Thread: capacitors on 3hp motors?

Hi Doug,

lots of variables here, there are many things that could cause to trip the overload switch.

Double capacitor type induction motors were developed for acceleration under full load conditions. The starting torque of single phase induction motors is far from brilliant (a lot worse than the startup behaviour of three phase fed motors), so an additional capacitor was added to improve things a bit. In Europe, where three phase power was already widespread in the 19th century and thus favoured for all heavy or industrial applications, the double capacitor motor was never used, made and sold on a large scale. It is a typical American development by General Electric, Westinghouse, Marathon, Baldor, Century and many other builders.
The Japanese, traditionally impressed by all American technology, had the designs copied by Hitachi, Matsushita, Toshiba and many smaller firms. Other Asian countries again copied those products in their own way. There are only a handful of European firms making them (AEG, SSW, Läuffer, MEZ) and the best known application is a large belt sander for parquet flooring, which has to start up and accelerate with its full weight leaning on the belt.

DC motors have no difficulty in finding a startup direction. Their fields are powered with either magnets or coils and their rotors are wound and fed through the commutator, which arranges magnetic attraction of rotor pole shoes when they get near the field pole shoes and reverse polarity to generate repulsive forces when the rotor pole leaves the field pole again. Startup torque of DC (and AC) brush motors is very good, since magnetic forces are in direct relation to the amount of current through the coils and startup current is higher than running current.

Induction motors have a rotor without external powering. This rotor is a massive iron cylinder with hollow channels running along the outer surface, parallel to the axle in the center. Bars of aluminium are cast inside the hollow channels and on either side there is a ring, connecting the ends of all bars. When you picture the iron mass away, you are left with two rings connected with longitudinal spokes (the bars) and this unit resembles a running cage as used for pet hamsters or squirrels as the play- and exercise gadget in their cages. Thus the name "squirrel cage rotors" was chosen and this has turned into a worldwide acknowledged technical discription. An induction motor must be seen as the secundary part of a transformer, with the stator windings and the field iron making up the primary part. The magnetic field as generated by the stator coils, is absorbed by the system of bars and rings, which makes up groups of short-circuited secundary windings. These secundary windings generate a counteracting magnetic force, which is only possible due to the constant polarity changing of AC. When DC is fed to an induction motor, it won't budge and it is merely an electric brake.

When the rotor is stalled, the magnetic field absorbed by the short circuited secundary rotor windings is largely converted into heat, since the stalling prevents the conversion into mechanical energy. In this situation, the same physical condition exists as in a conventional transmormer with a short-circuited secundary coil. But when the rotor iron is allowed to move, the counteracting magnetic force as generated by the secundary windings is converted into mechanical energy and torque. Three phase motors have thre coil groups in the stator, so there is natural tendency for the rotor to follow the changing of the field strengths as provided by the three sinuswaves. With only a single AC-phase, there is a problem. The field direction in the rotor changes in portions of 180 degrees or half a rotation. The rotor windings hesitate because there is no real given sense of direction. The motor will buzz but needs a push in either direction to start and keep running in that direction.

The first and simplest squirrel cage motors had to be started up by hand. One had to crank up the pulley or had to pull the drive belt a bit to generate some rotation, and then engage the power. Some decades later, auxiliary windings were added in the stator to divide the main poles into large portions for the main primary coils, and smaller or "split" pole portions for the auxiliary coils. By creating a time lag between the peak currents in the main and auxiliary coils, a sense of direction was created for the rotor to follow. One way to create such a time lag, is a capacitor. Current through a coil already flows when the sinus wave has not yet reached its peak, but the charging current of a capacitor increases when the voltage increases, so the current peak through coils fed through a capacitor lags behind the current peak in the directly fed main coils. And presto; there is your sense of startup direction inside a single phase induction motor. Which is only fractional of the starting torque that can be provided by the changing fields in a true rotary pattern, as provided by three phases.

The first auxiliary windings in induction motors were not laid out for continuous use and were rather makeshift. They were either operated by a centrifugal switch or by a special two-part switch, with one part being a regular on-off toggle and the other part a sprung toggle for the aux windings. The mechanical part of a centrifugal switch is mounted on the motor's axle and works like a throttle governor on a steam engine. Weights fling outward when the rpm increases and retract a ring that normally pushes and closes electrical contacts. When the motor is up to speed and the ring fully retracted, the aux windings are automatically shut off from the mains power. The same is achieved (in a way) by the double switch. The two toggles (meant for main and aux coils) are to be moved at the same time, with the main coil switch remaining in the on-position and the aux switch springing back to off position, as soon as the user lets go of the switch. The double switch is the cheapest solution, since the user is also used to compensate for the centrifugal system that had otherwise provided automatic action. In this forum i posted a few pictures of such switches as used on vintage induction motor orbital sanders by Festo and Elu.
With the aux coils switched off, the motor runs on its main field coils only, which is a good recipe for miserable efficiency. It is a typical solution for simple budget "fractional horsepower" (= output below 1 hp) motors, with 1/4 hp and 1/3 hp versions being the most ubiquitous. For a mere 1/4 hp (185 Watts output) these motors can take as much as a greedy 400 to 550 Watt input, which in the 21st centrury is no longer a justifiable value.

The motor with a continuous duty aux winding set sure improved things. But it was costlier to build, since the aux coils had to be more bulky. This system also needed really dependable capacitors, capable of enduring truly continuous rating. This was only possible after sufficient advance of insulation materials (somewhere around the 40's). This motor type uses its aux windings for starting and to increase torque and efficiency during running. A good example is a Miele washing machine motor of this type from the 60's, which is still hiding somewhere in my storage shed. For 1/3 hp ( 250 Watts) it merely takes 410 Watts input, and that's 61% efficient, which is very good for a small induction motor that shows the technical advance of half a century ago.

Your motor type is a combination of both systems above. Is uses a running capacitor plus an additional starting capacitor through a centrifugal switch, to improve starting torque. After having told this lengthy story, it is easier to explain all options that could be wrong with your motor.
The motor could have a single set of aux windings, that is used for efficiency improvement capacitor through the running, with the same ste of aux windings getting the extra starting jolt from the startup capacitor, which is shut off after acceleration. But the motor could also have two sets of aux windings; one generously built set for continuous rating and one makeshift set for short term startup only. To determine this, you will have to look for information on this motor or find out how the wiring ruins inside ther motor runs and exactly what portion is switched on/ogff by the centrifugal switch.
When the startup capacitor is shorted, the motor tries to acclerate on its running capcitiro, which will probalby not suffice. so the motor keeps hanging in its accelartion curve (= it will not reach full rpm), but the startup coil set with the shorted cap will drw a huge current in the mena time. Which means that the overcurrent switch will trip in less than a secind. So there is littel chance of that being the cause.

When the running cap is shorted, there is loss of torque and poor efficiency, with speedy heating of the motor and triggering of the oovercurrent switch after a few seconds. That sounds more like your situation. To determine this, you need to at least loosen one terminal of the running cap and measure across it with an ohmmeter. When the needle moves and slowly goes back to zero, it advertises a small charging current but no serious short circuit. In that case the cap has a good chance of being okay, although testing with a few volts is in a different league as continuous 240 volts duty.
One or more stator coils may also have been damaged. Smell will help you determine that. It begins with the mild smell of merely heated air, up to the sharpness of something smouldering or burning.

To be able to tell you more than this, i would love to see some pictures.

greetings

gerhard

Or, centrifugal switch not operating?

capacitor test?

A Simple Way To Test Capacitors.

Hi Doug

Take the motor to a motor repairer, there is Whites and Phartingtons out your way.

Yes, that's probably the best advise. Taking the thing apart and finding out its connections and the true state that all the components are in, requires expertise. I must have taken more than a hundred electric motors apart through the years (and put them back together in working order again, of course) and i've even rewound some, but in my first years i knew a fraction of what i know now. So if in doubt, there's no harm to have a repair shop look into it.

As for the centrifugal switch, yes, it could fail, but that doesn't happen often. When the mechanism is stuck due to soiling, rust, lack of lubrication or mechnical damage, it remains fixed in one position. Either the weights remain flung outward and the push ring remains retracted, even at standstill, leaving the electrical contacts opened. Or the weights are stuck in the rest position and do not fly outward during rotation, leaving the push ring in permanent contact with the electrical contact strips, keeping them constantly closed.

With the contacts constantly opened, the motor is no longer aided by the extra jolt from the startup capacitor and will have trouble accelerating under load. It may not reach its intended rpm and will draw more current than it nominally would. This will lead to the overcurrent protection switch to trip. Your ears will help you to determine if this is situation is at hand, since the "clack"-sound of the centrifugal switch, which will normally accompany the startup cycle of the motor, is absent. Furthermore, the motor sounds different as before, it runs slower and there will be the typical smell of hot air eminating from the cooling air slits before the protector switch trips.

With the push ring keeping the contact closed at all times, the startup capacitor will remain switched-on permanently for as long the motor itself it switched on. The "clack"-sound will also be missing and you can also hear the ring sliding against the contact strips during rotation, since it is not retracted by the stuck centrifugal weights.
With the second capacitor constantly switched-on, the power intake will be very high and the protector switch will soon cut in. When the extra startup cap feeds an extra startup aux winding (which is not laid out for continuous use but rather a few seconds' worth), the aux winding stands a high chance of getting burned after a few occurences. I guess that's another valid reason for having the motor examined by a pro.

good luck!

gerhard

Thanks for all the responses so far fellow forumites...

Gerhard:
Mate, I am humbled that you took so much time and effort to make the first reply to my post. I have read that post in full four times and different parts of it much more often. You have taught me so much about how single phase motors run and I am in awe of your knowledge. I hope one of the mods post it in the forum library. It is GOLD.

I will probably read that post several more times before I have fully grasped its content.

Bonox:
Thanks for the link to the capacitor check info. I have saved the link and there is a lot of great info there.

Phil Spencer:
I googled Whites and Phartingtons and all it found was your post on the woodwork forums. do you have any other contact info?

Dan:
you could be right, time will tell.


In any case Gerhard has made it painfully obvious that my knowledge gap here is much bigger than I thought so I will probably take Phil's advice at this stage if I can find a Whites and Phartingtons or similar; I am just too busy to muck around with it much more. Can anyone recommend a good, honest, reliable electric motor repairer, preferrably close to Hoppers crossing?

Thanks again to everyone for their help

Originally Posted by doug3030

Thanks for all the responses so far fellow forumites...


Phil Spencer:
I googled Whites and Phartingtons and all it found was your post on the woodwork forums. do you have any other contact info?

Here we go Doug try these

Bob White Electrix
38 Sunliner Drive
Truganina
03 9398 5928

Phartington electrix
Units 13-14, 100-104 Pipe Road
Laverton
03 8368 2881

Dowding & Mills
334 Boundary Road
Laverton North
03 9369 7913

Gday Doug

Just came across this thread and am not sure how you went with your problem, but here's a couple of suggestions.
Buy yourself a multimeter that measures capacitance (microfarads indicated by µF on the display), remove your capacitors one at a time and measure across the 2 terminals. Each capacitor should have it's capacitance printed on the side of it (eg 30µF). If the reading on the multimeter is different by more than +/-10%, then the capacitor is more than likely cactus.
If you have no other need for a multimeter, then don't buy one. Chase down a local lecky and ask him to measure it for you. I'm sure he won't charge you for 2 seconds work?????
By the way, I'm not surprised you were bamboozled by Gerhard's posts! I'm a lecky and I felt like I was back at TAFE, in a daze, listening to my lecturer again.
WAY too much info!!!!!! Sorry Gerhard

Berto

Originally Posted by Berto

Gday Doug

By the way, I'm not surprised you were bamboozled by Gerhard's posts! I'm a lecky and I felt like I was back at TAFE, in a daze, listening to my lecturer again.
WAY too much info!!!!!! Sorry Gerhard

Berto

I don't agree with you, I think Gerhard's description is excellent, almost textbook stuff.
You can never have to much info as to the workings of things if you wish to understand them, although I do admit sometimes it can be heavy going.

Hi all,

haha, thanks for coming to the rescue, Bradford! Berto's comment is outnumbered by the number of people wanting me to be elaborate, but in essence he's right. My texts are way too long and i walk into that trap again and again. Maybe it's because English is not native to me and i don't know the proper jargon for many technical things. That way my sentences grow too long while trying to find the right explanation, and i get entangled in the syntax. When i reread my own yarns after a while, i think "geez, what a pompous ####, that can't be me", and i'm not, really, but i sound like it. So Berto's got a point all right.

He's also right about having the measuring done by an electrician. That's simple and cheap and low risk. But doing it yourself is not so diffcult, even with a simple meter as long as it has a resistance measuring option.

First you must get at the capacitor's terminals and for each cap, decouple one wire (soldering, unscrewing, unplugging, whatever the situation demands). With some part of the system possibly connected in parallel to the cap, any measuring will be too uncertain, so by decoupling one of its wires, you are sure to measure across the cap itself and nothing else. Be sure that the meter has a good battery and set it in the kOhm-setting. Hold the probes against each other and turn the adjustment wheel such that the needle is on the right part of the scale, exactly on the zero line. Now hold the probes against the cap's terminals. The needle will at first swing to the right. That's when the cap charges itself with the voltage level of the meter's battery. After a few second, the voltage level in the cap will reach the same value as that of the battery and the meter wil return to the left part of the scale. If it rests at the "endless" setting (figure 8 sideways), the internal resistance of the cap is so high that there is no leak current whatsoever. My experience is, that such a cap can still be trusted, unless it shows outside traces of damage, like indentations. Theee traces can be the beginning of trouble.
Back to the metering. When the needle point towards a few hunderd KiloOhms, there is a tiny leak that may worsen and grow into a weak spot in the insulation layer, that will eventually short out. Remember that the meter's battery is something between 1.5 and 9 volts (which will cause no short) and that the few hundred volts of the motor's operating condition is another story entirely. When i measure a cap and it shows anything other than "endless", i tend to replace it just to be sure and to avoid possible expensive motor damage.

Should the caps be okay and should you still be puzzled about the blown fuses, remember that the starting current of an induction motor can be 6 times or more that of the nominal running current. So this behaviour may be quite normal for this motor and be merely a bit too much for your fuse board. A true 3 hp motor has an output of 2.2 kW and for that it needs an input of at least 3 kW, which at 240 volts is something like 13 to 15 Amps. At the moment of switch-on and rotor standstill, the peak may very shortly be something like 80 to 100 Amps! The starting cap causes an additional short burst amp flow, adding up more to an impressive total.

Don't take my word for it, watch these sample movies on induction motor startup current on YouTube:

[ame="http://www.youtube.com/watch?v=YnN8vl2U7V4"]http://www.youtube.com/watch?v=YnN8vl2U7V4[/ame]

[ame="http://www.youtube.com/watch?v=hferHPRirXA"]http://www.youtube.com/watch?v=hferHPRirXA[/ame]

Both motors run on 3.3 kV industrial current and drive huge centrifugal fans for highrise building airco purposes. The first example is in the USA and the second in Japan. The fact that these motors have to spin up huge masses doesn't matter; nor does the fact that they are three phase and spun up with automatic star-delta switches. The main issue is, that the needle bangs in redline to the scale's right at rotor standstill, and only returns back to a decent value when near nominal rpm is reached. By which i mean to say: your compressor motor may behave normal after all. But -granted- you can only be sure if you have measured the caps, the coils and the centrifugal switch.

good luck and greetings from a freezing Holland

Gerhard

???? what happened to the links i typed???

Never mind, as an alternative, start up YouTube first, copy/paste one of the codes below into the search window and have YouTube look up the corresponding movies for you to play :

YnN8vl2U7V4
hferHPRirXA

Hi they and good evening to all I have a 3hp. Motor but the markings on the capacitor fade off of the can not any but need to change them what can I do to get the right capacitor it 230 volts 3hp motor carry two cap and its on a Clarke compressor any help will do good.thanks.

Originally Posted by Stonedan

Hi they and good evening to all I have a 3hp. Motor but the markings on the capacitor fade off of the can not any but need to change them what can I do to get the right capacitor it 230 volts 3hp motor carry two cap and its on a Clarke compressor any help will do good.thanks.

My motor had two of these, although I am trying to figure out if these are the correct ones that "should" be in this motor.

Vanguard
BC-378
378-454 MFD
125 VAC 60 Hz 6329612
For Motor Starting Made in U.S.A.

That is what is printed on both capcitors.

My question is this:

Should both be the same. Gerhard, gave a great explanation of theory. But, I need object data in regards to numbers...

Should both capacitors be exactly the same or one (being a start) be the one above and the second (being the 'run'/constantly on when the motor is running) be different (if so then what am I looking for - word for word as printed on the label of a proposed capcitor[s])?

As I have noted on my AC unit the VAC on the capacitors state upto 600 VAC or so, for the pump. Although, technically this is a 220 volt system. So, the VAC is irrelevant. The microfarads is what matters...

I have always been told to think of electricity like water. Technically the microfarads is in regards to flavor of water, per say, 45 microfarads could be considered a cherry flavor and the volts is not relevant like the container you keep the water in (size [volts] or shape [AC/DC]), you still only have cherry flavor (XX microfarads). Minus the wear of the capacitor, which life is about 3-5 years for accuracy of capacitance. Which is why you should check them in that span of time, 5 or so years. Test them as noted above, in previous posts.

But, like my AC unit, one capacitor is a start for the pump (35-55 microfarads), and the second is for the fan (5-7.5 microfarads). The first is on and off as noted by Gerhard, at 3/4 motor speed, while the second is constantly on for the fan motor.

Thanks,
Omar J.

I am a registered nurse by trade and tinker with this stuff because I LOVE IT!

Originally Posted by jacoboomar

. . . . I have always been told to think of electricity like water. Technically the microfarads is in regards to flavor of water, per say, 45 microfarads could be considered a cherry flavor and the volts is not relevant like the container you keep the water in (size [volts] or shape [AC/DC]), you still only have cherry flavor (XX microfarads). . . .. !

Given the level of knowledge displayed in this post I would suggest contacting a qualified electrician about this.

Being a nurse is not going to help if your hands are held paralysed by the high voltage.