HP rating for the PC-7518 motor

[miker@xmission.com]

Does anyone have factual data on the HP rating of the Porter-Cable 7518 motor at each of the five speeds? In my testing, the motor seems to lose most of it's torque at the lower speeds.

I've sent a series of emails to Porter-Cable asking for that information. They are "unable" to help me. However, did send me this suggestion: "We have consulted with our bit supplier and determined that the feed rate of 100 IN. per minute with a single flute 3/8" diameter. cutter cutting at a depth of 3/4" is excessive for the 7518. It is therefore likely that the problem is related to the application rather than tool performance. The Model 7518 is designed for a hand held operation where the feed rates are generally slower. The only advise we can offer, since the tool is being used in an application it is not designed for, is to slow down the feed rate and increase the RPM of the router in the cut. We feel that a two or three wing cutter at 18,000 to 23,000 RPM and a feed rate of 22-38 inches per minute would enable the 7518 to do the job."

I found their suggestion contrary to my very limited experience with the Shopbot as well as being contrary to the feed/motor speeds recommendations made by members of this forum.

[ron brown]

Mike,

I do have some factual data on motor current draw. I recently bought a "Watts Up Pro" , a recording watt meter.

For what (watt?) it is worth, the large PC VS Router will pull a peak of 1900 watts on start at high speed. Running at the lowest speed and taking light cuts, I have seen 300-400 watts.

I plan on bringing it to the Jamboree. I will also bring some actual graphs of cutting files I have run and notes provided I have time.

I will flatly state that the PC router does not produce a constant 3.25 HP.

HTH,
Ron

[kerrazy]

Correct me, as I know you will, HP and Torque are two different beasts.
Also the RPM and torque at the end of a winged cutter will vary depending on the outside diameter of the cutter. the centre point of the shank is where I would guess PC gets its data for RPM and this will be considerably different with a 3/4 inch or 1" wing cutter.

[Dan B. (Unregistered Guest)]

Sorry but RPM does not change with the diameter of the cutter. Surface speed will change with the diameter of the cutter.This is why you run a large cutter slower.

[ron brown]

HP = RPM X Torque with correction factors

A revolution is 360 degrees EOS.

Torque is measured at a known distance from the center of rotation.

In most electric stuff horsepower is measured in watts with numbers between 740 and 750 being considered 1 HP. I've always used 746W = 1HP

Ron

[gerald_d]

746 Watt does not equal 1 HP for power tools!

Strictly, Ron is correct for his conversion factor, but the guys who measure watts and horsepower for power tools do it differently....

Americans use horsepower and measure the peak current that the tool draws and calculate the peak horsepower consumption from there. Because alternating current is used, the peak occurs only 60 times per second.

Europeans use Watts, they also measure consumption, but they smooth out the sine wave of the current, so they get an answer about 1.4 times less.

The power output to the cutter is much less than the consumption.

Electronic speed controllers work by reducing the power input to the tool. Generally, at half speed you will only get a quarter of the torque.

But, to get back to Mike's question at the top - I think that the guys you spoke to are drastically underestimating the PC. Yes, you lose a heck of a lot of torque if you use an electronic speed controller. If you had used gears or pulleys, you would actually be increasing the torque - that is why your car has a gearbox and not only a throttle pedal.

[gerald_d]

Here (http://www.mrotoday.com/mro/archives/Uptime/PowerToolFM2002.htm) is a bit of an explanation about power tool ratings. Some compressor guys have been caught out (http://www.aircompressorsettlement.com/index.php3) with their inflated horsepower ratings - will the router guys follow next?

[miker@xmission.com]

It would be *very* interesting to see the watt reading at each of the five speeds on the 7518. (All tests being run at the same feed rate through the same material.)

Right now, it appears that if I use the 7518 motor, that I'll have to either run it at the higher speeds, to get the needed power, and accept the fact that the cutters are going to fry, or slow the motor down and take much lighter cuts. Since time is money, I assume that the best course to follow, at this time, is to run the machine at a high feed speed and factor in a shorter cutter life.

(I'm still renting time from a local Shopbot owner until the programming skills and practical experience prove that buying the machine and renting a building to house it comfortably is practical - that machine is big!)

Of topic, last Saturday I had to cut hundreds of end-on-end dovetails (hand-held router, Leigh jig). I cut about 25% of the dovetails before the bit was dull. After I put in a new bit, I reduced the speed on the DeWalt router to near minimum and finished the job without further bit problems. When I cleaned up the sawdust, I noticed that the slower speed gave me chips and that the highest speed had given me dust. Then, when I counted the fifty-odd bits that I've set aside for resharpening *someday*, I realized that running the router too fast has cost about $1,000 in dulled bits. The unfortunate thing is that all of the router books that I have that were written by well known authors - Patrick Spielman, Pat Warner - suggest running small diameter bits at full speed with light cuts. My experience in cutting those dovetails was that slowing the speed down and making *healthy* cuts gave me longer bit life and dovetails with less tearout; however, the tearout issue might be related to using a better technique as the job progressed.

[gerald_d]

You can get good comparative "Watt" readings by simply measuring the current (in amps) for each of the speed settings. (Watts = Volts X Amps and the voltage stays constant)

[dvanr]

It might be more useful to measure the brake horsepower at the bit end (shaft). ( much like boat engine builders measuring at the propeller) Using a watt meter indicates the total energy being consumed, not what is being converted to useful motion. The best way would be to use a jig based on the work of Prony (http://inventors.about.com/gi/dynamic/offsite.htm?site=http://www.physics.purdue.edu/demo/1K/pronybrake.html). Also known as dynamometer. I have seen a simple jig based on this measuring HP at the shaft for Model Airplane engines.(I'll try and find it). Knowing how much power is going in and how much is actually useful would be a good measure of the quality of a router.

[gerald_d]

Dick, you are getting into the territory of efficiency - output power(shaft) as a percentage of input power(electrical). My guess is that all brands of router have very similar efficiency indices, but that is not to say they have similar qualities.

A prime quality factor would be the insulation and cooling of the copper windings. How long can the motor run at high power before the windings overheat, the insulation breaks down, a short-circuit occurs and it "burns out"? Then the other quality factors are, brush and bearing life, bearing run-out, shaft flexibility, commutator life, collet design & life, etc.

I think that what I am trying to say is that I wouldn't bother with dynamometer (or wattmeter) readings before selecting a router. An interesting exercise, yes - if somebody else did it and published the results here.


PS. My local power tool repair guy showed me how they check a tool that a customer claims is "under-powered".... Lock the shaft of the motor, switch on for 1-2 seconds taking the current reading, multiply these amps times the voltage to get watts, see if the calculated watts equals or exceeds what is stated on the plate.

[dvanr]

Gerald, I doubt that router brands have the same efficiencies , it is not just about bearings and copper. How the armature and the field are wound and assembled has a lot to do with how efficient the motor will be and the heat given off by a router. And I think a really big part is marketing, ads,and product placements. (Hi Norm!)

Compare picking the best router ( hp wise ) to picking a fast car. Would you pick a fast car because it guzzled more fuel? No, you would get in haul out a watch and see how fast you could go over a set distance ... or you would use a dynamometer.

If some one did publish dynamometer results comparing a range of routers I think there would some long faces in marketing depts. ( no more smoke and mirrors )

As for testing motors by locking them and measuring the current, thats a static test on just two out of 8 ,10 or 12 poles . What if the problem is with one of the other poles not connected to the brushes at that position? ( If it was a car engine your mechanic would only be looking at 1 of the 8 fuel injectors)

A dynamic test at the shaft is still the best way if you want to find the real answer to being "under powered"

Now I have to get back to building my Shopbot table while the exchange rate is on the improve!!


Cheers

PS. I have tried to find the test jig I viewed on the web, I have had no luck finding it. ( Which just drives me to distraction)

[gerald_d]

Dick, for my sins, I actually worked in the motor industry where I was the manager responsible for dyno testing and we had engine dyno's, roller dyno's, trailer dyno's and my pet favourite, a supercharged dyno truck. Torque and horsepower calcs and graphs covered my desk. (1981 to 1984 - before personal computers)

But, during the purchase of each of the 8 cars and 6 pick-up trucks in my life, horsepower was never a consideration. The test-drive, recommendations of friends and my experience of the dealer service is what sold me.