The selection of the moulded case circuit breaker, MCCB, feeder circuit on a motor goes way beyond that of simply selecting a product on current rating.
Smaller motors have been wired for direct on-line starting, DOL, and so the high starting current inrush will trip the MCCB during this time if not correctly chosen. There is also quite a variation on the motor characteristics between the manufacturers. This is especially so in recent times with high efficiency motors becoming highly popular. We need to also note there is a vast army of motors toiling away which come from a time this was not an issue.
Motors which have been refurbished by way of a rewind/repair will also not be as per original made specifications.
Star-Delta motor starting techniques which have been applied on the larger motors to reduce the inrush currents also do not escape the inrush issue.
Without going into details on the staring method itself where the reduced voltage, star connected, being applied to the motor is at some timed interval changed to full, delta connected, in order to reduce the inrush.
For that ever so brief moment between methods the voltage is removed and reapplied to a rotating motor. The motor is either being pulled into following the applied voltage or slowed to catch the next behind voltage. Being pulled to follow brings with a serious inrush current however brief. Winding rotation has some benefit if they can be changed.
One of the methods that can be used to reduce these types of inrushes is to have closed transition resistors which are suitable resistors which stay connected during the change over time. They are not very heavy duty as the change over is rather brief in any event. However, in the many years in the electrical industry I have yet to ever see these in action.
Motors should only be started according to their specifications, e.g., six starts per hour typically. So, approach your MCCB supplier for guidance on the correct selection. These may not always be 100% effective but at least some of the guesswork has been taken off the contractor if things go awry.
These charts have been built up during many years of both theory and practice by the manufacturers and often contain additional info on say the contactors that can be used. This is also true for when replacing a MCCB with either an alternative model/brand or when the replacement unit is not the same model being replaced.
MCCB’s for drives
Electronic starter systems, Drives, have come a long way from the early versions and these days most if not all make use of variable frequencies to control the motor. Generally, some care is still required but having a MCCB at 20-25% rating above the motor full load current, FLC, should be good. How ever the current draw at the range of frequencies may have an effect on the MCCB by virtue of the technology employed.
Modern drives offer so much more than just starting by way of additional inbuilt protection that the MCCB is really downgraded to an isolation device.
Earth leakage types of MCCB’s used for these applications may be more trouble than worth as the drive/frequencies may trigger nuisance tripping. It could be argued the risk of a problem from the MCCB to the drive is low for shorts/EL issues as it is housed within the DB zone under ideal conditions?
Technology: a subject all on its own
In brief, circuit breakers make use of three types of technology which require some thought as to what fits the situation best. Electronic types will require setting up to fully apply protection.
Notes: So, in summary, approach your MCB/MCCB supplier for their motor recommendations and keep these handy when applying circuit breakers in the field. It will save you time and earn respect from your customers!
By Kevin Flack
Flack has recently retired from industry and believes he still has valuable in-depth knowledge of the South African electrical landscape due to wide experiences gained in his 45+ years working career. He is keen to explore the training arena so that this can be passed on to the current youth so as to benefit their practical ability in the electrical field. In these trying COVID-19 times Flack can provide online training to keep your staff up to date on all aspects of circuit breaker deployment. Email kevin.flack@outlook.com
