V4.17 - 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.

V4.16 - 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

V4.15 - 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

V4.14 - And finally, leveling the tables

Our friend, the jointer is still on our hot list this week, but I think we’ll be finishing it up. So far, we’ve talked about what the jointer is used for. Then we checked the tables for flatness and being level with each other. Then we talked about setting the knives. Last week we talked about the importance of a good fence. The last thing on our list was how to level the tables. Leveling the tables always requires a good machined steel straight edge and a sent of feeler gauges. Don’t try this with a carpenter’s level or an aluminum straight edge…the tolerance of those items is nowhere near the accuracy that is needed.

So, let’s get after it and discuss leveling the tables. There are three main jointer designs: 1: Parallelogram, 2: Wedgebed & 3: non-movable outfeed.

The Parallelogram design jointer originated outside the USA, but many industrial jointers use this design. This design has the table pivoting on 2 rods that have eccentric bushings on each end. The secret to adjusting the tables on this design is found in the 4 eccentric upper bushings of each table. (that’s a total of 8) These bushings allow you to raise or lower the tables, front & rear or side to side. You cannot raise or lower them by the corners, tho. All that does is create a binding that stops movement of the table.

The wedgebed design was the most common design for many, many years when jointers were first invented. It’s still in use. This design has the tabkles sliding up and down on machined ‘ways’ and they are held in place with gibs and lockbars. The secret to adjusting the table of a wedgebed jointer is found in the gibs and lockbars of the bed ‘ways’. Loosening the gib screws and lifting the tables above where you want them is the key. You lock the gibs screws back down to hold the table there and then slowly loosen them and just at the point that the table gets where you want it…you stop and re-tighten them. This design does have one inherent problem. As the jointer is used, dust can lodge inside the ways. Over time, the dust will pack in and create a wedging effect that will cause the tables (usually only the infeed table) to go out of alignment. At times it is necessary to disassemble the gibs and clean the ways before adjusting.

The non-moveable outfeed jointer is usually reserved for the most basic jointer (if you want to read that as ‘the cheapest’ - that’s OK). I’m sure this design came about because it is the least expensive to make. Most jointer operations do not need a moving outfeed table, but there are times it might need adjusting. This design secret is that the outfeed table usually has 3 mounting points and they are mounted with rubber bushings that can expand or contract- depending on how tight the mounting bolt is.

Send your questions or comments to:
Toolsmartz@bellsouth.net and we’ll see what we can do to help you.

V4.13 - But what about the fence?

We’re still talking about our friend, the jointer. So far, we’ve talked about what the jointer is used for. Then we checked the tables for flatness and being level with each other. Then we talked about setting the knives. I can see two things we left out: Setting the fence and how to level the tables.

Let’s deal with the fence first. A good, flat, properly adjusted jointer fence is a critical feature of a decent jointer. Now, if I were you, the first question that arises is: ‘what does he mean by “good” fence and a “decent” jointer?

That's a couple of good questions, I’m glad you asked.

In my opinion a ‘good’ jointer fence is one that is made of cast iron. I have seen some very low-cost jointers with fences that were made of extruded aluminum. Yes, these were on 4-inch or 6-inch hobby-style jointers but the fence is such a critical piece that even on those kinds of jointers, an aluminum fence can kill your accuracy.

A ‘decent’ jointer is one that I can set up properly and expect it to stay adjusted and not have to fuss with it each time I want to use it. An aluminum fence is usually not that way. The heat and the cold and the movement can all affect the way an aluminum fence acts. Cast iron fences are much more stable and they tend to stay adjusted.

OK, so we have ‘good’ and ‘decent’ defined- here’s what to look for on ANY jointer fence. 1) You want it to be flat. Flatness is always checked by using a machined straight edge and a set of feeler gauges. You check across the width, then lengthwise, then ‘X’ it. Usually a .005 to .010 ‘out of flat’ is within specifications. I’ve seen many aluminum fences be out .020 to .030. That is not ‘good’. A fence that is not flat can cause all kinds of problems and you’ll never get a good jointed edge when using one. For instance, let’s say that you are edge jointing a 1 x 4. You set the fence up, you make sure it is square with the infeed table, but you forget to check to see if it is square with the outfeed table. (truthfully, the only reasons it would not be is because the fence is not flat OR the tables are not aligned properly.) You start your joint and when you get done, you discover that the edge is twisted. So, you have to junk that piece of wood. Another one of those not ‘good’s I was telling you about. 2) The second item on our ‘good’ fence is Repeatability. In other words, when you adjust it properly at 90 degrees, and then move it to 45 degrees, you can return it to 90 degrees and it will BE 90 degrees. Cast iron fences are good about this- aluminum fences… not so much.

Send your questions or comments to:
Toolsmartz@bellsouth.net and we’ll see what we can do to help you.