SKVA AND MOTOR STARTING
Generator sets are typically evaluated on the basis of their ability to start electric motors and accelerate them to rated speed. Although there is a generally accepted standard for this measurement, some manufacturers refer to a standard based on sustained voltage dip, while other industry leaders measure the instantaneous voltage dip. Since there is no direct relationship between sustained and instantaneous voltage dip, a comparison between the two may be misleading. Below is an explanation.
|MOTOR STARTING CAPABILITY|
Generator sets are typically evaluated on the basis of their ability to start electric motors and accelerate them to rated speed. Regardless of whether the motor has a load on it or not, starting it requires kV•A far in excess of the motor’s normal running kV•A demand. The motor starting capability, measured in terms of starting kilovolt-amperes (sKVA), is an indication of a generator set’s ability to start motors with minimal drop in voltage. When a generator is coupled to a synchronous driver (i.e., a nonslip motor tied to utility power), a generator voltage dip of 30% maximum is generally acceptable (20% in many applications). The voltage will recover to within ±2% of normal voltage as soon as the motor has accelerated to rated speed — normally about one second, or “60 electrical cycles”. Due to engine lug, diesel-powered recovery is longer but still occurs within seconds. The higher a generator’s sKVA for a given voltage dip, the better the generator for motor starting.
SUSTAINED VOLTAGE DIP VERSUS INSTANTANEOUS VOLTAGE DIP
Although there is a generally accepted standard for how motor starting capability is measured, some manufacturers refer to a sustained voltage dip value, which they explain as being the undervoltage which occurs for some period after motor starting commences. However, this value fails to quantify the absolute instantaneous voltage dip experienced during motor starting. Further, the method of calculating the value of sustained dip is largely undefined and does not account for the fact modern voltage regulators will most always return a generator to rated voltage after motor starting (assuming the engine can overcome the load).
Other industry leaders measure the minimum voltage immediately after the motor is switched on, known as the instantaneous voltage dip. This method assures that the actual absolute value of voltage dip is presented and generator starting capability is truly understood.
SIMPLE COMPARISONS MAY BE MISLEADING
This issue is further complicated by the fact that there is not likely to be a direct relationship between a sustained and instantaneous voltage dip, so a simple comparison between the two may be misleading. The difference in the methodology used for presenting motor starting capability between various manufacturers of generator sets must be considered when developing specifications. Simply specifying an sKVA value may no longer be enough to preclude misapplication in the field. Therefore absolute instantaneous voltage dip values should be utilized to ensure adequate support of sensitive loads.
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Ensure that you'll get comparable bids on projects you specify. (PDF 173KB)GUIDE TO VOLTAGE DIPS
Information from Caterpillar on sustained versus instantaneous voltage dips. (PDF 100KB)VOLTAGE DIP PRESENTATION
Presentation from Caterpillar on sustained versus instantaneous voltage dips. (PDF 200KB)DOWNLOAD ADOBE® READER®
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