AC drives hp rating?

OK, I'm getting ready to pull the trigger to buy one of the sort-of-low-cost
import drives, aka VFDs. It seems to me I've heard to buy a bit bigger than you
need. I have a machine with a 1 hp 3 phase motor, for which I could buy a 1 hp
drive for $119 or a 2 hp drive for $145. Is there any real reason to spend the
extra $26?
Thanks,
GWE
Reply to
Grant Erwin
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Yes! I guess you will use the machine with over-current from time to time (spinning up, heavy cuts). So you will want to have 20..30% more.
I have a VFD that has the same rating as the motor and am not that happy with it, except that it was cheap (1/3 price) and new.
Nick
Reply to
Nick Mueller
There is not reason to derate a VFD below the manufacturers ratings unless perhaps you'll be running it in Death Valley in July. The advice about derating VFDs is obsolete for the most part, it relates to using a 3 phase input, 3 phase output VFD with a single phase input.
Most of the small VFDs these days are rated for single phase input so there is no reason to do any derating. If the manufacturer who is providing the warranty says the drive is good for a 1 HP motor with 230V single phase input to the drive, it is.
Pete C.
Reply to
Pete C.
The VFD rating is for a 3-phase input. If you run single phase input, the VFD spec is derated.
My VFD manual says to derate the spec 50% when using single phase input.
I run my Hardinge lathe (with a 1HP Baldor 3-phase motor) from a 220v single phase dryer outlet in my garage. I purchased a 3HP rated VFD for the task.
Ed in Seattle
Grant Erw> OK, I'm getting ready to pull the trigger to buy one of the sort-of-low-cost
Reply to
cascadiadesign
The VFD rating is for three phase input, only if that is what the VFD is rated for. Many small VFDs are rated for single phase input and I think one or two aren't even capable of accepting three phase input.
Your manual for a VFD rated for three phase input. Not all are these days.
Take a look at
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and you will see quite a few small VFDs that are rated at single phase input.
Pete C.
Reply to
Pete C.
The key words in your posts are "almost" and "not all". The point is, check the mfg specs. Many have their manuals on-line.
In my case I decided to go with a rating that was 1.5 times what was required by the derated spec because the additional cost was marginal.
components designed for higher current than required will run cooler and last longer. My lathe is part of my livelyhood, so I view the small additional cost as an insurance policy.
Ed in Seattle
Pete C. wrote:
Reply to
cascadiadesign
Look at the full load current rating of the VFD. If it is equal to or more than your motor's nameplate current rating, you are safe. Most VFD's will carry a 10% overload without damage in 40° C ambient.
Randy
Reply to
Randal O'Brian
sort-of-low-cost
While quality units tend to be conservatively rated, low cost units tend to be rated right at the limits (think Sears air compressors...).
There is an old adage that holds, "It is better to pay too much for something than too little. You will usually find you paid only a little too much and are, therefore, out only that little. But, if you pay too little, you may well find yourself with something that won't do the job and are, therefore, out the entire price."
I'd say, spend the extra 26 bucks...
Jerry
Reply to
Jerry Foster
If the drive is rated for a given HP for 3 phase input, it needs to be derated by about 1/3. If it is rated for single phase input, it does not need derating. The better variable than horsepower is the motor FLA rating. A lot of companies fudge horsepower ratings. Compare this information from your motor nameplate with the VFD's data.
i
Reply to
Ignoramus23222
...Look at the full load current rating of the VFD. If it is equal to or more than your motor's nameplate current rating, you are safe.
Maybe, maybe not. A VFD converts the input AC (either 2 phase or 3 phase) to DC, then chops the DC to synthesize a 3 phase output. The output side of the VFD must meet or exceed the motor's ratings, regardless of the input ( 2 or 3 phase).
However, the derating is due to the design of the input side of the VFD, not the output circuitry. If you feed the VFD only 2 phase AC rather than 3 phase, more current flows through the input rectifiers. If the VFD is designed to handle the higher input current, all is fine. If it is not, then the VFD spec will say to derate the drive when using 2 phase input.
Again, check the manual.
Ed in Seattle
Reply to
cascadiadesign
opps .. mean to say "single phase", not "2 phase".
snipped-for-privacy@comcast.net wrote:
Reply to
cascadiadesign
You didn't say what kind of machine you are powering. The nature of how the load comes up to speed may affect what you need in a VFD.
Wes S
Reply to
clutch
Lots of variables as has been stated so far.
My experience is with two different applications with the same brand VFD used on both. The first application was on a rewinder for news paper. Lots of low speed running. We upgraded one size larger than we though we needed due to the low cost. Once we got it running I was really glad we did. It runs right at the VFD cutout limit most of the time.
The second was my friend who is powering a 14" Logan with a variable belt drive. Again we went one larger and it saved the day. The variable belt drive eats so much power that the larger VFD is needed to over drive the motor for short periods of time.
So I'd say go for the bigger one. You can always program it to cut out at the lower name plate rating of the motor. But keep in mind that motors often have to go over name plate rating for short periods of time depending on the nature of the work.
Reply to
Wayne Cook
On Thu, 18 Jan 2007 08:01:28 -0800, with neither quill nor qualm, Grant Erwin quickly quoth:
Yes. For the safety factor, and for a later date, when you replace the dead 1hp motor with a 1.5hp motor. ;)
I use the same logic for determining what model of appliance, TV, stereo component, etc. The models a step or three up from the bottom are simply better made, produce a cleaner output, have better specs, and definitely last longer.
- - - - - - - - - - - - - - - - - - Heart Attacks: God's revenge for eating his little animal friends
Reply to
Larry Jaques
... mmm ... most of the VFDs I've installed have had quite good short term overload capabilities, some as much a 150% for a minute
AWEM
Reply to
Andrew Mawson
The one I have has 20%. It's on a lathe. Especially with high revs (above 2000 RPM) and with the 3way chuck installed, those 20% extra are not enough to spin it up in 1 second or two. My next lathe will have a VFD that is clearly oversized. I learned my lesson.
Nick
Reply to
Nick Mueller
Yes. ALWAYS over rate the drives, particularly if you are running them off of single phase. In fact..in that case a 2hp would not be remiss.
Gunner
"Deep in her heart, every moslem woman yearns to show us her tits" John Griffin
Reply to
Gunner

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