AC vs DC, VFD vs SCR vs step pulleys

B

Bo T

Joined
Feb 12, 2011
Messages
2,520
Please correct my misunderstandings. I am thinking about a grinder build and I am trying to think through all of my options. I have 220 V single phase power available. So here are the pros and cons as I understand them.

AC step pully constant RPM: pro: Lowest cost, lowest maintenance, torque goes up as belt speed goes down, pretty much wire the motor then go.
con: 3 or 4 belt speeds, lowest motor efficiency @ 70% (might not have the lowest overall efficiency), takes more time to switch
belt speeds.

DC (brushed PM) step pulley constant RPM: pro: low cost to motor efficiency @ 81% (1.5 HP PMDC TEFC can be had for @$250), reversible.
con: Requires a full wave rectifier to operate on AC, higher maintenance (replace brushes), 3 or 4 belt speeds,
takes time to switch belt speeds.
notes: Unless I can build the appropriate rectifier for less than $100 it is probably better to go with variable
speed using SCR.

AC with VFD: pro: low cost to motor efficiency (3 phase @ 77-82%, these motors are close in price to 1 phase), variable speed, direct drive, smooth, 1P to
3P conversion.
con: VFD fairly expensive (@$400 for 1.5 HP NEMA 4), loss of motor efficiency as belt speed is decreased. 3P high efficiency motors (@ 86%)
fairly expensive. VFD fairly complex-more pieces to break.
notes: I think many VFDs can be wired to accept 1P or 3P, 110V or 220V input and set up to give 3P 110V, 208V, or 220V output. How good
are the warranties on VFDs? Might loose smoothness at lower RPMs.

DC (brushed PM) with SCR: pro: low cost to motor efficiency @ 81% (1.5 HP PMDC TEFC can be had for @$250), reversible, variable speed, maintains torque
as belt speed is decreased, direct drive, smooth throughout RPM range, SCR is simpler than VFD-less things to break.
con: higher maintenance on motor, SCR is still expensive (@$330 for 1.5 HP reversible NEMA 4), motor efficiency decreases as belt
speed is decreased.
notes: brushless PMDC motors are available with low maintenance and high efficiency but the prices are high. How long do the
motors (magnets) last on PMDC motors?
 
Step pulleys actually have a pretty high initial cost. You can get cheap ones, but you will see the difference. Good ones can cost as much as $700 for each pulley. But in the overall lifespan of the machine, yes much cheaper.

The VFD's now are very effective, and offer reasonable initial costs. Please don't confuse motor efficiency with the work output from the motor shaft. Your car burns more gas in the city than it does in the country too.
 
VS is the way to go if at all possible. VFD is a good way to do that. DC with a speed control is another. There are plus and minus on both, but the basic outcome is that both are good systems. Bader has used the DC system for many years, and KMG has used the VFD. I don't ever recall anyone saying they were unhappy with either.

If this is a hobby grinder, and cost is an issue, go with an affordable motor and a good set of step pulleys. If you have the funds, VS is the only good choice.
 
When comparing the step pulleys with a VFD setup, bear in mind the cost of the shaft, bearings, etc as well.

With a variable-speed motor, you can mount the grinding belt drive wheel directly on the shaft. The cost of the VSD and the space to mount it then stacks up against the cost of the pulleys, bearings, shaft, whatever engineering you may need to do to allow you to locate the shaft, release the tension to change the drive belt position and retension, and whatever value you put on the bench space to accommodate the extra bits.

Basically, there doesn't seem to be that much performance difference between DC and AC now, and AC seems to have become cheaper all round. Not so long ago, AC was cheap motors, expensive drives. DC was expensive motors, cheap drives. Now the AC drives are cheap, too.

I don't have any real DC drive experience, but I've been using VFDs since 1988 or 1989.

VFDs are pretty cheap. VFDs with dust protection good enough for grinding are not. I'm not aware of a UK dealer for the KBAC drives, and there aren't many other reasonably priced sealed drives over here (Invertek IP55 drives are about the best for price in the UK) so I built an IP20 (roughly equivalent to NEMA 1) drive into an IP66 (NEMA 4 equivalent) enclosure. I used a sealed socket for the power out and a trailing control box on "enough" cable. I can now use the one drive to run my lathe, the grinder and any other machine I might build in the future.

Although complex, VFDs don't actually seem to break often. Running 24/7 at work, we tend to get 5 years plus out of VFDs; not that far short of the motor life. That's about 50,000 hours. Even at 50 hrs/week, that's around 20 years of working life for most folk. If there's a cooling fan in the drive though, I'd recommend changing it every 20,000 hours, based on experience; $10-$20 every few years is good insurance.

Most VFD and motor manufacturers seem to recommend using 4-pole motors (rated for about 1750 RPM on 60 Hz) and running between 10 Hz and 100 Hz. I find that the simpler, V/Hz, drives tend to feel "coggy" below about 10 Hz. Below rated motor speed, VFDs provide constant torque. Above rated speed, constant power.

Sensorless Vector drives feel smooth down to 3 Hz and probably less; I've not had occasion to try less.

Below about 25 Hz, additional derating is usually needed on TEFC motors. It sounds worse than it is; in many real-world applications, the operator usually does the derating without even being aware of it. Grinding seems to be one of those applications. If you can get a TENV motor, derating isn't usually necessary

I don't know your circumstances, but suspect you are trying to overthink things, particularly with regard to efficiency. "Efficiency" has a precise engineering meaning, but most folk seem to use it when they mean "effectiveness" (sorry; I'm a pedant. I can't help it).

Unless you have data from drives or data-loggers on existing equipment, I'd suggest you are unlikely to have enough information on the actual working power usage to make a decent comparison of the efficiencies of the different options, so you'll need to judge on effectiveness. There does seem to be a consensus view that VFDs are effective.
 
I have both- AC VFD on my Coote, and a DC on one of my disk grinders. They both work well. The DC unit has a break, and it stops right now! But, speed reduction is not as smooth as the AC unit, but not a problem. I would never go back to pulleys. DC motors seem to be more expensive on the used market.

Milt
 
I pretty much echo Timgunn's sentiments. I think all things considered, AC VFD is the way to go. I have two in my shop, a TECO and a Vacon; one runs three machines and the other is dedicated to a grinder. Both work very smoothly. The TECO FM50 drive blew out on me once, I sent it back and the techs authorized free replacement. No other issues.
 
I agree that VS DC, VFD, or Step Pulleys are all going to work and work well. Is one going to be better
than the other two? Maybe. What I don't understand is the great overthinking of which will be better.
Personal choice. None of the above will create what time and learned skill will, and AFTER those two they
might make ones life a little easier.
Ken.
 
ron_m80 said:
Step pulleys actually have a pretty high initial cost. You can get cheap ones, but you will see the difference. Good ones can cost as much as $700 for each pulley. But in the overall lifespan of the machine, yes much cheaper.

The VFD's now are very effective, and offer reasonable initial costs. Please don't confuse motor efficiency with the work output from the motor shaft. Your car burns more gas in the city than it does in the country too.
Click to expand...

$700!! EACH! Those must be some heavy duty pulleys. Are those just larger, like 10x10? The ones that I have seen would be 4x4 at @ $50 each plus the shafts, bearings, and the additional work of setting up the bearings. I am guessing that when everything is said and done the cost for an inexpensive 4x4 pulley set up would be about the same as a VS SCR. I've had cars that got 27hwy and 14city and others that 30hwy and 22city. With the electric motors, on a month to month basis it won't amount to a hill of beans. But assuming I put the same load on each motor my guess was that over years, if one motor had a higher efficiency at lower speed then it might make a difference.
 
kc custom said:
I agree that VS DC, VFD, or Step Pulleys are all going to work and work well. Is one going to be better
than the other two? Maybe. What I don't understand is the great overthinking of which will be better.
Personal choice. None of the above will create what time and learned skill will, and AFTER those two they
might make ones life a little easier.
Ken.
Click to expand...

There are thinkers and there are doers. I'm more of a thinker. And since this is a hobby I don't need to produce a lot of knives. Over the last yeat I've spent a lot more free time learning about the metallurgy of simple knife steel than I have grinding knives. I also spent a lot of time building a PID controller and setting up a small kiln for HT. In that same period of time I've only ground out 7 blades with 3 designs. So, I'm hoping to finish my first knife this year. I did make a throwing knife out of mild steel to test the balance and handling characteristics. And I've put handles on a number of blades, which makes me a cutler (I guess). At any rate, I have a strong desire to understand the different setups for running grinders. A part of this involves examining them in some detail.

Now, if I wanted to sell knives then I would have to agree with you, I'd be wasting a lot of time.
 
I've started my shopping for a grinder as well.

The other thing you should consider is future upgrading. If you go pulleys and 1PH motor to start, that means an entirely new motor will need to be purchased along with a VFD to upgrade.

I can see the attractiveness of a VFD setup, and will probably go to that in the future. But if you are like me at all, shelling out even $1000 is a pretty steep investment so I'm starting my build as CHEAP as possible.

I got really lucky last week and on Friday found a local guy (Who has built 2 VFD type grinders) and got a steal on a basically brand new 1PH 2HP leeson, and 3 step pulleys for $150.

IIRC i priced a brand new chinese motor and pulleys for ~$300 shipped... high shipping costs in Canada are also a symptom but it's the way it is!
 
He meant $70 per pulley. That's for good balanced ones. Wolffbite makes a good point about the additional cost of upgrade at a later date being a deterrent. I also advise people to buy or make direct drive grinders for similar reasoning, even if going single speed to start with. It's a lot easier to retrofit a direct drive grinder with C-mount for variable speed.
 
Bo T said:
There are thinkers and there are doers. I'm more of a thinker. And since this is a hobby I don't need to produce a lot of knives. Over the last yeat I've spent a lot more free time learning about the metallurgy of simple knife steel than I have grinding knives. I also spent a lot of time building a PID controller and setting up a small kiln for HT. In that same period of time I've only ground out 7 blades with 3 designs. So, I'm hoping to finish my first knife this year. I did make a throwing knife out of mild steel to test the balance and handling characteristics. And I've put handles on a number of blades, which makes me a cutler (I guess). At any rate, I have a strong desire to understand the different setups for running grinders. A part of this involves examining them in some detail.

Now, if I wanted to sell knives then I would have to agree with you, I'd be wasting a lot of time.
Click to expand...

This sounds awfully familiar. In the last couple of years, I've built 5 electric HT ovens, some electric crucible furnace/salt pot thingys and several gas forges. Most are at the long-term-testing stage with guys that actually make knives. I've also packaged some VFDs with enclosures and remote controls for use with grinders. One of these days, I'll get around to making an actual finished knife myself.

For decades, DC was the only real electrical option for variable speed. When VFDs first appeared, that started to change and VFDs have been gaining ground ever since. Now, it's probably fair to say that VFDs and 3-phase motors are the cheapest, easiest way to get varaible speed using off-the-shelf equipment.

For occasional use, any of the drive systems you mentioned in your original post will get the job done, and you can evaluate them based on what best fits your circumstances. If, as seems to be the case, you are looking at ways to drive a grinder for your own use and are not limited by the commercial constraints that would restrict the choice of a professional knifemaker to something that can quickly be replaced, it may be worth considering mechanical variators as well. Cost new is likely to be prohibitive, but they do come up secondhand.
 
I used step pulleys for quite a while with a KMG grinder and they worked well. However, last year I purchased a TW-90 grinder which has a VFD. WOW what a difference!! It gives so much more control it is amazing.
 
timgunn1962 said:
For occasional use, any of the drive systems you mentioned in your original post will get the job done, and you can evaluate them based on what best fits your circumstances. If, as seems to be the case, you are looking at ways to drive a grinder for your own use and are not limited by the commercial constraints that would restrict the choice of a professional knifemaker to something that can quickly be replaced, it may be worth considering mechanical variators as well. Cost new is likely to be prohibitive, but they do come up secondhand.
Click to expand...

I briefly considered a clutched transmission and pretty much discarded the idea. Is there a small (sized for .5 to 2 HP) infinite ratio clutchless variator that could be coupled to a constant speed motor? I saw a lot of variators for scooters listed on the net but I have little clue on how they work. I'd probably be the only kid on the block that had one.

I am an opportunist so I would buy a good used step pulley set up if it was a very good price. New, a variable speed PMDC set up with an Iron Horse 1.5 HP motor is about $100 less expensive than a comparible VFD AC set up. That is with the reversing switch. Although the VFD is more sophisticated, I would probably have no need for the added capabilities.

It seems that a 90 VDC SCR will drive any 90 VDC motor that is at or below its maximum rated HP. Does the VFD have the same capabilities? I can imagine setting up a 1.5 HP belt grinder, a 1 HP disk grinder and a .5 HP buffer off of the same VS controller using a double pole triple throw switch.
 
I have a VFD that runs two 3hp and one 2hp motor no prob. 3hp is the max rating on it. The KBAC has a jumper you can set to select horsepower I believe, the TECO I have just runs max rated or lower. I use a series of triple pole double throw switches to select.
 
The VFD will run motors smaller than the VFD rating; I watched the panel builder we use at work test a 110 kW (150 HP) drive panel on a 1/8 HP motor a few weeks ago.

Where the VFD scores particularly well is in being able to run motors faster than their rated speed.

With DC, the voltage determines the speed.

With AC, the frequency primarily determines the speed. Below rated speed, both voltage and frequency vary, providing constant-torque capability, similar to the DC setup. Above rated speed, the voltage is already at the maximum rated voltage for the motor, so no longer increases with rising speed. The frequency, however, can continue to increase up to the maximum set in the drive. Between motor rated speed and maximum speed, the motor provides constant-power capability.

There are limits: losses tend to increase as the frequency gets further from the motor design frequency, but 10 Hz to 100 Hz is the generally accepted range. I've gone to 120 Hz on several occasions, and 150 Hz once, though my guess is that the motor wouldn't have given full power at these speeds. In that particular case, it didn't matter, as the motor was probably oversized by some margin.
 
You must log in or register to reply here.
Back
Top