VFD running at 0 Hz 2

[cmahaffey]

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

 

[itsmoked]

I'm guessing, but the result may be essentially DC fed to the motor to lock it up. No revolving field at all.

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

 

[roydm]

Shouldn't the V/Hz parameter drive the output voltage to zero also?
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

 

[Skogsgurra]

As you know, the motor voltages are composed from PWM voltages that varies to form three sine voltages that are 120 degrees apart. Depending on motor frequency, these three voltages form a rotating system with a frequency that is determined by the speed setpoint.

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

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[cmahaffey]

Thanks to all for the response...I did say that I used a clamp on, but in fact I was reading from the display.

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??

 

[jraef]

.I am assuming this is a built in saftey feature for a lot of VFD mfgrs.

This isn't a "safety feature" of the VFD, it is a choice someone made (or failed to make) in setup of the VFD. You have to specifically instruct the VFD as to how you want it to respond to a loss of speed command. The typical default is to have it go to the Minimum Speed setting if the Run command is still active. So if nobody altered the default setting (and this WAS the default), but they set the Minimum Speed as Zero, then the VFD is doing what it was told to do. But you have a choice to tell it to coast, go to a different speed, remain at the last commanded speed, and often a few other options I can't think of off the top of my head.

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

 

[Skogsgurra]

Yes cmahaffey, you are reading my answer partly correct.

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

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScumPunk]

Gunnar/jraef,
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

 

[jraef]

Actually, there is no specific standard to which that applies. It can range from anything between 20 and 60% depending on the motor design. I have always used 20% as a benchmark only because I have never seen it lower than that, but I have seen 4 pole Design B motors pulling 35% of FLA with no load before. I just did a start-up on a 12 pole 350HP Design A motor and the no-load current was almost 60% of FLA. Generally, nobody cares much about the NLA of a motor because what is the point of running it with no load? So consequently it is difficult to find data published on it. The only time I have seen No Load Amps on a nameplate was on some big synchronous motors being used as condensers (to improve power factor), so there never was a load connected to them and therefore the NLA was worthy of consideration.


"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

 

[ScumPunk]

jraef,
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