Generator Sizing for a Block Plant Off Grid: Capacity, Surge and Margin

Generator sizing for a block plant off grid starts with the total installed power of the plant, not with the label on a single machine, and it ends by checking the starting surge of the largest motor. A plant that adds up its motors and orders a set of that size will trip on the first start. The real numbers are smaller than they look: our QST6-15 carries 34.4 kW of installed power and runs on about 30 kW of site supply, while a QST1800 at 104 kW needs a different electrical room altogether.

Why is the generator smaller than the sum of the nameplates?

Because a nameplate is a peak figure for a motor that does not run at peak all day. On a block line the mixer, the hydraulic power unit, the vibrators, the conveyors and the compressor follow each other around one cycle. Only part of the installed power is drawn at the same moment, and that overlap, not the total, is what the set has to carry.

The gap is easy to see on a small automatic machine. The QST6-15 shows 34.4 kW installed, and roughly 30 kW of site supply is enough to run it, because the vibrators and the hydraulic pump are seldom at full draw together. Sizing from the nameplate total alone buys a bigger set than the plant needs, and the fuel bill follows the set size every single day.

How do you add up the load for a block plant?

Count every load, not only the block machine: the mixer or batching unit, the belt conveyors, the hydraulic power unit, the vibrators, the compressor, the water pump, curing fans and lighting, and the workshop or office if it sits on the same set. Then apply a demand factor rather than using the raw total, because the loads that genuinely run together are fewer than the list suggests.

For the four machines we build, the installed power is 34.4 kW on the QST6-15, 44.9 kW on the QST8-15, 57.6 kW on the QST1200 and 104 kW on the QST1800, which develops 280 kN of vibration force and 31.5 MPa of hydraulic pressure. Those four figures describe four different generator rooms, and none of them is the whole load list on its own.

Add the starting surge last. A motor started direct on line pulls roughly 2.5 to 3 times its running current for a few seconds, and the set has to carry that without letting voltage fall far enough to drop a contactor or reset the control panel. A star delta starter or a soft starter lowers the surge, and it is cheaper than a larger generator.

Then compare your total with the rating that matters. Diesel sets are sold with several ratings, and the one you can use for a long production day is the continuous or prime figure, not the short term peak. Keep a margin above the calculated load so that a second machine, extra lighting or a bigger compressor does not put you back in the market for another set.

Which generator capacity matches which machine?

There is no single answer, because the generator follows the plant rather than the model. As an order of magnitude, a plant built around a QST6-15 at the 34.4 kW installed figure and about 30 kW of site supply sits in a much smaller class than a plant with a QST1800 at 104 kW where the mixer, the conveyors and the curing area run on the set as well.

The practical method is to size the set from the machine's electrical data sheet plus your own ancillary list, then check the result against the set's continuous rating. A set that only just covers the load on its one hour rating will not survive a 10 hour production day, and the day it fails is usually the day a large order is waiting on the yard.

What happens when the generator is undersized?

The first symptom appears at start up: voltage and frequency dip, the cycle stutters, a contactor drops out, or the control panel resets in the middle of a pallet. In the plant the operator notices wet block faces and short fills before anyone notices the generator at all.

Repeated low voltage heats the motors and shortens their life, and hydraulic pressure sags during pressing, which shows up as lower density and weak edges. An oversized set has its own failure mode: run far below half its rating for long periods, a diesel engine burns incompletely, deposits build up and service intervals shorten. Between the two sits the working range, with enough margin for starting and enough load to keep the engine happy.

Two other symptoms get blamed on the machine when the set is the cause: a press that will not hold pressure at the end of a long day, and a mixer that slows when the vibrators start. Both are load steps the generator is failing to absorb, and both are fixed at the electrical design stage, not by adjusting the mix.

A generator checklist to fill in before you order

Site voltage and frequency come first: tell us whether the plant is on 380 V, 400 V or 415 V, and whether the supply is 50 Hz or 60 Hz, because the motors and the control panel are ordered for one of them. Altitude and the hottest working month matter as well, since both reduce the power a diesel set can deliver.

Then list what runs at the same time, the longest continuous working day in hours, the cable distance from the set to the machine, how fuel reaches the site and how often it can be refilled, and whether grid power may arrive later. If a second machine is planned in the next two years, say so before the set is chosen rather than after.

One more item that costs nothing and saves a week: agree who is responsible for the changeover, the earthing and the protection settings between the set and the plant. On an off grid site those answers usually live with the local electrician, and the local electrician needs your machine data before the set is delivered.

Send your site data and we will confirm the configuration

Send the machine model and count, the ancillary list with their ratings, your site voltage and frequency, the altitude and the hottest working month, the daily running hours, and how fuel and spare parts will reach the site.

We will confirm the machine configuration and the documentation set that matches your power supply with the quote, including the electrical data you need to hand to the local supplier. The generator itself is usually bought locally, and a correct data sheet from the machine side is what makes that purchase right the first time.