Grinder motor confusion

H

Hubert S.

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Dec 14, 2019
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I am looking for a 3-phase motor for a 2x72 grinder I am building. I want to get a 2 HP 56C TEFC motor and a KBAC-27D. After reading a bunch of threads here and other places and looking at motor specs online, I am more than a little confused.

I have read that it is better to get a 4-pole 1800rpm motor and run it up to 120 Hz vs. a 2-pole 3600rpm at 60Hz in order to get more power. Is this really a concern? Anybody out there with a 3600rpm motor wishing they had bought an 1800rpm?

What has me even more confused are the specs given for different motors. There is an 1800rpm Leeson motor that specifies 10:1 turn-down ratio, speed range 600-4,000 rpm and 30-60Hz all at the same time. I really don't know what to make of that. Can this motor be operated at 180rpm as indicated by the turn down ratio, or is 600rpm the minimum, or the 900rpm indicated by 30Hz? Maybe I am misunderstanding what the turn down ratio is. It seems to be all over the place for different motors, ranging from 2:1 to 1000:1. I have seen a couple of specs showing different ratios for constant vs variable torque, but I guess the variable torque is what matters for a grinder.

I am probably over analyzing this, but I would appreciate if somebody could shed some light on how to interpret these motor specs and maybe give some recommendations of what to buy.
 
You get a little bit of extra torque on the lower speeds of an 1800rpm motor than a 3600rpm, but at the end of the day, it's probably negligible. Some guys like a little extra "head room" with a 3600 rpm motor when running ceramic belts, as they need a minimum SFPM (5000 or more) to get the most out of them. You can achieve the minimum speeds needed with an 1800 rpm motor, but you'll need at least a 5" drive wheel, if not 6". IIRC the numbers, a 5" gives you just under 5000 at 120hz.

That said, most of us are doing 90% of our grinding at at around half those speeds or less (especially at higher grits), so the fuss about getting the most out of you ceramics is really only applicable to when you're really trying to hog some steel.

At the end of the day, either a 2 pole or 4 pole motor will spin your belts for you and do what you want to do.
 
Drew summed it up nicely, but I'll add one more comparison. I'm running 3600 RPM on my grinder, and 1800 RPM on my disk grinder, and I think 1800 on horizontal, but not sure it could well be a 3600 rpm motor. You'll never know the difference once it's setup.

Allow me to state I've set about 4,000 RPM as max rpm with a 4" drive wheel that gives me just a tad over 4,000 SFPM belt speed. While many grind with ceramic belts at 5,000 and 6,000 SFPM speeds, that's just more than I wish to do. 4,000 SFPM with a 36 grit Ceramic belt will really throw some sparks!

Think of it this way, assume that 100% speed is 3600 rpm for both 2 hp motors (running the 1800 motor at 120 hz). The 1800 motor WILL give more torque and power at 50% speed, it will have 2hp available at 1800 rpm while the 3600 rpm motor will have only 1hp available (just an assumption), BUT, at 50% speed most likely only 1/2 hp is needed to grind. At 100% speed the 3600 rpm will have the full 2hp available while the 1800 rpm motor running at twice rated speed will have just barely slightly less than 2hp available, but not enough less you'll ever be able to tell the difference.

All the specs you're confused about are for industrial applications where the motor must hold up for 100% duty cycle, running 24 hrs/day, 7 days/week. For our 2X72 grinder use, those specs mean very little. Either motor will run down to 300 rpm (or slightly less) just fine. The only problem is the fan not blowing enough air for cooling. That usually isn't a problem because the motor is usually running only a few minutes before stopping in grinding.

My decision for a 3600 rpm motor was because the cost was about $40 less and a few lb less weight on motor making the finished grinder a bit easier to move around. When bolted to a stand and staying in same place the weight isn't a factor.

Here's a link to Ironhorse motors which many of us use due to cost. https://tinyurl.com/spbwf4r
Note the 2 hp 3600 rpm motor is $165 shipped while the 1800 rpm is $222 shipped.
 
Drew and Ken, thank you for your explanations. I figured it probably would not make a big difference. I had not really thought much about the drive wheel size, thank you for pointing that out.

Ken, I looked at the Ironhorse motors and ruled out the 1800 rpm because it is two inches longer than their 3600 rpm motor. I am building a tilting grinder and don't want to raise the pivot point by two inches if I don't have to. The Leeson I linked to is only 1/2" longer, but costs almost $400.

I will see what I can find for a drive wheel and then figure out what motor to get.
 
I replaced the 2 hp 4 pole 1800 rpm motor on my grinder with a 3hp 2 pole 3600.
The idea was to increase belt speed on my vertical and use the 2 hp on a horizontal machine.
What I have found is that the 2 pole motor is not smooth at certain slower speeds. So much so that there is a range I cannot use. The whole machine shakes. This was not a problem with the 4 pole motor.
I have since learned that the limiting factor regarding motor speed is the centrifugal force on the windings and that the limit on the 2 and 4 pole motors is the same.
So if I was making the decision now, I would get the slower 4 pole and just bump up the hz
 
In context of induction motors driven by VFDs.
I suppose its squirrel cage bars could be considered windings. But they are pretty much immune to centrifugal force.
I think overspeed mechanical limits/failure is more likely result of bearing failure or rotor balance.

Fwiw, Large series wound brushed motors. Windings or commutator can be damaged by overspeed. They must never be allowed to run without appropriate mechanical load. Otherwise they can spin up until it explodes. But if lucky a winding comes off first to shut itself down.
limiting factor regarding motor speed is the centrifugal force on the windings
Click to expand...
 
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Mike Read said:
I replaced the 2 hp 4 pole 1800 rpm motor on my grinder with a 3hp 2 pole 3600.
The idea was to increase belt speed on my vertical and use the 2 hp on a horizontal machine.
What I have found is that the 2 pole motor is not smooth at certain slower speeds. So much so that there is a range I cannot use. The whole machine shakes. This was not a problem with the 4 pole motor.
I have since learned that the limiting factor regarding motor speed is the centrifugal force on the windings and that the limit on the 2 and 4 pole motors is the same.
So if I was making the decision now, I would get the slower 4 pole and just bump up the hz
Click to expand...
You've got VFD setting problems. That will happen from time to time with 2 pole OR 4 pole motors. From my understanding the vibration issue isn't a factor of 2 pole or 4 pole, but just happens.
 
Makes sense Lieblad - so my motor supplier was talking rubbish - which doesn't surprise me :(

So if you are using a VFD and can run either motor at any speed, is there any practical reason to choose one over the other?

Ken H Any idea what settings might be wrong? It is a few years since I set it all up and cannot remember what I did other than I read the manual carefully and went through every parameter.

I had assumed it was the 2 pole motors lower electrical frequency resonating with the grinder frame.
 
I use an Ironhorse 1800 rpm, 1.5hp DC motor from Automation Direct (link below) on my Ameribrade grinder. I paired it with a DC motor controller (link below). I use a 7" drive wheel to get the speed I need. It operates on 110v AC. The torque from this motor is amazing, especially at slow speeds and the reverse feature on the controller comes in handy when I use my grinder as a sharpener (Reeder sharpening jig copy). The speed control is sealed but the wire terminal strip is exposed (not good for metal dust contamination). At the moment I have electrical tape covering the terminal but will probably drop it all in a sealed box eventually. KB also makes a DC speed controller that's in a NEMA enclosure, but it's a bit pricier. My whole motor/speed control combination only set me back $360.

https://www.automationdirect.com/ad...ors_(up_to_2hp)/general_purpose/mtpm-1p5-1l18

https://www.ebay.com/itm/120-VAC-To...:OiQAAOSwAmVck46n:sc:USPSPriority!38469!US!-1
 
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When I started this thread, I was hoping y'all would clear up my confusion about motors, not add to it!

volsr1, the DC motor looks nice, but it is about 3 1/2" longer than the AC motor I was looking at. I don't want to make the tilt stand that much taller. I did email Ameribrade about their wheels to confirm that they have crush sleeves and they do. I will order their set with a 7" drive wheel, just because the big wheel looks cool and bigger is always better. I plan to get an 1800 rpm motor and then set the max speed to whatever I feel comfortable with on the VFD.
 
On the 2"x72" I just put together I opted for a 2 pole 3HP 3 phase, I have no vibration issues anywhere in the rpm range from my VFD. I will be adding an external cooling fan as my research has shown me that rpm's under 40% might not allow the built in fan to move enough air to cool the motor well enough for good longevity, better safe than sorry.
 
I looked at some more motor specs and some show performance graphs from near 0% to 150% or 200% of the nominal speed. I am assuming that means no extra fan would be required, but you never know.

I am also trying to find specs for how much clearance is required for the fan to get enough air to cool properly. If anybody knows, or could measure the clearance on their tilting grinder when in the horizontal position, I would appreciate it. My current plan is to have about 2 1/2" of clearance.
 
Mike Read said:
Ken H Any idea what settings might be wrong? It is a few years since I set it all up and cannot remember what I did other than I read the manual carefully and went through every parameter.
Click to expand...
Mike, One of the things to do is check which speeds the vibration occurs, then in the settings you can set VFD to skip those speeds i.e. if vibration occurs at 1200 RPM, skip 1150 to 1250 rpm. That's small enough doubtful the skipped speed will be noticed. I've never had the issue and I have both 1800 and 3600 rpm motors. From all the reading I've done, this seems to be fairly uncommon and 2 pole vs 4p doesn't seem to factor in.

V volsr1 : What prompted you to go DC route rather than the normal AC 3 ph route? Does your motor have brushes? I didn't consider DC due to the extra cost. My 2hp motor 'n VFD were around $225 shipped for both.

Simple check to determine if an extra fan is required, lay your hand on motor from time to time while in use and see how hot it gets. If it gets uncomfortably hot to touch, then by all means add a fan.

2-1/2" of clearance should be more than enough for air flow - again, in use touch motor with hand. Bet it never gets much past mildly warm.

7" drive wheel will do just fine with an 1800 rpm motor. at 200 rpm you'll around 350 SFPM belt speed which should be slow enough for any slow work you want. 1800 rpm will give around 3300 SFPM, and speeding motor up to 3000 RPM will provide around 5500 SFPM, a good bit faster than I care to grind, but will really make a ceramic belt work correctly.
 
Thank you, Ken. I think I will set the VFD to 2,700 rpm or so to start out with and go from there.

Most of the 4-pole motors I have looked at are a lot longer than the Leeson, so I am leaning towards purchasing that one despite the price tag.
 
How long is that Leeson 4 pole motor? The Ironhorse 2 hp is shown here. The "C" is from tip of shaft to back side of fan covering. The shaft is 1.88" long, so it looks like the 3600 rpm would be about 10" long from face of motor to rear of fan enclosure?
C = 13.9”; 2hp MTRP-002-3BD18
C = 11.9”; 1 to 2hp MTRP-xxx-3BD36
Look at bottom of 4th page: https://cdn.automationdirect.com/static/specs/ironhorsetprs.pdf
 
Ken, I went DC due to the cost. I used a wood lathe one summer with a DC motor and really liked it. I couldn't find a 3ph AC with vfd for less than the motor/DC drive I got. I should have shopped around more as I see some pretty good deals on ebay. Yes it does have brushes but they are rated for 2500 hours.
 
Ken, I ordered the 7" drive wheel earlier today, so I want to get an 1800rpm motor. I was looking on electricmotorwholesale.com, they have four different Leeson 56C TEFC 2HP 1800rpm motors for under $400. I think they are all between 10" and 10.5" long, the most expensive one at $390 being the longest one. The two cheaper ones have a turn down rating of 2:1, the two more expensive ones 10:1. The one for $390 I linked to in my first post has the contradictory specs. None of the model numbers given are in the Leeson catalog from Regal Beloit. I will contact electric motor wholesales tomorrow and see if they have any more detailed specs. All of the cheaper motors I have come across (Ironhorse, Hallmark, Weg) are about 1 3/4" longer.
 
Now I'm wondering what I've been missing since I have a max belt speed of 3300 fpm. I've ground using a 60 grit norton ceramic and it seems to work fine but I've had no experience running it at higher speeds. I'm also planning a home made 10-12" disc sander build. I may move my DC motor to the disc project and either get a 3600 rpm DC motor (if it exists) or 1800 rpm AC 3ph motor with vfd (running at 120 Hz for 3600 rpm) for my 2x72.
 
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Just to update, the information on the website was indeed incorrect. I contacted them and their rep initially gave me another set of specs that did not match, but they have updated their website and the motor specs are consistent now.
 
volsr01, get that speed up around 4,000 to 5,000 SFPM belt speed and really put some pressure on the metal to ceramic belt and you'll be amazed how many sparks will be created, and how fast the metal will grind away. I run max at 4100 SFPM belt speed and that is enough for me.

Hubert, what specs changed on motor?
 
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