Key recommendations to facilitate the proper selection of generator:
- Receivers powered by a generator set should be divided into single and three phase.
- Determine the power in kW for each of the receivers, using their nameplate or manual.
- Add powers of the individual receivers, which will run simultaneously. Power of three-phase and single-phase loads should be accumulated separately.
- Select the unit, which power is greater than the total power demand of receivers working simultaneously.
Power of the single-phase generator units is defined in kW at power factor cosφ = 1. Sometimes manufacturers give this power in kVA at a power factor of less than 1. Choosing a unit that power must be converted to kW, using specified power factor.
Power of three-phase generators is known as reactive power in kVA at a power factor cos φ = 0,8. When selecting the generator reactive power should be converted to the active power in kW. For example, if the manufacturer gives the power unit equal to 10 kVA, the active power then will be: 10 kVA x 0.8 = 8 kW.
Single-phase resistance: those, which at the start don’t have any start-up currents, but their total power should not exceed 80% of rated power unit (eg bulb, heater, iron, electric kettle, etc.).
Single-phase induction: refers to any device equipped with electric motors. In this case, during start-up there is starting current, which value can be specified on the nameplate or in the manual. If there is no such information it is usually taken to be three times the rated power of a powered device. The same power value is assumed in case of current-sensitive devices, such as UPS power supplies. In this case, choose a generator or inverter generator with an electronic voltage regulator (AVR).
Three-phase resistance: there have practically no starting current, but their total power should not exceed 80% of the aggregate (such as heater, electric stove, etc..)
Three-phase induction: refers to any device equipped with electric shorted motors. In this case, running them there is a significant startup current, which value can be specified on the nameplate or in the manual. Depending on the type of connection start-up can be light or heavy. Light start occurs in the case of a delta-star connection type of electric motor and typically ranges from 2 to 3 times of the powered device rated power. Light duty is also when the electric motor is started by soft-starter or device to facilitate start-up. Heavy duty occurs when the electric motor connection is direct. Then the starting current can be up to six times the rated power. Frequently, this problem occurs in case of powering water pumps, compressors and refrigerators.
- Single-phase and three-phase generators can be rapidly charged with power no greater than 60% of rated power, then they can be charged with receivers up to 80% of their rated power. The best way to power the receivers is in sequence, one after another, in a small interval. The first powered receiver should have the greatest power consumption, followed by the ones with lower consumption in order from highest to lowest.
- The three-phase generator sets with low power in case of simultaneous power of single-phase and three-phase receivers can occur an imbalance at different phases. Asymmetry is allowed up to 30%. Above this ratio less charged phase will have a higher voltage, which may damage the receiver or powered generator itself. Therefore, you should try not to use this kind of supply.
- When powering single-phase receivers from the the-phase generator, you can receive power of not greater than 60% of its rated power, for example in a three-phase generator with a power of 6 kVA you can receive single-phase power not greater than 3.6 kW.
- If you have only single-phase receivers, although you have three-phase electric instalation distributed among the floors, do not use three-phase part.