I have a chiller with three compressors which are started using star/delta starters with open transition. The transition time is 20ms.

The motors are 400V. Two have maximum absorbed currents of 232A, LRA in star of 634A, LRA in delta of 1980A, transition current from star to delta of 1485A; one has a maximum absorbed currents of 281A, LRA in star of 692A and LRA in delta of 2247A, transition current from star to delta of 1685A.

When sizing fuses / overloads / power cables etc, do I need to be mindful of the peak transitional current, or is it not significant since it is only present for such a short time? Should I only consider the actual star or delta currents?

 

Responding to Tim Mitchell's Fri, Nov 20, 2000, 11:35am query:

Before recommending cable and protective design criteria, please provide the following:

(a) Are you sure that transition is only 20 ms (2 cycles @ 50 Hz)?
Reason: maximum asymmetrical first-cycle peak could reach 4,800 Amperes.

(b) What are other motor parameters, such as: HP; speed or number of
poles; normal starting and acceleratig torque?

(c) Do motors have to reaccelerate upon restoration of power source following a power source "dip?" Or upon power source restoration
following a short duration interruption? Or will they always be "tripped" off line for the conditions noted?

(d) What are approximate motor feeder cable lengths? 3, 3-1/2; or 4-core cables? 3 single core? Insulation type; overall sheath-type, if any?

(e) Available short circuit duty, or transformer size, supplying power to the starter location? Is 440 volt supply neutral solidly earthed? Or is earth-fault current limited by a neutral resistor?

(f) Starter type: mechanical or vacuum contactor? Air circuit breaker?

Regards,
Phil Corso, PE
Trip-A-Larm Corp
(Deerfield Beach, FL)

 

> I have a chiller with three compressors which are started using star/delta starters with open transition. The transition time is 20ms.

You do realize that a contactor hasn't got a hope in hell of meeting this time. I think the absolute fastest contactors I've seen are in the high double digit to low triple digits of milliseconds (90-150 or more) -- you won't notice much on that first step at all.

> When sizing fuses / overloads / power cables etc, do I need to be mindful of the peak transitional current, or is it not significant since it is only present for such a short time? Should I only consider the actual star or delta currents?

I would like to know why you are considering contactors over even the cheapest of solid state starters. I may be a bit biased (I design starters for Benshaw Canada (http://www.benshaw.com) but I haven't been able to come up with a good reason to use contactors in new designs in this day and age. Yeah they're cheap but the tips die and the torque shock to the motor and equipment costs you more in maintenance.

I'm not up on my fuse sizing but your overload is sized for the motor. i.e. a 150HP motor gets a 150HP overload (the overloads are rated to withstand typical locked rotor currents for the duration of the overload. If you're using bimetallic overloads you need to remember that they're fairly inaccurate, even if you go with the temperature compensating ones. We've seen them drift over a +/- 20% range in a fairly cool and dry ambient!

It might be worth your time to just call for a quote. Don't just listen to me though -- call Benshaw, ABB, A-B, Cutler-Hammer... I don't know of many people who recommend contactors these days, not even solid-state contactors.

 

We sub-contract our starter work, but I have not considered transitional currents before when specifying compressor/motor details to panel builders. 20mS star/delta changeover seems mighty quick to me! However some screw compressors (small semihermetics) do not like long changeovers. I've seen changeover times of between 1 and 5 seconds on the majority of plant. I'll have stab of at guessing the motor size.. 232A FLC say about 120 to 135Kw - I personally would not put soft start on this size of compressor. Soft start for us (and our customers) usually makes economic sense at around 250Kw and up. Saying that, we have new tax breaks to encourage businesses to improve electrical consumption - typically motor efficiencies and VSD, so we are expecting more soft start/inverter enquiries in the future.

 

The motors are 400V. Two have maximum absorbed currents of 232A, LRA in star of 634A, LRA in delta of 1980A, transition current from star
to delta of 1485A; one has a maximum absorbed currents of 281A, LRA in star of 692A and LRA in delta of 2247A, transition current from star
to delta of 1685A.

When sizing fuses / overloads / power cables etc, do I need to be mindful of the peak transitional current, or is it not significant since
it is only present for such a short time? Should I only consider the actual star or delta currents?

Size all equipment and cable for Normal FLC ie 232+232+281, however you may wish to take into account that not all 3 will be running at full
load, i suggest you check with your local authority for applying a demand factor, general rule of thumb is 80% of total, if it is critical
load then i would go fo 100%, protective devices should be selected upon FLC, check time withstand of starting condition with the time
characteristic to see that the devices will not operated given starting currents.

Check protective devices for disconnection time regarding short circuit condition and make sure that you have discrimination with upstream
protective devices