I recently came across previous postings on this thread. Several very misinformed individuals advocated rapid transition, but exceedingly insignificant, times... of the order of 20, even 50 milliseconds.

The purpose of star-delta starting is to reduce current inrush... perhaps to reduce burden on the power supply! Or, perhaps to "soften" the mechanical shock to the equipment lineup. Regardless of the intent, 20 ms represents about one revolution of the motor while 50 ms represents about 2 1/2 revs. Hardly enough to impact on starting duty!

Regards,
Phil Corso, PE
(Boca Raton, FL)

 

Good point Phil, and don't forget contactor travel time. Any mechanical switch takes time to move, and in some apps that time has to be accounted for. 20mS is just too short. I've done quite a bit of work in Europe where they Wye-Delta start most motors, even small ones. Its most typically done with pneumatic timers piggybacked on the contactors. These timers aren't anywhere near that fast, but the method works just fine. Personally I dislike the pneumatic timers and IEC contactors, but the point is that slower switch times work just fine, and with too fast a switch time, why bother with the extra contactor.

 

Typically star delta relays will have a "Dwell" time of 20 to 100mS. This is the time that is allowed to elapse between the star contactor being de-energised and the delta contactor being energised. The star contactor will be energised for as long as it takes for the motor to get upto speed... several seconds.

Simon Cooper
Foxtam Controls
England.

 

Phil. The misinformed people know more than you do. The 20 milliseconds is the time between star and delta NOT the star time. The 20 milliseconds allows one star contactor to be completely out before delta comes in.

Regards Alan Case

 

Exactly my point. The fast transition time negates the reason(s) for the wye-delta starter!

Regards,
Phil Corso, PE
(Boca Raton, FL)

 

This 20-50 ms is nothing to do with the revolution of the motor. It is the time that is necessary to turn off the star contactor, to prevent a short-time short circuit. By the way, we use TESCH time relays that have a dead time of 100 ms, which is safer and during this 100 ms the motor will not lose its speed.

 

I don't understand, Phil.

The alternative to Wye-Delta start is massive mains and big transformers. Much of the "glitches" that take place in the 110 volt single phase domain are IR drops from big power hogs coming on-line in Delta. Granted, the harmonics of fast transition Wye-delta starters leave lots to be desired, but that should be controllable.

Two instances, when there is a high penalty for instantaneous 275% of run current draws, and when repeated motor starting leads to overheating, are
good recommendations for this conservative European technique!

=>Jim<=

 

the millisecond times you refer to are the dwell times for star contactor changeover to delta contactor and this is necesarry for arc supression at the contacts.this is after the start timer in star has timed out ( 6 to 12 Seconds) also and this is most important the contactors for star delta starting are not rated at full DOL currents typically the KIM & K5M swtchgear is rated at 0.58 X Ie and K3M switchgear at 0.33 x Ie so milliseconds changeover at start condition would be be disasterous for the delta contactor K5M DOL contactor would have to be at least AC3 rated.
i seem to remember that there is a KW limit on what local eletricity authorities will allow for DOL starting. any way you should ask them just to make sure.hence the use of star delta in europe.

good luck
jeff

 

Responding to Hakan's Sat, Jul 6, 1:0pm reply:

Obviously one of us has misunderstood the other. If I'm at fault then it's my turn to utter the usual engineer creed "what I meant to say was"...

If the elapsed time between start (breakaway @ t=0) and run (delta contactor closed @ t=Tr) is actually milliseconds, then the rotational speed attained @ t=Tr, is virtually nil. However, sufficient breakaway torque develops to exceed starting torque. But, because there is no appreciable speed gain there is no corresponding increase in motor impedance. Thus, @ t=Tr when the "run" contactor closes the second peak of inrush current and torque will be virtually the same as if it started as a delta motor @ t=0!

Please note, the above scenario presumes "open" transition configuration and neglects dwell-time. This is the period between opening of the star or "start" contactor and closing of the delta or "run" contactor as defined by Simon Cooper.

In conclusion, a star-delta controller is unwarranted!

Regards,
Phil Corso, PE
(Boca Raton, FL)

 

Responding to "Jim" Hebbard's query:

First, a historical view.
In the past, source/mains restrictions dictated the use of reduced starting-current technology. A users' option were, in general, limited to "country" preference. In Europe wye-delta was used often. Here, in the US wye-delta was an unusual choice, principly, because of the need
to run six wires vs three. Auto-transformer, primary-resistor, primary-reactor were more common. For industrial plants, the choice was
based on economic evaluation.

Today's view.
Of course, you probably realize that I am of the "old school!" There is nothing simpler than an "across-the-line" starter... even used one for
starting a 22,000 Hp induction machine. However, I am not against the use of W-D. But, I believe its choice should not be across the board,
especially if the start-time is of the order of 0.1 to 0.2 second. Such a short start time before switching to "run" results in an insignificant
current reduction. If many drives are being considered, then, the monies budgeted for W-D, will be better spent by enlarging mains (definitely not the method I would pursue) or increasing the source's short-circuit level.

My future outlook.
It is obvious to me that if supply restrictions are impossible to overcome, then, soft-starting (lower frequency) is the way to go. BTW, lower frequency starting was also used in the 30' and 40's in the US.

Regards,
Phil Corso, PE
(Boca Raton, FL)