The loadbank test puts stress on a standby generator. Stress is good. By loading the engine you increase the heat load and pressures inside the engine’s combustion chambers and minimize the effects of “wet stacking”. With diesel this extra heat tends to burn out leftover carbon that can block the action of valves and rings and reduce compression and as a result reduce power output. It does the same for a gas engine although to a lesser degree.
Loadbank tests benefit the standby generator in other ways as well.
The safety shutdown system
The safety shutdown system normally has four main shutdown functions. They are high engine temperature, low oil pressure, overspeed, and overcrank.
The overcrank system prevents damage to the batteries and starter motor by limiting the time the starter motor is allowed to crank the engine. This is a timed function controlled by the engine controller.
The cooling system
The high engine temperature safety causes the engine to shutdown in the event the cooling system cannot cool the engine sufficiently. Things like clogged radiator, failing fan belt or idler, blocked or restricted air flow, failing damper motor, poorly designed installation, and failing coolant pump can bring about a high engine temperature shutdown. Also, a failing sensor that triggers the shutdown too early, a failing control board, or a short in the signal wire, can cause a problem
By loading the engine during a loadbank the added stress on the cooling system can make developing problems apparent and allow the operator to repair any issues that come up before the system is needed for an emergency.
The exhaust system
The extra load provided by a loadbank test causes the engine to burn significantly more fuel than it might during low or unloaded weekly testing. The extra fuel burned raises the exhaust gas temperature and as a result raises the exhaust piping and muffler temperature.
This is never good.
Exhaust gas temperature can reach in excess of 1000 degrees F for both gas and diesel systems. During a loadbank test, the operator has the opportunity to see:
if the exhaust system is too close to a sprinkler head
if the thimble is properly installed or missing
if any structure is too close to the hot piping
if something has been inappropriately placed on the exhaust piping as happened in the above photo
If the heat rejected to the air in the room raises the temperature too much for the radiator to do its job. Normally these systems are only allowed a 200 F temperature rise
There is also the matter of exhaust gas pressure. Most engines cannot tolerate much backpressure before causing engine damage or effecting performance. A loadbank test can bring these problems to light as well as show up leaks.
The voltage regulation and excitation systems
A generator makes electricity by spinning a big electro magnet called a rotor inside of a set of coils called a stator. The output voltage is maintained by increasing or decreasing the strength of that magnet.
The strength if the magnet is controlled by a DC voltage produced either directly from the voltage regulator (direct excitation) or indirectly through a brushless exciter (brushless excitation). Most systems are brushless.
The voltage regulator is normally a solid state device subject to heat, moisture, and vibration; all three conditions are common within the standby generator mounting. The exciter uses diodes or rectifiers along with coils of magnet wire and is subject to the same problems that degrade the voltage regulator plus centrifugal force.
A loadbank test adds stress to these systems and will often bring to light a developing problem during a controlled test rather than during an emergency.
Generators and automatic transfer switches as well as their appurtenant devices employ high voltages that can hurt you. Do not attempt to work with this equipment unless you are qualified. Observe all rules and cautions found in the manufacturer’s manuals as well as NFPA 70E, Standard for Electrical Safety in the Workplace.
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