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Simple Guide to Choosing Generator to Motor Size

How to choose a generator motor size

One question we are always asked when we answer the phone to a customer is, what size generator do I need?

Unfortunately 9 times out of 10 it is not as simple as guesswork on their annual consumption. The main reason for this is motors have two types of starting methods, DOL (Direct online) and SD (Soft start or Star-Delta). Most consumers will calculate the running kW and use PF of 0.8 work out what kVA sized generator they need. This does work on a lot of sites but is not always the best way to calculate a generator. This is why here at CPS offer site visits, either through ourselves or with our appointed dealers.

Once on-site, we will calculate the customer’s power rating and evaluate the size of the generator. Below is a mocked-up idea of a Power rating evaluation, to give you an idea of what we offer. These figures are mocked figures at SD start to give you an idea of how we calculate. Other companies will do things differently, but we find that everyone will be on a par with the end figure.

We also offer the customer a number of solutions as we understand not all motors will start at the same time, plus you will have the added hassle of oversizing your generator, when a lot of the motors may run in sequence. We tailor our reports for our customers so they know the facts and figures before they decide on the right generator for them.

Below is a power Rating Evaluation sheet we fill out on every site visit we attend. This does not give us a definitive answer on what size generator you need as sizing a generator has many implications, but it gives us a good starting ground to explain to a customer what the maximum sized generator he is going to require.

Using our power rating evaluation to help give the client options

We understand that sizing the generator to all motors will oversize the generator, which can have detrimental effects on the generator, but at the same time, under-sizing will have an impact not only on the engine but also on the application and circuit breaker. It is about finding the middle ground and educating the customer. If the customer only has one option and that is to start every motor on the premises, then there is not a lot of educating needed for the client. If you can educate the customer on the best practice for starting certain motors firstly. Then you can offer the client a smaller sized generator which would be more cost-effective and better for the generator. We have had a project where we have had to over-size the generator due to the initial start-up of several motors at once. The solution was to add a dummy load on to the generator. This helps with his consumption and efficiency of the engine.

Some Notes for Sizing Generators

If you size the generator too small will cause voltage fluctuations and damage to the generator or equipment winding, a Diesel generator will need to run between 30% and 75%. I always find as a rule of thumb that anywhere between 60% and 75%  rated load for best efficiency and longevity of your Diesel Generator

Rule of Thumb

Note 1 D.O.L. – Direct Online starting (high current starting).

Note 2 S.D. – Star Delta (smooth starting med-high current).

Note 3 – A dummy load or extra load will need to be applied to avoid glazing once running.

One horsepower (1 hp) equals 0.75 kilowatt i.e. 20hp x 0.75 = 15kW

Always warm up your generator before you apply load, we offer water jacket heaters to assist your generator and make sure it is not starting cold.

When running in sequence make sure your motors are run from smallest to largest if possible. This will also stop your circuit breakers tripping

If you have single-phase load then have an electrician balance the load across all phases.

When the motor is operating, after the start, the requirement will be 1kva for 1hp i.e. 20hp – 20kva used when running.

Size the generator to run between 60-80% full load rating of the generator

Amps available on a generator per phase = Kva x 1.39. i.e. 20kva = 28amps/phase over 3 phases.

If you are looking for some advice on sizing a generator, then why not call our helpful team on 01757428140 or alternatively email info@cps-generators.com and we would be keen to size the right solution for your home or business.

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