Discuss Fault finding in the Industrial Electrician Talk area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

R

robbie4

Got a couple of questions that i need help with.

I have a single phase capacitor start induction motor that has been running fine , but i turned it off for a while and now when i turn it on again it is not running and it is just humming ?

Three phase motor turns on but it is quite slow and doesn`t want to get up to normal speed but after a while the overloads in the starter operate?

Thanx
 
off top of my head and the depth of my brain:
1 .capicitor is gone (not split phasing)
2.you got it in star not delta
these are the first places to look
 
Wayne is on the right track

For single phase:

the motor has the capaictor in series with the start winding which puts it out of phase with the run winding (do you know CIVIL?)

It is either left in circuit, or commonly it will have a centrifugal switch on the end of the rotor which siwtches the capacitor and start winding out of circuit when it is up to speed (this is called intermitently rated)

What happens is, the contacts in the centrigual switch become pitted and burned, so the capcitive circuit will be open circuit on start up. Depending on wther or not a load is connected, you can spin the rotor by hand sometimes and it will 'catch' the run winding

To test the capacitor you need a dvm with bar scale in addition to the numeric display.

Basically a capictor is open circuit when charged, and short circuit when discharged

take the capacitor out and connect it ti the dvm on volts range, if it is charged the voltage will be displayed on the dvm, it will also discharge through the dvm and the voltage will drop

fully discharge it (use a resistor accroos the terminals to speed it up)

now select ohms on the dvm and connect it the capaictor, things will happen VERY quickly which is why you need the bar scale, because the numeric display is not quick enough to register what happens.

So the capictor is discharged and short circuit, when you connect the dvm is the scale will immeditely drop to zero (resistance) and quickly rise again as the battery in the dvm charges the capaictor. It will will slow down as it reaches full charge, but the first rapid drop and rise will happen in a microsecond

if this happens, capacitor is ok, if it dont, its 'donald ducked'. Generally though, a blown capacitor will be literally 'blown' (the case will distorted) or just stink to hell

ok the three phase motor:

assuming its a squirrel cage with a star delta starter

you should have three contactors, the main, the star and delta contactors

you will reognise the star, because the star point will be on it (normally a shorting link accross all three phases or wire links)

When you start, main and star comes in (both 'clunk' at the same time)

then a timer will drop out star and bring in delta (double 'clunk') it then runs up to full speed

i am assuming its an automatic not an old 'manual' in which case YOU are the timer

just watch the contactors, or if you cant reallly see, safely put your hands on them and 'feel' the clunks

if you had an actual motor problem such as a burnt out winding you would get different symptoms, such as motor starts in one DOR, or the opposite DOR, or just sits in one position and hums until the trips blow.

Let us know how you get on
 
newi was right!
when i say in star, check within the terminal block ,if its 6 terminals are they connected star or delta?
 
it would be unsual to connect in star or delta at the terminal block.

If you are using a sterter you would normally have six windings (twp per phase) which you do in star and delta at the starter (thats why the star point is at the contactor)

Genneys would normally be connected in star (or even delta) at the terminal block unless its got a single / three phase change over switch, which normally changed them from three phase star, to two phases in parralel with one series (which is why single phase ratings are normally 2/3 of three phase rating instead of 1/3)

sorry, I am rambling on again...........
 
some motors have the six terminal arrangement which allows your motor to be configured in star or delta while on DOL starting
is the 3 phase motor connected to a DOL starter or star delta starter?
 
some motors have the six terminal arrangement which allows your motor to be configured in star or delta while on DOL starting
is the 3 phase motor connected to a DOL starter or star delta starter?

Thats correct wayne. if the motor has the "links" in the delta config ,volt drop caused by the high starting current can cause it to fail to get up to speed

for dol starting it is normal to connect the motor in star

hope this is of some help:)
 
i also seem to recall star configuration will make it run slower but take more current
its been years since i was taught about voltage /current / kirchoffthree phase .....im rambling like shakey;)
 
It usually says on the motor rating plate what connection should be, IE 240v star (or star symbol), or 415 delta (or triangle). Best have a look at plate and see what it says.
 
some motors have the six terminal arrangement which allows your motor to be configured in star or delta while on DOL starting
is the 3 phase motor connected to a DOL starter or star delta starter?

many would have the 6 terminal arrangement, which you need for the starter to be able to re-configure the windings from star delta, most larger motors i have worked on would have 12 terminals, becuase they use two windings per phase

It usually says on the motor rating plate what connection should be, IE 240v star (or star symbol), or 415 delta (or triangle). Best have a look at plate and see what it says.

hi spud, dont see where you get the 240V star 415V delta from

If its a three phase motor its 415 from line to line wether its in star OR delta

Thing that changes is the amount of current being drawn

Cant think of an application where a motor would be started in delta

If it its small enough to get away with a DOL (generally about 3kw or less, then it would need to be in star, if it is big enough to need star delta then it would have to start in star


Plus gents, we are assuming its a star delta contactor (ie a squiirel cage motor)

Could be a three phase resistance starter with a wound rotor induction motor

Could be a synchronous reaction motor, could be an auto syncronous induction motor with with banks of contactors switching resistors out (which is entriely diferent to star/ delta)

or it could be a cage rotor, but using a soft starter with a BPC (by pass contactor) and the BPC has failed

could be running of an invertor

either way, he has implied that this something that has 'happened', which is why you could discount any wiring/confuguration faults

my money is its a automatic star/delta running a squiirel cage, and the change over timer has failed

C'mon fella, give us a clue!!!!!:p


*Shakey rambles off into the distance*
 
Last edited by a moderator:
Yes chaps lets start simple, thats what i was getting at, check the plate to see if it is connected as per what it says to make sure the links are right and work from there.

Once encountered a newly wired motor running from a star delta starter and the spark who wired it left the motor links in, and wondered why it kept tripping on start up !
 

Reply to Fault finding in the Industrial Electrician Talk area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock