CALCULATING POWER NEEDS
THREE PHASE MOTOR LOADS
Soft starters and VFD’s are widely used to minimize the starting kVA effects of motor loads on generator sizing. Soft Starter is a general term that describes a motor starter that reduces the power required to start a motor. There are many varieties of soft starters on the market; some of the common types are primary resistor, autotransformer, part winding, wye delta and solid state.
Understanding the operational characteristics and settings of each type is extremely important when calculating skVA. Some soft starters have the ability to start at low voltage and ramp up to full voltage. In this configuration, generator excitation and governor response is such that there are no voltage or frequency transients. Some soft starters’ ramp voltage until a preset current limit is reached. This preset current limit will determine the size of generator required.
Two of the most widely used soft starters are autotransformer and solid state. Auto Transformer styles have voltage tap settings, and solid state have current limit settings, each corresponding to a reduction in the skVA required to start the motor.
Auto Transformer: Taps set to provide 80%, 65% and 50% of the line voltage and provide a 20%, 35% and 50% reduction in skVA.
Solid State: Adjustable Current Limit Settings between
150-600%600% = across the line starting (no skVA reduction)
300% = a 50% reduction in skVA
150% = a 75% reduction in skVA
Looking at our 100 hp example above, a solid state soft starter with a 300% current limit setting would reduce our skVA from 630 across the line, down to 315 skVA, resulting in a smaller genset being used.
Variable Frequency Drives (VFD) vary voltage and frequency, allowing them to control motor starting, stopping and speed rates. They are generally used to ramp up during the starting sequence and to control speed during operation. When a VFD is used, the generator no longer sees the motor as the load; the VFD becomes the load that the genset recognizes. So calculating the running kVA or kW of the VFD is required for genset sizing. We would use this equation to determine the running kW:
The current drawn by VFD’s is nonlinear (having harmonics), which causes a distorted voltage drop across the reactance of the generator. Since VFDs are nonlinear, you must include additional generator capacity when sizing to keep voltage distortion to a reasonable level of approximately 15% total harmonic distortion (THD) or less. The larger the generator, the greater the reduction in impedance of the power source (generator), which in turn, reduces the effects caused by harmonic current distortion.
For older six-pulse VFDs, a typical generator up-sizing factor would be 2X the running kW of the drive. If it is newer 12 pulse or pulse width modulated (PWM) type (or includes an input filter to limit current distortion to less than 10%), then you can reduce the sizing factor down to 1.4X the running kW of the drive. Given the need to upsize generators to compensate for harmonics, it sometimes offsets any reduction in starting kW/kVA that users were hoping to gain by adding a VFD.
Proper sizing of the generator will save users money on fuel consumption and rental fees. Owners can avoid costly repairs by not having their gensets applied in undersized applications and experience wet stacking. Ensuring power cable lengths and gauges are sized to meet the voltage and amp draw of the application is another important consideration. To avoid any issues, consult with a certified electrician before renting, buying or connecting a generator to insure you comply with all federal, state and local electrical codes.
When you properly size a generator, you minimize risk and downtime from power disruptions. Considering project costs, an optimally sized generator will prevent lost revenue due to an unforeseen halt in production, cancellation of events from being unable to secure sufficient power sources, and delays in construction due to a lack of power. Over or under sizing the generator for your job, could end up adding unforeseen costs to the project. Size it right from the start and you will reap the benefits of your mobile power.