Page 46 - Energize February 2022

TECHNICAL

must cater for start-up kVA which can be four to seven times the connection of transformer-based loads but this may disturb the
normal loaded motor current, with current peaks approaching 10 other sensitive loads. The alternative is to use a UPS designed
times normal current. to limit inrush current but this can only be applied where
The battery will also be required to deliver current at start-up the transformer-based load is the only load on the UPS. The
that could be four to seven times the normal full load current. general solution would be to size for the inrush current of the
Increased battery current results in a drop in battery voltage transformer.
which could affect the performance of the inverter, so the battery
characteristics need to be taken into account when sizing for Condenser input converters
start-up current. Battery voltage must not drop below inverter Converters, including variable frequency drives (VFDs), which
input limits, and this may require a larger battery size than what incorporate condensers, can exhibit inrush current on start up. In
would normally be the case. some VFD designs current is limited on the application of input
power by the design of the drive’s pre-charge circuit. The DC bus
Motor and drive types capacitors are soft charged based on an RC time constant such
There are three classes of motor which will be encountered in an that the current at power up will be well below the motor’s full
industrial network; load current. For other converters, the actual inrush current
• Direct online start: The motor is started by connecting directly of the load needs to be determined. The inrush current of a
to the full voltage. Start-up current can be four to seven times converter will generally be of shorter duration than that of a
the full load current with current peaks of ten times the full motor application. Many converter circuits incorporate inrush
load current. current limiting stages.
• Soft start: The motor is connected via a soft start controller
which limits the start-up current and the start–up torque and Non-linear loads
speed, or rate of acceleration. The system does not use variable Although most equipment using electronic converters has
speed control. The start-up current can be varied to suit the circuitry that limits the crest factor, there may still be some that
start-up torque required and generally does not exceed 500% produces high current peaks. Figure 4 shows a typical non-linear
of the full load current (Figure 3). When sizing for soft start load input current waveform. The high current peaks in this
motors, the actual start current setting needs to be taken into device can cause overloading of UPS circuitry and need to be
account, or where this varies with application, the maximum taken into account when sizing the UPS.
start current available with the soft start should be assumed.
• Variable frequency drives: The motor is connected via a speed Load calculation
control device which generally incorporates soft start as a UPS ratings are normally quoted as kVA and not current so the
function. start-up current needs to be converted to a kVA figure when
sizing. In general the maximum current resulting from switching
Motor drives and soft starters can also cause harmonics in the on of motors and other devices needs to be determined.
input current which can affect the sizing of the UPS. The presence
of harmonics requires derating of the UPS. • Single loads: The single load is the easiest to calculate. Sizing
will be based on steady state maximum load and start-up load.
Transformers • Multiple similar loads: A more complex case, the maximum
Switching on of down-stream loads connected via transformers load will depend on the operating schedule of the loads
results in inrush currents which can be five times the full load and must cater for steady state full load as well as start-up
current. One solution is to use the static bypass switch for loads. Extremes are loads running, and all loads starting up
simultaneously. If the operating schedule is not known, the
worst-case scenario should be assumed.
• Mixed loads: This is even more complex as the inrush current
of each load needs to be determined as well as the operating
sequence.

Figure 3: Soft start up currents (Eaton) Figure 4: Non-linear load current ( Solid State Controls)

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