Drive size vs Motor size

[travish]

We purchased a used roll forming machine unseen, (I had nothing to do with the purchase) sent it to an outside source to be tooled up, and also had an outside source to build the controls for the machine. The company tooling up the machine told us that there was a 40hp drive motor on the machine. We told the company building the controls it had a 40 hp drive mtr. this is all 3 months ago. The tooling company finished and shipped the machine here this week I go to look at the machine and right away see that the motor is larger than 40hp.Looks like a 60, the name plate is unlegible, I can read the current rating, it shows 71amps for the high voltage current which is close to a 60hp at 480v. the machine does have an old (very old) 40hp vfd in it. I was able to get a good part number off of it, and determine that the drive is a 40hp. The motor remains a mystery. So the machine did run from a 40 hp drive. If a 60hp was only doing 40 hp of work I guess the 40hp drive would run it???

We told the control company it was 40hp, they are ready to ship the controls and have a 40 hp "VFD" installed in the cabinet. So the question is, would you leave in the 40 or try to get a 60 installed before shipping?

 

[Jraef]

You theoretically can run a large motor from a drive rated for a smaller one. The risk is, a motor is a "dumb animal" in that it knows not what it is supposed to be limited to, it will "try" to respond to any change in load by increasing torque and thus pulling more current. If that current exceeds the rating of it's own winding conductors, it catches on fire. But when taken the other way, if the load NEEDS 60HP and the drive cannot deliver it, you have two choices:

1. You can program the drive (in most cases) to limit the output torque to correspond to the DRIVE'S rated maximum current, typically done by setting up a "current limit" or "torque limit" value. How the drive typically responds to that is to limit the speed command, overriding whatever your control system is trying to accomplish, because lower speed = lower HP. You have to decide if that is appropriate for your operation.
2. Option two is to have the drive trip off line to protect itself. Again, you have to decide if that is appropriate. For your operation.

But keep in mind that in some cases, a machine designer also just might be taking advantage of that lower HP issue, i.e. the machine is designed to NEVER run above 40Hz, so the motor will never need to be more than a 40HP motor. He chose a 60HP motor in order to get the 40HP at the 40Hz maximum operating speed.

 

[travish]

your last point was a very good one, that may indeed be the original intent. this machine runs very slow.

so somewhat off subject just as an example: 1 hp drive and 1hp motor am i correct in saying that a typical VFD run above 60hz is constant HP and torque begins to fall off when you get above 60hz

thanks for your input Jraef

 

[Jraef]

... so somewhat off subject just as an example: 1 hp drive and 1hp motor am i correct in saying that a typical VFD run above 60hz is constant HP and torque begins to fall off when you get above 60hz

Absolutely correct. The V and Hz need to change at the same rate to maintain torque. So once you run out of V and continue to increase Hz, the HP can no longer increase. But because HP is a shorthand notation of "torque at a certain speed", as you keep increasing speed and the HP remains constant, the torque is, by definition, decreasing.

 

[petersonra]

We have run into similar situations. It will probably work just fine.

However, it probably does not meet code.

 

[travish]

why would it not meet code, everything down stream is protected for over current by something up stream. the max current is progamable in the drive, so it will trip before the motor or conductors would burn up. it looks like the drive is protecting the motor and itself by stoping if it sees an overcurrent condition and there are properly sized fuses ahead of the drive for short ciruit protection.

 

[petersonra]

why would it not meet code, everything down stream is protected for over current by something up stream. the max current is progamable in the drive, so it will trip before the motor or conductors would burn up. it looks like the drive is protecting the motor and itself by stoping if it sees an overcurrent condition and there are properly sized fuses ahead of the drive for short ciruit protection.

because the vfd is sized to the motor nameplate HP, not the motor current, or the actual HP used.

I did not suggest there was anything actually dangerous in doing so.

In any case, you would have to run conductors to the motor sized for the actual NP HP.

 

[Besoeker]

But keep in mind that in some cases, a machine designer also just might be taking advantage of that lower HP issue, i.e. the machine is designed to NEVER run above 40Hz, so the motor will never need to be more than a 40HP motor. He chose a 60HP motor in order to get the 40HP at the 40Hz maximum operating speed.

To get 40HP at 40Hz you'd need the same torque and thus current that you would to get 60HP at 60Hz.

 

[mike_kilroy]

....the machine does have an old (very old) 40hp vfd in it. I was able to get a good part number off of it, and determine that the drive is a 40hp. The motor remains a mystery. So the machine did run from a 40 hp drive. If a 60hp was only doing 40 hp of work I guess the 40hp drive would run it???

We told the control company it was 40hp, they are ready to ship the controls and have a 40 hp "VFD" installed in the cabinet. So the question is, would you leave in the 40 or try to get a 60 installed before shipping?

How about sharing the old 40hp drive part number and CURRENT rating? Different mfgrs can have significantly different current ratings on similar hp nominal rated drives. Lots of guesses can be made but bottom line is what is current rating of the old drive that used to work? As last post points out, to get 40HP (@66% of the 60hp motor's rated speed - or any speed below base) DOES still require the same motor nameplate current of 71 amps. So either the old 40hp drive is rated 71 amps or higher, and thus will produce the motor rated torque at all speeds up to base speed rating, or if rated lower than 71amps it will not. How much less is calculable if the no load amp draw of the 60hp motor is known.

So to your specific question, should the new control folks put in 40hp or 60hp drive, if you want same performance as the old 40hp drive, you need to supply at least the same CURRENT rating from the new drive as the old.

 

[travish]

OK,
petersonra I do understand what you said about the drive being sized to NP hp I agree thanks for your input.

let me see if I understand the other comments, back to the example 1hp mtr and drive, basic freq drive not a vector drive or anything special

Below 60hz = constant torque but hp is dependant upon the speed (hz) ?
Below 60hz we are varying the voltage and freq, as long as the v/hz curve is linear (60hz@480v 30hz@240v 15hz@120v etc) it takes the same amount of current to produce "x" amount of torque at 15hz,30hz or 45hz. Torque and current are the same as NP, speed is 1/4, 1/2 or 3/4 of mtr NP. the hp is less because the speed is less, and hp has a time factor in the calculation? so would the hp also be 1/4, 1/2 and 3/4 of NP hp?

Above 60hz we are only varying the freq not voltage so the speed goes up, torque goes down, hp stays the same, the motor is moving faster but we are able to do less torque.

So in the HP calculation normally speed increase would have increased the hp, because there was a decrease in the amount of torque there was less work done by the motor. Speed went up at the same rate that torque went down so no net gain in HP

Is that correct?

 

[Besoeker]

OK,
petersonra I do understand what you said about the drive being sized to NP hp I agree thanks for your input.

let me see if I understand the other comments, back to the example 1hp mtr and drive, basic freq drive not a vector drive or anything special

Below 60hz = constant torque but hp is dependant upon the speed (hz) ?
Below 60hz we are varying the voltage and freq, as long as the v/hz curve is linear (60hz@480v 30hz@240v 15hz@120v etc) it takes the same amount of current to produce "x" amount of torque at 15hz,30hz or 45hz. Torque and current are the same as NP, speed is 1/4, 1/2 or 3/4 of mtr NP. the hp is less because the speed is less, and hp has a time factor in the calculation? so would the hp also be 1/4, 1/2 and 3/4 of NP hp?

Above 60hz we are only varying the freq not voltage so the speed goes up, torque goes down, hp stays the same, the motor is moving faster but we are able to do less torque.

So in the HP calculation normally speed increase would have increased the hp, because there was a decrease in the amount of torque there was less work done by the motor. Speed went up at the same rate that torque went down so no net gain in HP

Is that correct?

Pretty much. From zero to 60Hz base speed you can get rated torque.
Constant V/f ratio.
Power is speed times torque so available power is proportional to speed. Constant torque region.

Above 60Hz and rated voltage you can increase the frequency but, from nominal supply voltage, you can't increase the volts.
Constant power region.

 

[mike_kilroy]

OK,
hp has a time factor in the calculation? so would the hp also be 1/4, 1/2 and 3/4 of NP hp?

yep! you got it! all good statements. only thing confusing me is your italicized question above about time factor; not sure what you mean by that. second question quoted is of course true.

 

[travish]

Mike, the time factor, I thought that HP was some amount of weight, lift to some height, in some amount of time? so if we run slower we lift the same weight, same height but took longer to get there, didn't we do it with less hp?

 

[mike_kilroy]

ok, sure. didn't realize that is what you meant (I was thinking you meant the motor could only put out the hp for shorter period of time). sorry.

 

[travish]

thanks for all the help guys

 

[John120/240]

Mike, the time factor, I thought that HP was some amount of weight, lift to some height, in some amount of time? so if we run slower we lift the same weight, same height but took longer to get there, didn't we do it with less hp?

travish one Horse Power is the amount of energy that it takes a average horse to raise

33,000 pounds one foot in one minute. Or 3,300 pounds 10 feet in one minute.

You can use any multiple that equals 33,000 lbs/minute. You know 746 watts=1 HP

 

[Besoeker]

travish one Horse Power is the amount of energy that it takes a average horse to raise

33,000 pounds one foot in one minute. Or 3,300 pounds 10 feet in one minute.

You can use any multiple that equals 33,000 lbs/minute. You know 746 watts=1 HP

Or 550 ft lb per second.

I know the numbers for Imperial measurements - the were seared into my brain at an early age.
But how much easier SI units are.

One Watt is a force of one Newton moved through one metre in one second.
It still gives you power as rate of doing work. But without the need for conversion factors.