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02-04-2010, 21:22   #1
Randyleprechaun
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STAR/DELTA MOTOR STARTING

Hi,

I need some advice on sizing and placing an overload in a star/delta circuit.

I have my own idea on how and where but just want second opinion.

The motor in question was wired up by me about 2 years ago and was working fine till the other day. it now trips the overload when the delta contactor pulls in.
So here goes.
  1. The motors plate shows delta symbol and says 31.5 amps.
  2. When I first wired the motor up 2 years ago, it came with an old star/delta starter(intregrated to machine) which I ripped out and replaced with new telemecanique gear.
What I'd like to know is:
  • Does this motor definetly require star/delta starter.
  • If so, what should the thermal overload be rated at and where should it be placed.
As I said, I have my idea, but I'd like to get other opinions.

If you have any further questions please post them and I will answer.

I can't understand how it was working fine for 2 years and now is causing probs

Thanks
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03-04-2010, 00:47   #2
M cebee
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hey.

i've been off the industrial maintenance for a good while so had to refresh my memory

http://www.wiringmanual.com/motor039.html

the first schematic is the overload in the 'motor line' and the amps is 0.58 the rated current.

afaik that prob offers the best protection


not sure what the prob is. have you checked the delta contactor?

Last edited by M cebee; 04-04-2010 at 10:04.
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03-04-2010, 07:53   #3
superg
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The last picture in M cebee's link is how we wired star delta in tech very recently.

The overload should be set for the motors full load current
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03-04-2010, 08:59   #4
2011
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Quote:
Does this motor definetly require star/delta starter.
Hard to say if it "definetly" needs one without more details, but it is better to have one than not. What rating is the motor?

It is better as it reduces the starting current to about 1/3 of what it would be starting in delta.

Starting it in star is a bit like starting a car in 1st gear instead of 3rd (and then changing up a gear whaen it goes to delta).

Quote:
  • If so, what should the thermal overload be rated at and where should it be placed.
The overload should be set to the full load current of the motor as superg said.

The overloads normally have adjustment on them. Perhaps you just need to tweak them a bit.

It would be no harm to check a few things with the motor for example:

1) Can it rotate freely?
2) Measure the resistance of the windings. They should all be about the same.
3) Carry out an insulation resistance test (meggar).
4) Has the load changed?

Good luck!
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03-04-2010, 09:14   #5
ikb
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http://www.wiringmanual.com/motor039.html

What a very useful link. I have an original K.M handbook from the 70s, which I carted around in my toolbag when apprenticed, Have tried to get an updated copy, many times over the years- without success.... The Interweb,... Information at your fingertips.
As with Mr C, I am also a long time away from plant maintenance.
Your issue may well be that the thermal O/L are just tired out.
You must first rule out any motor issues.Easiest way is to measure the currents to the motor when running (Line current is ok) All should be even.
If this is the case, Unload the motor, (ie. disconnect whatever from the drive pulley/ shaft) and check again.
It is possible that something on the driven end of the shaft is jamming/ seizing causing the motor to load up.?
DO NOT BE TEMPTED TO UP THE OVERLOAD SETTING.
What size motor is it? (kw/hp).
To be honest,Overloads wear out, are easy to change, and- provided all motor currents are even and within spec- are a good place to start.
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03-04-2010, 09:19   #6
ikb
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Quote:
Originally Posted by 2011 View Post
Hard to say if it "definetly" needs one without more details, but it is better to have one than not. What rating is the motor?

It is better as it reduces the starting current to about 1/3 of what it would be starting in delta.


It would be no harm to check a few things with the motor for example:

1) Can it rotate freely?
2) Measure the resistance of the windings. They should all be about the same.
3) Carry out an insulation resistance test (meggar).
4) Has the load changed?

Good luck!
+1...very good advice from 2011.
Lets know how you get on.
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03-04-2010, 09:40   #7
M cebee
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Quote:
Originally Posted by superg View Post
The last picture in M cebee's link is how we wired star delta in tech very recently.

The overload should be set for the motors full load current
for 'motor line' and 'delta circuit' it will be 0.58*FLA


for 'mains supply line' overload protection or a 'motor circuit breaker' it will be 1*FLA

if the overload is sized according to FLA it will be approx 18amp if placed in series with the motor windings(motor line,delta circuit)
they're similar except with 'delta circuit' the overload is out of circuit for starting

Last edited by M cebee; 03-04-2010 at 18:05.
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04-04-2010, 15:16   #8
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If the overload is in the main motor contactor (Q11 in m ceebee diagrams) then it should be set to just over half motor full load amps, as when the motor is running, half will go down this part of the circuit when the motor is running in delta, if it is put into the other contactor (Q15) which closes to change to delta then its also set to just over half of motor full load amps but is not in the circuit until delta change over, so facilitates heavy load or slow start ups. If overload is in main contactor supply then its set to full load amps.

Changing the overload to a new one will probably fix it, but do an amps check on the part of circuit where the overload is, and compare it to the overload setting. If the amps is lower than the setting then new overload needed. If amps is higher then the load on the motor may have changed somehow, maybe by the machinery its driving not as free running as it was, or the motor itself is not running as freely as it was.

probably the overload itself needs replacing though.

Last edited by Bruthal; 04-04-2010 at 18:12.
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04-04-2010, 15:20   #9
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Quote:
Originally Posted by superg View Post
The last picture in M cebee's link is how we wired star delta in tech very recently.

The overload should be set for the motors full load current
The last picture shows it in the delta change over leg there by the look of it, so it should be set to 0.58 as he said, i would of thought.

Last edited by Bruthal; 04-04-2010 at 15:23.
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04-04-2010, 16:01   #10
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plate shows delta symbol and says 31.5 amps.
  1. When I first wired the motor up 2 years ago, it came with an old star/delta starter(intregrated to machine) which I ripped out and replaced with new telemecanique gear.
What I'd like to know is:
  • Does this motor definetly require star/delta starter.
  • If so, what should the thermal overload be rated at and where should it be placed.
As I said, I have my idea, but I'd like to get other opinions.

If you have any further questions please post them and I will answer.

I can't understand how it was working fine for 2 years and now is causing probs

Thanks[/QUOTE]

It would need a star delta starter as it looks in and around a 20kw motor or just above if its 31.5 amps at 400v. Where the thermal overload goes depends on the motor startup required, if its a heavy slow build up to speed then it would be put in the delta contactor that closes when its changing to delta so it is only in the circuit at changeover and set to just over half the full load current. I never done it this way though.
It can be put in the main line contactor and so set to the full load amps, or in the motor line contactor and again set to just over half the full load setting, and in this position as in the main line it will be in the circuit during startup. Any time i made up star delta starters this is where i put them as far as i can remember anyway. Most motors i connected in recent years used VSD`s.
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04-04-2010, 16:01   #11
M cebee
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i notice on some manual starters they recommend a multiplier of 0.9

if the motor 'service factor' is 1

http://www.sprecherschuh.com/downloa...8_KT4_v309.pdf

afaik if the service factor is above 1 it should be on the nameplate

some large motors also operate well below FLA and reducing the overload setting is possible without causing nuisance
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04-04-2010, 16:05   #12
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Its all 1kw radio control model motors these days for me, with mini VSD`s or ESC`s as they call them.
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04-04-2010, 17:51   #13
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Quote:
Originally Posted by robbie7730 View Post
The last picture shows it in the delta change over leg there by the look of it, so it should be set to 0.58 as he said, i would of thought.
No idea.I've never wired one in industry just in tech and We wired them that way and were told the overload should be set at FLC.

Maybe its down to the type of overload used as to what it should be set at.

Last edited by superg; 04-04-2010 at 17:54.
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04-04-2010, 18:06   #14
Bruthal
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Originally Posted by superg View Post
No idea.I've never wired one in industry just in tech and We wired them that way and were told the overload should be set at FLC.

Maybe its down to the type of overload used as to what it should be set at.
If you look closely at the star/delta power circuit you will see that in star all the motor current flows through the main motor contactor, and when it changes to delta it now has 6 wires feeding the windings so the motor current splits in 2, half through the main motor contactor and half through the delta one, so if the overload is on one of these contactors it must be set at around half the full load current or just above.

If full load current to motor is 31.5 amps then it will be about 10 amps in star through main contactor, and in delta it will be about 16 amps through main contactor and 16 through the delta contactor, so the overload on one of these would be set at about 17 or 18 amps

If however its on the main supply to the first main contactor then its set at full load current at 31 amps.

Last edited by Bruthal; 04-04-2010 at 18:19.
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04-04-2010, 18:25   #15
M cebee
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Quote:
Originally Posted by superg View Post
No idea.I've never wired one in industry just in tech and We wired them that way and were told the overload should be set at FLC.

Maybe its down to the type of overload used as to what it should be set at.
no .there may other variables such as service factor and the motor/overload characteristics but the multiplier of 0.58 will still apply


with a service factor of 1 the motor windings can handle 18 amps

Last edited by M cebee; 04-04-2010 at 18:30.
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