Consequently, the degree of generator over‐sizing for voltage distortion is significantly reduced, leaving only the initial or impact kVA as being of major concern. If the connected load consists primarily of motors controlled by Fairford Electronics soft starters, the tolerance to voltage distortion is high (approx. Providing the generator AVR is immune to voltage distortion (see 4 below), the question is limited to the distortion tolerance of the connected equipment, including the soft starter. Since the starting kVA can be readily calculated from the motor/load characteristics, the main additional influence is the allowable level of voltage distortion.
This value should be as low as possible contingent with an economically viable package which meets the starting kVA demand plus any limitation in voltage distortion imposed by the type and sensitivity of other connected loads. In this case, it is even more important that a motor with a high LRT is used.ģ The characteristic which determines a generator's ability to respond to sudden and large demands in energy output is its "magnetic stiffness", and it is usual to consider the generator's sub transient reactance (Xd) as the indicator of the supply source impedance.
Where a high breakaway torque is required, such as a high friction load, it may be necessary to strengthen the generator in order to deliver enough energy to overcome the initial "stiction". Although these loads place little demand on the power supply at start up and acceleration, it is important that the high inertia loads such as fans, flywheels, crushers are identified, because these applications frequently require an extended length of time to complete the acceleration process. When controlled by a soft starter, this type of motor usually accelerates comfortably and smoothly in approximately 3.5 x FLC.Ģ Many loads have low breakaway torque requirements as, for example, centrifugal pumps, fans, rotary compressors, chillers etc., and can often be turned by hand when stationary.
Most reputable manufacturers produce motors with these characteristics.
An ideal motor would have a high locked rotor torque (LRT) of say, greater than 2 x rated full load torque and a low locked rotor current (LRC) in the region of 5.5 ‐ 6.5 x full load current (FLC). The essential elements in deciding the size of a generator are: ‐ġ The characteristics of the largest motor to be started (Speed/Torque/Current relationships).Ģ The characteristics of the load of the largest motor to be started (Speed/Torque relationship).ģ The sub‐transient reactance of the generator (X”d).Ĥ The nature of the generator automatic voltage regulator.ĥ The sensitivity to supply voltage distortion of any non‐motor loads which remain connected to the generator during the motor starting process.Ħ The soft starter power circuit configuration.ġ Since the objective is to provide an optimally sized system, it is important the motor has an advantageous speed/torque characteristic since this will significantly lower the demands made on the generator during motor starting. Since the capacity of the generator is usually small compared with that of a typical public or large‐scale private supply network, these machines are frequently characterised by having relatively high source impedance.īecause of this constraint, consideration has to be given to the effects of soft starting a motor on the generator and on other types of equipment that is also connected to the generator output terminals during the motor starting process. In these situations, the electrical energy is frequently derived from a local generator set driven by a prime mover such as a diesel engine, hydraulic turbine or similar device. There are many instances where it is necessary to start a large motor or motors from an isolated power supply as for instance, on board ship or at a construction site etc.