Can I supply a Variable Speed Drive with a diesel generator supply?

Many essential processors have backup generators to ensure continued operation if the mains supply should fail. However, if variable speed drives are involved in the process, it is essential to make certain that the drives will still work on the generators supply. If the drives fail to operate the first time emergency power kicks in, then the investment in backup power will have been of little use.

A backup generator will start up on supply loss, run up to speed and then be able to accept load – not seamless, but all in a relatively short time.  This may seem like an ideal application for a variable-speed drive, as starting large motor loads direct on line would cause significant volt drop. However, the supply from a standby generator of this nature will almost inevitably be much weaker than the mains supply that it is temporarily replacing.

The main issue is that the fault level of the generators supply will be a lot lower than the network fault level, perhaps only one-tenth. This means that the distortion caused by the harmonic content of the drive must be considered.

In practice, most generators will cope with a voltage distortion of about eight percent, for a single unit, compared to the five percent which a 400 V public electricity supply is limited to. Filtering may be a good idea, but careful consideration is recommended – check with the drive manufacturer for the most appropriate filter type for your application.

Some filters are set up on the basis of the lower impedance seen on the mains and need to be checked that they will not resonate when used on a generator; the only types that do not resonate are closed loop active filters and active rectifiers, but in general they do need a restart, to re-identify the supply. It is important to check and to test out the equipment, rather than to wait for the first outage.

Another point is that a generator is normally rated in kVA at 0.8 power factor and the prime mover is generally rated to produce only the kW of the system plus its losses. It is again important not to exceed the kW rating of the generator, or at least obtain the manufacturer’s advice.

And remember using a variable speed drive to control motors fed by diesel generators reduces the power required by up 40%, this means up to 40% less diesel fuel required to run your generator. A huge potential saving and as a variable speed drives only pulls motor full load current on start up then the actual generator size can be vastly reduced.

This is due the current capacity of the generator only having to be rated to the Full Load Current of the motor rather 7 x full load when using a standard Direct On Line motor starter.

So by using variable speed drive with your diesel generator you have reduced the size of the generator and hence rental costs along with diesel running costs. Plus you are doing your bit for the environment.

Quantum offer variable speed drives for hire to match with temporary generators sets. We have over 1000 variable speed drives ex stock from 15kW up to 1500Kw all mounted in IP54 enclosures, with Emergency Stop Circuit and Line Contactors ready for despatch to you.

Call us any time day or night for technical expertise or emergency response, we ‘Guarantee’ to respond.

For more information or to discuss your particular application in details call Quantum Controls on 0330 9000 247 or [email protected]

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