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The first step in sizing and selecting a generator set is to establish project parameters.
From the load analysis it is possible to establish:
|Maximum transient load step||SkVA|
|Maximum transient load step||SkW|
|Maximum (peak) transient load||SkVA|
|Maximum (peak) transient load||SkW|
|Maximum (peak) transient non-linear load||SkVA|
|Running non-linear load|
The generator site rating must exceed the running load requirement.
The transient load step capability must exceed the maximum transient load step requirement.
The power, including the overload capability of the set, must exceed the maximum (peak) transient load requirement
The next and most important step in sizing a generator set is to identify every type and size of load the generator set will power. In general, when non-linear loads are present, it may be necessary to oversize the alternator.
Following is a general discussion of how various loads and electrical factors affect the sizing of generator sets.
Power factor (PF): The inductances and capacitances in AC load circuits cause the point at which the sinusoidal current wave passes through zero to lag or lead. Lagging power factor, where current lags voltage, is more generally the case and is a result of the inductance of the circuit. Power Factor is the ratio of kW to kVA and is expressed as a decimal figure (0.8) or as a percentage (80%). Three-phase generator sets are rated for 0.8 PF loads and single-phase generator sets for 1.0 PF loads. Lower PFs require larger alternators or generator sets to properly serve the load. Caution should be used whenever applying generator sets to leading power factor loads. Only slightly leading power factor can cause generator sets to lose voltage control.
Single-phase loads and load imbalance: Single phase loads should be distributed as evenly as possible between the three phases of a three-phase generator set in order to fully utilize generator set capacity and limit voltage imbalance.
Peak loads: Peak loads are caused by loads that cycle on and off, such as welding equipment, medical imaging equipment, or motors. Taking cyclic loads into account can significantly increase the size of the recommended generator set despite painstaking efforts to place loads in a step starting sequence.
Motor loads: Calculating specific motor loads is best handled by sizing software which will convert types of motors into load starting and running requirements. For this discussion, however, it is sufficient to broadly characterize loads as high-inertia or as low-inertia loads for the purpose of determining engine power needed to start and accelerate motor loads.
Motors over 50 HP: A large motor started across the line with a generator set represents a low-impedance load while at locked rotor or initial stalled condition. The result is a high inrush current, typically six times the motor rated (running) current. The high inrush current causes generator voltage dip which can affect other systems. The manner in which generator voltage recovers from this dip is a function of the relative sizes of the generator, the motor, engine power (kW capacity) and generator excitation forcing capability. Depending on the severity of the load, the generator should be sized to recover to rated voltage within a few seconds, if not cycles. Various types of reduced-voltage motor starters are available to reduce the starting kVA of a motor in applications where reduced motor torque is acceptable. Reducing motor starting kVA can reduce the voltage dip, the size of the generator set, and provide a softer mechanical start. However, these starting methods should only be applied to low-inertia motor loads unless it can be determined that the motor will produce adequate accelerating torque during starting.
Alow yourself a sufficent KW for future expansion and development.
Mechron FZE is pleased to announce our new Load Monitoring and Measurement service. This new service will provide our clients with 30 day demand recordings on real time.
We will come to your facility and install our Fluke meters. After 30 days, we will return disconnect the meter(s), download the data, and generate a complete and easy to read power summary report.
Our state of the art Reliable Power Meter will also record other important power parameters such as harmonics and transient impulses. This service will ensure that you are getting full KW/$ out of your Generator, backed against fuel charges we will know the performance and condition of your machine and perdict operation costs.
For more on Generator sizing please contact our aftersales team for assistance.