i installed a 20 QF1 C1 cbi circuit breaker for some aircons.amp...while commisioning the aircons i noticed a buzzing sound coming from one of the circuit breakers...
there are 3 x daikin invertor 9000 BTU aircons connected to 1 x 20 amp C1 circuit breaker...which in theory should be no problem.
while discussing the sale to the customer the aircon rep indicates that they only draw between 4 -6 amps...low current energy saving etc...you know sales talk...so trusting the rep as he sells these units on a daily basis...bad idea i should have done my homework like i normally do...the units get installed and i read the full load current on the side of the condenser unit...full load current 8 amps per unit...x 3 = 24 amps...now suddenly my installation is under rated...working on 6 amps full load current is all 3 were running at the same time for a short period of time wouldnt have been a problem...but running at 24 amps even for a short period of time now becomes and issue.
so thinking it was just a faulty circuit breaker i collected a new one and replaced it...wow great service the customer thought...but unfortunately the new circuit breaker made an even louder buzzing sound...now the customer is starting to doubt my abilities...so i remove the new circuit breaker and replace it with an older type C2 white circuit breaker...turn the aircons on to 16 degrees and run all 3 together...no buzzing sound...so i decided to collect my fluke 435 and monitor the power while the units are running...now things really start going pear shaped...the reason for the buzzing sound is the extremelt distorted current sine wave...with a crest factor of over 3.
i also decided to clip my current clamp which can detect 50/60 hz frequencies or you can switch it over and it can read higher frequencies a quick way of checking current harmonics.
i noted a 1 amp difference in the current...current thd is over 40 %
i ran the units for a couple of hours and downloaded the info...if anyone wants to see screen shots.
i also ran one unit by it self to see what would happen still made a buzzing sound...so the bottom line is whether i installed 3 seperate circuit or 1 circuit i would have still had the problem with the circuit breaker buzzing...my problem now is eventually the white lever C2 breaker will fail because of the startup
and anyone buying invertor aircons beware they dont have filters and feed back into the electrical system...causing all sorts of problems to other equipment on your premises.