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How does someone gauge the power of a DC or AC motor?

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Chris TickTock07/11/2019 21:22:07
222 forum posts
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Hi, I openly admit to asking thisc question in almost total ignorance, but you have to start somewhere.

I am investigation attatching a 50mm diamond disk to a 400 rpm motor. I have looked on ebay and found a motor of suitable rotation speed it is 12v but how does one gauge if the torque would be sufficient. This applies to both Ac and DC so any help gratefully received.

Chris

Bill Davies 207/11/2019 21:43:07
118 forum posts
10 photos

I suggest you look at similar commericial products, with similar dimensions and speeds. The free running speed is not the same that for as maximum power, which is 50% of that speed. 400 RPM suggests some kind of gearing, it is very low for AC and DC motors unless a powerful motor is used at much lower than its rated speed.

Bill

Robert Atkinson 207/11/2019 21:44:43
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Hmm,

You don't need to know the power of the motor, you need to know the torque required to turn the disk. This will depend on what you are cutting and how fast you want to cut it.

More information please.

Robert G8RPI.

not done it yet07/11/2019 21:58:46
3556 forum posts
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Someone doesn’t. The motor manufacturers supply the information. Those that design motors have the experience and testing facilities to provide the true result and supply that information on the rating plate. Mechanical power out is a function of Electrical power in and power wasted (mainly as heat but will include cooling fans if fitted). Known or expected efficiencies are the name of the game.

Motors sold into commercial ventures are often sold on efficiency and longevity - lifetime costs.

Many items are mis-represented by some manufacturers who naughtily quote figures which might even be gross input power - just like compressor suppliers who quote ‘swept volume of air’ but conveniently forget that not all that air is transferred to the outlet - the figures work OK when the outlet is at atmospheric pressure but most certainly not when close to the maximum working pressure!

Start by looking at the maximum current at 12 volts. That will give you the power if the efficiency were 100% - which it will not be, obviously. Then there are different power factors to take into account (obviously none for DC motors). Cheap motors are likely to be very inefficient - as well as not long-lived.

Watch out for motors where the electrical input is the same as the output power claimed.

Torque is merely a function of speed and power.

Chris TickTock07/11/2019 22:05:51
222 forum posts
8 photos
Posted by Bill Davies 2 on 07/11/2019 21:43:07:

I suggest you look at similar commericial products, with similar dimensions and speeds. The free running speed is not the same that for as maximum power, which is 50% of that speed. 400 RPM suggests some kind of gearing, it is very low for AC and DC motors unless a powerful motor is used at much lower than its rated speed.

Bill

Thanks Bill, I think I will need a powerful motor geared with a reduction ratio to acheive enough power to give correct torque at the approx speed when under use. As the diamond disks I intend to use were made to run on small jewlers lathes I think I should look at the spec of my Sherline and other similar lathes to give a benchmark. The other alternative would be to use a more powerful motor with some degree of speed control.

Chris

John Haine07/11/2019 22:11:53
2693 forum posts
138 photos

You can overthink this. If you have a Sherline, bung the disc on a mandrel, fit it on the lathe, start cutting and see what happens. If the discs are designed for small jewelers' lathes (watchmaking lathes?) the Sherline should be well able to do it. Success in engineering is often just down to trying stuff out.

Bill Davies 207/11/2019 23:16:04
118 forum posts
10 photos

I think John has the right idea. But I've crunched some numbers, so I've overthought it already.

I note that 110mm diamond tile saws use 450-500W motors. The disc speeds are approaching 15,000RPM (probably busting speed, rather than cutting speed). Allowing significant losses, I assume the power at the disc is 300W. Converting to Imperial units, which I assume are more familair to most people here, we have 0.4HP available.

Unless I've cocked up (it's late) I get about 0.14 ft-lbf torque, or 1.28 lbf at the periphery of the grinding disc. A 50mm disc would have about twice that force at the periphery. I don't know Chris' application, but cutting ceramic tiles must absorb quite a lot of power. I'm not sure why Chris chooses 400 RPM.

I hope this offers some useful info,

Bill

Edited By Bill Davies 2 on 07/11/2019 23:16:31

duncan webster08/11/2019 00:24:17
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2267 forum posts
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Speaking in complete ignorance (as usual) but 400 rpm sounds very slow for a 50 mm diamond grinding disc

I have a 100mm CBN disc and the manufacturer recommended 3000 rpm

Jeff Dayman08/11/2019 04:53:13
1656 forum posts
42 photos

Not sure why OP wants to reinvent the wheel by using a slow DC motor for grinding. Motor may be pooched already if running at only 400 rpm. By the time you find a 12 VDC supply like a battery charger, connect it and test it with the motor, and ask on here re power and how to do it, you've gone to a bit of trouble for something that's probably not fast enough. I'd go AC bench grinder if I were you. Life's too short. Many cheap bench grinders w AC motors available at minimum cost (new or used ) that will drive the diamond wheel perfectly at about 3000 rpm as it should, with decent power for light or heavy-ish grinding. Due to cost of diamond wheel I suggest lighter cuts to make the wheel last. 400 rpm far too slow in my opinion.

Robert Atkinson 208/11/2019 07:28:40
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400 forum posts
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OP has said they are cutting discs, not grinding. Still need to know what is being cut and speed expectations and why 400RPM. If it to reduce heat then faster cutter speed with slower feed might still be a better choice. The heat put into the work piece (opposed to at the cutter interface) is proportional to the rate of material removal, which includes depth of cut, cutter speed and feed rate. If work piece heating is the issue and it's a dry cut, look at it's mass and thermal capacity of the materiel and the maximum temperature you want. Basic physics (how long does it take to boil a litre of water?) will tell you the maximum time you can cut it for a given power input (torgue and speed) This will give an idea of the maximum motor power and feed rate. It will be conservative because of losses etc but will give an idea if the motor is too big for your limits. It the feed rate turns out to bee far too slow you need to rethink e.g. wet cutting.

Robert G8RPI.

JasonB08/11/2019 07:34:29
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His other thread on diamond grinding and lapping will give an idea of use

Chris can you post a link to these discs, I can't find any with google. Do sound a bit more like diamond coated discs for lapping stone and glass for lapidary rather than shapening.

Like others 400rpm sounds slow at that diameter if you are using these to shape your own tools they it will take forever. If they are just to tough up a previously shaped tool then may be OK and slow speed will not generate heat. At this speed the cutting speed won't be far off my Tormek but I would not use that for initial shaping, 67.6m/min on the Tormek vs. 60m/min on the little discs

By way of power the Tormek turns a 250mm wheel with 200W input power but drive system plays a big part in things.

 

 

Edited By JasonB on 08/11/2019 07:52:12

Juddy08/11/2019 08:30:24
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63 forum posts

If you only want to spin a 50mm disc I would go for something like this: **LINK**

I have one of these and for the price its very good. Showing out of stock at the moment but Aim Tools seem to have a good turn around on stock and I expect its available from other places.

SillyOldDuffer08/11/2019 10:36:07
4843 forum posts
1018 photos

As Bill said the best way is to copy someone else. Find the specification of a commercial machine, and buy a similar motor.

To design from first principles its necessary to clear up confusion about Work, Power and Torque. Be warned, the scientific and engineering definitions of 'Force', 'Energy', 'Work' and 'Power' are far more specific than the same words used loosely in day to day chat. It matters.

Work is the amount on energy needed to achieve a task. It's measured in Joules and time taken is not considered.

Power is the rate at which work is done. It is measured in Watts, where a Watt is 1 Joule per Second. If a 1000W motor takes 10 seconds to complete a job then 100W motor will take 100 seconds to deliver the same result. Both motors do 10000 joules of work. Time is considered!

Torque is the measure of turning force in Newton Metres and time is not relevant.

Although power and torque are intertwined they're not the same thing, and the motor type chosen impacts on both. For example, an electric stepper motor has excellent torque even when stopped between steps, whereas a single phase motor has poor torque at low speeds and improves as it speeds up. The relationship between torque, power and speed isn't linear, and each motor will have graphs showing the relationship over a range of speeds and loads. Very often these graphs aren't provided and have to be asked for. Although maximum Torque is not achieved at maximum power and neither are achieved at maximum speed, reasonable performance can be expected somewhere in the middle.

It takes a certain amount of work to cut metal. Table XIX of my 1947 Newnes Engineer's Reference Book gives examples, all in Cubic Inches per Horse Power per Minute:

  • Mild Steel - 0.75
  • High Tensile Steels - 0.87
  • Cast Iron - 1
  • Soft Brass - 2.3
  • Hard Brass - 0.9
  • Aluminium - 3.0
  • Duralumin - 2.7
  • Magnesium alloy - 3.5

One Horse Power is about 735W

So, given the rate at which it is wanted to remove a particular metal it's possible to calculate the minimum output power needed from the motor.

However, there are many booby traps. One is optimum cutting speeds vary by metal, as does the ability of cutters to cope with the heat generated. Carbon steel (like a file) loses hardness at low temperatures, say 120C; HSS is much better, staying hard up to a low red-heat, though this is not recommended for long life; carbide out-performs HSS by a factor or at least two, major advantage. As cutting at high temperature softens metals, considerably less work is needed to remove metal when carbide is used brutally, saving the big boys lots of money. But getting the best out of carbide needs fast powerful machines, not often found in a home workshop. Other limits apply, for example the brazing on a brazed carbide tool melts before the carbide tip. Diamond is an interesting one. Carbon has a very high melting point but the advantage disappears in air because Carbon burns. Likewise the relatively poor temperature characteristics of the diamond disk and bonding agent prevent diamond performing to maximum potential. Further, the need to achieve a particular speed and the torque required to maintain a particular feed-rate are as interesting to the designer as a motor's power output. Another booby trap is the machines ability to deliver power to the cutting point - the drive-train and air resistance consume power. Not only does motor efficiency varies by speed and motor type, but many motors are rated to work in bursts and need time to cool-down before the next effort. Putting a continuously rated motor on intermittent work wastes money, overloading an intermittently rated motor risks burning it out. And it would be unwise to fit a motor sized to meet a minimum calculated load only by working flat out - prudent to add a safety margin!

In practice, machine tool designed take all these factors into consideration and the usual result is a somewhat over-powered motor for the machine, and a bunch of limitations, often related to cost! An industrial machine will be expensively built to do a lot of work quickly, while a hobby lathe won't. The Sherline is a small hobby lathe with a tiny motor, emphatically not designed to remove metal at high-speed on a production line! May not matter - within limits, hobbyists manage well within a range of motor powers even on the same machine, for example early Myford lathes used 1/3HP single-phase motors and upgraded later to 1/2HP. Some owners have upgraded to as much as 1HP 3-phase. From this you can conclude that 1/3HP is a little too small, and 1HP is plenty. 1/2 or 3/4HP single-phase is 'sensible', 3HP would be too much.

If the diamond disk is for sharpening cutters, the amount of metal removed is low, and a small fast motor plus patience would be appropriate. Cutting removes a lot more metal and generally the operator wants it done quickly. This requires a much bigger motor: my bench grinder and angle grinder are both 750W at 3000rpm. For fine work, my Dremel is about 150W max, speed variable between 5000 and 30000rpm. I don't use the Dremel to cut paving slabs!

As this is complicated, and there's more to worry about, Bob's advice is rock-solid. Copy someone else!

Dave

Grindstone Cowboy08/11/2019 11:03:06
139 forum posts
1 photos

Clearly our Old Duffer isn't as silly as he claims yes Great explanation!

Has the first part of this conversation disappeared for everyone else as well, or is it just me? First post appears to be from Bill now.

Rob

Chris TickTock08/11/2019 11:27:25
222 forum posts
8 photos

Thanks Guys, there is a lot of information set cutters.jpga special thankyou to the old Duffer for the huge contribution...need time to digest that.

firstly i will upload the disks note they are from eternal tools and are sold as diamond grinding disks to be run with light pressure and at low speeds. From research I take the speed to be around the 400 rpm. What i am minded to do is convert a n old bench grinder with a speed say at 3000 rpm and link it via pully to another pully of suitable ratio to get the speed near 400 rpm which will afford all the torque I need and I doubt other than a simple ratio calculation that will be all the mathematics I will hopefully need. Not to say that I am over looking any advice here I am not and will read it to see what is useful.

The problem then shifts to the spindle and bearings on the additional reduction spindle. Any pointers as to where to get suitable bearings appreciated. obviously if i can get a variable speed powerful motor I can simplify the design. The reason as i understand it for running at the slow speed is heat can deteriorate the grinders surface. Another option for me is to be a crappy old lathe and adapt that with an angled table.

Chris

JasonB08/11/2019 11:39:19
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I'd just get a cheap or even slightly more expensive spindle and run that via a belt off your lathe motor so you have the vari speed.

Chris TickTock08/11/2019 12:17:39
222 forum posts
8 photos
Posted by JasonB on 08/11/2019 11:39:19:

I'd just get a cheap or even slightly more expensive spindle and run that via a belt off your lathe motor so you have the vari speed.

Thanks Jason,

Funny enough I contemplated using the cheap spindle. If it would hold up it might do but the ratio of pullies is dictating a lot, for instance if using a step doen ratio of 10:1 the spindle would be carryong a largish pulley..I dont think I want to use my lathe with the diamond grinder but instead make a separate device.

I have never come across the head powered spindle. I take it pullies go on one end but what goes on the other? I am awaiting the arrival of the cutting disk and arbour before thinking again.

Chris

Chris TickTock08/11/2019 12:29:29
222 forum posts
8 photos

Can anyone help with the feasability of adding a spindle using bearings as shoen. I need to make a spindle to take a suitably sized pulley. That hopefully I can do, but I have never used bearings so I need something to hold the spindle in place. For a starting point here is an RS stock nos 750-8930 that has an internal diameter just under 16mm. Using these as bearings what other components would be beeded apart from the spindle and pulley/

Chrispillowblock bearings.jpg

JasonB08/11/2019 12:59:37
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Posted by Chris TickTock on 08/11/2019 12:17:39:
Posted by JasonB on 08/11/2019 11:39:19:

I'd just get a cheap or even slightly more expensive spindle and run that via a belt off your lathe motor so you have the vari speed.

I have never come across the head powered spindle. I take it pullies go on one end but what goes on the other? I am awaiting the arrival of the cutting disk and arbour before thinking again.

Chris

Pully goes on the end with the keyway and the other end has a tapered hole and external thread with closing nut to take ER16 collets which would grip a simple arbor for your wheels

Chris TickTock09/11/2019 13:52:10
222 forum posts
8 photos

Hi Guys,

Rather than spend too much time making something there is a piece of kit here: It is a 500w CNc motor with speed control and 13 E!! collets. My only hesitation is in not understanding what they mean by idle speed stated as 2000-12000. Baring in mind the DC input for this motor has a range of 12v to 100 are they talking about the no load speed? My concern is I need to run my diamond lap disks around the 400 rpm so 2000 would be too high.

chrismotorkit.jpg

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