Huanyang VFD current draw measurements.

[Edward]

Hi guys,
Some of you may be interested. I set up the VDF with a potentiometer and the 2.2kw spindle on the bench at 15000rev. The Amp setting on the panel was saying 1.5A

Then I used my ammeter on the mains power cable and it actually measured 0.8A. I trust this reading to be more accurate than the VFD panel reading.

Naturally this was without any load, i.e. not cutting anything, so the current will go up under cutting conditions, nevertheless I thought it was interesting to see how little current it draws and also to show the disparity between the panel reading and an ammeter reading.

I also measured the noise level (again without load) using a digital decibel meter. My room level is 41db. The fan is quite noisy, at around 53db (1 meter distance) and with the spindle on at 15000 revs it read 62db. I was quite surprised that it is so quiet on the bench.

Edward

[Edward]

I also just set up the analog voltage control and ON/OFF of the VFD from my Kflop/Kanalog controller and it all works nicely. I see no fluctuation in the speed as it is, on the bench, nice and sweet. The speed variation between what it says on the VFD panel and my own reading with a tachometer was practically bang on too.

For the on/off relay I've used a cheap SSR from RS, part number 720-3956. It only costs £6.62 and it operates at 24V. No diode needed.

Edward

22766

[magicniner]

The effective voltage to the motor may not be the same as mains and you're driving an inductive load so power factor is also in play ;-)

If it were a Siemens, Hitachi etc. I would trust the VFD, I don't have and probably never will have experience of tertiary VFD brands, I require good documentation and support in any hardware I use.

[Neale]

Out of curiosity, are you also able to measure the analogue voltage coming out of the Kanalog? I was looking at my CSMIO+HY VFD combination yesterday. According to Mach3, the demanded speed was 15000; the VFD said 15500. Error of about 3% - not a big deal, and I can't think of many situations where that would matter to me. I would expect the spindle speed to track that pretty closely, given the motor technology and the fact that the motor speed display will be derived from the VFD digital electronics. When I did a similar test while initially configuring the machine, I seem to remember that there was a slight error in the analogue output from the motion controller which probably explains any discrepancy but I'm not sure I would trust my Maplin digital multimeter that well anyway.

But it's nice to know that it does what you expect! As for current readings - as Nick suggests, you don't know exactly what and how they are deriving these numbers, but it might be interesting to compare off-load and full-load readings.

[Edward]

Yes, I actually measured the voltage with my multimeter out of the Kanalog before I connected anything. So for example, out of a requested 5V, I got a reading of 5.04V. The analog conversion is not 100 per cent linear, but pretty close. Then with the software you have a multiplier that you can adjust, so when your code says for instance, s12000, you also get the 12000 on the VFD. But then if you want S15000, you may get 15100revs, as I say it's not linear, but in practice it's of zero importance to the cut, I think it's the same with any other boards really, some may have more linear conversion than others throughout the 0-10V scale.

I think the current reading that's most relevant is the actual current draw at the entry point to the VFD, i.e the mains power. I used a clamp meter, which I think is fairly accurate, though it's not one of the super expensive ones. I am curious too as to how much more current it will draw whilst cutting, as this could give you a good idea of how much you can push your cut settings in order to find when you are close to the limit, and maybe also a give you a glimpse at the current draw at start up and stopping? I guess this could also determine the fuse rating you use.

Edward

[EddyCurrent]

I think the current reading that's most relevant is the actual current draw at the entry point to the VFD

relevant to what ?

Regarding current, read this; http://www.industrial-electronics.com/output_devices_amplifiers_valves_relays_variable-frequency_drives_stepper_motors_servomotors/AC-drives-FAQ-2.html

[Edward]

Thank you, that kind of throws light on the difference between the panel reading and input current reading.

Edward

[Clive S]

relevant to what ?

Regarding current, read this; http://www.industrial-electronics.com/output_devices_amplifiers_valves_relays_variable-frequency_drives_stepper_motors_servomotors/AC-drives-FAQ-2.html

Hi Eddy I was waiting for you to chime in, Good read.

[Edward]

Relevant to what, you asked?

Well, for instance, the current at the input is relevant to me as I want to have some idea of how much current the whole system is likely to draw in total when at full blast, including 3 servos, air compressor, water pump and other minor peripherals. I have two mains rings in my workshop, so I may use the second ring for the compressor, which I haven't bought yet, but it will be the largest - silent type- I can safely get away with.

Edward

[magicniner]

Well, for instance, the current at the input is relevant to me as I want to have some idea of how much current the whole system is likely to draw in total when at full blast

Motor Stall Current multiplied by one over the VFD efficiency or the maximum rated current draw for the VFD will give you a far better idea of the maximum current under heavy working conditions than spinning up the motor and sticking a meter on the mains input.
Maths and manufacturers data are your friends here, not a meter and a finger in the air ;-)

- Nick

[Edward]

If you took manufacturer's info from certain devices, my Chinese servo for instance, you wouldn't even want to plug it in your 13A fused socket:) Steppers are another example. While some people spend their precious time solving complicated equations to do with inductance, back EFM and speed graphs, all I need to know - not being of a brainy disposition - is, can I plug it in with a reasonable margin of safety?

BTW, do you happen to know the Motor stall current x 1/VFD efficiency of the typical Huanyang/2.2Kw combo? I just can't find it anywhere in the instructions:)

Edward

[magicniner]

BTW, do you happen to know the Motor stall current x 1/VFD efficiency of the typical Huanyang/2.2Kw combo? I just can't find it anywhere in the instructions:)

Motor stall current should be on the manufacturer's spec sheet for the motor, VFD efficiency should be on the manufacturer's spec sheet for the VFD but you do to some extent get what you pay for.

If these are not available then ball park figures can be used from a device of similar specification from a manufacturer that does publish technical data and you'll still be closer to the number you are trying to get than you would be with a clamp meter, spindle running with no/moderate load.

- Nick

[Edward]

Hmmm, unfortunately there are too many variants that affect the actual current draw of a particular device, for instance the forces applied at the cutting end. And often practical experience will tell you more than a myriad of calculations.

And when I do some heavy cutting, I will observe the current to see where I am in relation to the max current set for the device, it will be interesting to know, that's all.

I only measured the input current with no load just out of sheer curiosity, nothing else, I am not trying to defy science;)

Edward

[magicniner]

Hmmm, unfortunately there are too many variants that affect the actual current draw of a particular device, for instance the forces applied at the cutting end.

Incorrect
0/10

[m_c]

The problem with monitoring the line in current, is it's relation to motor current draw varies depending on the motor speed.

The motor will only see the full mains voltage when running at the rated speed. Any speed below the rated speed, the voltage applied is proportional to the speed i.e. 50% speed, 50% voltage.
What this means, if using a 2.2kw spindle rated at 220V with a 220V supply for example, is if you're running at maximum torque, at rated speed you'll be pulling 10A with the spindle ideally seeing 220V, and using 2.2KW. Now if you half the speed, the voltage drops to 110V, but the current remains constant at 10A, and you're only using 1.1KW. The supply voltage is still 220V, and due to the wonders of the switching in VFDs, the VFD will only draw 1.1KW/5A from the supply (that's assuming 100% efficiency - there will be some power loss).

Now from a spindle monitoring point, the supply current draw is useless, unless you're always going to combine it with the motor speed to get some kind of meaningful figure.


As for manufacturer's recommendations for supply fusing. They are based on worse case scenarios. Nearly all VFDs (and that includes servo drives, as they are essentially a VFD) have huge instantaneous switch on current surges. When first powered up, they appear as a short on the supply until their internal capacitor bank charges.
Then due to the switching frequency, you can get very high currents (why noise filters should be fitted), that can cause fuses to fail. Even though the average current will never exceed the rated power, there could be spikes many times the average current, which can be worsened if multiple drives share the same unfiltered supply, due to noise/frequency harmonics. In extreme cases, harmonics can cause wiring to burn out due to overheating, yet not trip any protection system.

[Edward]

Thank you m_c, for the detailed explanation.

I am aware of the current spikes on switch on, but if we leave that aside for the moment, I thought that the power line current was also proportional to the current drawn on the motor side, for instance, I thought that if the VFD panel showed an increase of 30% in current, then the inline current would also increase by the same rate. I have not had a chance to observe that. Obviously if that isn't the case, in terms of monitoring the current, the VFD measurement is the one to monitor. Thanks

Edward

[m_c]

It's a common thing that people don't understand the fundamentals of motor current and voltage.
Current = Torque. Voltage = Speed.
As motor speed increases, back EMF increases, so you need more voltage to push/drive the required current through the windings.
As load increases, due to the motor poles dragging further behind the magnetic fields, it takes more current to pull/push the rotor along.

If you were to run a motor at say 10% rated speed, but still apply rated voltage, you would burn out the windings, as due to the lack of back EMF to limit current, excess current would flow through the windings.
That's why VFDs reduce the voltage in proportion to commanded speed.

Then combine that with the glorified switch mode power supply that is a VFD, and the lines can get very blurred, but in nutshell, Power out = Power In.
So supply current = (motor voltage * motor current) / supply voltage.
That obviously doesn't allow for any inefficiency, but I'd guess that even cheap VFDs will still have efficiency ratings in the high 90s, as they don't generate any major amount of heat.


You could use the supply current to calculate spindle power, however without knowing the speed, it's relatively meaningless figure, as max spindle power is proportional to speed, so you end up with a percentage of a proportional power. And that's before you consider any inefficiency in the VFD.
That's why the VFD motor current figure is far more useful, as it corresponds directly to motor torque, and is measured after any VFD inefficiency, so you can quite easily convert to a percentage of available torque.

With the KFlop, you could pull that data from the VFD using Modbus, then show it in KMotionCNC.

[magicniner]

Nearly all VFDs (and that includes servo drives, as they are essentially a VFD) have huge instantaneous switch on current surges. When first powered up, they appear as a short on the supply until their internal capacitor bank charges.

Yet if you follow general industrial practice and leave them powered on, controlling the motor with signals and not VFD input power switching, the ramp-up options give you soft-start options which dramatically reduce surge allowing multiple machines to be run safely on circuits which would not normally allow this.

I'd appreciate links to technical write-ups of examples of harmonic wiring burnout due to absence of filters as the only machine I run a filter on is my CNC mill where the control system is sensitive and requires a filter and I have 6 other VFD systems in the workshop,
Regards,
Nick

[Edward]

With the KFlop, you could pull that data from the VFD using Modbus, then show it in KMotionCNC.

Thanks m_c. That would be quite handy, as the VFD will be some distance from the computer. However, learning all about Modbus is a little beyond me at this stage;)

I have my second Kflop now for the router I'm building, it took a little while to be familiarised with it sufficiently to get it fully going, with some simple C code for the probe and so on, but it's been very reliable.

Edward

[m_c]

Nick, this link is the first one I've found that explains a bit about the hazards - http://www.ecmweb.com/content/fundamentals-harmonics

I'm in no way an expert, but the problem with the higher order harmonics, is the higher the frequency passing through a conductor, the more the current travels towards the outside of the conductor. IIRC it's called the skin effect, and is why the insulation is far more likely to fail, despite wiring being adequately sized for the rated voltage/current.

[Doddy]

Hi guys,
Then I used my ammeter on the mains power cable and it actually measured 0.8A. I trust this reading to be more accurate than the VFD panel reading.
Edward

You mentioned later that you're using a clamp meter. Quick question - did you position this around a single core of the supply. or the 3-core mains cable? If the latter, is your clamp meter designed for 2/3 core cables?, or a basic clamp meter? If the latter, then your reading is inaccurate.

[Edward]

You mentioned later that you're using a clamp meter. Quick question - did you position this around a single core of the supply. or the 3-core mains cable? If the latter, is your clamp meter designed for 2/3 core cables?, or a basic clamp meter? If the latter, then your reading is inaccurate.


It's a clamp meter designed to be placed around a single core, for example either the live or the neutral, and used as such. I have various dial ammeters that I could also have used, but for a quick cursory look, the clamp meter was more convenient.

Edward

[Edward]

"Hmmm, unfortunately there are too many variants that affect the actual current draw of a particular device, for instance the forces applied at the cutting end."



Incorrect
0/10

Puerile as puerile goes. But whatever:)

[magicniner]

No mention of Drive, VFD or Inverter Drive?

[magicniner]

"Hmmm, unfortunately there are too many variants that affect the actual current draw of a particular device, for instance the forces applied at the cutting end."

That's quite obviously utter bunkum, otherwise nothing electrical other than simple resistive load systems in industry could ever be built to a specification without years of iterative testing, fettling and re-testing.

0/10 seems extremely apt as it happens ;-)

You're saying that something calculated every day by professional engineers cannot be calculated - you're full of it and don't like that you are demonstrably incorrect, I suggest you get used to it as you seem hell bent on continuing the pattern :D

[Edward]

Hmmm, you are waffling and sidetracking and you know it, you know very well what I was referring to. This silly point scoring and displays of bravado don't do you any favours. It makes you look bad.

I suggest you apply your energy and knowledge (which I'm sure you have in abundance) to better use, as practically all the other knowledgeable posters do, to whom I am grateful.

End of story, I let you have your last word, because of course, you must.

Edward