HT Motor Protection

did u use a star delta starter for a 3300 V motor?Normally all MV motors are DOL or other meethods like auto transformer.I have nvere come across a star delta starter for MV machine.By the way who is the manufacturer.It is no doubt possible using suitable contactors etc

Oh, this was in the 70's when i was a young engineer with not much experience, and not too many people who knew more . The customer's chief engineer thought he was better than Edison and Tesla combined, and shot me down when i suggested timidly a Korndorfer Auto-transformer starter (which i had read about). He went and did what he wanted. Now of course, one would use vacuum contactors in closed-transition ATS.

Poetic justice prevailed and i became his boss in 1998 when GE bought both our companies. Pity was that i realised i had no animosity left in me anymore . He did remember the incident and casually mentioned that all later starters were ATS..no hint of apology mind you Well, c'est la vie !

3.3 KV set at 8 seconds and it is perfect. So you can set 16 seconds . That will be best solution.

You don't need to bother asking me Guest#4.

If it is DOL, the possible reasons for the tripping have already been given by other eminent and professional engineers on this thread. The main thing is that the OP should get a solution to his/her problem. That is why we are here.

It is a known fact that motors take 5 to 6 times the full load current while starting. You have to program your relay for this. What is important is that the time it takes to reach the full load.

If your system is snsitive, then you can use equipments like soft starter to reduce the starting current

Please let us know the following:

1) Is this problem cropped up recently? if so how long motor was in operation after commissioning ?What are the settings in the motor protection relay.

If the motor was in operation for quite some months/years and the problem is recent one then in this instance we have to check the connections especially CT connections for open or short.

2)If this is new installation please furnish the relay details,CT details,Setting details

Settings may be wrong particularly (49) Thermal settings.

If you

furnish all the details like motor capacity,voltage,time constants of motor,protection relay make and type,CT ratio etc , may be to the best my ability i can help you.

ramvinod

you are lucky.the motors normally take 6 times + tolerance which works out to 7.2 times.This in line with IEC.However it is common practice to ask for motors with LRC limited to 6 without tolerance and now a days 5 without tolerance.Special motors with starting currents limited to 3.5 PU are also available.You need to have the characteristic matching the motor requirement.Normally a thermal withstand curve both in hot and cold conditions will be given by the manufacturer to select a suitable relay.so relay change is the option.Motor cannot heat within this time.

Generally, starting current is not come into picture in thermal protection, only over current or starting supervision come into picture during starting protection. please check protection relay, it may be malfunctioning. By-KANU

Dear Mr. Meditronhari,

You have not referred about starting of the motor - whether it is started against load.? or it is started and then loaded.?

Then another Important Factor is the Gd^2 VALUE of the motor - which determines the Acceleration and time taken to reach rated speed. Longer the time taken to reach Rated Speed or Full Speed, Higher Current flow duration also will be longer, causing temp.rise in the winding, relay etc.

A reply in this aspect also may help to locate the probem precisely and seek remedy.

Thanks,

sdr710