potentiometer

I believe this is what your friend meant:

You're supposed to use all three terminals of your potentiometer. You probably connected only two. Your drive's manual should tell you where to connect the third leg.

First of all, it would be nice to have more info on your motor. There are several types of AC motors. But in general : With a potentiometer you change the current that flows through the motor. An AC motor's RPM is NOT determined by it's current, but by the FREQUENCY of the current. So you'll never be able to have the RPM changed by a potentiometer. The magic thing you need is a frequency convertor (wich will be controlled by a potentiometer...). There are several possabilities available on the market.

The RPM of an AC motor is determined by the number of poles of the Stator windings.

It is very difficult to control speed of AC motor by the voltage regulation. Most control circuits of low cost designs only vary the voltage. A potentiometer aka variable resistor is the very crude form of voltage variation.

tijit

Well, yes I know that potentiometers are not used to directly control the speed of a motor . A variable frequency drive (VFD) can have a potentiometer to control its frequency output and thus control the speed. I just assumed that our friend overlooked that bit of detail in his post.

Obviously he's already got it connected and tried it. He didn't say that it blew up so it's obviously not directly connected.

It is probably a single phase induction motor (connected across 2 phases - Phase to Phase to achieve the voltage needed).

They are (used to be at least!) quite difficult to control the speed of, especially without the load as they will try and follow the frequency of the AC., so simple voltage control could prove difficult.

For small home applications a universal motor would be best (as in mains drills etc, easy to control speed!), for industrial or heavy home use, stay with the Induction motor (assuming it is one of these as you did not say!), and use a Variable Frequency control, I believe they are called VFDs.....or similar.

We need better infos from you before we can be more exact, sorry.

You hardy can control the speed of an unloaded motor. The potmeter probably controls a triac circuit which can change the flow angle of the current. If you use a universal motor, that will decrease the available torque so the loaded motor will slow down. In case of an induction motor the situation is more sophisticated: the speed of the motor mainly determined by the frequency. The normal operational point of the motor is on the steepest part of the torque-RPM curve, on the right hand side of the maximal torque point. When you decrease the flow angle, you also decrease the torque but in addition, by the higher presence of harmonics, you push the max.torque point to the direction of lower speed. That means that the right hand side of the torque-RPM curve will be less steep so the speed of the motor will better depend on the actual load and the same load will result a lower speed.

So: first you have to check the triac circuit if it works properly, and you have to load the motor.

There is something that I think it is important to be mentioned,

This motor is in a german tensile testing machine which was made in about 1950s, so all the circuits are electrical and there is no electronic parts in the system,

For a tensile testing machine, the load on the motor is varying according to the degree of the tension in the specimen but the machine must keep constant strain rate (constant motor rotational speed) to maintain constant rate of extending the specimen.

If you look at my post #3, you will see that I mention that there are no real 2 phase motors, either single or 3 generally. Theoretically, there could be more than three, but never 2.....!!!

Can you please explain to us what you mean exactly with regard to 2 (two) phases as it may mean I have to change my mind about something if you are correct!

"the load on the motor is varying according to the degree of the tension in the specimen but the machine must keep constant strain rate (constant motor rotational speed) to maintain constant rate of extending the specimen."

If this is the case, why are you trying to change the speed of the motor?

If the motor is slowing down when it is not supposed to, perhaps the motor needs to be replaced.

If the speed of the AC motor is supposed to be variable then some electronics other than just a potentiometer must be used.

Regardless of whether or not grounding the machine improves the function of the potentiometer, it should be grounded for safety. Your voltage is probably 220 volts or greater which can be very deadly under the right conditions (or should I say wrong conditions). Ground the machine first before messing around with it gets you killed!

You have now received from us at CR4, far better information than you have given us, or should I say, in spite of the information you have given us.

Now would be a good time to either give us more info, if you need more help, or if you now have enough infos, let us know so that we can forget cudgelling our brains further for you.....

Dear all

first I am sorry Andy for making you angry. I just need some advice. The photos in the past two posts were the potentiometer ans the board for the potentiometer. The motor is shown and the way of wiring is also shown.

what i am sure of that

1. it is an AC motor

2. It needs minimum two phase to work (one phase did not start the machine and 3 phase did not make the potentiometer make any variation in motor speed)

Also I need the motor to change its speed because it is a tensile testing machine and every specimen needs a certain speed to be tensioned.

I am not angry, I never was, only a complete moron loses his cool when Blogging, sadly there are enough of those around.....not all on CR4 either if you have a good look at the internet!!

Sometimes I am naughty and point out their failings and they go ballistic!!! thats their problem, if they cannot take a joke, they should never have joined!!

But:-

I am hard and accurate though, as I see no reason not to "call a Spade a Spade!" (Old British proverb), and you then know exactly what is required.

I also have the feeling that you do not read and understand our replies in some way........which was why I said "In spite of the infos you gave us...."

This is still true, we are still missing several important pieces of information!!!

May I point out that as far I am aware (I just looked through again quickly), the following has still NOT been stated!

1) The local voltage in your area of the world, please also include the frequency in your answer.

2) The Voltage stamped on the maker's plate on the motor, in fact a photo of this might have helped us more than what you did give us!!

Thats for starters.

I would also like to point out that the Pot in your question, either runs very hot when in use (it is a fairly high wattage pot though, but still....!) or has at some point burnt out and probably been replaced, look at the panel around the pot!!

Or it has taken up the Tobacco habit!

I would also like to read the markings on the motor connector block, if it has any.

I also mentioned before that there are technically speaking, no 2 phase motors, either single phase or 3 phase only.

I also mentioned that it was possible "you might need to connect a single phase motor across 2 phases", to achieve the correct voltage that the motor requires. This still a single phase motor. I believe I gave examples. You have not mentioned this at all!!! WHY NOT? IT IS VERY, VERY IMPORTANT!!!!

Guessing only, the colors shown on the motor connector, could be 3 phase.....this is ONLY a guess......!!!!

Also you mention connecting for testing as a 1) single phase, 2)"double phase"!! and 3)as 3 phase. HOW????

Would you please explain this in the smallest of detail please......normally something would have been burnt up by now I feel!!!!!!!! You cannot usually connect in anyone of three ways to anything!!! I am flabbergasted!!!

The dangers you are exposing yourself to with this manner of working could mean that you may remove your Genes from the world's Gene Pool suddenly......as I have the feeling that you are not qualified for such work......and this cannot be a terribly difficult problem for anyone who has the proper training! This is most probably a fact, if you get upset when I call a Spade a Spade, you are not thinking straight either!!!

If I am right, please be so kind as to stop working on this electrical equipment asap and get a qualified person there. Watch over his shoulder only......

Please goto:- http://darwinawards.com/

Then you may better understand the dangers you are exposing yourself to......and have a Macabre laugh at the same time!!!

My qualifications are that I was a RN Apprentice and I worked on supply & Generation on many types of NATO warships at both 220volt DC and 440Volt AC 3 Phase 60Hz.

I have repaired Generators, switchgear (sometimes "LIVE") as well as ancillary equipment.....I have had my share of shocks too, but luckily only once from 440AC, DC shocks are nothing so long as no arc is also present, we just shrugged them off.....if anyone is interested, I could explain in detail why that is so.....but please do not try it out!!

After a 440AC shock and if you are still alive, you need at least 30 minutes till your brain starts running the normal program again, awful.....!!!

Its as though you were sitting in the palm of a Giant and he decided to clap his hands.

My shock was from both hands, down the each side of my body and out through my sweaty overalls and my rear end, into a huge metal pipe I was sitting on. I nearly fell the 15 feet into the bilge, but both my legs were jammed between the side of the ship and the pipe, it saved me...

It did not pas through my heart area......luckily for me.

From Hand to hand shock, you are usually dead, but could be revived if they get to you quickly. The same goes for R hand to L Leg, or the opposite, that crosses the heart area. Even having the shock I had and a weak heart......BINGO!!!

"Or it has taken up the Tobacco habit!" Funny...

Just to muck things up a bit, there ARE indeed such things as 2 phase motors. 2 phase power is still in use in some really old industrial areas of the US, primarily the oldest sections of Philadelphia. Beyond there is it extremely rare however and I seriously doubt this is a 2 phase machine as well.

Generally though, potentiometer control directly to a motor winding is not usually a valid method of controlling speed, but if the motor is specailly designed for it, it can be a method of controlling torque. If it is a high slip motor, often used for shaker table applications, it could provide some limited torque control capability.

Now, to lend my voice to this situation however, you can get better help from us by posting a clear picture of the motor nameplate, a clear description of what your incoming power is (ask your utility if you are not sure) and if you can, maybe a closer picture of that control box. There is probably more to that box than you may realize. A schematic drawing would be great if you have it.

I am fully prepared to be wrong as one always should in this life (especially on CR4!!), but I had a look at the web and mostly the2 phase motors in question are stepper motors.......could it be that our friend has a 2 phase stepper motor!!

This would fit in both with the type of control he needs and the fact that it is quite easy electronically to produce the necessary phases.....probably using a pot as well!!

Also, stepper motors have been around for at least 50 years, so that would also fit in with the age of the machine!!!

A true 2 phase induction motor would appear to be a single phase, capacitive start motor, without the Cap!! Eg, two separate phases.

The possibility that he has such a specialized system is to my mind, almost 99% certain to not be such an animal!! He would also need a generator of a very special nature too!!

If I read the Runes correctly at this point, we need a whole lot more valid info (as requested by several people) before we can progress.....so Mr Blogger, please deliver!!