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On a FB forum there is a discussion ongoing about using VFD's as the control for a power hammer.  As always, there are those who insist that it can't be done.  The main issue (for this side of the argument) is that the speed control of a VFD causes power losses at the motor, and for that reason a VFD is not a good choice to replace a clutch on a power hammer.

 

My experience with VFD's is limited to my belt grinder.  In that application, I can't say that I have noticed a loss of power.  Does anyone here have more experience with VFD's in industrial settings?

I think the idea is replacing the mechanical clutch on a hammer with a VFD.

Just before he died (literally days before) I spoke with Grant Sarver about using a VFD to replace the clutch on a hammer.  He said that even a few months before he would have said that it was not, but that someone had showed him one doing just that.  We never got to explore the idea further.

Does anyone have real information on the subject?

 

Geoff

"The worst day smithing is better than the best day working for someone else."

 

I said that.

 

If a thing is worth doing, it is worth doing badly.

- - -G. K. Chesterton

 

So, just for the record: the fact that it does work still should not be taken as definitive proof that you are not crazy.

 

Grant Sarver

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That is an interesting proposition Geoff. My hammer is a helve hammer so to slow the beat I use light pressure on the treadle and that slips the belt a bit. This is something I may look at as even at lower power the only thing I can see is that the "whip" is going to be reduced and for some operations that may well be an advantage. 

Von Gruff

http://www.vongruffknives.com/

The ability to do comes with doing.

 

 

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Variable Frequency Drive, my understanding is you are changing the frequency of the electricity instead of the voltage so you don't loose "power", just reducing the speed.

Logically there must be some losses somewhere

Edited by Gerhard Gerber
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I am not an expert on VFD, they are a bit of electronic magic in a box to me.

As far as I understand a VFD is not that great at speeding up quickly, They need to be programmed to start up slowly or else there is a chance to fry the delicate electronics.

The drive on my belt grinder takes a few seconds to get to max surface speed.

So this would not be that great for a hammer where you want quick acceleration.

 

I do think that it could work on a hammer to control the max speed for instance, and then engage it with a clutch.

In my mind this is overcomplicating things, a properly tuned flat belt or clutch can give great control and this has worked for at least 150 years

 

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I have my VFD on my hammer set to ramp up the speed very quickly. It is at full speed in less than 2 seconds. I can see it working for controlling the speed of the hammer. The only problem I see is when I'm running the hammer all in one operation I find myself starting slow and rapidly increasing the force of the blow. That's something a VFD cant do. You basically set your speed and that's what you have until you adjust the speed on the VFD. I wouldn't want to be trying to adjust the speed while trying to hammer something home! I think if you are just drawing out stock it would be okay but if you were using top tooling I believe it would hinder the performance.

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Different kind of hammer here, so that doesn't directly apply, but I have my pneumatic hammer on a VFD and like Jeremy said, although it's fine to control my fixed speed (i.e., what would be your max speed on a mechanical with a clutch), I don't think I would want to link the treadle to the speed controller, that would be too laggy for top tooling, which would be my main reason to do it in the first place.

 

For me this is even limited to about 75% of the normal speed: if I go under that, the pneumatic action doesn't have enough force to lift the ram back up all the way and it start to creep down with every blow until it sits on the bottom die. It's still nicer to work 25% slower with top tooling though and being able to go a bit faster than normal on occasion is also nice, so I don't regret doing the conversion (especially since that means it's ready to run as-is once I'll be in europe).

 

Another thing to consider is that motors will see reduced lifetimes if they mostly run slower than about 70% of their rated speed. Part of the issue is the reduced cooling from the fan, but I've also read that it will cause or increase internal sparking (which a VFD can make more likely in the first place, even at rated speeds). It looks like "spark resistance" for newer AC motors is now fairly standard, but that does not help the cooling issue.

 

I can't speak from experience to the loss of power on a mechanical hammer, but I would expect it once you reach significantly slower speeds, like it clearly happens on my grinder. As a replacement for a clutch, the speed of dynamic speed adjustment sounds like the main issue to me. Then again it wouldn't surprise me if someone out there is working with that limitation and has learned to live with it.

Edited by Francis Gastellu
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<Deep Breath>

I am an electrical engineer, and spent a fair bit of my career in industrial control systems.  That being said, "Motors" and "Electronic Industrial Drives" were two classes that I struggled with, and enjoyed the least.  However, I probably have more correct knowledge on the topic than the average bear so I'll try to explain what I can.

 

First, a bit about horse power - Torque, speed and power are all tied together by the laws of physics.  You can only choose two, then the third one is defined for you.  At a given speed and power, the torque available is fixed and not up to debate.  Similarly, pick a speed and torque if you wish, and the required power is defined already.

 

There are a lot of different types of motors.  DC Permanent magnet, BLDC, universal, etc.  They all have different torque/speed characteristics.  The power rating on the name plate of the motor only occurs at one specific speed.  That means at the predefined speed, the motor is designed to develop enough torque to develop the rated power.  Generally speaking VFDs only work with 3 phase induction motors.  You can look those torque speed curves up if you wish.

 

In general, if you apply more load to a motor running at full speed it slows down until the torque required by the increased load balances with the speed to result in a power level that is within the capacity of the motor.  However a 1hp motor bogged down to 50% speed under load is probably producing less than 1hp because that isn't the optimum designed operating point.  Depending on the motor design, and how far away from the rated speed you are, the motor may still be running close to its rated power.  If you go to the extreme and stall the motor, you are producing torque, but no power.  There is a power curve that is defined by the motor design that you can't violate.

 

However, and this is a big however, slowing a motor down with a VFD is not the same as slowing a motor down with a load.  Here is an easy to understand example: If I set the VFD on my grinder to 100 rpm, I can easily stall the motor by pressing on the belt with my thumb.  If I were to plug in my grinder motor straight to power without a VFD it would run at full speed.  If I then tried to load that belt down with my thumb until the motor was only running at 100rpm, it would shred my thumb down to the knuckle in a heartbeat.  Even if I stalled the motor with some sort of clamp, and tried to hold the belt in place with my thumb, I wouldn't be able to.

 

Why is this?  The VFD isn't magic, and can't alter the laws of physics.  It allows you to shift the torque speed curve a little bit, but you are still limited by what the motor can do.

 

 

 

 

 

 

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-Brian

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Knowing nothing about power hammers and obviously very little about VFDs, I found everybody's replies very interesting.

 

@Brian Dougherty is my basic understanding about how a VFD even 70% correct?

Quote

However, and this is a big however, slowing a motor down with a VFD is not the same as slowing a motor down with a load.  Here is an easy to understand example: If I set the VFD on my grinder to 100 rpm, I can easily stall the motor by pressing on the belt with my thumb.  If I were to plug in my grinder motor straight to power without a VFD it would run at full speed.  If I then tried to load that belt down with my thumb until the motor was only running at 100rpm, it would shred my thumb down to the knuckle in a heartbeat.  Even if I stalled the motor with some sort of clamp, and tried to hold the belt in place with my thumb, I wouldn't be able to.

 

Why is this?  The VFD isn't magic, and can't alter the laws of physics.  It allows you to shift the torque speed curve a little bit, but you are still limited by what the motor can do.

I found this very surprising, don't have the means or inclination to test for myself, but it begs the question why a VFD at all?  
I can understand the usefulness of variable speed, but with such losses I would expect more people to look at alternatives like pullies.

 

This also changes my expectations about the motor I wanted to use to build a disc grinder somewhat, no previous experience with what electrical motors can or cannot do.

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VFD's are useful in that you can run 3ph motors on 1ph power.  In the US it's nearly impossible to get your local power company to pull 3ph into a non-industrial zoned property.  So we have to fiddle around with phase converters of some kind.  I have a VFD on my grinder and a big rotary phase converter for the press and a couple of other machines.

VFD's also can give you speed control (depends on the VFD) and some other programable features like soft start and stop.  What I was thinking about was using a VFD hooked up to some sort of peddle to control my hammer, as opposed to the slack belt clutch I used when I built the hammer in 2000.  Essentially a direct drive to the crank shaft (my hammer is a crank style based on a Champion).  If it could give me single high power blows, that would be great.

OTOH, it works fine as it is and the press does most of precision work, so it's mostly a thought experiment.  And look at all the cool stuff we've learned!

 

Geoff

"The worst day smithing is better than the best day working for someone else."

 

I said that.

 

If a thing is worth doing, it is worth doing badly.

- - -G. K. Chesterton

 

So, just for the record: the fact that it does work still should not be taken as definitive proof that you are not crazy.

 

Grant Sarver

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VFDs are very handy, and I am a huge fan of them.  As long as you are running at speeds that are "in the zone" for your particular motor, you would never know the difference in power. 

 

However, people frequently think they can run a motor at 10% speed and still get the rated power out.  That simply isn't going to happen.

-Brian

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There are even 1-phase to 1-phase VFDs, though they are rare. I just bought one to convert 50Hz power to 60hz for my engraving compressor, which would otherwise lose just enough CFM in Europe to be under the specs required by my pneumatic engraving machine (this doesn't work for all types of load, but as far as I can tell, this should work for a compressor, *fingerscrossed*).

 

Also, even though slowing down the motor will result in power losses, it remains useful because you can still do the work, you just need to reduce the load (i.e, don't press so hard on the belt). It takes correspondingly longer to do the job, but you're a lot less likely to overheat a blade that's already been heat-treated. There are also cases where I just want to grind a tiny tiny little bit of material, and that's a lot easier to control with a slow belt.

 

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11 hours ago, Brian Dougherty said:

VFDs are very handy, and I am a huge fan of them.  As long as you are running at speeds that are "in the zone" for your particular motor, you would never know the difference in power. 

 

However, people frequently think they can run a motor at 10% speed and still get the rated power out.  That simply isn't going to happen.

How do you know where that zone is?  My VFD has a range on the display of 1-100, practically only starts moving after 20 and feels uncomfortable faster than 70, now you have me worried I might not be running them fast enough....?

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1-100 is probably in % of full speed.  However, it could be in Hz (Hertz) in which case 100 would be darn near twice the rated full speed.  Hz is less likely in this case, but I'd double check if I were in your situation.

 

If the motor is performing acceptably at the speed you are running at then there is nothing to worry about.  You'll know when you are out of the zone when the motor fails to provide enough power to do what you want it to.

 

I certainly didn't mean to alarm you with all this.  There really isn't anything to worry about.  Just don't expect a 1hp motor to deliver 1hp at really low speeds by using a VFD :)

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-Brian

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52 minutes ago, Brian Dougherty said:

1-100 is probably in % of full speed.  However, it could be in Hz (Hertz) in which case 100 would be darn near twice the rated full speed. 

I looked it up and Namibia uses 50Hz motors so if his adjustment on the VFD is in Hz then 100 would be twice the speed.

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On 4/22/2022 at 3:22 PM, Jeremy Blohm said:

I looked it up and Namibia uses 50Hz motors so if his adjustment on the VFD is in Hz then 100 would be twice the speed.

Correct.
 

The place I bought the kit from supplies a chart that IIRC allows you to work out belt speed and nothing else.

When I picked up a problem with a bearing in the platten's one wheel, the already built "flat grinding attachment" suddenly because a "low speed polishing attachment" that I broke by running it too fast.

Going purely by what I see on videos and what works, I run then at about 60 for normal grinding.

 

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