How to limit variable speed drive power draw 1

[Ares]

During power outage when a 40 HP fan (on a VSD) is on emergency supply I would like to limit its power consumption arbitrarily to 25% nominal to avoid the generator overload.

What is the best, foolproof way to do it?

 

[jbartos]

Suggestion: The fan speed-torque curve reflecting the motor shaft load can be used to determine about 40% motor shaft load less the motor internal loss (via motor efficiency). Then, the fan speed might be reduced to the corresponding speed or slip.

 

[jOmega]

Not knowing what the actual BHP of the fan is and at what speed, as a rule of thumb...for a square law load torque.... 50% speed is 25% load

(.50 x .50 = .25)

Since many of the VFD's today have the ability to operate at PRESET, pre-programmed speeds, selected by an contact closure, I'll bet you an come up with a scheme that automatically selects the fixed speed (<50%) whenever the generator is put on line... probably need a contact closure that operates whenever the generator is made to go live.... and feed this to one of the VFD's I/O points that causes the preset speed mode to function.

 

[Ares]

Thank you,
The pre-programmed speed - selected by an contact closure is all I need.

 

[GSI1]

Set the Current Limit on the VFD to the level you need and enable slip compansation on the VFD. This will limit the output current to the lower level. The slip compansation will limit the motor speed. If you need power loss ride through, increase the number of caps on the VFD buss. Don't forget to set the VFd to Freq Search on start up to prevent overload while the fan is spinning down.

 

[jOmega]

GSI1 ...
Not wanting to rain on your parade, but those suggestions are fraught with misgivings.

1st, current limit is an overriding function. It overrides slip compensation which is designed to increase speed as a function of load current increase.

Ergo, you cannot be in current limit and.... have slip compensation controlling speed. Current Limit on most PWM type VFDs serves to reduce the output voltage and frequency inorder to limit the current. Speed is sacraficed in the process.

2nd..Increasing the number of capacitors in the DC Bus is never a good idea because it can cause:
- Premature failure of the VFD .. particularly the precharge circuit.
- percussive failue in the power circuits in the event of a fault on the dc bus our output circuit. More caps = more stored energy which the VFD is most likely not designed to handle in a fault situation.
- violation of agency approvals, such as U.L. CUL, CSA, etc...
- Longer discharge time of the dc bus.... violates NEC, UL... etc...

... and we didn't even mention the issue of input protection coordination.....

Just not a good idea....

 

[GSI1]

Just to let you know I have used this method in the past and it works with no ill effects.
The slip compansation allows the output freq of the drive to fine the best running point if you don't uenable the drive will attempt to hole the set freq. and some times will become unstable. as for ther extra caps at least severial drive companies use this method all the time for power loss ride through. the same drive can also be connected to run directly on a DC buss. that will work as well.

 

[jOmega]

I hear what you're saying, GSI1.

But I also know a guy that uses the back wall of his garage to stop his car when he pulls in at nite.

Works for a while .... but .....?? ? ?



As you said....
&quot;..... if you don't uenable the drive will attempt to hole the set freq. and some times will become unstable. &quot;

Do you really think that is a good solution ?


...and the extra caps... I guess as long as you've never had a drive blow up or burn up or have a consequential catastrophic failure.... you'll just go on believing this too is a good solution ... But when the carbon is a flying .... what will you say then.... ?????

Kinda like the fellow that used a lit match to check for gas leaks... got away with it for a while... didn't see anything wrong with doing it...
He's no longer with us....and his family misses him.

 

[mdonner]

I implemented the idea described in iOmega's reply in several projects some years ago (VFD digital input to triger preset speed, and turn on the catching circuit.
Rule of thumb for fans:
Power ratio is the cube of speed ratio, flow ratio is the speed ratio, pressure ratio is the square of speed ratio.

 

[HarryDampers]

Firstly work out what speed will give you 25%load. As outlined above , kW falls as per the cube of the speed . If your motor draws full load at full speed ....then 25% load should be somewhere around 63%speed

Then (when the Generator kicks in) you need to switch the speed reference from the variable reference that normally comes from your PLC or potentiometer ....to a fixed reference that will drive the fan at 63% or whatever speed gives 25% load.

I am sure that there are several ways you can think of to switch the reference over when your power fails . Probably the simplest being to use a relay that is energised as long as their is three phase voltage available

 

[jbartos]

Suggestion: There is a plenty of room in the VFD engineering and design. If the manufacturer has an industry standard label affixed to the product, then the product is officially meeting industry standards. There are differences in VFD engineering and designs varying from the manufacturer to manufacturer, at least to some extent. The VFD schematics are often considered proprietary. Essentially, what I am writing about is that there may be someone who worked for a Detroit City car manufacturer and consequently, that car manufacturer may be preferred to have its products sold by that someone.

 

[jOmega]

JBartos

Please provide references to the &quot;Industry Standards&quot; that VFD designs must meet, as stated in your last post.

Also, would you please provide reference to &quot;Industry Standard Label&quot;, to which you refer in your last post, that would assure the VFD is meeting the &quot;Industry Standards&quot; for VFDs

Could you also cite specific VFD manufacturers that :

1) comply to &quot;Industry Standards&quot;,

2) what particular standards they comply to, and

3) affix a label to their product stating that they meet &quot;Industry Standards&quot; and which standards they meet.

Oh, and could you provide some additional words that explain what you meant by: &quot;There is a plenty of room in the VFD engineering and design.&quot;

I'm having a hard time understanding the meaning of your post.

Thanks.

 

[jbartos]

Suggestion to the previous posting: Visit

http://www.benshaw.com/literature/specifications/18p_vfdspec.pdf

for:
1.02 Applicable Standards:
A. The VFD(s) shall be designed, manufactured, tested and labeled where indicated to conform to the following industry standards and specifications:
1. ANSI
2. CSA
3. EEMAC
4. IEEE 519, latest edition
5. NEC
6. NEMA ICS2
7. OSHA
8. UL 508C, listed and labeled

etc. for more info

 

[jbartos]

Cont'd:
Reference:
1. Ned Mohan, Tore M. Undeland, William P. Robbins &quot;Power Electronics, Converters, Applications, and Design,&quot; 3rd Edition, John Wiley & Sons, Inc., 2003,
Section 14-6 Variable-Frequency Converter Classifications
Figure 14-18 Classification of Variable Frequency Converters:
A) PWM-VSI with a diode rectifier
B) Square-Wave VSI with a Controlled Rectifier
C) CSI with a Controlled Rectifier

 

[jbartos]

Suggestion: There appear to be engineering challenges in VFD areas as far as multiphase motors are concerned, e.g. 15-phase Chorus Borealis. Visit

http://www.chorusmotors.com/technology/motortutorial1.pdf

http://www.chorusmotors.com/technology/simulator.shtml

etc. for more info