Motor Starting Problem

MCCB is undersized, change to 600 Amps.

Maybe the motor has not accelerated fully in 8 sec (Is the load a high-inertia one)? If so, when transition takes place, it may still act as DOL and trip the MCCB. What is the magnetic setting of the MCCB? Is there a "transition delay" in the starter ? (a small delay like 100ms between star contactor drop and delta contactor pickup)....

Thanks for your comment. There is no transition delay in the starter. I will try with extending the time delay by another 2 seconds... is this OK?

i am afraid that increasing the main dekay won't help.

The "transition delay" timer specially designed for ΥΔ starters has TWO timing functions :

1. The main timing function to change from star to delta, usually site-adjustable to suit the acceleration time of the motor.

2. A "transition delay" timing function which delays the pickup of the delta contactor after the star contactor has dropped. Usually 30ms for small motors, going up to 100ms for large ones. This delay ensures that the arcing in the star contactor has completely died down, and its arcing zone is completely de-ionised, before the delta contactor picks up. Without this delay, you may have a short-circuit. This short-circuit will trip the MCCB magnetically.

If this is the reason, you have two options.

a) Buy a standalone Star/delta timer (electronic, DIN rail mountable) available from several good manufacturers. Pneumatic timers are also available, but they are usually contactor-mountable, and you need to ask the original starter supplier if he has such a timer.

b) You can wire an interposing relay, which is given a command to close when star contactor drops, and its main contacts switch on the delta contactor. This will introduce a small (unknown) delay, but should do the job.

i would recommend (a). But to check if this transition delay is really the culprit, you can wire up a slave relay as per (b) assuming you have some relay lying around in the workshop.

Good luck, please feel free to ask if something is not clear.

Yes. It may solve the problem. Why two seconds ? Try more time.

It can be like connecting a generator that is out of phase with the line.

You ain't wrong there chief.......

Houston............we have a problem!!!!

WOW!

I've seen shafts sheared clean off, but never anything like that.

I've seen shafts sheared clean off,

Broken crankshafts can be the result as well.

I'll start by asking a question, has this motor run OK in the past? If it has then something has changed, it's up to you to find out what.

If it's a new install I'd go with mountk2's advise to begin.

Thanks for your comment. The compressor motor could started for just 6 months without any starting problem, but later it started tripping during the transition from Star to delta. But once it tripped I need to wait for another 20 minutes and then it starts normally without any problem. I am operating this motor since 2008 and is still in operation except this kind of starting problem once in a while. Normally I start this motor twice in a week and will continuously for next two days. Expect your further comments. ka.jaison

Can you remember any maintenance work done after the first six months?. Check the timer,connections and contacts of the starter

Whey/Delta starting? I thought it was obsolete in 70-ties when frequency inverter allowed huge electricity savings during starting up. Who keeps this obsolete start-ups?

My suggestion is to scrap the junk and install proper frequency converter so no current surge impacts rest of the installation. 160 kW is not so difficult to control.

Dear User,

I have noted your query. One possible solution is as under.

At the time of change over delta-contactor should operate first, then after 2-3 secs star contactor, so as to avoid open rotor..is this happening??

i am surprised. If both Star and Dela contactors are ON at the same time, there will be a dead-short-circuit of the mains. In fact, star and delta contactors are interlocked electrically (and preferably mechanically) so that they are never ON at the same time.

There are special closed-transition ΥΔ starters, but that is another story...

Can you give the circuit diagram of what you mean? It will be interesting for many of us.

Wouldn't like to be around when it starts!

Not after my escapade with the 800HP star/delta.

Is it a newly connected motor?(may be new motor or the existing motor installed after overhauling). If yes, in your case there could be a chance of wrong connection. i.e for delta connection one windings start end to be connected to other winding finish end(R1Y2, Y1B2 AND B1R2).If it is connected for example R1Y1 then it will lead dead short during delta changeover. In star mode it will not give problem and it will start normally.

Dear Sir, This is Manikandan from chennai. Incomer MCCB is tripping while switching on one of the Star/Delta closed transition starter during S/D changeover. It's a new installation. I beleive that It could be a wrong connection like you mentioned.Can you please help me in sending the drawing of this kind of connection in S/D starter.Your immediate replym is highly appreciated. my mail ID is maniking1@yahoo.co.in.

Regards

K.Manikandan

Give details of MCCB like make,Im,Ir,adjustable or fixed etc. Check the peak current during changeover from star to delta. Increase the time delay between star and delta. Try to start on no load,see whether tripping. Check the setting of magnetic trip.If 500A MCCB also trips inform manufacturer and get his advice.

Is this a new installation?

If so does the circuit breaker trip the instant that it changes over to delta, or is there a slight delay?

It the circuit breaker that you are using a D curve type?

8 seconds for that size of motor seems a bit to short, has the motor reached a minumin of 60% full speed when it changes to delta?

Help us to help you by giving me the answers to these questions.

Cheers

Joe

Extend the time duration of timer from 10 to 15 sec Please check the Delta Connection of motor Check breaker adjustment current

Warning, increasing the transition delay can make it WORSE as well! With too much delay, the motor is off-line too long and actually slows down too much, the magnetic field decays too much so the flux penetration is gone and when re-connected, it may as well have been started DOL. This is why I hate Star-Delta, everything has to be just right for it to work properly.

The fact that it only happens occasionally points to the issue of the leading / lagging transition phase angle I mentioned above. You are so close to the ragged edge of acceptability that the line phase angle at the moment of transition is making a difference one way or the other, something you will have ZERO control over. Try rolling the conductors, but otherwise learn to live with it if you are unwilling to fix it.

JRaef is absolutely right. The main reason for softstarters being chosen over ΥΔ. i knew this phenomenon as "frozen rotor"....here is an extract from a paper...

_{The transfer from star to delta momentarily removes the motor from the supply. During this time the motor is under the mechanical influence of the rotating load and, at the instant of disconnection, current will still flow in the rotor bars due to the time delay necessary for the magnetic flux to die away. Therefore, there is a residual flux "frozen" on the surface of the rotating rotor, which cuts the stator windings, generating a voltage whose frequency dependson the rotor speed. If the load inertia is small, such as in a pump, or if the friction is high, therecould be a significant loss of speed during the time the supply is disconnected. In this case,when the reconnection to delta is made, a large phase differential can exist between the supplyand the rotor fluxes.This can give rise to very large current surges (as much, or more than full-voltage locked rotor current), together with massive transient torque oscillations. (These oscillations can be as much as five times full-load torque.) Although the effects described are only present for a very short period of time (about one fifth of a second), they are sources of great stress and damage to the whole drive system, and where frequent starting is necessary, invoke high maintenance costs.There are methods of control, for example, the closed transition starter, which eliminate or reduce the reconnection transients. However, such starters are expensive and have reliability implications; for these reasons they are not widely applied. The star-delta starter also has disadvantages due to the restricted starting torque available (if you need 40% LRT to breakaway,you can only increase the motor size, or revert to direct-on-line). Combined with the severe effects of the re-switching surges, and the additional costs of bringing six conductors from the motor to the starter instead of only three, star-delta only offers an imperfect solutionto the problem of starting the induction motor.}

Here is the full paper.basic_soft_starter_principles.pdf

JRaef and i rest our case.