The inrush current at motor start up has only a minor impact on peak demand kilowatt (kW) and energy use—measured in kilowatt-hours (kWh). During start up, inrush current contributes to higher energy usage during the very short starting time for the motor. For example, a 44 kW motor (50 horsepower) would consume five times the normal operating kW, or 220 kW.
A premium-efficiency motor might exhibit a 10-times higher than normal power spike. However, these start-up spikes will only occur for a few seconds during start up. Motor starting does have an impact on increasing kW demand and kWh energy usage, but the effect is not significant over the 15-minute interval typically used to meter for peak demand charges.
A soft-start device can reduce the inrush current. The soft-start feature protects the motor against over- and under-voltage conditions and eliminates the high impact or jerk starts that cause mechanical wear and damage. A variable-frequency drive (VFD) automatically limits start-up power draw to only 10 to 15 percent more than rated power draw. However, the cost premium for VFDs over a soft-start device makes it difficult to justify the investment if energy savings is not also considered.
If the motor has a soft start feature, then the 44 kW motor may consume only three times the normal operating current spread over a longer period of time, say 15 seconds. The demand meter calculates pulses received over a 15-minute interval (900 seconds). If the motor start takes 15 seconds, the amount of starting current will be less than an across-the-line hard-start motor, but the average energy consumed will translate to about the same amount as for an instantaneous-start motor, thereby giving a similar kW demand peak rating for that 15-minute period.
Image source: U.S. Department of Energy