What's new
What's new
By:
Advanced search…

3HP VFD to Power 5HP 3PH Motor?

E

ElementalMaker

Plastic
Joined
Dec 14, 2019
Location
NE USA
I've searched but haven't found any definitive answer to this question and it seems to vary quite a bit depending upon the situation.

I just purchased an old Logan 2535-2VH lathe. It had a 2HP motor in it but it was removed before purchase. I have several working 5HP 3PH treadmill motors. These are beefy units. I am considering using a VFD to power one of these and give me some good control of the lathe.

Would a 3HP VFD power one of these 5HP motors? Since the machine doesn't require more than 2HP I'm not worried about machine power or taking heavy cuts. I just am not sure if a 3HP VFD can reliably run a 5HP motor, even if I wont be using all the power the motor is capable of.

Insight much appreciated.

Thank you,
Bob
 
You have understood the issue just fine.
A 3Hp VFD will provide power up to 3 HP. They are current limited.
Your 5HP motors have a slightly higher internal loss than a 3HP motor at the same output, but I don't believe you would notice.
On the other hand, if we're to actually be able to make chips that took 3HP to cut (which I doubt very much), then your 5HP motor will barely raise a sweat :)
I've run big motors on smaller VFDs without any issues at all - but must admit I have never managed to hit the limit of the VFD using them.
Connect it all and set the display to show you the output current every now and then. That will tell you the real story quite quickly.
Please post your findings for future reading and reference.

Cheers
Joe
 
Joe, many thanks for the helpful and insightful answer. I appreciate you confirming my suspicions on the subject. I will give it a go and make sure I'm not pushing the VFD current on heavy cuts. Will update as to how it turns out along with some restoration photos.

Cheers to the Holidays and New Year.
 
ElementalMaker said:
I've searched but haven't found any definitive answer to this question and it seems to vary quite a bit depending upon the situation.

I just purchased an old Logan 2535-2VH lathe. It had a 2HP motor in it but it was removed before purchase. I have several working 5HP 3PH treadmill motors. These are beefy units. I am considering using a VFD to power one of these and give me some good control of the lathe.

Would a 3HP VFD power one of these 5HP motors? Since the machine doesn't require more than 2HP I'm not worried about machine power or taking heavy cuts. I just am not sure if a 3HP VFD can reliably run a 5HP motor, even if I wont be using all the power the motor is capable of.

Insight much appreciated.

Thank you,
Bob
Click to expand...

Bob,
My advice is not to do it. It should always be the other way around where the the VFD is rated higher than the motor it powers. The reason for this is that the back EMF from the larger motor windings can fry the electronic circuit drivers regardless of the driver type (SCR or IGBT) and that there is no current limiting capability on most VFDs. The VFD will try to supply the current the motor is requesting. Like Joe says, it might work if the VFD is using a soft start feature that limits high inrush current and the motor load never exceeds 3 HP. If no inrush control is present or the load exceeds the VFD rating, you will kill the VFD.
 
steve-l said:
Bob,
My advice is not to do it. It should always be the other way around where the the VFD is rated higher than the motor it powers. The reason for this is that the back EMF from the larger motor windings can fry the electronic circuit drivers regardless of the driver type (SCR or IGBT) and that there is no current limiting capability on most VFDs. The VFD will try to supply the current the motor is requesting. Like Joe says, it might work if the VFD is using a soft start feature that limits high inrush current and the motor load never exceeds 3 HP. If no inrush control is present or the load exceeds the VFD rating, you will kill the VFD.
Click to expand...

I respectfully disagree with several assertions here. First, back EMF is generated T-lead voltage which will be the same for two motors rated at a given voltage even if one is 1hp and the other 100hp and will have no effect on the switching device.

While a 5HP motor will have higher inductance and require more magnetizing current than a 3HP motor, generally 3HP and 5HP drives are built upon the same frame size drives and use the same devices for switching current to the stator windings. The primary difference in drive capacity within a given frame size is in the current sensing LEM modules.

Even so, the switching devices will most certainly be IGBTs and have built in circuit protection for instantaneous over current.

This is true for Siemens, Control-Techniques, Rockwell, WEG, Yaskawa, TECO, and ABB drives that I am aware of . . . no doubt this is true for many others.

My concern would be for any treadmill motor rated at 5HP . . . this sounds odd to me in that most treadmill motors I have seen are much smaller and of a permanent magnet design. If you indeed have a 5HP unit, make sure you set the drive up for the proper motor type (PM or induction).

One way to determine if the hp rating is realistic is to compare current ratings at a given voltage. A 5 hp motor operating at 230V should draw about 12 Amps continuous.
 
My educated WAG here is that it should work. I concur with Motion Guru.

I suggest that you check the manual for your VFD.... if you can find a mode that is "current limiting", which DOES NOT "trip off" on an overload, that would be the best to use, simply to avoid the hassles of restarts etc. Many VFDs DO have that, and will hold at a given max current, but continue to drive the load.

The biggest problem you may have is with starting up a heavy high inertia load in the lathe. You will probably need to start such a load slower, in order to avoid hitting the limit. But not TOO slow, so that it does not time out at limit current, which may shut the unit doen even with the current limiting setting.

I doubt you will hit the limits of the VFD with most work, but it is possible to do.
 
motion guru said:
I respectfully disagree with several assertions here. First, back EMF is generated T-lead voltage which will be the same for two motors rated at a given voltage even if one is 1hp and the other 100hp and will have no effect on the switching device.

While a 5HP motor will have higher inductance and require more magnetizing current than a 3HP motor, generally 3HP and 5HP drives are built upon the same frame size drives and use the same devices for switching current to the stator windings. The primary difference in drive capacity within a given frame size is in the current sensing LEM modules.

Even so, the switching devices will most certainly be IGBTs and have built in circuit protection for instantaneous over current.

This is true for Siemens, Control-Techniques, Rockwell, WEG, Yaskawa, TECO, and ABB drives that I am aware of . . . no doubt this is true for many others.

My concern would be for any treadmill motor rated at 5HP . . . this sounds odd to me in that most treadmill motors I have seen are much smaller and of a permanent magnet design. If you indeed have a 5HP unit, make sure you set the drive up for the proper motor type (PM or induction).

One way to determine if the hp rating is realistic is to compare current ratings at a given voltage. A 5 hp motor operating at 230V should draw about 12 Amps continuous.
Click to expand...

It's OK to disagree, but you're passing out bad advise to the OP. If the OP bought a new VFD and used it on a larger motor than its rating and it should happen to fail, do you think the guarantee would still be valid? These things have power ratings on them for a reason. exceeding
the manufacturer's use envelope is just dumb.
 
Hi steve-l,

The point of my post was to simply correct errors of logic in arguments against using a 3HP drive on a 5HP motor.

Back EMF is of no consequence when mismatching drive and motor size.

Switching devices used in 3HP drives and 5HP drives are identical among the vast majority of drive manufacturers and additionally are equipped with built in over current and short circuit protection.

The OP asked a question about what concerns there might be and the answer you offered was inaccurate and unhelpful.
 
I think he was concerned about not back EMF, but energy stored in the windings. If the stored energy is released back, as it is every switch cycle, he is worried that it may be too much for the commuting diodes in the drive.

The switching modules are indeed "optimized" or "value engineered" to the lowest cost, but there are limits even to that, with any decent manufacturer.

Not likely to be an issue, as the drive supplies only so much current, which is a known factor used in the design, plus the overload characteristics of diodes are much better than transistors of most any type. Diodes are the toughest things in the SS world, in general, partly due to their internal simplicity.

it is normal for the current in the motor windings to be "continuous", so the diodes need to be rated to take that for the duty cycle required for their function. Therefore the stored energy should not be an issue.

In any case, I have run motors up to nearly 2x the drive rating with no problems, and I would not expect any. Above that, and the magnetizing current requirement may suck up most of the VFD output and leave nothing useful remaining for power producing current
 
While Im not nearly as technical as motion guru or jst, I can offer my experience.

Im currently using a 3hp actech to drive the 5hp motor on my 325 quincy.
It wont start it up snap on, but I want to limit current inrush anyways. So a 5 second ramp on, while the hydraulic unloaders are waiting to oil pressure is just about perfect.
External pressure switch triggers the VFD ramp on\off.
I've left it for as long as a full week while on vacation (and running a massive surfacing program on the mill).
From my reading, piston compressors are hard on VFDs.

Been running this configuration for a couple of years now, with no issues. Seems to be rock solid reliable.
If you can deal with a nice ramp on, youll love the control the vfd can offer.
 
You must log in or register to reply here.

Similar threads

millrdmillworks
  • Locked
Huanyang 5hp VFD
Replies
1
Views
359
JST
JST
S
Torque difference between 1PH and 3PH motor
2 3
Replies
55
Views
3K
JST
JST
S
A/B motor control
Replies
3
Views
283
mksj
M
tnmgcarbide
will using an overrated VFD damage my motor?
Replies
5
Views
986
ignator
I
H
Running a 3 phase motors using VFD
Replies
1
Views
165
Karl_T
Karl_T








 

ABOUT PRACTICAL MACHINIST

With more than 10.6 million unique visitors over the last year, Practical Machinist is the most visited site for metalworking professionals. Practical Machinist is the easiest way to learn new techniques, get answers quickly and discuss common challenges with your peers. Register for the world’s largest manufacturing technology forum for free today to stay in the know.

Learn more about us.

STAY CONNECTED

Facebook Instagram YouTube LinkedIn Pinterest Twitter Tiktok

Register Become a forum member. Register today.

Sign Up Sign up for our e-newsletter.

© 2024 Copyright Practicalmachinist.com. All Rights reserved.

Back
Top