trying to understand 3 phase motor/capacitor installation

[qaz]

I'm a qualified mechanic but always tackled small electrical jobs without incident. wonder if i can get some advice re a 60's potters wheel.

it consists of :
-a 220-250 volt 3 phase, c/s 50, motor
-an largish oval capacitor [with a resistor between the 2 contacts] some distance away from motor. A 2 core cable runs from the switch box directly to the capacitor.
-a switch box on outside with 2 buttons, one for on and the other i assume is reset or off.

when switched on it runs extremely slowly for 10 seconds then stops, if you wait for a minute and press what i assume is the reset button, it will start again.

during the 10 seconds it runs you can stop it with one finger at which point it reverses direction.

any help in understanding this installation and why its not working would be much appreciated
m

 

[westward10]

A few pics would help especially the motor rating plate.

 

[Pete999]

Are you sure it's a 3 phase motor, from your description it sound like a single phase job can't be sure though without a pic of the rating plate.

 

[qaz]

here you are
thanks

 

[westward10]

Where do the wires from the capacitor go to.

 

[qaz]

directly into the switch box , no where else

 

[westward10]

Is it running from a single phase supply by any chance. Be interesting to see inside the box if you can take a pic with the power OFF.

 

[telectrix]

i think that's a 3 phase motor. 415V in star, 230V in delta

 

[qaz]

yes we are 240 volts. what does 415V in star mean , pls
will get pic today

 

[Pete999]

i think that's a 3 phase motor. 415V in star, 230V in delta

Agree just read the rating plate OP never mentioned the 380 Volt bit

 

[Pete999]

Is it running from a single phase supply by any chance. Be interesting to see inside the box if you can take a pic with the power OFF.

3/4 HP 0n 3 Phase does sound OTT Like Westward said a pic of the inside of the box, or better still a pic of the motor connections. BUT MAKE SURE THE MOTOR IS ISOLATED PRIOR TO TAKING ANY COVERS OFF
If it is SP then the symptoms suggest either a failed start or run capacitor, or a faulty Centrifugal switch, may need to call a Sparky who is familiar with Motors and the associated control gear.

 

[westward10]

yes we are 240 volts. what does 415V in star mean , pls
will get pic today

It means it will run with 240v in delta connection and 415v in star connection but both supplies need to be three phase. It is possible someone has fitted the capacitor in an attempt to make it run from 240v single phase, possibly.

 

[qaz]

if u still need pic of motor connection just ask. im not familiar with short versions of terms, if u all mind would use whole words if poss,.
thanx

 

[qaz]

It means it will run with 240v in delta connection and 415v in star connection but both supplies need to be three phase. It is possible someone has fitted the capacitor in an attempt to make it run from 240v single phase, possibly.

thanx for clarification . previous owner said that it just stopped working one day

 

[Pete999]

It means it will run with 240v in delta connection and 415v in star connection but both supplies need to be three phase. It is possible someone has fitted the capacitor in an attempt to make it run from 240v single phase, possibly.

Either way get a pro to help.

 

[qaz]

inside cover plate

 

[qaz]

Either way get a pro to help.

wheel is of limited value, i will persist a bit longer if people here can spare more of their valuable time

 

[Pete999]

wheel is of limited value, i will persist a bit longer if people here can spare more of their valuable time

What ever floats yer boat

 

[westward10]

Assuming the green wire was connected under the terminal cover how many more wires are in that cable.

 

[qaz]

so to re cap its a 3 phase more running from a 1 phase supply, so, it must have been converted, hence the capacitor. is this pretty certain from what people here can see?

and if it is are thee any pointers as to what may be the fault, i may investigate using a single phase motor
thanx

 

[qaz]

Assuming the green wire was connected under the terminal cover how many more wires are in that cable.

yes it was. 3 more, red, blue , white.

will be back i bit later...its a sunny day here

 

[Pete999]

yes it was. 3 more, red, blue , white.

will be back i bit later...its a sunny day here

As it is in sunny N/ton

 

[westward10]

The problem is the whole set-up is somewhat dangerous and Heath Robinson. It is possible it has never functioned correctly or could be a duff/incorrect capacitor. Either way getting a three phase motor to run from single phase is never practical as the motor will never run efficiently.

 

[PEG]

Every time i have a potters wheel problem,a long haired ghost from the eighties,turns up,behind me....right weird...

 

[qaz]

The problem is the whole set-up is somewhat dangerous and Heath Robinson. It is possible it has never functioned correctly or could be a duff/incorrect capacitor. Either way getting a three phase motor to run from single phase is never practical as the motor will never run efficiently.

yes i accept that, thankyou . sorry to re post this question but is the following correct assumption on my part: its a 3 phase more running from a 1 phase supply, so, it must have been converted, hence the capacitor. is this pretty certain from what people here can see?
thanx

 

[westward10]

It is difficult to say without physically being there but the inclusion of a capacitor which is not required for a three phase squirrel cage induction motor suggests an attempt to run it from a single phase source.

 

[sinewove]

Are you sure it's a 3 phase motor, from your description it sound like a single phase job can't be sure though without a pic of the rating plate.

I'm a qualified mechanic but always tackled small electrical jobs without incident. wonder if i can get some advice re a 60's potters wheel.

it consists of :
-a 220-250 volt 3 phase, c/s 50, motor
-an largish oval capacitor [with a resistor between the 2 contacts] some distance away from motor. A 2 core cable runs from the switch box directly to the capacitor.
-a switch box on outside with 2 buttons, one for on and the other i assume is reset or off.

when switched on it runs extremely slowly for 10 seconds then stops, if you wait for a minute and press what i assume is the reset button, it will start again.

during the 10 seconds it runs you can stop it with one finger at which point it reverses direction.

any help in understanding this installation and why its not working would be much appreciated
m

According to my motor textbook,the capacitor for a 1 to 3 phase conversion on a 3/4 h motor is 30mfarad.It may be worth fitting a new capacitor if you feel confident.
Regards.S

 

[qaz]

According to my motor textbook,the capacitor for a 1 to 3 phase conversion on a 3/4 h motor is 30mfarad.It may be worth fitting a new capacitor if you feel confident.

thanks , i will make note of the suggested capacitor but agree it may be a botch, however it apparently ran well for many years on single phase . it was built in the 60's not 80's as Peg mentions in his joke, which i can't fathom

providing i can find a single phase motor that physically fits is the rest straight forward ? [assuming i know something about electrical systems] Regards.S

 

[Deleted member 26818]

In the photos showing the contactor connections are some data plates.
Can’t see them too clearly, but in one, you can clearly see 240V.

 

[qaz]

In the photos showing the contactor connections are some data plates.
Can’t see them too clearly, but in one, you can clearly see 240V.

is thaT GOOD?

 

[qaz]

i appreciate it may be a botch, however it apparently ran well for many years, so i may try to fix it , but im also very interested in fitting a single phase motor, is that straightforward?

 

[marconi]

See if you can trace the wiring to come up with a diagram which looks like the star one on the right (Steinmetz circuit):

 

[sinewove]

is thaT GOOD?

As others have said,the 3 phase motor must be connected in delta to work off 240v.The capacitor produces another phase.Live and neutral go to 2 motor terminals and the capacitor goes between live and the other motor terminal.Nowhere near as efficient as running on true 3 phase.
Regards,S

 

[westward10]

It doesn't produce another phase it induces a shift in phase to one of the windings effectively kick starting the motor.

 

[Deleted member 26818]

I think it would be an idea to have a look at the other data plates which are partly shown in the photos of the contactor terminations.

 

[qaz]

I think it would be an idea to have a look at the other data plates which are partly shown in the photos of the contactor terminations.

thats interesting, am starting to see the complexity of it. are the data plates the ones inside the plastic box, i couldn't see the mention of "240v"
am starting now to see where that "centrifugal switch" might come in

 

[Charlie_]

Does the motor spin freely, no bearing issues?
Do you know how to test a capacitor?

 

[qaz]

yes its free and bearing ok. i would luv to know how to test the capacitor.

i'm wondering if theres a clue in the fact [see first post] that the motor runs extremely slowly and for 10 seconds, at switch on [with no torque]

something I'm interested to know is why , in the first place would they use a 3 phase motor?. wheel probably built early 60's

 

[Charlie_]

Before my time but perhaps at that time most of the motors were manufactured the same but their individual control methods varied depending on application..or it could just be an hadoc build from a very resourceful electrician.

Few ways to test the capacitor;
Digital multi meter on the capacitor setting.. I don’t like this method

Do you have an analogue multi meter (not digital, type with a needle)

Set it to ohms and apply probes to corresponding terminals on capacitor, needle will move up high and then drop. If nothing happens then it’s not charging..

Other way is to get say a 9v battery, apply to the capacitor and charge it up.
Set meter to dc V and the reading should be about 9v and then it will discharge quickly through the multi meter

 

[PEG]

Has this machine got,or has it had,a speed control pedal?

I would be tempted to convert to single phase,as this is your supply,and add a pedal....

If i can remember correctly,from Mrs. Furness's pottery class...a speed control pedal,and a clay shaving tool,scraped on the side of the wheel...makes it sound like you are driving a formula one car...
Add a bit of exaggerated Murray Walker commentary...and voila! a trip to the headmaster's office...

 

[marconi]

Re qaz #38: 'why would they use a three phase induction motor in the first place?'

I think a possible reason might be found in the torque-speed plot below, but first study the plot for the three phase motor supplied by a Steinmetz circuit at my #32 - as the rotational speed reduces so does the torque.

For a single phase induction motor, as the speed reduces from normal running speed the torque reduces until the centrifugal switch closes whereupon the torque jumps up because it is boosted by the auxiliary winding. I am not a potter but I suppose this is undesirable - the potter would prefer to control the speed with the torque he applies with his hands and without the jump up in torque ever happening.

 

[qaz]

Before my time but perhaps at that time most of the motors were manufactured the same but their individual control methods varied depending on application..or it could just be an hadoc build from a very resourceful electrician.

Few ways to test the capacitor;
Digital multi meter on the capacitor setting.. I don’t like this method

Do you have an analogue multi meter (not digital, type with a needle)

Set it to ohms and apply probes to corresponding terminals on capacitor, needle will move up high and then drop. If nothing happens then it’s not charging..

Other way is to get say a 9v battery, apply to the capacitor and charge it up.
Set meter to dc V and the reading should be about 9v and then it will discharge quickly through the multi meter

right, i'll test with an analogue meter today and post results. to answer another question the speed control is via a rotating cone , so no pedal

 

[qaz]

[ I am not a potter but I suppose this is undesirable - the potter would prefer to control the speed with the torque he applies with his hands and without the jump up in torque ever happening.

not quite like that, unless i'm mis reading you. the potter needs continuous, smooth torque , but yes, certainly a jump in torque would not be good[/QUOTE]

 

[qaz]

I think it would be an idea to have a look at the other data plates which are partly shown in the photos of the contactor terminations.

i cant find that photo, which is it pls

 

[qaz]

update: i foolishly tested the capacitor with motor off but still plugged in, i got a flash at the probes

the capacitor tested ok with analogize meter

for a couple of attempts the motor behaved as before i.e. running for 10 secs then off. then to my surprise it ran continuously but still at the extremely slow speed exactly like before. it also smokes quite nicely as it runs [it may have smoked before but not for long enough the notice]
thanks in advance for any more input

 

[James]

it looks like something is missing from the L2 connection on the top of main contactor. could you have a 3 phase motor being used with a single phase supply incorporating a start capacitor, a run capacitor and a centrifugal switch to change from start to run?
if so then it looks like the run cap. is missing.

 

[qaz]

the short answer is "i dont know". i get the jist [ish] of what u say but i'm not familiar with these matters. i had a look round and cant see where another capacitor might have been.

where is the contactor and centrifugal switch pls
thanx

 

[Pete999]

it looks like something is missing from the L2 connection on the top of main contactor. could you have a 3 phase motor being used with a single phase supply incorporating a start capacitor, a run capacitor and a centrifugal switch to change from start to run?
if so then it looks like the run cap. is missing.

I gave you a disagree James, solely because I seriously doubt any self respecting Electrician that professes to understand how a poly phase motor works would attempt to stick 230 V on said Machine. a Poly phase motor does not incorporate a start and run winding or a Centrifugal switch on it's shaft, why would it, a Polyphase motor. incorporates DOL, S/D, there are some modern soft starting method which I have no experience of, or in some cases a wound Rotor to aid the initial starting.
Types of Single Phase Induction Motors | Single Phase Induction Motor Wiring Diagram - Electrical A2Z - http://electricala2z.com/motors-control/types-single-phase-induction-motors-single-phase-induction-motor-wiring-diagram/

 

[me2]

It looks to me that the capacitor has been used to create a 2nd phase with a delay of 90degrees I have seen in the past a few times . If faced with this problem I would typically suggest to the customer to replace the capacitor setup with a Variable speed drive with a single phase input and 3 phases output . I would before hand carry out a full motor test to see if motor can cope with the higher frequency switching from the drive.
If insulation test is quite low I wouldn't bother.
Hope this help. But realistically if you are unsure please pass this job on to an electrician . safety is Paramount.

 

[Richard Williams]

The capacitor has died, on startup it stores a charge which looks like a low voltage on one set of windings.
That makes sure the motor runs in one direction only at startup.
The resistor (looks about 47 meg ohms) is there to drain the charge when the motor is not running.
Make sure to replace it