trying to understand 3 phase motor/capacitor installation

[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

 

[M4SSO]

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

Either way get a pro to help.

Sounds like you are using a three phase motor on a 240 Volt single phase supply with the motor connected in the delta connection. The capacitor is used to create the effect that a three phase rotating field would create from a three phase supply. (Steinmetz delta connection with a single capacitor)

https://i.stack.Upload the image directly to the thread.com/ME2aD.png

 

[M4SSO]

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

What you are describing is a split phase motor which is a single phase device not three phase, the capacitor in this case is connected to a separate start up winding which is at 90 degrees to the main winding. Split phase motors can be reversed by reversing the connections on the start or main winding in relation to each other.

The motor he has is three phase with a capacitor connected to simulate the third phase on a single phase supply.
In all likelihood it is probably the capacitor that has failed, but he would still need to check the motor thoroughly for possible winding faults eg short or open circuit.

 

[ex 2 Base]

With the supply disconnected short out the capacitor terminal. That little resistor is should do it for you, but they do fail and so do cap
You have a DELTA connected 3 phase motor running on single phase, this arrangement is quite common in home workshops. The symptoms you describe suggest that the cap has failed and so the motor is overloading tripping out the starter overloads.
Motor connections are, Live to any one of the motor terminals, Neutral to another, either way round. One of the cap leads to either of the previous two connection and the second cap lead to the third connection on the motor. The cap looks to be an ex fluorescent Pf correction capacitor, which are pretty reliable. Like every thing in life they do fail, so first of all try a replacement capacitor. You may know an electrician stripping out old five foot fluorescent fittings and fitting Leds, he will have a lot.

 

[Chris S]

Another vote for binning the capacitor, run/start switch and buying a single-phase to 3-phase speed controller. They are simple enough to wire in and you will get full-torque soft start, jog, fine speed adjustment. Usually can wire in external potentiometer speed control if there is a pedal. Check the 3 motor coils are all good resistance first before spending, say 15-25 ohm.

Something like this 0.4kW / 0.5HP (bit small for your 3/4 hp)
RS £82.25+VAT
https://uk.rs-online.com/web/p/inverter-drives/1223410/
or 0.75 kW / 1HP which is more suited to the motor £85.31
RS PRO Inverter Drive, 1-Phase In, 0.01 → 599Hz Out 0.75 kW, 230 V ac with EMC Filter, 11 A RS510, IP20 | RS Components - https://uk.rs-online.com/web/p/inverter-drives/1223411/

Worth reading the manual to give you some idea.

 

[qaz]

its suggested by some people here that i replace the cap, but i did test it. does the test result mean anything ?

also no ones mentioned the fact that the motor smokes, which i reported, is that significant ?
thanx

 

[me2]

As mentioned in previous posts , the motor windings must be tested so see if there is any damages to the insulation . You ll need an insulation tester and carry the test at 500v .If you do not have the equipment nor knowledge I suggest you pass this on to an electrician .

 

[qaz]

As mentioned in previous posts , the motor windings must be tested so see if there is any damages to the insulation . You ll need an insulation tester and carry the test at 500v .If you do not have the equipment nor knowledge I suggest you pass this on to an electrician .

thanx for that. now i have a last question: its suggested by some people here that i replace the cap, but i did test it, result with a meter was ok. does the test result have any bearing on the advice to replace it

 

[M4SSO]

its suggested by some people here that i replace the cap, but i did test it. does the test result mean anything ?

also no ones mentioned the fact that the motor smokes, which i reported, is that significant ?
thanx

If the motor is smoking then there is every likelihood that the motor windings are damaged, this is probably because when you start the motor it trips on overload, you have then attempted to restart it on more than one occasion without allowing it to cool down sufficiently. Heat will damage the motor windings. If a motor trips on overload (momentary overload) it should be given sufficient time to cool down before attempting to restart it i.e. both the thermal overload and the motor. If it cannot be started within a couple of attempts then there is something seriously wrong and a proper investigation should be carried out.

In most industrial applications when a motor trips on overload the operator maybe allowed to attempt to restart it once. If this attempt fails then he is prevented from any further attempts by the drive automatically locking out, this feature is to prevent the motor from burning out by constantly being asked to start and failing.

 

[qaz]

If the motor is smoking then there is every likelihood that the motor windings are damaged, this is probably because when you start the motor it trips on overload, you have then attempted to restart it on more than one occasion without allowing it to cool down sufficiently. Heat will damage the motor windings. If a motor trips on overload (momentary overload) it should be given sufficient time to cool down before attempting to restart it i.e. both the thermal overload and the motor. If it cannot be started within a couple of attempts then there is something seriously wrong and a proper investigation should be carried out.

In most industrial applications when a motor trips on overload the operator maybe allowed to attempt to restart it once. If this attempt fails then he is prevented from any further attempts by the drive automatically locking out, this feature is to prevent the motor from burning out by constantly being asked to start and failing.

thanks, thats very interesting and obviously important, it might never have occurred to me.

as the speed control is by the cone [shown] i will just fit a single phase motor which will run continuously at one speed. if i can adapt it to the cradle [shown]

i want to thank everyone for their input, ive learned a lot from your comments
m

 

[ex 2 Base]

thanks, thats very interesting and obviously important, it might never have occurred to me.

as the speed control is by the cone [shown] i will just fit a single phase motor which will run continuously at one speed. if i can adapt it to the cradle [shown]

i want to thank everyone for their input, ive learned a lot from your comments
m

Capacitors need testing at full working voltage , in the case of 240 volts its about 400, a digital multi meter is not suitable.