V4.22 - So, your motor just up and quit?

This week we have a typical scenario from a client: “My 17 year old JJ-6CSX was running fine and then just wouldn't start. My suspects would be the motor capacitor or the starting switch, but I'm out of my element here. Any way to determine which one--or if it's something else?”

Translated, it would read like this: “My 17 year old JET jointer quit working and I’m not sure what’s wrong. I’m no good at electrical troubleshooting- HELP?” Just for the record, we are talking about a 1 horsepower single phase induction motor, which has a start capacitor.

Fortunately, troubleshooting a motor’s electrical problem is really not that difficult, but there is a standard way of doing it. First off – Keep It Simple.

Look for the easy stuff first.

1: Make sure you have power to the outlet. Try plugging the motor into a different circuit-one that you KNOW is working. Sometimes a breaker trips for no apparent reason.

2: With the power cord UNPLUGGED- check the wiring connections and make sure that one of them has not simply come loose.

3: If you KNOW there is power to the outlet – does the motor hum when you flip the switch? If it hums, that means the switch is working because there is power getting to it.

3A: If there is no humming- you’ll need to go deeper. But FIRST- UNPLUG THE POWER CORD. Then you will need to test the switch with a meter OR use a test cord that will bypass the switch, so you can plug it straight into the outlet and see if the motor runs. IF the motor runs using the test cord, replace the switch and you should be fine.

3B: Let’s say the motor does hum. Your problem is in the start capacitor or the centrifugal start switch. FIRST- UNPLUG THE POWER CORD. Now, you will need to find the centrifugal start switch. This usually requires some disassembly of the motor to find that switch’s contacts. Once you find them, vacuum the dust out if there is any. You can also blow it out with compressed air. Once you know that the switch contacts are clean, dry and contacting each other properly, try the motor again. If it still only hums, you probably have a bad start capacitor. You can replace that, OR you can take the motor to a motor shop for further testing and repair.

Motors are rather simple objects, but look for the easy stuff first. I had one customer who went thru everything on the motor, only to find out that his multi-outlet breaker strip had tripped out because of a little rainwater getting in it. (He had it on an open window’s sill) I promise he felt really dumb after that one.

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

V4.20 - V-Belts and heat; part 2

Last week, we started talking about a fellow’s Powermatic planer that has a 3 V-belt drive system and he thinks the belts/pulleys are getting too hot. From the tone of his post, it reads as if he is amazed that there is any heat at all. My contention is that, yes, there is heat, there will BE some heat and it really isn’t unusual… for these reasons…picking up where we left off:

Just running with that and thinking out loud, with a 4500 rpm cutterhead and 3" OD cutterhead pulley (2.85" pitch diameter), I get around 3350 ft/min belt speed. If it takes a 5 lb pull on the belts to move the system at a steady speed, that's about .5 hp [375W or 1270 BTU/hr] lost in the belts and bearings (mostly the belts, assuming ball bearings). Scale that up or down if 5 lb isn't right, but that's a pretty big power loss running under no-load conditions (1/10th of the 5 hp I think that machine's motor is rated for).

Not surprising, as there are three A-section belts in there, but pretty big nonetheless. Additional power is lost through air movement and sound generation, but I suspect that's minor. But losing 1/2 hp (more or less) to heat in the drive should make for a noticeable increase in temperature, and with little skin area on those sheaves, they'll have a hard time shedding that heat, at least until they're hot enough to reach equilibrium.

Power loss in the belts may actually be higher at high speeds due to the viscoelasticity of synthetic rubbers, where the faster you deform it, the harder it resists deformation, so at high speeds it's harder to flex, and therefore wastes more energy. But for sure, the bend/unbend events are very fast, and with a 30" pulley to pulley center distance (eyeball estimate), each belt bends and straightens over 1000 times per minute (twice per round trip). Cogged AX-series belts won't heat as much, and serpentine belts far less still (they're so thin, there's little stretching/compressing of the 'rubber'). But the sliding of the vee into and out of the grooves will still be there for all belts, so some heat is generated there too, no matter what.

A possible resolution would be to reduce the belt tension a little. As long as they don't slip under heavy cutting, there's no harm, but frying the belts if the cutterhead stalls wouldn't be a good thing. I'd just run them as-is, and if they need replacing some day, use AX belts for the higher efficiency and lower heat generation.

The bottom line is that heat is going to happen. How much is too much? If the belts start melting – yep, that’s too much.

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

V4.19 - V-Belts and heat

As with most writers, it is sometimes difficult to come up with a subject for a column. What I have found that seems to be working is to just stay in touch with my friends on the internet forums and use their discussions as a basis for my next column… it seems to be working out fairly well. This week’s column fits in real nice because I was once the Technical Service Manager for the group that owns Powermatic. So, let’s start with the issue and then we will determine the resolution.

The question from a poster was: “I have a Powermatic PM15 planer with 3 drive belts. The belts seem to run very hot. The pulleys on the cutter head and motor get hot enough that you can’t hold your hand to them. The pulleys are aligned within 1/16 and are tensioned with about 1/2 inch deflection, as per the manual. I called Powermatic and talked to a tech and he said it was normal for the belts to run hot. Anybody have any input on this? This is an older used machine, not a new one. I would appreciate any ideas on this, thanks”

'Too hot to touch' is rather meaningless when it comes to stuff like this, but it's used all the time. I say “meaningless” because what might be ‘too hot to touch’ for Bob, might only feel mildly warm to Sam. Without actually giving a measurement, who really knows? Yet, people say that to describe something that they perceive to be a problem. I was curious as to just how much heat is generated by a v-belt, as I'd never actually looked into it. I couldn't find any data on it from the major belt manufacturers, so the physics of 'how hard is the pull, and how fast is it going' seems to give a reasonable answer and it seems to be in the general neighborhood of what you'd expect, at around 10% power overhead loss from friction and flexing, give or take, though 10% would depend on how big the motor is, and the belts and pulleys don't know that. They just generate heat when running at speed and when under load.

Just running with that and thinking out loud, with a 4500 rpm cutterhead and 3" OD cutterhead pulley (2.85" pitch diameter), I get around 3350 ft/min belt speed. If it takes a 5 lb pull on the belts to move the system at a steady speed, that's about .5 hp [375W or 1270 BTU/hr] lost in the belts and bearings (mostly the belts, assuming ball bearings). Scale that up or down if 5 lb isn't right, but that's a pretty big power loss running under no-load conditions (1/10th of the 5 hp I think that machine's motor is rated for).

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

V4.18 - Electric motors Part 4

We left off while talking about working a motor pretty hard...

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, or run it at the higher voltage. With twice the voltage and half the current, you get 1/4 the percentage of voltage drop through the same supply wiring, all other things being equal. With the torque curve sagging as the square of the voltage ratio, you get what amounts to 4 times greater torque curve stiffness compared to the lower voltage.

You can see that in the reduction in start-up time that folks often report, and in the apparent increase in power. It's not actually an increase in power; it's more like removing inhibitors to the motor operating along its design curve. It's a bit like running an engine at high altitude for a long time, where it's wimpy but you're used to it, then bringing it down to sea level and being amazed at how much power it has. It's not that it somehow gained power - it's just operating how it's designed to operate. If you have a continuous, light load on it, you wouldn't notice a difference, and it wouldn't matter, since it can drive the load without strain even at high altitudes. But push it as hard as it can go, and it won't go as hard at high altitudes, and may load up and start overheating.*** (*** My friend has to allow for the thin air with diesel hydraulic power units that he designs for use in Mexico City. Unless they're turbo-charged, where the high altitude torque rating at speed doesn't change much, which is one reason they use them. Same with some piston airplane engines.) Drop the altitude, and suddenly it seems to be a new engine. It's not, of course, but you're not choking it any more, much like supplying an induction motor with voltage within its design parameters by fattening up the conductors, or raising the voltage.

So the bottom line is: No, technically an induction motor will not be more efficient at the higher voltage, but the effects of voltage sag in the supply are minimized when operating at the higher of the two voltages. That may or may not be meaningful. Use short wiring of adequate size, and don't push it to nearly stall, and the voltage won't sag that much, nor will you experience any real gains. But it doesn't take much to drag down a big motor at low voltage, and it's a very slippery slope. Running at the higher voltage flattens that slope considerably.

If you stuck with this series, your knowledge of motors just got bigger.

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