All you Kneed to Know about Generators

Sizing up a backup Generator, and what to consider


Take your time and get it right the first time 


· Resistive or Inductive Loads? inductive loads are those loads which consume reactive power. In layman’s language, all the loads which have rotating parts are inductive loads like fans, motors, etc.  Resistive loads are typically used to convert current into forms of energy such as heat. Common examples are heaters, lights, electric jugs. The key difference between inductive loads is that an inductive load depending on its starting method (direct online, star delta or VSD) may require up to seven times the rated current to start up and as such needs to be taken into account when sizing a generator.

· Soft-Start or Star Delta: Star-delta or soft starters limit the voltage on start to reduce the ‘inrush current or excessive high current draw required to start inductive motors. It is highly recommended to install a star-delta or soft starter onto any large inductive loads where possible to reduce the impact load on the engine and alternator. Soft starting methods generally mean you can downsize the generator saving in capital expenditure and reduced maintenance costs.

· Direct online Motor: Direct on Line or DOL motors are inductive motors with no restriction on the starting voltage. A DOL motor starts in one step and therefore may require up to seven times the running current to start the electric motor. High impact loads are not recommended on a generator, and in some instances, the generator may need to be more than doubled in size to start DOL motors. We recommend installing Star Delta or Soft Starting to reduce the high impact load.

· Don’t oversize Generator:  Diesel engines don’t do well without a load. Light loading a diesel engine results in the exhaust are becoming full of soot. The soot is a symptom of a bigger problem (the ecological impact aside) in that the poor combustion—in layman’s terms—gums up the engine. It will require an overhaul sooner rather than later. And then there is the fuel efficiency issue. The big diesel engine will probably use more fuel just to idle than a 30kW generator at half load. Why waste all that fuel?

· Get generator sizing correct: Size matters, the most important thing to consider when sizing a generator is the high inrush currents associated with starting electric motors. Under or oversizing a generator are both a major issue. Therefore it is paramount that all of the above are carefully weighed up before buying or hiring a generator.

· Contingency: Future proofing and contingency is an important consideration, but too much contingency can cause you a headache down the track. The old rule of thumb advice “need a 60kVA Generator, add on an additional 20%” is a starting point. However, it requires full detail on what you’re trying to achieve currently and into the future to avoid under loading the generator.

· Seek expert help:  unless you have studied and installed electric motors for a living, then we strongly recommend speaking to your preferred supplier/installer.

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