star/delta motor mystery

[powerdown]

i work in bread baking.
on one of the mixing machines during a 24 hour day and 200-250 mixes it will trip the main mcb-in the switch room-around about 12-15 times.
checking the current with an identical mixer when kicking into delta the problem mixer pulls 340 amps and the alright mixer pulls 160 amps.
the motor directions are ok. the timers are set at 11 secs with a 0.05 lapse time.
can anyone offer any reasons why we have a difference.

 

[jbartos]

Suggestion: The mixer may be experiencing a wear. It shall be tested for its parameters, e.g. winding resistance/impedance, insulation resistance/impedance, etc. An electrical motor repair shop could solve the problem.

 

[skills]

The contactor that changes configurations is likely waering and maybe reacting slower than it should, this would cause such problems. Also check all its contacts for wear. It isn't very likely that the motor is at fault, at least as compared to the transfer contactor failure. I saw a delta/wye starter that held the delta while closing the star and it sounded like a bomb when it did that.

 

[busbar]

skills raises an interesting point. If one contractor hangs intermittently during transition, then effectively a three-phase bolted fault may occur. Some Y-D contactor sets are electrically interlocked only, where others have mechanical AND electrical interlocking.

It is advisable to troubleshoot the problem before having to reset a tripped breaker; else a violent failure and significant outage time may occur in the associated electrical components.

 

[jbartos]

Suggestion: The contactor contact malfunctions could cause protective devices to trip.

 

[powerdown]

Thankyou for the helpful advice thus far.
i have replaced the star contactor which looked a little suspect but not 2 bad.
checked the motor for resistance and insulation using a 1000 volt megger.
i have checked the mechanical interlocks and timer which are ok.
so any other ideas would be appreciated.
incidently the mixer has a 75 kw motor and when starting the current kicks in at about 320 amps and quickly drops to 45 amps before rising to 90 amps, when the delta kicks in. and the mixer runs at about 112 amps happily thats if the mcb doesn,t trip at y/D changeover.
when in this instance do u feel that the delta should pull in.
bare in mind that at the time the mixer starts the mixing tub is flour and water.
also should the 0.05 sec timer be used to create a lapse between the Delta kicking in b4 the star or hold the Star in for 0.05 whilest the delta kicks in.

 

[Shortstub]

Powerdown,

1) Can you determine if the motor has properly accelerated to nominal or near nominal speed?

2) What is the setting of the main mcb in the switch room?

3) Can you confirm that the motor's breaker does/doesn't trip?

 

[Shortstub]

Remember, Y-D starting was never intended for severe starting-duty, ie, large inertia, or increasing load torque during start.

You never mentioned how long the problem has been going on. But, if an old installation, could the problem be related to the hiring of new employee, or employees? Or could the filling method have changed?

I know you're not loafing, and its probably costing a lot of dough, but you did mention current measurements! How about voltage?

 

[jraef]

Several issues to add for investigation:
1) There is ALWAYS a transition spike when switching from Y to Delta using an Open Transition starter. If you are not sure, it is Open Transition if there are 3 contactors. Closed Transition needs 4. If you have 4 contactors you will also have a transition resistor, and it might be bad.
2) This spike from Open Transition is well known to cause breaker tripping. The magnitude can be different between 2 identical motors because of differences in connection of the leads, even though they run in the same direction. There is a phase-shift when transitioning and the spike differs based on if the voltage leads the current or vice-versa. "Rolling" of the connection may help, but don't attempt it if you don't know what this means.
3) The MCB will build up a "thermal memory" if it is an older design, meaning that it takes less and less to trip it each time, exacerbating your problems. This only applies if your breaker is tripping on Thermal (long time) trip as a result of load current. If it is tripping on magnetic (instantaneous) trip then it is probably the transition spike.

One good solution is to use solid state starters instead of Y-Delta. No spikes, infinately adjustable, standard motors and connections.
Quando Omni Flunkus Moritati

 

[CB2]

With the main mcb tripping as much as you have reported, I would for sure check the line and load connections. Use a thermal camera or pull that yearly maintenance, disconnect/clean/tighten to proper torque.

Next check the settings on the breaker.

 

[jbartos]

Suggestion to powerdown (Electrical) Feb 19, 2003 marked ///\\i have replaced the star contactor which looked a little suspect but not 2 bad. ///Good idea.\\checked the motor for resistance and insulation using a 1000 volt megger.
///Good, was it in MegaOhm range?\\i have checked the mechanical interlocks and timer which are ok.
///Good.\\incidently the mixer has a 75 kw motor and when starting the current kicks in at about 320 amps and quickly drops to 45 amps before rising to 90 amps, when the delta kicks in. and the mixer runs at about 112 amps happily thats if the mcb doesn,t trip at y/D changeover.
///It looks that the motor connected in the star connection is overloaded.\\\
when in this instance do u feel that the delta should pull in.
bare in mind that at the time the mixer starts the mixing tub is flour and water.
also should the 0.05 sec timer be used to create a lapse between the Delta kicking in b4 the star
///If the star is already overloaded, this would not help. It would just place the motor more into motor starting in delta connection with the heavy shaft load and potentially facing the motor counterelectromotive force.\\ or hold the Star in for 0.05 whilest the delta kicks in.
///This would call for the closed transition wye-delta starter.\\\

 

[busbar]

With a power-rated AC mutimeter, a millivolt-drop test can be applied to contacts and terminals of starters and circuit breakers as an indicator of heating from excessive resistance.

http://ecmweb.com/ar/electric_testing_contact_quality/index.htm

 

[powerdown]

Everyone, thanks very much for the benifit of your wise advice and experience.
since changing the star contactor, on tuesday morning, apart from 1 or 2 trips we experienced after -which may have been a result of incorrect timer settings- the main mcb has not tripped.
i hope this thread has been of assistance to other members.
again thanks powerdown.

 

[Marke]

One thing to consider is that there are major differences between apparently like motors in regard to starting characteristics. If one machine is not up to full speed when the changeover occurs, this will result in the characteristics that you describe. I would suggest that lengthening the start time by a small amount may make a difference, allowing the motor to accelerate to a higher speed before changeover. Essentially, the timer should be set to allow the motor to reach full speed in star. If it can not reach full speed in star due to a lack of torque, then set the timer at the time when the motor ceases to accellerate. The lack of start torque can be a function of the motor, or the machine.
Best regards, Mark Empson

http://www.lmphotonics.com

 

[snpyon]

The best bet, if it is determined that the y-d contactors are satisfactory, and usually 9 out of 10 times, the fault lies in the winding, believe it or not, the motors on mixer, especially those of hobart, and others, are prone to dielectric failures of the winding, which means the resistance form winding to ground (through the insulations) is so low that basically when heat rises on the ambient op, then the resistance by theory should rise as well, when that happens the current rises, and overcomes the dielectrics of the insualation, and causes a "trip" of the breaker mains.

My suggestion would be to have the motor taken down and checked, and also while the winding is being checked, make sure they replace both bearings, as these also comprise of over 40 % of downtime, due to bearing failures.