The Role of Load Banks in Generator Testing : The Growing Need for Non-unity Power Factor Tests

By Chris Dodds on 19th May, 2015

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The Role of Load Banks in Generator Testing : The Growing Need for Non-unity Power Factor Tests

T&D UK would like to thank Karl Sullivan, Operations Director at Optimum Power Services for this article on the role of load banks in generator testing.

Optimum Power Services are one of the UK's leading providers of generator rental and loadbank testing equipment with over 30 years in the market covering all aspects of power testing.

This post focuses mainly on the need for non-unity power factor tests.

To contact OPS, Telephone +44 1322 381726 or email to


Optimum Power Services

The Growing Need for Non-unity Power Factor Tests

Load testing is an important part of the proving of generators at the time of manufacture, commissioning and later in the life as part of managed maintenance. Load banks are used in the manufacturing plant and on site for acceptance tests to prove that a generator will perform when required, with its corresponding enclosure, cooling system, fuel and exhaust systems in place.

Standards and test criteria are well established, but there are misconceptions about what non-unity power factor loading really means, and the need for resistive and inductive load banks to truly prove generator performance. 

The StandardsISO 8528 

ISO 8528 (BS7698) part 6 is the standard for test methods of engine-driven generators defining both the functional test and an acceptance test. Functional tests must always be carried out and usually occur at the manufacturer's factory.

The standard defines three performance classes - G1, G2 and G3, with each performance class having a different criteria for a range of characteristics of the generator. Acceptance tests are optional, may be done on site and are often witnessed by the customer or his representative.

In all cases tests must be done with reference to the agreed specification of the generator. Prior to operational tests, environmental data must be recorded and a preliminary inspection is specified. This encompasses safety checks, earth connections and guarding, insulation tests, fluid levels checks etc. On initial start-up, the emergency stop system must first be checked, followed by frequency, voltage and phase rotation checks, and an inspection for leaks and vibration.

Only after these preliminary checks are load tests started. These include load duration tests or 'heat run', with recording of steady-state voltage and frequency followed by load acceptance tests, when transient responses to load changes are recorded.

Other tests can also be specified, for example cold-start load acceptance, simulated motor starting loads and synchronised parallel running.

Pic : OPS Delivering A 10.8 Tonne Loadbank For Testing Purposes

Optimum Power Services Loadbank Installations

Non-Unity Power Factor Testing 

In practice, almost all generators see a non-unity power factor load when in normal use and most include inductive and motor loads. As a result the majority of generators are designed and rated at a lagging power factor, usually 0.8.

When engineers and consultants are involved in specifying a power supply for a project they require that a set is tested to the standards and at the nameplate rating: i.e. non-unity, or resistive/inductive load testing. ISO 8528 specifies that test reports should note if tests have been done at a power factor which is different than the rated one. In most instances tests done with a purely resistive load can be considered incomplete.

Why Test At Non-Unity Power Factor? 

As stated, generators are largely designed and specified at a power factor of 0.8, and therefore the engine is not capable of delivering full kVA at unity power factor. For example a 1000kVA generator rated at 0.8 power factor, would only be able to deliver 800kW into a purely resistive load.

Testing using a resistive load will usually result in a full load test of the prime mover (i.e. the engine), but not of the alternator, which will be tested to only 80% of its rated current. This means that the alternator and its control system are not tested to their rated limit.

A non-unity power factor load affects the way that an alternator responds to load because with inductive loads the load current is not exactly in phase with the output voltage. The field within the magnetic circuit of the alternator is distorted and the automatic voltage regulator (AVR) and excitation circuit must provide a higher current to maintain the set output voltage.

The relative losses within the alternator increase when operating at non-unity power factor meaning more heat dissipation within the alternator laminations and windings.

The result is that the alternator will be running significantly cooler when the generating set is tested solely at unity power factor. This is both because the current is lower and because the current is exactly in phase with the voltage (i.e. unity power factor). The thermal performance of the generator as a whole will not be tested as it would if the rated, non-unity power factor load were applied.

There are engineers who test generators that consider this is not too important, trusting the alternator is of proven design. Their main concern is to show that the prime mover is in a serviceable condition, and is able to accept load without instability.

Without doubt resistive-only tests do give useful data, but they cannot give the whole story. The electrical parts of the generating set, the alternator and ancillary components such as circuit breakers, current detectors, connections and wiring, meters and instrumentation, are clearly not being tested to their limit when a resistive-only test is done.

Video: Optimum Power Services RBS Project - Supplying 4MVA @ 20kV Reactive Loadbank Equipment For DRUPS Testing


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