31st Jul 2009, 09:56 AM #1
Can any electric motor gurus help with a wiring question?
I have a half horse single phase clutch motor (ex sewing machine) that I'd like to use and I am trying to sort out the wiring.
Ho Hsing Machinery Co, Taiwan. Type STK-3. 4 Pole.
It's wired with a five-core, double insulated main cable and a figure 8 (dual wire) cable as well. A single, 6 uF cap is also present.
White and grey of the main cable have 6.2 ohms between them, while the cap is wired in series with 30 ohms across the red and black conductors. The fifth conductor is earth.
The figure 8 cable has no marking to differentiate one conductor from the other and a 0 ohm short exists between the conductors. The conductor diameter is identical to the main cable at ~1.2 mm.
Disassembly reveals a squirrel cage rotor. There was no apparent centrifugal switch (is it likely to be buried inside the rotor?), so I'm not sure how to treat it. With a single capacitor, I'm guessing it is more likely to be cap-start (start winding different gauge and number of turns to run winding) than permanently split (identical gauge and number of turns) as there are two different resistance values...
I have been tempted to just connect the R and series RC neworks in parallel to 240 v A + N and see what happens, but since I didnt find a cut-out switch, I'm a bit concerned about burning out the start winding.
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2nd Aug 2009, 02:30 PM #2
If you can't find a centrigual switch, it's a Permanently Split motor. The different resistance of one winding may indicate damage to the winding...all the more reason for an insulation test.
You can test "winding to winding" but if the insulation is already damaged, this test may cause even more damage.
If it's an old motor, your best option is to take it to a motor rewinder & have them test it for you.“I do not think there is any thrill that can go through the human heart like that felt by the inventor as he sees some creation of the brain unfolding to success... Such emotions make a man forget food, sleep, friends, love, everything.” - Nikola Tesla.
5th Aug 2009, 01:16 AM #3
Hi from Amsterdam,
the windings between the white and grey lead, with the resistance of 6,2 Ohms, probably constitute the main coil group. A thicker wire gauge often goes together with lower resistance in the case of a capacitor type induction motor.
If i were to test this, i would use an Amp meter and my ears and my nose. A single phase induction motor is able to keep running on its main coils only, as soon as it is helped on its way in one direction. So i would personally connect the white and grey lead to an extension cord with a plug (with the Amp meter connected in series), give the rotor a whirl and plug the motor in. Be sure though, to be able to unplug it within a second, if necessary. You have only seconds to judge the next feats and to act upon them.
The sound should be the usual whirr without too much of an AC hum, the current at no load is supposed to be between 1/4 and 1/3 the nominal amount under load (as stated on the type plate). Your nose kept close to the cooling air vents should immediately detect the typical smell of heated air. It is not yet a burning smell, but the typical smell given of by heating elements or a heated linen iron. In this is the case, you have chosen the auxiliary coils, which will heat up very quickly if connected directly to the mains. The sound will also be different; the whirr will be more silent and the buzzing part of the sound will be louder. Pull the plug immediately to avoid damage.
If there is no heat smell you can carry on, check how quickly the motor heats up when run in this way for a while. It should take minutes before any warming up can be felt on its housing exterior. The temperature rise is usually mentioned on the type plate. With no load, this value will not be reached for at least 5 minutes, if ever at all. Keep checking the Amp intake, it should not increase.
The auxiliary windings are often thinner than the main windings, only when the induction motor is wound in three phase mode with a center star point and the capacitor connected in "Steinmetz"-mode, all windings have the same gauge.
Lacking a centrifugal switch, the motor will probably be a typical "capacitor motor", meaning that the capacitor permanently feeds the auxiliary coils (unless there is some sort of starting relay involved). In the case of a cont.cap motor the aux coils are not mere starting coils, but are part of the permanent running system to improve efficiency and torque. You can double check by reading the print on the cap. Since professional workshop sewing machine motors run all day (and sometimes at night as well, in shifts), the capacitor is bound to display something like "continuous duty". If not (like "1.7% duty cycle"), the cap is not for continuous use and there is a component in the motor missing (like a start-up relay; it doesn't necessarily have to be a built-in centrifugal switch).
If i am still right so far, than the capacitor is meant to be connected in series with the aux coils between the red and black lead; you can connect one of the cap's terminal to either the red or the black lead. This whole assembly of aux coils plus cap is connected in parallel with the white and grey lead. If you were to switch the red and black wire, you could choose the sense of direction in which way the motor runs. But before you do that, closely examine the clutch system, there may be threaded axle components involved that may loosen themselves and get damaged, if the motor spins the wrong way. But there may also be a printed arrow on the motor to point out which way it is meant to turn, so you're all right when it does so conform that arrow. A motor with an arrow is usually meant for specific use in one running direction only.
80 microFarad is enough to feed windings that must produce a power output of about 1000 Watts, for an AC frequency of 50 Hertz. Fed by 6 microFarad, the auxiliary windings will deliver 75 Watts, which is about right for a motor of 1/4 to 1/3 HP (300 to 550 Watts total intake).
If your total line-up is ready, check if the motor starts up at its own accord. It should do so quickly and smoothly, the sound must be clean without too much of a hum, and the acceleration must display clear strength. A sluggish start indicates a fault. Keep checking the current intake and stay alert for odd sound and smells and heat. If the motor is under nominal load (you will know this when the Amp intake equals the value as stated on the type plate), the rpm loss should be between 5 to 10% max. If the rpm drop is more severe, then there is still something wrong with the balance between torque and current consumption. Winding damage may have occurred at some point, hopefully not during these trials.
There aren't many scenarios for things going wrong. You either pick the main coils right the first time, or you pick the aux coils by mistake. The motor can run on its main coils very long, and its aux coils will fry within 10 seconds. When you are able to avoid the latter, you have a high chance of succeeding.
It is logical for an industrial sewing machine to choose an induction motor with a permanently connected capacitor instead of one with a pure start-up winding and a short term cap and centrifugal switch. Sewing machines start up under zero load, but experience duty cycles of engaged load (clutch to the machine drive) during continuous running. A permanently connected auxiliary winding is the most energy efficient solution for this purpose.
If you are not entirely sure about this and are only willing to take the least amount of risk, take apart the motor again. I read that you've already done so in the past, so that should be no problem. Follow the coloured leads as best as you can and watch where they enter the copper wire coil sets. You may succeed in deducting if a certain lead disappears into a coil of thick or thin wire. You only have to be sure about one colour, since the Ohm meter already told you what colours go together. If a red or a black wire seems to belong to a coil with thin wire, then you are automatically sure about the white and the grey lead. There are only two sorts of coils involved, there's a 50% chance.
Lots of success!
6th Aug 2009, 02:12 AM #4
some additional info on the post right above here.
The pic shows a typical single phase induction motor stator with auxiliary windings. These are easily recognisable by their thinner copper wire and smaller size. If the amount of windings in the aux coils are considerably less than those in the main coils, then they are short peak duty coils, meaning they have to be switched off immediately after start-up by a relay or a centrifugal switch. These coils are particularly vulnerable when connected directly to the mains power, they will burn within seconds. Aux coils permanently connected to a capacitor have more windings, their bulk can be 1/3 to 1/2 the size of the main coils.
The Steinmetz connection mode is meant to operate "3 phase wound" motors on one phase mains power. Because a true 3 phase motor has very poor torque and high losses when running only one phase (up to 66% power loss), operating a true 400 Volts motor on 230 Volts single phase must be regarded as a makeshift emergency measure only. The delta mode combined with a capacitor is the reason for the poor efficiency.
There are many single phase induction motors around, wound in a "3 phase way" but being true single phase motors nonetheless. This is purely done because of several advantages. There is only one gauge of copper wire needed, which is the same for all three coil groups. The winding process and the filling of the insulated stator iron slots are much easier to perform and the terminal connections are always easy. In a stator with coils made from thinner wire, it is detrimental to mistake the aux coils for the main coils. With all coils being the same, it doesn't matter which coil is used as main coil or as aux coil. So the Steinmetz connection mode is rather used to build cheaper motors, by adapting the cheaper 3 phase winding layout for one phase operation.
Important: in the post above i mentioned a capacitor value of 80 microFarad for every kiloWatt of auxiliary coil power. I checked the text books to be sure and it turned out to be 70 microFarad! Apologies for this mistake, it goes to show that it is good to check that all figures stored in your memory do not deteriorate or change with age!
All the best
6th Aug 2009, 11:21 AM #5
Thanks for your suggestion elkangorito, but I've since paid more attention to the stator windings and there are two distinctly different wire diameters so it can't be permanently split.
gerhard. What can I say besides thankyou. Responses such as yours are what I really appreciate in forums. Helpful, informative and of benefit to others, not just me. Excellent stuff.
As mentioned above, I took a closer look at Stator windings.jpg to find different wire gauges. What I have found is that the motor will run but not start with white and grey (low resistance, main coil) energised. The series RC network of aux windings and 6 u cap is wired within the loom (the black and yellow wires on the RHS of Stator windings.jpg are from the cap terminals) but terminate in the red and black wires in the mains cable as mentioned in the first post. I switched the aux coil into the main and found that the motor appears happy to start and run (for a couple of minutes at least, before I got bored and turned it off) but a vibration starts up if the aux is switched out when running.
Motor plate.jpg, cap.jpg and the Disassembly_x.jpg photos may also be of interest. To clarify "You can double check by reading the print on the cap.", cap.jpg (and Disassembly_x.jpg) was an initial photo I took with my phone, so I apologise for the poor quality, but it reads:
I haven't yet done load, direction change (reversing aux 'polarity') or current draw tests.
Would you suggest that I am safe to use this thing with the aux and main windings permanently connected?
Thanks again for your excellent contribution.
6th Aug 2009, 08:45 PM #6
your photos are excellent and informative. The presence of two different wire gauges has been established, this is no Steinmetz method three phase wound motor. The coil group between white and grey and measuring 6.2 Ohms, has been established to be the main coil group. The motor can run for minutes on it without overheating, which is another fact that would confirm it. The capacitor has a black and yellow lead and has no printed indication that hints towards a limited time of use; the + 10% and -5% are merely manufacturing precision tolerances, the capacity may vary between 6.6 and 5.7 microFarad. The term "capacitor motor" on the type plate is usually chosen for motors running continuously on their single capacitor, which doubles as start-up help and eficiency and torque improver during running. There is no trace of any centrifugal switch or relay components, which would further lead to suggest that the capacitor is meant for continuous use. The size of the aux coils is more than half the bulk of the main coils, which is often the case for aux coils that are continuously powered during running. In my post above, in the pic left from the Steinmetz schematics, the aux coils are relatively puny in comparison. These are typical short peak action coils, which must be disconnected immediately after start-up, since their sparing layout will not survive powering for more than a few seconds.
There's one thing remaining that got me puzzled. You mentioned that the red and black lead were led outside the motor into the 5 core cable, together with the green and white and grey, whereas the yellow and black leads from the capacitor vanish into the motor and are somehow connected inside. Why would they do that? It's illogical. No electric motor on any sewing machine ever reverses, that can be arranged through the mechanism in the sewing machine itself. Half a horse is quite hefty for stitching, these workshop sewing machines are very speedy and heavy duty machines, built to process very thick fabrics, like denim, in a multilayered way. If a stitch needs to change direction or is repeated to and fro (e.g. for buttonholes, i know because my uncle worked in such a workshop), this is arranged by a littel lever, which quickly reverses the transport teeth underneath the fabric. Reversing the entire motor would take too much time, especially bulky induction motors like these are too sluggish and consume too much power while doing so. So why lead the red and black wire outside the motor and leave the yellow and the black ones from the cap inside it? When you measure 30 Ohms between the red and black lead, does that reading remain constant? In that case you have indeed measured the "bare" aux coils without the capacitor connected in series. With the cap in series, your meter's needle would move a bit and fall back to zero soon, after the cap would have filled itself with the few volts from your meter's batteries. Without the cap, the 30 Ohms would remain a constant reading.
Let's assume that this is the case. As you already mentioned yourself, the motor keeps running normally if the white and grey leads are connected to the mains. From your story i deduct that you connected the red and black leads parallel to the white and grey leads and found that the running becomes cumbersome and vibratory. Is that deduction right? If so, i can tell that this is normal behaviour, only with the capacitor connected in series there is the necessary 90 degree phase shift in the aux coils to truly help making the motor's magnetic field a decent rotary magnetic field.
Anyway, for my money the motor should be connected as in the schematic i drew below. I used an imaginary "connection block" with 5 numbered terminals to keep some oversight in the situation. Number 1 speaks for itself, that's just the earth wire.
Nummer 2 and 3 are used for the main coil leads white and grey. In Europe we use different colours, but from what i've gathered is, that in the Pacific region the vintage USA colour codes are still in use. That code mentioned green as earth, white as neutral and black as live. The Japanese have always been very fond of American technology, so they copied most of it, including many of Edison's original values. They still use 100 Volts (and 200 for industrial three phase use, which by now they must regret for all the thick copper strengths required for that) and they also copied the green, white and black. I've seen that in many Hitachi electric motors, which in Europe are used in many lift door and drill press and airco systems. It seems that the Chinese and Taiwanese in their turn copied these Japanese designs, again including these lead colours. So in my schematic i took the liberty to connect the white main coil lead to the mains neutral wire and the grey lead to the black live terminal, which must be about right. Let's assume that the red and black wire are indeed from the aux coils. It is a sure thing that these coils must be connected to the mains in series with a capacitor. This whole lot (aux coils plus cap) is to be connected in parallel with the main coils. So you could screw or clamp the white lead together with the red wire, make sure that the yellow cap wire is connected to the black one rom the aux coils, and that the cap's black one is clamped together with the grey one from the main coil . To reverse rotation, just swap the two aux coil leads on 4 and 5.
How am i doing so far, Elkangorito? I'm just a bloody amateur, you're the pro! Join us again, haha!
Last edited by gerhard; 6th Aug 2009 at 08:55 PM. Reason: Addition "on 4 and 5" in last line
7th Aug 2009, 09:06 AM #7
Hello again, gerhard.
You've summed that up rather well. The only point I would clarify is that junctions 5 and (part of) 3 on your schematic (nice work there, by the way) are inside the motor, bundled up with the enammelled copper to insulated flex connections. See connections.jpg. This made me a bit reluctant to pull it apart for fear of damaging the enamel and causing a short.
I have modified your schematic (schematic_modified.jpg) to show what I mean. The two diagrams are electrically identical, but the modified version shows the actual locations of the various connections.
You are on the right path by suggesting that a multimeter will have trouble with a series RC network. For those unaware, capacitors only pass AC. DC will flow momentarily if the cap is initially discharged, before falling to 0 A when the cap is fully charged. Multimeters measure resistance by passing a small amount of DC through the device under test, so a falling value is to be expected for a resistance measurement across the red and black wires. To remove the effects of the series capacitance, I measured the aux coil across the red lead and the yellow terminal of the cap.
Your comments on a sewing machine motor not needing to reverse seem quite appropriate and I wonder why the manufacturer bothered to bring four wires out of the motor when two would do. Having said that, I'm happy they did in case I want to reverse it at some stage in the future.
And for one last question: can anybody think of why the grey fig 8 cable, seen mostly out of focus in connections.jpg crossing over the yellow and black cap leads would have been included in a sewing machine? It disappears into the heatshrinked connector bundle and has 0 ohms between the conductors.
Thanks again, gerhard.
7th Aug 2009, 08:06 PM #8
Perhaps the grey fig 8 cable is connected to a thermal switch embedded in the motor windings. This should read 0 ohms under normal conditions and open circuit if the motor overheats.
Last edited by Chas; 8th Aug 2009 at 07:50 AM. Reason: Correction
7th Aug 2009, 08:59 PM #9
Interesting idea. I assume you mean open circuit on overheat. (Zero ohms being a short circuit)
10th Aug 2009, 10:14 PM #10
yes, the way you drew it is very logical. Well, it would seem it's about settled now.
I can only think of two reasons for the red and black wires to be led out of the motor. One reason could be that these sewing machines are also made in left hand use versions. Everything would be a mirror image and with the motor hanging underneath in mirror fashion, it would need to rotate the other way.
The other reason may be, that the aux coils are short term start up windings after all, which -judging from their bulky size, the continuous duty cap and the specific term "capacitor motor"- i think they're not. The amount of heating up after several minutes of running under load, will sort that out soon. But i saw one of my old induction motor driven orbital sanders last weekend, and these have those peculiar double switches. The toggle lever really consists of two parts instead of one. When flipping the switch, both parts are flipped "on" and activate their own contact mechanisms underneath. But one of the lever parts is designed to stay in the flipped position and the other part is designed to flip back immediately by a spring, when let loose. Imagine the unsprung part of the switch to be connected to the main coils and the springing-back part to be conected to the aux coils. There is your short term "automatic" switching mechanism; no centrifugal system or peak current relay, but the human himself doing the work.
It's not a good picture, but it was the only digital readily in stock, depicting such an orbital sander with double switch. The switch itself is hardly visible, but you can imagine what it must look like. It's a double toggle of which one half springs back upon letting it loose. The blue notice on top states that the switch must be let loose as soon as the motor has reached full speed. This was an Ebay pic from the auction in which i won the sander, it has since been cleaned and restored. Looks smashing, when the polished aluminium really shines again. It's an Elu type MVS17W, made in 1962.
Well, that was just a thought, i couldn't come up with any other reasons. If the motor makes pronounced forced sounds with the aux coils in continuous opration, then they may indeed meant for short term use. A power intake meter reading will confirm this, if it is true.
And hi, Chas,
yes, a thermal switch is a logical idea.
12th Aug 2009, 08:39 PM #11
Thanks again, gerhard.
I plan on installing this motor in a lathe (an old Hercus, similar to the top photo) this weekend. If I had the time, it would be fun to set the clutch up with a foot pedal, but I don't think that's going to happen. The rotating inertia of the chuck would probably prematurely wear out the clutch anyway, so I'll just leave it permanently engaged for now.
I'll keep you posted as to whether it works fine (the original motor was only .25 hp and the lathe will only see light work, so at least there should not be any lack of power) or if the auxillary winding blows up.
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