V5.52 - Merry Christmas and so long

Well, by now I have had my neck surgery and have a pretty good idea how it turned out (it was scheduled for 12-19). No matter how it did, I offer Praise and Thanksgiving to my God in Heaven and to His Son, Jesus.

What this does tho, is knock me out of doing these articles and perhaps now is a good time to just end them. When I started this venture 5 years ago, I had hoped that I would get our local readers a little ‘stirred up’ about power tools and get some good feedback. Sadly, that never happened. Only a couple of people ever wrote me and when I responded to their comments, I never heard from them again. Oh, well.

I want to say a huge ‘thank you’ to Pete and Kaye Doughtie who took a chance on a complete rookie and allowed me to try this effort.

5 years… a LOT has happened in 5 years, but some things remain the same. The Great God I AM is still on His throne and Jesus is still Lord and King. Barry Soetoro still has not produced a valid birth certificate or Social Security number. Democrats are still the party of choice for the pro-death (at least for the unborn) crowd and more racists than were ever in the South. They have become so skilled at creating a crisis to control the news (aka: Wag the Dog) that the low information voters don’t even notice it anymore. (ex: Gruber’s hearing was bad news, so to distract, they release a report about torture from TEN years ago and get that all stirred up again) Republicans are still the party of spineless “leaders” and the party of last resort for those who oppose the internal destruction of our country.

Muslims still pretend to be offended when someone recites the truth about their ‘perfect man’, mohammed. I find it incredibly interesting how simply stating the facts is so unnerving. I suppose that is just a result of trying to keep the sheeple fooled.

5 years in the future… I hope and pray that all Believers have been snatched away (Raptured, if you will) and that we are at home with our Savior. If we have not been caught up, I can tell you this; Christian persecution will still be increasing, Democrats and RINOs will still be serving their own interests, not ours. Gasoline will be more expensive than ever… and Muslims will still be beheading innocent people. Not a very pleasant picture is it?

I’ll leave you with this final thought. Jesus IS coming back. Maybe today…maybe tomorrow…or maybe in 5 years. It is my hope and prayer that everyone who reads this repents and Believes in the One true Son of the Living God. Please be ready to meet Him face to face.

As always, if I can be of assistance to any of you, I can be found at toolsmartz@bellsouth.net . God Bless you all -30-  

V5.51 - Electric motors Part 3

We left off while talking about the voltage sagging on its way to the motor…

As long as the applied load doesn't exceed the torque associated for the rated hp at the rated speed, a continuous rated motor will work fine for continuous operation within the voltage range at the motor leads. The supply wiring can normally deliver the current without suffering excessive voltage sag at the motor leads, but if the run is very long, a bump up in size or two will take care of that. Where the problems start is with voltage that sags too far, causing the current to rise too high, like a refrigerator or air conditioner during a serious brownout (the compressor torque load is fixed), and when manually loading a motor, like with common woodworking machines.

Since you don't know how hard you're working the tool, folks tend to push them pretty hard, easily way past the continuous output rating of the motor. Induction motors can normally output anywhere from 150% to 300% of rated power, with three-phase motors generally able to handle larger overloads, but that's just a generalization. As the load goes beyond rated load, the current also goes beyond rated current, and with the rise in current, the voltage will droop. When the voltage droops, the torque curve becomes more depressed, so the rotor runs slower, causing greater current to flow, and with greater current flow comes greater voltage droop, depressing the torque curve further, slowing the rotor, . . . and so on until equilibrium is reached.

If you don't believe me, do some heavy ripping and get a feel for how it performs, then put your saw on a long extension cord and try again. Add another long extension cord, and so on. With enough cord, you'd have trouble ripping a Popsicle stick. Startups will also get longer as cord is added.** (** Startups are just extreme overloads to the motor - from zero rotor speed up to near rated speed results in 'starting current', usually 5-8 times rated current, which can be calculated from the letter in the "Code" or "kVA Code" box on the nameplate, the rated hp, and the voltage it's configured for.)  Remember that the torque curve sags as the square of the voltage ratio, so small reductions in voltage have a disproportionate effect on current and max torque. To prevent excessive current problems, you need to make sure the voltage drop over the supply wiring isn't excessive. For 'normal' loads, like fans and pumps and whatever, that's not a problem. Just size the conductors to keep the voltage within range. But if you're going to push the motor for all it's worth, like with a contractor saw, where you work it well beyond its continuous power rating, you'll want to think about keeping the supply wiring short and heavy.

Send your questions or comments to:

Toolsmartz@bellsouth.net and we’ll see what we can do to help you. 

V5.50 - Electric motors Part 2

We left off our discussion by saying that the source voltage is not usually the voltage that makes it to the motor. That's important because induction motors are designed to operate at exactly their nameplate voltage (120 or 240V, in this case), and all of the published parameters are based on exactly that voltage at the motor leads. They also are designed to operate within a range of voltage, above and below that rated voltage, but the current, power factor, efficiency, internal heating, and torque curve will vary (not usually for the better) at voltages other than the nameplate voltage. As the voltage at the motor leads droops, the torque curve also droops, but as the square of the actual voltage to nameplate voltage ratio, meaning voltage at the motor that's 10% lower than the rated voltage will depress the torque curve to 81% of design (.9 squared). A 20% voltage sag results in a torque curve depressed to 64%.

Here's where it gets complicated. Motor current, at least within the normal operating range, depends on rotor speed. The rotor is always trying to spin at the synchronous speed, which is 3600 rpm, or 1800 rpm (and slower, like 1200, 900, etc.), depending on the motor. It can't ever reach that, because the rotor spinning less than synchronous speed is what induces current within the rotor, forming magnetic fields within the rotor which interact with the stator magnetic fields, providing the torque (hence the term 'induction' motor). The slower the rotor goes, the greater the speed difference between the stator fields and the rotor fields, and the greater the current in the stator windings, the greater the strength of the resulting magnetic fields, and the greater the torque. When you put a load on the motor (by cutting wood), you can hear the motor slow slightly. Slightly, because the difference between full-load and no-load is only about 4.2% [(1-(1725/1800)) * 100%].

The slower the motor spins, the greater the current through the windings. BUT, if the torque curve is depressed due to saggy voltage, for a given torque load, the motor will spin slower. Slower rotor means higher current to maintain the output torque. So lower voltage = higher current = greater winding heating.

This isn't a problem for a motor operating within 'normal' conditions, as in torque at or below rated, ambient temperature at or below rated (it's on the nameplate), and voltage at or within tolerance (+/-10%). In fact, the reason your motor is rated 115/230V for operation on a 120/240V system is in recognition of voltage sag over the supply wiring (includes 200V for 208V systems, 460V for 480V, and 575V for 600V, which is common in Canada).

Send your questions or comments to:

Toolsmartz@bellsouth.net and we’ll see what we can do to help you. 

V5.49 - Electric motors for woodworking

This week, and for maybe a few more, we are going to talk about electric motors that are used on woodworking machinery. This came about because one of the forums that I frequent had a good discussion going on. A good friend of mine from up in New York is an electrical engineer and he had some fine comments. I’ll change some terminology so that it’s easier to understand. Many woodworking machines are equipped with motors that can be wired for a low and a high voltage. These are called ‘dual-voltage’ motors. Most of the time those voltages are 120volt and 240volt, so that is what we will be discussing. The initial question was: “Will my motor somehow be more efficient if wired for 220v? - but if not, then why would anyone bother to run it 220v, when there are always 120v sockets around?"

Dual-voltage single-phase motors use two main (run) windings that are connected in series for 240V operation, or parallel for 120V use. Connecting two equal impedance windings (impedance means- resistance. In other words, the windings have some resistance) in series results in half the supply voltage being felt across each of the two windings, and the current through one winding is exactly the same as the current through the other winding. Half of 240V is 120V, so each winding 'sees' 120V and some amount of current (depending on the % of rated load, but for consistency, we'll just stick with rated load and rated current at rated voltage).

When one reconnects them for 120V, the two windings are in parallel, so the supply voltage is split off to two windings together, and they both 'see' 120V. But since they're in parallel, and the impedance of each individual winding is still the same, the current in each winding is still the same, but there is twice as much current through the supply wiring. Half the supply voltage equals twice the supply current, as compared to the 240V series-connected arrangement.

So assuming the supply voltage is exactly that of the motor's nameplate voltage for both cases (115V or 230V for NEMA compliant motors), the current at full load will be exactly what the nameplate shows* (* There is a tolerance on this, of course, but that just muddies the water even more.) and for that matter, the efficiency, power factor, and torque curve will be exactly the same. The motor won't know the difference. In theory. Where the difference shows up in practice is that no circuit has zero impedance, so with any current flow, the voltage at the motor will not be the same as the voltage at the source. The higher the current, the greater the voltage difference between any two points on the circuit, or more importantly, at the motor leads.

Send your questions or comments to:

Toolsmartz@bellsouth.net and we’ll see what we can do to help you. 

V5.48 - HAPPY THANKSGIVING !

“Whereas it is the duty of all nations to acknowledge the providence of Almighty God, to obey His will, to be grateful for His benefits, and humbly to implore His protection and favor; and Whereas both Houses of Congress have, by their joint committee, requested me to "recommend to the people of the United States a day of public thanksgiving and prayer, to be observed by acknowledging with grateful hearts the many and signal favors of Almighty God, especially by affording them an opportunity peaceably to establish a form of government for their safety and happiness:"

Now, therefore, I do recommend and assign Thursday, the 26th day of November next, to be devoted by the people of these States to the service of that great and glorious Being who is the beneficent author of all the good that was, that is, or that will be; that we may then all unite in rendering unto Him our sincere and humble thanks for His kind care and protection of the people of this country previous to their becoming a nation; for the signal and manifold mercies and the favorable interpositions of His providence in the course and conclusion of the late war; for the great degree of tranquility, union, and plenty which we have since enjoyed; for the peaceable and rational manner in which we have been able to establish constitutions of government for our safety and happiness, and particularly the national one now lately instituted for the civil and religious liberty with which we are blessed, and the means we have of acquiring and diffusing useful knowledge; and, in general, for all the great and various favors which He has been pleased to confer upon us.

And also that we may then unite in most humbly offering our prayers and supplications to the great Lord and Ruler of Nations and beseech Him to pardon our national and other transgressions; to enable us all, whether in public or private stations, to perform our several and relative duties properly and punctually; to render our National Government a blessing to all the people by constantly being a Government of wise, just, and constitutional laws, discreetly and faithfully executed and obeyed; to protect and guide all sovereigns and nations (especially such as have shown kindness to us), and to bless them with good governments, peace, and concord; to promote the knowledge and practice of true religion and virtue, and the increase of science among them and us; and, generally to grant unto all mankind such a degree of temporal prosperity as He alone knows to be best.

Given under my hand, at the city of New York, the 3d day of October, A.D. 1789.” – George Washington.

This, my friends, was the first Thanksgiving Proclamation and it serves as a prime example (one among thousands) of our nation’s CHRISTIAN heritage. Godless Democrats (who boo’ed God Himself), liberals and so-called “Progressives” will try to tell you otherwise. But they are lying.