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tony ennis
01-25-2008, 02:10 PM
Ok, I have a VFD rated at 1HP.

I was going to put a 3/4HP motor on it to keep from working the VFD so hard. I hate to run anything at its max rating.

A guy I know will give me a 1HP motor as long as I can use it. I don't want to take it unless I am sure - it would be unneighborly.

Will I stress the VFD by running a 1HP motor on it? Or can I run it at lower RPMs and be gentler? Or does it not matter, and I'm worrying too much (as is my nature...)

Alistair Hosie
01-25-2008, 02:22 PM
if it's rated at 1 horse and you use 1 horse I can't see any probs you might run it slightly under the rating and it may need not be run 24 hours a day without a cooling of point but I can't see the problem.I should think max rating is pretty secure.Alistair Ps ask John Stevenson he's good on these.

QSIMDO
01-25-2008, 02:27 PM
Hitachi told me the new models don't need to be down rated.
Don't know what you have but maybe the same deal?

macona
01-25-2008, 02:40 PM
There is no stress put on a device by running it on what it was intended to run on.

SGW
01-25-2008, 03:01 PM
It's designed to run a 1 HP motor. Don't worry about it. In fact, in all likelihood you'll never put a full 1 HP load on the motor, anyway.

hoof
01-25-2008, 04:43 PM
Is you power going into the VFD single phase for a three phase device? If thats the case you may want to reconsider the the 1 HP idea. The VFD's I have used in like manner need to be down rated near 1/1.73 full current. Reason being the DC supply to "re=manufacture" the three phase AC will not be able stay as high as it should. The othe thing to consider is that it is unlikely that you will need to draw that kind of current. The current is a function of the load placed on the motor. And at the end of the day, VFD's have a great deal of survival instinct built into them. The do a great job of protecting themselves ant the motor the supply as long as the instructions are followed.

macona
01-25-2008, 05:35 PM
Very few VFDs under 5HP need to be derated. Especially if they specify single phase input. My VFD on my mill (Altivar 18) is rated to put out I think 120% of it rating for 20 seconds. All on single phase.

On drives intended for three phase I have usually seen a 1/3 derating. 10hp on a 15hp drive.

Teco even makes some drives that will run 240v 3 phase motors on 120v input. I believe they max out at about 1 HP.

tony ennis
01-25-2008, 08:29 PM
I got this VFD (http://web2.automationdirect.com/adc/Shopping/Catalog/AC_Drives/DURApulse_-_GS3_(230_-z-_460_VAC_Sensorless_Vector_Control)/DURApulse_-_GS3_Drive_Units_(230_-z-_460_VAC_SLV)/GS3-21P0). On that page is a link to more information.

I'll be feeding it 220v 1ph power.

I can't imagine getting anywhere near this motor's capacity running a Craftsman 12" lathe.

SGW
01-25-2008, 09:06 PM
I'm pretty sure the derating business applies only if the VFD is intended for 3-phase input. If it's designed for single-phase input, no derating should apply, as macona says.

Forrest Addy
01-25-2008, 09:13 PM
Just because yoiu have a 1 HP motor doesn't mean you'll run it at 1 HP full time. 90% of the time the machine will loaf along at much less than full load. Beside VFD's have several overload protection features. You can't overload the VFD or hurt the motor even if you tried provided the various set-up parameter have been properly set.

lazlo
01-25-2008, 10:34 PM
I'm pretty sure the derating business applies only if the VFD is intended for 3-phase input. If it's designed for single-phase input, no derating should apply, as macona says.

That's right -- the only time you have to de-rate is when you're using a 3-phase input VFD on single-phase input.

I'm not quite sure why that is, since you're rectifying the 220V, whether its 1 or 3 phase... I'll have to look at the 3-phase rectifier in the morning...

rdfeil
01-26-2008, 02:55 AM
Tony,

The GS3 drive is rated for 3 phase input so you will need to derate it if you use single phase input. You can use it on a 1 Hp motor but with single phase input you run the risk of damaging the input diodes in the drive if you actually load the motor to a full 1Hp. If you haven't bought it yet the GS1 or GS2 drives are a better choice in general as they are rated for single phase input. Also, don't worry about running a drive AT its ratings, they are made to do it 24/7. I have many running that way and the only failures I have had can be traced to incoming power surges but, do watch the ratings and abide by them.

Good Luck,
Robin

Lazlo,

Take a look at the earlier thread (a week or so ago) where we were giving advice on this same thing. My last post there explains the derate because of the single / three phase thing. If you have any questions feel free to PM me or start a new thread and I will do my best to answer.:D Same offer to all:)

R.

Forrest Addy
01-26-2008, 03:38 AM
Here's some other points that takes into consideration the electrical behavior of induction motors and the strategy of running three phase rated VFD's run from single phase.

A FVD produces sine coded PWM variable frequency output whose effective volts are in proportion to the set frequency. Lower the frequency and the volts drop in proportion. If the motor amps stay constant, the total VA the VFD delivers to the motor drops. The PWM of the VFD output has a transformer-like effect on the demand as the motor frequency is reduced. Thus reducing the motor's frequency when its connected to a full and constant mechanical torque load reduces the VFD's demand from the line.

A FVD's nameplate ratings are seldom identical to the motor connected to it. Quite often there is a "cushion" or reserve rating allowing a connection to motors having relatatively large service factors.

The VFD's DC supply filtering has an effect on the performance of a three phase rated VFD when its connected to a single phase supply. Sometimes it's not only the diode ratings that limit the motor HP of a three phase rated VFD. The allowable ripple current from the filter capacitors also has an effect. If the DC buss dips below the triggering setpoint you may have nuisance low volts faults. Another problem is the filter caps may overheat and fail if the VFD is run at full Hz and full load motor Amps for an extended period.

When rating a motor and a VFD for a particular mechanical load you have to go through a little analysis where the motor ratings, the VFD ratings, and the three phase Vs single phase ratings are cranked together. Quite often a rule of thumb like "you have to de-rate a three phase rated VFD by 2/3 or 1/2 for single phase etc" may or may not be valid depending on circumstances.

Much depends on the motor's duty cycle. Without some analysis, a VFD purchaser may be stabbing in the dark. Its all too easy to over spend or underbuy without a process of self informing and analysis.

Rules of thumb are handy but a careful assessment of the equipment you propose to connect and how you would be most likely to load it AND if you can sacrifice a tiny bit in performance to take advantage of a deal (or freebie stuff) will lead you to the most bang for the buck.

Generally speaking in a home shop machine tool where its run convervatively most the the time the motor will see only light loads and the VFD will not be stressed. Apply a full motor load then the VFD still will not be stressed provided the motor is run at reduced frequency. It's only when the motor is run close to 60 Hz that the derating has to be carefully considered. But only when the VFD is three phase rated. If the VFD is single phase rated you can run it 24/7 to full ratings without a drop of sweat.

If it was easy anyone could do it.

tony ennis
01-26-2008, 11:02 AM
I'm sorry but this is all new to me. Good information here but it's dense.

I am looking at the manual now. The VFD will accept either single- or three-phase 200-240VAC +- 10%, 50/60Hz +- 5%.

I'll be running mine on single phase.

Output:
The max motor output is 1HP / .75kW
Maximum output voltage is three phase 200-240 (proportional to input voltage.)
Rated output current is 4.2A.
Rated output frequency is 1 to 400Hz.


a VFD purchaser may be stabbing in the dark.

Guilty as charged, but I didn't know there were considerations, I just thought you plugged it in! :p


AND if you can sacrifice a tiny bit in performance

I can. I was preferring a 3/4HP motor. But the price is right on this one.



Apply a full motor load then the VFD still will not be stressed provided the motor is run at reduced frequency. It's only when the motor is run close to 60 Hz that the derating has to be carefully considered.

I take it my rated output frequency of 400Hz is something entirely different?


...But only when the VFD is three phase rated. If the VFD is single phase rated you can run it 24/7 to full ratings without a drop of sweat.

I don't know if mine is single or three phase rated. Perhaps single because that's what I'm feeding it? Or three, because the VFD *could* handle it, electrically, if I so desired? Of course the former is more likely.

Well either way, I'm off to get the motor now, my wife and I are going to make a day of it.

Paul Alciatore
01-26-2008, 05:31 PM
As Forrest was saying, your motor will likely not be running at a full 1 HP very often. It all hinges on the current being drawn as it is that current that the VFD must supply. Motors will only draw the current needed to meet the load imposed on them. A motor with little or no load on it will draw minimum current and will be loafing along at maximum speed. As the load increases, as with a heavy cut, the current it is drawing will increase and it's speed will decrease.

When the load becomes so large as to stall the motor, then the current will be at a maximum and you will probably start to smell smoke. Then the VFD would probably be overstressed, but it should have protective devices (fuses, breakers, or electronic shut down circuitry) to prevent any damage. Besides, you wouldn't likely just leave it on, smoking and humming.

So, with either a 3/4 or a 1 HP motor, it is the load that determines the current being drawn and all else being equal, they will draw about the same amount of current with the same load. You would actually be better off with the 1 HP motor as it is less likely to burn up with heavy cuts as compared to the 3/4 HP one. The stress on the VFD would be the same with the same load and all else being equal. Of course, the operative phrase here is "all else being equal". That includes many factors.

Personally, I would go for the 1 HP. If you are really concerned about the stress, you could install a current meter on the motor leads and monitor it as you cut.

SGW
01-26-2008, 05:38 PM
Um, I wouldn't try cranking it up to 400 Hz. You'd be running the motor at nearly 7X its design RPM.

tony ennis
01-26-2008, 06:17 PM
We're back. The guy gave me two motors. One is the 1HP Baldor. The other is a 1/2HP Rockwell. Both are three-phase. The Baldor is enormous. It must weigh 50 lbs. I believe it may be heavier than the 1 1/2 HP 1-phase Baldor that was on the lathe. The Rockwell is what I expect.

The guy was quite a gentleman and invited my wife and me to stay for green beans and cornbread.