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VFD for rpm increase?

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rjredline

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#1
Hi all. My old LeBLOND lathe has a max RPM of 500. Is it safe to use the VFD to increase? And how high of a frequency is ok? 1 hp 3 phase motor.

Thanks!
 

Cooter Brown

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#2
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MozamPete

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#3
More rpm = less torque.
Not necessarily. In an induction motor torque is related to slip - the difference in speed between the rotating magnetic field and the actual rotor rotational speed. A good VFD should be able to give near full torque at any speed from standstill to well over rated speed.

I don't think the motor will be the limiting factor, it will be the spindle bearings. If the original motor/pulley combination could only do 500rpm then likely the spindle bearing were only designed for 500rpm.
 
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Ulma Doctor

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#4
the motor can take 90 Hz or slightly more for short periods of time without damage, that equals about 750 rpm
 

Nogoingback

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#5
Another consideration is whether your motor's windings are VFD rated (for temperature). Older motors usually aren't.
 

markba633csi

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#6
It's temperature and also dielectric breakdown. Variable frequency drives generate voltage overshoots/spikes that can exceed the breakdown
rating of old insulation- modern "inverter rated" motors use heavy Formvar or Mylar insulation to protect against this.
Mark
 

Briney Eye

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#7
A new inverter-rated motor rated for 60hz/1725rpm will be quite comfortable at 120hz/3450rpm. After all, the bearings are most likely the same as in the equivalent 3450rpm motor, and they've likely balanced the rotor on the same equipment at the same speed. Your motor might not be, let alone your lathe.
 

mksj

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#8
A good VFD should be able to give near full torque at any speed from standstill to well over rated speed
Torque falls off above a motor's base speed because the V/Hz ratio cannot be maintained above the motors base speed. So torque falls off in a somewhat non-linear fashion above this point for standard motors. What is not often factored in is that a machine with a fixed speed motor, to achieve the same machine spindle speed needs to be geared/belted up and thus torque is reduced based on the change in motor to spindle ratio. There are some variations on this where you have a dual voltage motor, say 230/460, you wire it for 230 but run the VFD to 460V. BLDC and other motor variants may also be able to maintain full torque within the operating design of the system. Hp falls off in a linear fashion below the base speed, one reason why VFD machines often oversize their motors vs. a comparable geared fixed speed motor machine. VFDs can also boost torque/current to the motor for short periods of time, so you will often see 150% boost for 1 minute. An older motor should be set to 100%. The speed range that a motor can maintain torque below it's base speed varies by motor design. Typical older motors might have a 2:1 constant torque ratio so 30-60Hz, newer inverter rated motors maybe 10:1 so 6-60Hz, and vector motors have a 1000:1 or greater so full torque down to 0 RPM.
Typical Motor Curves.jpg

Most VFD have an upper frequency range of somewhere in the 400Hz range, but for all practical use with a 1750 RPM motor one should stay with an upper frequency of around 90-100Hz. This has more to do with performance fall of above this frequncy as opposed to motor bearings. A newer 3500 RPM motor, I would not exceed 70-75Hz. Vector motors can maintain full Hp up to their maximum operating speed, for a 1750 RPM motor this is usually around 6K for motors 3Hp and smaller. As others have mentioned, older motors/insulation do not do well with the voltage spikes generated by VFDs, and also the higher carrier frequency which generates more heat. There is also a difference if the motor is a dual voltage motor, such as a 230/460, since the factory wire insulation is able to better handle the voltage spikes if run at 230V. Too maintain the longevity of an older motor the carrier frequency should be kept at 2kHz, and I probably would not exceed 75Hz as an upper limit, a newish motor than maybe 90Hz for short periods. There are VFD motor filter chokes which can tame the voltage spikes, they tend to be expensive, but you can pick one up used or NOS for a fraction of list. On an older motor I would run it in a V/Hz setting as opposed to sensorless vector.
 

markba633csi

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#9
Pretty much covers it, good tutorial
Mark
 

rjredline

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#10
Wow. Lots of info.
I think I will keep it simple like Cooter mentioned. Might speed it up some but watch the spindle bearings.
Thanks for all tour input.
 

Hawkeye

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#11
I've been running my 2 HP lathe and 1 1/2 HP mill/drill from 10 Hz to 90 Hz for some years, with no problems.
 

RandyWilson

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#12
Wow. Lots of info.
I think I will keep it simple like Cooter mentioned. Might speed it up some but watch the spindle bearings.
Thanks for all tour input.

I too run 10 to 90 Hz. I do it not to exceed the spindle rated speeds, but so I don't have to constantly change the drive belt. The VFD plus back gear has gotten me the range I want without touching the belt. Now if I had a real heavy cut that required being towards the extremes of the VFD, I wouldn't hesitate to change belt positions. But I haven't needed to yet
 

stupoty

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#13
you should be fine pushing the rpm a bit. I did find my vfd can slow the lath down quite fast which has caused the chuck to unscrew, so I've set my decelerate time to a fair bit longer than standard.
 

pacifica

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#14
A line/load reactor between vfd and motor is easy way too solve spike issues for an older motor. Look for MTE on ebay,
 

magicniner

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#15
A good quality modern Inverter Duty motor will safely run faster than any published specification might hint.
I have run a 1Hp, 2 Pole, Compact Frame, Electro Adda motor at up to 7000rpm for a couple of years in my bench top CNC mill without any issues at all.
I have run a 1Hp, 4 Pole TEE (Turkish made) motor in my Myford Super 7 at up to 3000rpm on a daily basis for over 5 years with no issues.

It's probably best to follow the data sheeps until you have enough experience to make your own decisions based upon experimentation though.
 

machPete99

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#16
If motor is normally 1800 RPM at 60 hz you can potentially get it up to 3600 at 120 hz. Most VFDs can do this, I have my lathe set up this way.
If motor is already 3600 you probably don't want to push it any faster.
 

randyjaco

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#17
A VFD will only decrease the input frequency from 60Hz down to 0Hz. That will only make your lathe run slower.

Edit: I didn't know there were VFD's that push more than 60Hz. I love google sometimes. More rpm = less torque.

http://www.electriciantalk.com/f28/running-motor-vfd-past-its-hertz-rating-19715/
That's not true of my VFDs. They will run to at least 120 hz. I would not run my motors long at that rate, but I can double the speed according to the Tachometer. I usually do not go above 150 percent. I don't drop below 50 percent either, because the torque drops significantly. But between 30 and 90 hz all run well.

Randy
 

macardoso

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#18
VFD's can run above the motor base speed no problem, although you will lose torque. Couple of things to watch for (as stated above) running a non-inverter duty rated motor at any speed with a VFD can damage the motor. VFD's generate high voltage spikes which can short between the windings (this is exacerbated the further away from the motor is from the VFD). Second, the motor bearings and mechanical structure of the windings are rated for a particular speed. Running beyond the rated speed can quickly degrade bearing, however going too fast can cause a catastrophic failure of the windings from centripetal force. I've seen that happen first hand. Inverter duty motors typically have a maximum speed on the name plate and have specially designed rotors to handle the higher speeds.

All said and done, you probably can push the top speed a little but don't go crazy.
 

magicniner

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#19
going too fast can cause a catastrophic failure of the windings from centripetal force.
That's very interesting as none of my Induction Motors have rotor windings and I have yet to see one that does ;-)
Induction motor rotors are made in exactly the same way for 4 pole motors as for 2 pole motors and they don't fit different bearings for the same frame size in 2 & 4 pole motors.
Applying some engineering logic this strongly suggests that Inverter Duty 4 pole Induction Motors can be safely run at 200% of rated speed.
And that is indeed the case ;-)
 

macardoso

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#20
That's very interesting as none of my Induction Motors have rotor windings and I have yet to see one that does ;-)
Induction motor rotors are made in exactly the same way for 4 pole motors as for 2 pole motors and they don't fit different bearings for the same frame size in 2 & 4 pole motors.
Applying some engineering logic this strongly suggests that Inverter Duty 4 pole Induction Motors can be safely run at 200% of rated speed.
And that is indeed the case ;-)
Depends on the motor... most induction motors are the “squirrel cage” type where the rotor is constructed with copper bars in a cage shape. Some motors like the ones in your washing machine as well as specialty high speed induction motors will have wrapped wire windings. Regardless I referred to them as windings. If you’d prefer, let’s use the word rotor. The same principles apply
 
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