DC Motor questions

L

little mac

Joined
Dec 3, 2005
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122
I am beginning my build on a KMG clone and at some point in the near future I will need to make a motor selection.
Can y'all tell me what I need to know about DC motors and their controllers or what kind I'm looking for. The little research I've done so far has taught me that they are not rated in HP, but voltage. But I can't find a direct corrolation betweem voltage and HP. I was told a 120 volt motor was about 1 HP, but being a bit distrustful of salesmen, that might have been because that's what he had on hand to sell me.
To muddy the waters further, there are alot of types out there and they all seem to have differing charactoristics. Perm magnet, servo, 3-phase, brushless, ect.
From what I've read here, I need to shy away from those from treadmills.
I've worked in the stoneage of electronics for many years, but my exposure to dc motors and their controllers has been almost non-existant.
 
3 Phase is certainly an option and as I'm guessing a possibly cheaper one. However, the Dc motor is likely to be lighter weight. My employer is doing some experimenting on a DC motor project that I maybe able to scavenge some prototype controller designs from.
There are some advantages to the lighter weight. I could possibly adapt other equipment to run from the same motor and just move the motor as needed. 3 phase motors are heavy and alot of work and modification will have to go into arranging a mount for such a beast. I'd love to convert his vertical wood bandsaw to run on a vari speed dc set-up. I got several metal cutting blades at home that will fit it. Also when it comes time to have my own saw at home, used wood saws are alot easier to find and afford than a metal saw.
A dedicated motor to every job would be ideal, but may not be affordable at this time, so I'm exploring options.
 
If a salesman told you that a motor is based on voltage and that means something in Horsepower, either they really don't know their products, or they're a lying sack o' shit.

Voltage and horsepower are separate issues

DC in variable speed setups have higher torque when used at lower speeds than AC with VFD's

90 VDC and 180 VDC motors are common


A popular controller
NEMA 4 Enclosures
KBPC-240D

with 220 VAC input and the optional heat sink, it will run a 2 HP 180VDC motor

http://www.kbelectronics.com/kbsearch/descriptions/popup_kbpc_240d.htm

web
http://www.kbelectronics.com/catalog_nema_4x.htm


manuals
http://www.kbelectronics.com/data_sheets/kbpc_240d.pdf

http://www.kbelectronics.com/manuals/kbpc_240d_manual.pdf


I believe that Stacy Apelt prefers and uses DC variable speed motors over the VFD controlled AC motors

Searches will find you some good threads on this.
 
Yep,
I like the low speed control, and the high speed too. Torque is very even.
DC motors are rated in HP, just like any other. However, the specs are often given in voltage and current, as well. The info might read: DC Motor, 3HP,180VDC, 14.5 Amps, 1750RPM, Permanent Magnet..... That would be a perfect unit for a grinder. Here is one like that on ebay:
http://cgi.ebay.com/3HP-180-VOLTS-P...5|66:2|65:12|39:1|240:1318|301:1|293:1|294:50


You want a minimum of 1.5HP for a grinder, 2-3 is really nice.
The motor usually runs on 90 or 180VDC, (similar to 120/240VAC motors) but the power developed is twice as much at 180VDC. The drive unit will run on 120 or 240VAC to determine which it delivers . I have several DC motors, and have never had a problem.

Using a VFD and a 3 phase motor AC is also an option, but the DC has worked so well for me, that I never have had one.

The magnetic field is generated by either a Permanent Magnet or an electric winding (field). It doesn't really matter which it is. The controller has the output for the field. PM is simpler, probably.

The controller has a ballast resistor that must be matched to the motor size ( and can be changed). It is often a plug in piece called the "HP Module". Make sure you have one that matches the motor HP.

Stacy
 
My KMG runs on a 3hp baldor, 180volt, wired for 220v, single phase. I fully agree that at least a 1.5hp is needed for a DC motor on a grinder application. I've worked on many grinders with 1 hp DC motors, and they are just plain doggy concerning power.
I purchased my motor off Ebay, and ordered the controller (Baldor BC160) locally. Together the total was less than $500. It was one of the best investments I've made in equipment.

Don't discount the VFD setups....if I were doing it again, that is the route I would go. The 3-phase motors are much smaller than their DC counterparts, and especially this 3hp DC monster I have. From what I have seen of 3-phase motors, its likely that you'd never have to replace it. Not long ago I was curious and looked up what my motor (the 3 hp DC Baldor) would cost to replace... I about fell off my chair! New, its $2,345!

If at all possible, I would say to get a capable motor, and wire the motor for 220V. For years I ran my 1 hp Wilton Square Wheel on 110V, and could bog it down fairly easily. When I finally rewired it for 220V, it was a totally different machine....much stronger.

Just take your time and educate yourself. Then make a decision based on your needs.
 
It almost took an act of congress to bring 3 phase into our shop to run a rolling mill. Are you sure you have access to 3 phase?
 
skybone said:
It almost took an act of congress to bring 3 phase into our shop to run a rolling mill. Are you sure you have access to 3 phase?
Click to expand...
The VFD with 3 phase motor these guys are talking about run on 220 single phase. Phase converter is in the controler.

I have VDC and VFD in my shop and both work very well.
 
There are two types of VFD's for 3-phase motors I've come across and I've never been able to get straight in my mind which is which, whether either would work on a grinder as well, or not. Unfortunately I never remember the technical language for the two... it's something like "constant torque" vs "variable torque". If you know this stuff, would you be willing to explain it, please?

Mike
 
Mike, I don't know the diff. I just got the 2 HP 3 ph motor with VFD controler from Rob
when I ordered my KMG. Since I know little about all this, I like to order a package that
will work :)
 
Thanks for the replies folks. Sounds like Ac motor might be the best overall way to go for me. I think I can put together a 3-phase converter with idler motor a bunch cheaper than a DC set-up. There are plans online to build your own converter. That would cut the cost significantly. I can use pulleys for now and up grade to a vfd later if necessary.
I just needed to make up my mind, as it will effect how I build the back end of the grinder.
Thanks again
Rick
 
The reason that a DC motor may feel weak relative to an AC motor of the same hp rating is that the AC motor will inherently put out rated torque from zero rpm to 100% rate rpm. The output torque of a DC motor will inherently fall off as the speed is reduced below 100% rated rpm.

The variable torque control mode is for applications where the speed is ramped up and down a lot like an elevator or conveyor belt system. The most basic and common control scheme in a VFD is constant torque and that is all you need for the constant speed applications like a grinder.

Pulleys will be the least expensive way to get variable speed.

Phil
 
pso said:
The reason that a DC motor may feel weak relative to an AC motor of the same hp rating is that the AC motor will inherently put out rated torque from zero rpm to 100% rate rpm. The output torque of a DC motor will inherently fall off as the speed is reduced below 100% rated rpm.

The variable torque control mode is for applications where the speed is ramped up and down a lot like an elevator or conveyor belt system. The most basic and common control scheme in a VFD is constant torque and that is all you need for the constant speed applications like a grinder.

Pulleys will be the least expensive way to get variable speed.

Phil
Click to expand...

Phil,

Thank you very much (thank you too, Don).

Phil, I think you made the difference in the VFD types stick permanently this time... something to do with the way you said it. I can't tell you how much I appreciate getting this straight after all these years.

I do understand pulleys, even really good ones, are cheaper. I've been able to find 3-phase motors for so cheap, adding a VFD makes the whole package pretty easy to swallow.

Mike
 
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