VFD running at 0 Hz
VFD running at 0 Hz
(OP)
I have question for someone more familiar with AC Motors/VFDs than I. We are using a VFD to drive an AC motor for a Rotary UPS...I noticed if the drive loses the 0mA reference signal (0-20mA control loop) while the drive is running, it automatically "brakes" the motor...I am assuming this is a built in saftey feature for a lot of VFD mfgrs. Without knowing much about what exactly was going on, I got to toying with the drive and some meters around and noticed when this happens:
The drive is still running with the max current setpoint, in this case 100Aac @ 0 Hz...I am assuming this is the full torque at no speed benefit of VFDs which may be useful for drilling drawworks, hoists, cranes, etc (I have heard of this concept, just never seen in person)..My question is, how exactly is the drive achieving 0Hz with full current/torque?? What is physically happening?? What does the voltage waveform look like? Is this somehows shorting the windings/phases?? I am assuming since no "real" work is being done this 100Aac I am reading with a clamp on is purely reactive power??
Any comments/links that can help me clear this up in my head is greatly appreciated. I apologize if some of the questions are not properly worded or don't make sense.
Regards,
Colin
The drive is still running with the max current setpoint, in this case 100Aac @ 0 Hz...I am assuming this is the full torque at no speed benefit of VFDs which may be useful for drilling drawworks, hoists, cranes, etc (I have heard of this concept, just never seen in person)..My question is, how exactly is the drive achieving 0Hz with full current/torque?? What is physically happening?? What does the voltage waveform look like? Is this somehows shorting the windings/phases?? I am assuming since no "real" work is being done this 100Aac I am reading with a clamp on is purely reactive power??
Any comments/links that can help me clear this up in my head is greatly appreciated. I apologize if some of the questions are not properly worded or don't make sense.
Regards,
Colin





RE: VFD running at 0 Hz
Mainly, I wanted to warn you that clamp-ons are not going to be getting a VFD's outputs correctly. If you want correct you need to read it from the drive's display somehow. The waveforms are so fast that clamp-ons don't follow them correctly.
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD running at 0 Hz
It cant be good to pass 100 Amps through the windings with no cooling
I suspect the fact you see current at zero speed is a setting somewhere.
Roy
RE: VFD running at 0 Hz
At setpoint zero, there will still be three voltages with 120 degrees phase difference, but zero Hz frequency. It may be hard to visualize, but if you draw three sines 120 degrees apart and then look at them at any phase angle, you will see three voltages with certain values. Look at another time (angle) and you will see three other voltages - the motor sees the same voltages.
Since the frequency is zero, each of these voltages will, of course, be DC. But not pure DC, because the PWM is still active.
If you feel curious, you can hook up three lamps and have a look at these three voltages. Increase frequency slightly and you will see how the brightness varies periodically.
The motor will carry full magnetizing current (usually 60 - 70 percent of FLA) and consequently get very hot if not cooled by an external fan.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD running at 0 Hz
Skogsgurra - So if I am reading your response correctly the output @ 0 Hz is in fact a PWM DC signal across each phase?? The output is in fact regulated (via the control paramters) to be no more than 100A (DC I suppose in this case)...So the output is still modulated pulses as in the normal configuration, but instead all positive pulses??
RE: VFD running at 0 Hz
When you say "The drive is still running with the max current setpoint, in this case 100Aac @ 0 Hz...", that does not sound right. Usually a VFD has a Speed or Torque setpoint, not a current setpoint. You can have a current LIMIT, but that does NOT make the VFD try to put out a specific amount of current even if it is stopped. It tries to maintain a speed or torque output and if it takes that much current, then that is what it takes. But you can't make a motor pull more current than it wants to in order to accomplish its task at hand. With no load on a locked rotor, as Skogsgura said, it should only be drawing magnetizing current, which is typically around 20% of FLC. That would make your FLC somewhere around 500A. So are you saying that when the VFD is at Zero Speed from the loss of speed command, the motor current display reads 100A? That is a LOT of magnetizing current unless it is a big motor, i.e. 400HP or more.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: VFD running at 0 Hz
The current is DC. But it doesn't have to be positive and it definitely isn't 100 A in all phases. It all depends on what phase angle you happen to be in. Let's say you are at 0 degrees. Then your phase A current is zero, your phase B current is 100*sqrt(2)*sine(-120) and your phase C is 100*sqrt(2)*sine(120). Or, in other words: 0 A, -122 A and 122 A. The sum of the three currents is always zero, but the instantaneous sum of the voltages never can be in a PWM system.
What actual current you have is dependent on your control mode. In scalar mode and no special "curve" activated, you usually have full magnetizing current. And that is rated current times sqrt(1-PF^2), which - for a .8 PF motor will be 60 % of rated amps.
Vector mode and scalar with parabolic (fan) curve usually makes the standstill current a lot less.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD running at 0 Hz
I'm confused - Gunnar states magnetising current is typically 60% of FLA, while jraef says it is c.20% of FLA.
You both usually give very good, correct advice, but is it possible that you are both right?
Thanks,
Mort
RE: VFD running at 0 Hz
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: VFD running at 0 Hz
Thanks for clearing that up. On our motors (120W - 11kW) the average is probably around 50% of FLA. The smaller the motor, the higher the percentage.
Cheers,
Mort