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VFD for 2 motors 1

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tinotoro

tinotoro

Electrical
Nov 2, 2005
18
I got a comment from an inspector in Canada that a VFD should be specifically be rated to power 2 motor simultaneously if intended for that application (he is referencing the CSA 28-500 (3-d), which refers to motor controllers in general, not specifically VFD's). I can't find any reference for such a requirement and vendor documentation doesn't mention it.
Is there any specific requirement for VFD used to power two motors of same power rates simultaneously?
 
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That is a very common application. There are traction motors where two or four motors are fed from one VFD (Alstom is but one example) and there are conveyors with ten-twenty or more motors on the same VFD. The only thing that you need to be careful with is if you have high demands on dynamics, like in servo-like applications.
Re thermal protection, I would recommend NTC or similar sensors, even "Klaxon" will work, in all motors and that the VFD is inhibited or stopped if the mechanical arrangement needs it. Or just disconnect the failing motor, if that is OK in your application. But you should make sure that you don't switch of all motors. That is not considered best practice.But have seen that work without problems, too.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
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Sure! Klaxon is for cars in France. I would drop the s, though. Klixon.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
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I thought a klaxon is one of those old horns that goes 'ah-ooo-gah' and means, among other things, "Dive! Dive! Dive" in old submarine movies; do they still use those on cars in France?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
 
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Didn’t Cap’n Kirk fight the Klaxons?

I’m not sure of the question here. Are you asking if the VFD must be sized for the combined load? Why would that not be intuitive? Would you propose sizing the drive for one motor and trying to run two?

Or are you being specific to questioning the issue of being identically sized? Because that would not actually be necessary. So long as you need to operate them at the same frequency, the motors can be any size, any mixture. You must protect each one individually because the drive cannot differentiate, but the drive itself could not care less how the load is split up down stream.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
 
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Yes, the Kaxons... from the planet Kronos


tinotoro; That may be how you get the inspector on board. You point out that each motor has independent overloads. That said it is far better to have the drive discover a motor overloading before it gets disconnected by it's motor-starter contactor because interrupting the output of a VFD is bad ju ju. Hence, the individual temp sensors seen by the VFD or a relay or ??? that can tell the VFD there's an external error and to shut down.


Keith Cress
kcress -
 
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About Klaxons - they are gone from French cars. But the word lives on. There is also a verb, "Klaxoner" to go with it.
"Klaxonnez ce viellard-la!"

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
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Thanks for the responses. Actually it is a very simple system, it is a vibration screen with two 5HP motors. There are no operation complications, they start and stop simultaneously. Still, there is a provision in the Canadian code about 'motor control' for two motors, that it should be rated for this application. I don't think that it is applicable to VFD's. So, I wonder if there is such a thing as 'VFD rating for 2 parallel motors'.
I appreciate your inputs.
 
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Looks OK to me. What's the fuzz about? Tell the inspector to read the book!

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
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Unexpectedly, I was involved in a problem with, guess what? - parallel asynchronous motors connected to one VFD. Simplest possible Connection with a short jumper Cable from forst motor's p*ck*rhead to next. Around 100 cm Cable length. AND, there were bearing problems. Heavy EDM.

Suspect interaction between leakage inductance in the two motors together with double capacitance to ground. Will do measurements on June 4th.

Also see that at least one motor manufacturer recently amended their motor manual with a warning about this. So, there are problems that I wasn't aware of. Nor do most installers seem to know about this. But that is not what the OP's inspector was thinking of.

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
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That would be rule# 28.500,(3),(d)
You may suggest to the inspector that each motor is independently controlled by the motor protectors suggested by jraef and that the VFD is a separately derived source supplying a special power to the motor controllers.
Showing a printout of the drawing linked by jraef may help.
Alternately you may be able to obtain a declaration from the VFD supplier stating that the drive is suitable for supplying multiple, independently protected motors.
By the way, that rule has been added since I was designing systems with multiple motors on one starter.
We used a chart showing the ratio of starting current to running current for various sized motors. The ratio decreased as the motor size increased. Hence, ten one HP motors would have a higher starting current than one ten HP motor.
That may be part of the reason for the code change.
Another reason may be that control equipment manufacturers are heavily represented on the code committees.

Here is a Link to the document containing the page extracted by jraef.
Note: This does not address the issues brought up by Skogs.
Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Drives are so cheap that they're often not much more than a contactor and the hassle to install it. I'm not sure why one would bother trying to parallel two motors, especially with Skoggs pending doom.

Keith Cress
kcress -
 
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Vibrating screens with two motors probably are spinning off center weights in opposite directions. They tend to self synchronize and cancel on one axis so that the vibration is in one direction only rather than circular.
If the frequency is not exactly the same, one motor may tend to drive the other motor.
Feeding both motors from the same source is preferable.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Further to my previous post in regards to starting KVA per HP of various sized motors:
The Cowern papers show 5 HP motors as design H with 6.3 to 7.1 starting KVA per HP.
The Cowern papers show 10 HP motors as design H with 5.6 to 6.3 starting KVA per HP.
So the starting KVA of two 5 HP motors is a maximum of 71 starting KVA.
The starting KVA of one 10 HP motor is a maximum of 63 starting KVA.
A 15 HP motor is a design F with 5.0 to 5.6 starting KVA per HP for a maximum of 8.4 starting KVA.
Back in the day, we would use 15 HP rated equipment to start and control two 5 HP motors. (With individual O/L protection of course.)
The information includes the following note:
The proposed Design E motors, which will have very
high efficiencies, will have higher inrush currents than
the motors currently available. These motors will
require special considerations when sizing
circuit breakers and starters for these motors when they
become available. The 1998 National Electrical
Code incorporated some special provisions for these
proposed Design E motors.

If you do some research on the starting KVA per HP of modern high efficiency motors and present it to the AHJ he may be willing to accept an oversized VFD as suitable.
There is provision in the CEC for the onsite inspection and acceptance of non approved electrical equipment.
The AHJ may be willing to apply that procedure to your VFD if you can show good supporting documentation.
With an old seasoned inspector your chances are good.
With a young fairly new inspector your chances are not so good.
Cowern papers: see page 9 (.pdf page 16, see also .pdf page 33).


Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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