AC vs DC motors

Nick Schreiber

Nick Schreiber

Well-Known Member
Hey all! I am planning on building a 2x72 grinder in the near future. I got a set of plans that I am pretty excited about. My father in law is going to help me build it and while talking to him about the set up, I mentioned I was going to purchase a 2HP ac motor and a VFD. He offered me a free VFD that he has in his shop. He uses the same VFD on one of his lathes. The only caveat is that it is a DC VFD. Can anyone tell me what the pros and cons would be for taking this free VFD and buying a DC motor vs buying an AC motor and a VFD? Any thoughts, suggestions, opinions, etc, would be greatly appreciated!!!

-Nick
 
Most DC motors use brushes, which wear out and will require maintenance. A lot of DC motors are ventilated too. A 3ph AC TEFC motor is about as maintenance free of an item as anything in your shop. DC motors don't seem as common or cheap as AC motors either.
 
I suspect you could purchase a new 2 hp TEFC 3 ph Ironhorse motor and a Chinese NEMA 1 VFD for a good bit less than you'll find a DC 2 hp TEFC motor. Just checked this site: 1800 RPM, 2hp motor and $400. You notice the motor comes with replacement brushes, tells you they must wear pretty fast. You'd have to confirm the voltage was in agreement with the DC voltage VFD you've got also.

Here's the 2 hp, 3 ph, 3600 rpm motor for $165 shipped. while the 1800 rpm motor will cost you $215 shipped.
 
KenH said:
I suspect you could purchase a new 2 hp TEFC 3 ph Ironhorse motor and a Chinese NEMA 1 VFD for a good bit less than you'll find a DC 2 hp TEFC motor. Just checked this site: 1800 RPM, 2hp motor and $400. You notice the motor comes with replacement brushes, tells you they must wear pretty fast. You'd have to confirm the voltage was in agreement with the DC voltage VFD you've got also.

Here's the 2 hp, 3 ph, 3600 rpm motor for $165 shipped. while the 1800 rpm motor will cost you $215 shipped.
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I was thinking about that as an option too. Any opinions about 1800rpm vs 3600rpm?
 
3600 RPM is preferable. You want the higher RPMs when hogging material on your grinder.

Most 1800 RPM motors can be run at twice rated speed from a VFD, and will be just a tiny bit smoother at extremely low RPM, but in real life a 3600 RPM motor will be just fine. You'll never grind slow enough that it matters, but you will grind at high speed a lot.
 
KenH said:
I suspect you could purchase a new 2 hp TEFC 3 ph Ironhorse motor and a Chinese NEMA 1 VFD for a good bit less than you'll find a DC 2 hp TEFC motor. Just checked this site: 1800 RPM, 2hp motor and $400. You notice the motor comes with replacement brushes, tells you they must wear pretty fast. You'd have to confirm the voltage was in agreement with the DC voltage VFD you've got also.

Here's the 2 hp, 3 ph, 3600 rpm motor for $165 shipped. while the 1800 rpm motor will cost you $215 shipped.
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I have the 3600 RPM motor that KenH is referring to here. I've had it about a year along with a VFD. Not a single problem from either.

On a bit of a side note I've put brushes in many motors. Some are easy to do and others will drive you crazy. Just a thought.
 
I have a Bader III grinder that has brushes, their easily accessible. maybe once a year the motor will start to jog weird at low speed, I take the brushes out and lightly face them on a disc sander then it runs like brand new.

I also have an older square wheel grinder that I never use mostly because it runs like an unleashed lightning bolt at max warp speed. I've been thinking of putting a new motor on it like Ken posted and a VFD. it would make a good machine better if I could just slow it down.
 
J. Hoffman said:
An 1800 RPM motor is a 4 pole and will offer more torque. The VFD will allow you to run the 1800 motor at 3600.
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Yep, the 1800 rpm motor will have more torque at low rpm - where you don't need it. Torque is (5252*hp)/RPM. So with an electric motor your max torque is at lowest RPM where max HP is reached.

for 1800 rpm 2hp motor; (5252*2)/1800 = 5.8 ft/lb of torque at 1800 rpm, but when you increase to 3600 rpm you've got (5252*2)/3600 = 2.9 or about half the amount of torque at 1800 rpm.

for 3600 rpm 2hp motor running at half rpm let's assume it produces 1 hp; (5252*1)/1800 = 2.9 ft/lb of torque at 1800 rpm, but when you increase to 3600 rpm you've got (5252*2)/3600 = 2.9 or the same torque as the 1800 rpm motor. At 1800 rpm the 3600 rpm motor has only half the HP and torque of the 1800 rpm motor, but at 1800 rpm you're running slow and won't be using the power needed to "hog" metal.

Remember, it's not just the RPM, it's also the diameter of the drive wheel that gives the belt speed. You really want at least 4,000 SFPM belt speed for hogging, and ceramic belts really start to shine at 5,000+ SFPM speed. Ceramics really like heavy pressure and high speed.

A 4" diameter drive wheel will give just a tad more than motor rpm for a direct drive motor. I've got my VFD set to 70 hz to overspeed the motor a tad and get a tad over 4,000 SFPM. A motor should handle close to 5,000 RPM max.

I sure hope I got all the math correct.
 
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Ok, my take on 1800 vs 3600.

An 1800 motor will do 2355 sfpm with a 5" drive wheel. That's 4710 sfpm @ 3600. If I want to drop to 1000 sfpm, or half speed on my 1800 motor, a 2 hp motor is still about 1 hp. The 3600 motor would be about 1/2 hp, wouldn't it? Seems like there should be a gain across the full range with the 4 pole motor.
 
tkroenlein said:
Ok, my take on 1800 vs 3600.

An 1800 motor will do 2355 sfpm with a 5" drive wheel. That's 4710 sfpm @ 3600. If I want to drop to 1000 sfpm, or half speed on my 1800 motor, a 2 hp motor is still about 1 hp. The 3600 motor would be about 1/2 hp, wouldn't it? Seems like there should be a gain across the full range with the 4 pole motor.
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And you can run the 1800 RPM motor at 3600 RPM. Win/Win
 
Most likely you'll never know the difference in working with the grinder, 1800 or 3600 rpm motor. I'm running a 3600 rpm motor and have never came anywhere close to loading the motor at lower speeds.
 
KenH said:
Most likely you'll never know the difference in working with the grinder, 1800 or 3600 rpm motor. I'm running a 3600 rpm motor and have never came anywhere close to loading the motor at lower speeds.
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I think that's where the recommendations for motor HP come into play. I've been running a single phase 1HP 1800rpm Baldor since I started. Many folks would like say that 1HP is marginal, but that thing is unstoppable with a 7" drive wheel. That got me to thinking about why everyone liked 2-3 HP motors. I concluded that it must be to account for VFD usage. I'll be upgrading to a VFD and a 2HP 1800 rpm Baldor here in the next couple of weeks. If I had 3600 motor instead, I'd probably drop drive wheel size a little, but I'd use it without any concern.
 
tkroenlein said:
I think that's where the recommendations for motor HP come into play. I've been running a single phase 1HP 1800rpm Baldor since I started. Many folks would like say that 1HP is marginal, but that thing is unstoppable with a 7" drive wheel. That got me to thinking about why everyone liked 2-3 HP motors. I concluded that it must be to account for VFD usage. I'll be upgrading to a VFD and a 2HP 1800 rpm Baldor here in the next couple of weeks. If I had 3600 motor instead, I'd probably drop drive wheel size a little, but I'd use it without any concern.
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I'm the same. Had a Baldor 1hp on a VFD since day one and never feel like I need more. I have a 4" drive wheel and my speed range and torque are fine, never bogs down. I don't really use my grinder for throwing a 4 foot trail of high speed sparks hogging off tons of material, maybe that's the difference. I use a bandsaw for that.
 
Agree with everything above, but only want to add one caveat. If you are using a KBAC drive with 110VAC single phase input and getting 3 phase 220 VAC out, the output is derated. Meaning your 2HP motor is only getting about 1-1/2 HP out. That's one reason everyone suggests a 2HP motor / VFD setup if you are feeding it with single phase 110. A VFD can make 230 from 110, but it doesn't do it for free. There is loss involved.

Back to real life: All of this is largely academic. Grinders don't work that hard. We aren't talking about running two-ton pallets of material on a conveyor belt and trying to accelerate instantly and then stop them on a dime. If you're pushing into a belt grinder hard enough to bog it down I really have to wonder why. Just increase your speed and let the belt do the work. I'm not saying a grinder can't be bogged down, just that I work mine as hard as I would want to and I have never needed more power. I have made the grinder belt slip because I wanted to see how hard I could hog metal, but that was a purposeful exercise in silliness.
 
John Wilson said:
I have made the grinder belt slip because I wanted to see how hard I could hog metal,
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Yep, and that was belt slipping, not motor bogging down. I think we're all on the same page here. I went with a 3600 RPM motor to save about $50, and it's a bit less weight when I was moving my grinder around some. Now it sits in same place the extra weight of an 1800 rpm motor wouldn't be an issue, just the extra cost.
 
I want to thank everyone for all of their input and comments. I have read every comment and you have all provided me with a great deal of information that would be difficult to find without talking to the people that are into this wonderful hobby/art. From what I am gathering, I really can’t go wrong with either option. Either choice is a massive upgrade from my 1/2 HP 2x42 grinder than I’ve been using. Unless someone says something to change my mind I’m going to buy the 3600 RPM motor and a VFD, likely the KBAC.

Some comments came up about drive wheel size. Suggestions about drive wheel size? I’ve found aluminum drive wheels in various sizes but the most common seems to be 5”. Would that be an appropriate size or are there benefits to a 4 or 6 inch wheel?
 
Schreiber815 said:
Some comments came up about drive wheel size. Suggestions about drive wheel size? I’ve found aluminum drive wheels in various sizes but the most common seems to be 5”. Would that be an appropriate size or are there benefits to a 4 or 6 inch wheel?
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The VFD is the great equalizer. With a VFD the drive wheel size is not nearly as important. Having said that, physics still applies. A bigger drive wheel gives you more speed on the top end. When using pulleys this can be a concern, because going slow is more useful, more often, than going fast. A VFD unchains you from these restraints because you can now run any speed from a dead crawl to wide open and anywhere in between.

A 4 inch drive wheel is pretty standard. People go to a 5 inch for more top end.
 
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