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Dane Lance

VFD's and AC motors

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Before I go all crazy and start welding up steel to make a Jeremy Schmidt grinder, I have a couple questions concerning VFD's and AC motors that maybe someone can shed some light on.

I understand the basics of how they work, that I need a 3-phase motor and all that, but I'm not so clear on what I really need in terms of powering my grinder, or more specifically the "mode" of operation.

With a VFD you can run variable torque, constant torque, and constant HP, etc.  From reading it seems a constant HP mode would be best for a grinder (at least all the charts I've found say that).  But, it seems from further reading, that a "constant HP" mode is only achievable when running the motor above base RPM.  Essentially, you get two modes with the set up, "constant torque" which occurs for speeds from 0 to base RPM, then constant HP when running above base RPM.  Constant HP is achieved by varying the voltage once speed passes base.  

So, I guess the question is, do I go with a slower base speed motor (like a 1750 RPM), size the drive wheel such that base RPM at the motor is my "slowest speed" on the grinder, then drive it up from there?

Or, is all of this not really needed and just run it in constant torque mode (i.e. at or slower than base RPM) and not worry about it?  Seems running at slower than base RPM for any length of time will cause a lot of heat build up and not be good for the motor?

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Some 1700rpm motors can run at 3400 but not all. There can be mechanical limitations, such as dynamic balancing, that may limit the speed at which a motor can run. 

 

1700 motors got more torque at lower speeds but you don't really need this kind of torque since you'll be doing light work at those speeds. 

 

Imo, better get an inverter duty 3400rpm motor(they run cooler at low speeds) and 5" or bigger drive wheel.  

 

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Most of the wheel sets I'm seeing are 4" drive wheels.

I'll go with the 3450/3600 RPM inverter duty motor.  3450 RPM with a 4" wheel is around 3600 SFPM.  Assuming the motor will be ok and the KBAC VFD I'm looking at will do it, 30% of max speed is about where I figure the low end will be, so around 1000 SFPM.  I should be able to run the motor above max speed with the VFD too so, actual top speed should probably, safely, be somewhere around 25-30% over max as long as the VFD can maintain the V/f ratio.  This would get things in the  4300-4500 RPM range on the high end without cooking stuff.   I think that's a nice and useful speed range.

I've seen grinders running in the mid 5,000 speed range, if not closer to 6,000 and that seemed a bit much, if not scary.  

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My logic was to use a larger drive wheel instead of overspeeding the motor. Larger wheel meant less torque, but overspeeding also decreases the torque output. So I played it safe and bought a 6" drive wheel.

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1 hour ago, Dane Lance said:

I've seen grinders running in the mid 5,000 speed range, if not closer to 6,000 and that seemed a bit much, if not scary.  

 

If you want to get the best out of low-grit ceramic belts, 6000 SFPM is where it's at, if not faster.  But yeah, it's scary!  For handle work and other wood/antler/bone jobs you will want to drop to 800 FPM or below.  My three-speed grinder's slow speed is around 850 FPM, and even that can burn stuff if you're not careful, especially with a dull belt.  Most VFDs will let you work down to 20% or so before the torque drops below usable levels.  At 10% you can watch the belt jump every time the phase shifts...

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My cheap Chinese VFD can go down to 6hz but I've never felt the phases shift. Perhaps it's because it's fully electronic and the VFD simulates 6hz but behaves differently. No idea...

 

It has useable torque down to around 8hz, for light work of course. 

 

Mileage may vary depending on your stuff I guess...

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Posted (edited)

Dane, I think you're overthinking it a bit. Either "mode" is really fine for this application, IMO. I'm usually pushing hardest into my grinder when it's running at max speed (when I'm really hogging material). In my mind, this is where you'd want the most torque. If you go with a slower speed motor and double the speed and get into the constant hp "mode", then you're going to be losing torque at the high end. That's opposite to what makes sense to me.  I'm not sure you'll find anyone complaining about either setup though.

 

I think you'll be wanting more SFPM than you realize.

 

End up at 50% torque at 120Hz

vfdchart.png

 

That said, you can get more torque on the lower end with the slower 1725 RPM TEFC motor if that's the way you think you'll grind. I've never found it an issue. Usually when I'm running at slow speeds, I'm also not pushing into the grinder very much.

 

Particularly with non-inverter duty TEFC's? I'm getting to the limit of my knowledge of motors here. Most people say to get a TEFC but often don't specify inverter duty ones. I guess most people aren't too concerned with low-end torque. That or most of them today are inverter-duty already.

 

inverterduty.png

 

Edit: Just in case someone was wondering, those graphs came from the KBAC user manual.

 

 

 

Edited by Cody Killgore

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Posted (edited)

My understanding was that the inverter duty motors run cooler at low speed so they don't overheat after prolonged use at low rpm. 

 

Edit: though your graph could explain why I don't feel much torque loss and phase changing at lower speeds. 

 

Anyways, I'm glad I paid a little extra for an inverter duty...

Edited by Joël Mercier

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Yeah, I'm probably overthinking things, I usually do, mainly because the cost of a VFD and a motor isn't chump change and I want to make sure I get the "right" set up...plus learning new stuff is always good. 

An inverter rated motor is a definite (I've learned about that already).  Something else I learned, or at least read about...both 2-pole and 4-pole motors are typically balanced to speeds of 25% over base of the 2-poles, meaning both are balanced to about 4500RPM.  On a VFD, this equates to 75Hz on the 2-pole and 135Hz on the 4-pole.    Assuming the correct VFD, theoretically, the 4-pole motor is going to give you the widest speed range, but does running it at sub-60Hz ranges create too much heat? (more torque for same RPM means more power).  The question is, how much more?  Inefficiencies considered, I would guestimate it has to be something over twice as much, but is it verging on burning up the motor, or is it within tolerances?

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Posted (edited)

The VFD will effectively vary the Voltage and the Frequency together from minimum speed up to the rated speed of the motor. The maximum Current (in Amps) is whatever the motor is rated at, regardless of speed. Power can be thought of (in slighly oversimplified terms) as Volts times Amps. As the Volts go up, and the Amps stay the same, the power goes up. This is the "constant torque" part of the torque/speed/power diagram: Power is also Torque times RPM.

 

Once the rated motor speed is reached, the rated Voltage is also reached. The VFD cannot increase Voltage beyond this, but can still increase the frequency. Power is again Volts times Amps. This is the "constant power" part of the torque/speed/power diagram. Power is Torque times RPM, so as the RPM increases, the torque must reduce.

 

In real terms, the 1750 RPM 2 HP motor and the 3500 RPM 2 HP motor will both be giving 2 HP at 3500 RPM. Both will be giving the same torque because power is torque times RPM.

 

Most of the heat generated within a motor is down to the Current (Amps) being drawn. At full Current (Amps), the heat generated will be broadly similar, whether the motor is running at maximum speed or at minimum speed.

 

For TEFC motor, the cooling airflow moved by the cooling fan will vary with the speed of the shaft.

 

At full rated speed, there is plenty of airflow and the motor can run at full-load continuously. As the speed reduces, the airflow reduces and the ability of the motor to cool itself reduces, such that the motor will need to be derated at lower speeds.

 

Grinders tend not to be run at constant maximum torque, even at high speed, but particularly not at low speeds for a number of reasons: usually the operator has some degree of mechanical sympathy, workpieces get hot and need to be cooled down (during which the motor is under minimal load), belts get changed, etc, etc.  This means that in "our" application, the derating usually happens without anyone needing to think about it.

 

 

Edited by timgunn

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The "Inverter duty and TENV motors" curve in Cody's second graph will almost certainly be for Non-Ventilated motors and for fan-cooled motors with a separately-powered cooling fan: effectively a mains-powered, fixed-speed fan which maintains the cooling airflow regardless of the motor speed.

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Yep, timgunn is right. Those charts are from the KBAC manual. Went back and it does specify externally cooled motors in that section. Sorry, not sure how I missed that...

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Posted (edited)

Ah, timgunn, thank you!  That puts it all in perfect perspective.  I was ignoring the voltage variance by the VFD vs amps in the sub-60Hz range.  I understood it concerning the base+ speeds.

The trade off between the two different motors then, is that the 4-pole will have more torque in the sub-60Hz up to base speed, but lose in the torque department to the 2-pole beyond 75Hz.

The 4-pole does provide a broader speed range, 900RPM to 4500RPM (30Hz to 135Hz),  theoretically that is, if balance is the only limiting factor and what I read about manufacturer balancing holds true.  The 2-pole speed range would only manage  1800PM to 4500RPM (30Hz to 75Hz).  Additionally, the 4-pole has less torque in the upper range, but more in the lower range.  Almost seems opposite of what I really want.  

If frequency were theoretically unlimited in a motor, then at some point beyond base speed, the motor will  not be able to increase RPM once torque falls to that necessary to simply keep the motor (or the entire device it is attached to) turning as long as amperage remains constant and only frequency is increasing.        

I get torque and RPM, at least as it applies to HP in reciprocating engines, things get a bit funkier with electric motors and even more so with a VFD.    Will the AC motor operate the same on the VFD as far as back EMF and sync?  What I mean is, with no load, back EMF acts like an amperage throttle, so no load is less amp draw and synchronous, or near synchronous RPM.  Add a load, RPM drops, back EMF decreases, which allows more amps, motor makes more torque to try to get back to rated RPM.  This is more like a constant speed mode.  This is why it's often said you can replace a gas engine of a given HP with an electric motor of lesser rated HP.  In practice, however, you can get away with it only for intermittent use.  Continuous duty cycle needs a 1 to 1 swap.  Or, do 3-phase motors act differently than this?  

Or better yet, just find out what everyone else uses and steer in that direction.  This is making my brain squirm too much this morning.

Edited by Dane Lance

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