hp motor running at no load/full speed would use more than 3 hp just to turn its own rotor. 5-8 amps on the input and I may not have even questioned it. But again OP may not have been using true RMS meter and 3 amps may mean nothing.

I think I was doing the same thing as you. I too would think a 50HP motor even unloaded would be drawing more current, but you have to remember that the VFD is essentially correcting the power factor as seen by the line. So while we tend to guesstimate unloaded CURRENT at 20-40% of FLA when referring to being across-the-line, that's at a really poor PF. But when the VFD corrects that PF, the actual current will drop to what represents the real load (kW) being used. So until I looked at George and Besoeker's math I hadn't thought that out much, but it makes sense now that they forced me to think about it.

 

[GeorgeB]

I think I've posted it before, but Baldor has a published graph which really pulls this all together. They used a 15HP on a 208 supply, but the characteristics are similar for most 3 phase induction motors.

Power is in watts <g> but motor load is in equine units <bg>

It is interesting to guess where the watts line intercepts the zero load ... looks like ABOUT 500 watts ... certainly between 0 and 1kW. Full load is about 12-3/4 kW. On THIS motor, no load current is almost 60% of full load.

This graph, if you look at it and understand it, helps with almost all motor questions I've had presented.

 

[jim dungar]

Jim : I was pretty sure RMS played into it as far as accuracy goes but it was the difference between input and output that threw me. If using a true RMS meter would the output read lower? Our department is too cheap to provide a decent meter.

Effectively there are two ways to measure AC values; Peak to Peak and RMS. RMS is the value used most commonly in our industry therefore it would be extremely improbable to have a hand-held meter that did not display it.

But there are several ways to take the AC sine wave and determine its RMS equivalent value. The two most common are usually referred to as Average and True. Average reading RMS meters are the predominant type for most users. True RMS meters are better able to evaluate waveforms that have a lot of harmonics in them such as those associated with VFDs and other power conversion equipment.

An Average RMS meter on a drive input is nowhere near as 'inaccurate' as it is on the output, especially when the VFD is running lightly loaded or at reduced speed. Many people rely on the VFD display for 'seeing' voltage and current values instead of using a meter.

 

[petersonra]

these days most VFDS have current meters built in on both line and load side. just a matter of figuring out how to display it.

 

[Besoeker]

Power is in watts <g> but motor load is in equine units <bg>

Power is power. Both electrical and mechanical power can be expressed in Watts or multiples thereof.
You guys will get with the rest of the world on that .....eventually.

 

[Besoeker]

So until I looked at George and Besoeker's math I hadn't thought that out much, but it makes sense now that they forced me to think about it.

Thank you for that. But no forcing involved. If it made you think that can't be a bad thing.

 

[Besoeker]

I don't know if we will ever get rid of our units of measurement and fully go metric. I do know metric and know that just moving decimal place is all that is needed to convert from one unit to another, but been using feet, inches, cup, pint, quart, ounces, pounds, farenheit, for so long, it would have other difficulties.

I would not, and do not, suggest that you get rid of units.
There has been an adverse reaction to that here in UK. SI units were imposed. Unreasonably and unnecessarily so in my opinion.
Why do I have to buy fuel in litres rather than gallons? What exactly did that achieve? Confusion on fuel prices, deliberate or otherwise.
That somewhat angers me.
So, keep your units for as long as you can.


But I like metric (SI) in engineering because the units hang together. I don't need to remember that 33,000 ft-lb per minute is a horsepower.
Electrically one Volt and one Amp at the same instant is one Watt.
Mechanically one Newton force moved one metre in one second is also one Watt.

I like simple.

 

[Jraef]

I would not, and do not, suggest that you get rid of units.
There has been an adverse reaction to that here in UK. SI units were imposed. Unreasonably and unnecessarily so in my opinion.
Why do I have to buy fuel in litres rather than gallons? What exactly did that achieve? Confusion on fuel prices, deliberate or otherwise.
That somewhat angers me.
So, keep your units for as long as you can.


But I like metric (SI) in engineering because the units hang together. I don't need to remember that 33,000 ft-lb per minute is a horsepower.
Electrically one Volt and one Amp at the same instant is one Watt.
Mechanically one Newton force moved one metre in one second is also one Watt.

I like simple.

The only thing is, when "you people*" use the term watts with regards to motors, there is the inevitable confusion (among some) as to whether that is mechanical power output or electrical power input, which is not the same thing. When we say "Horse Power" we know we are always referring to the mechanical power output, then we are free to use the term watts for electrical reference only. Not a big deal really, just a convenience from my standpoint.

Mind you I'm an EE, not an ME who may also use the term HP referring to boiler steam, which is not directly mechanical. But I never talk to them anyway...

* meant as a good natured joke...

 

[weressl]

I would be very concerned testing 480 with anything other than a 600V/IV-1000V/III. In fact I wouldn't do it.

Isn't he measuring current only?

 

[kwired]

I would be very concerned testing 480 with anything other than a 600V/IV-1000V/III. In fact I wouldn't do it.

Isn't he measuring current only?

How about measuring current by reading the display on the VFD??

 

[Besoeker]

The only thing is, when "you people*" use the term watts with regards to motors, there is the inevitable confusion (among some) as to whether that is mechanical power output or electrical power input, which is not the same thing.

Not here. A 200kW motor is 200kW output. For a particular application output power determines what the motor can do. I'd see that as the starting point.
As I've said before, I'm old enough to have been educated in Imperial CGS, MKS, and SI.
I can do all but, out of choice, I use SI.
As you probably know, Mrs B is from your side of the pond. And likes to cook. I routinely have to convert units from grams to ounces and vice versa. No problem. I just know conversion factors. Despite my advance years, I'm blessed and cursed with a very good memory. Particularly for numbers.

Here's something I posted elsewhere:

Last week 600 miles in three days.
Bedfordshire, Hertfordshire, Bucks, Essex, Kent, Nottingham....and all points between.
Helluva week.
Then had the grand daughter for three days over the weekend.
I need to slow down. Not get out more.

But this memory thing gets me thinking at times. I don't need to know the co-ordinates of my old school. I just do. The registration plates of the cars my father had, all of mine. They're just stuck in there. Same with phone numbers. I have a few stored on my phone but rather than look them up I usually just key them in. Same with job numbers, part numbers engine capacities.....stuff.
The Austin Westminster had a BMC C series in line six pot job with a displaced capacity of 2912cc. I can remember my aunt's address from when I was maybe five years old.

The why makes me ponder. I have all that taking up cells in my brain. Is it reducing my capacity to absorb more?
I don't often fret over it. But, to some extent, it can be a burden. People, colleagues, get an unreasonable expectation that I can remember every detail of all projects.
That's hard.

The why remains.

* meant as a good natured joke...

Not taken any other way.