Motor energy audit and VFD retrofit business case for a spinning mill

Bhavani Spinners pays about ₹7 crore a year for roughly 95 lakh kWh. Its humidification plant, which keeps the spinning hall at the humidity cotton needs, is one of the largest loads after the ring frames. Today its fans run at full speed all the time and the airflow is controlled by dampers, and its pumps are throttled with valves. The maintenance manager has seen VFD retrofits at a neighbouring mill and wants to know if the numbers work here.

The plant has: - 4 supply-air fans at 37 kW, dampers set between 60% and 70% open; - 4 return-air fans at 22 kW; - 4 spray-water pumps at 15 kW with discharge valves half-closed; - 2 cooling-tower fans at 11 kW.

The mill runs 330 days a year, 24 hours a day. Energy costs about ₹7.80 per kWh on the HT tariff. The managing director will approve a retrofit only if simple payback is under 2 years and the savings can be checked afterwards.

You will build a motor register, analyse the spot readings an electrician took with a power analyser, model savings for each motor with the affinity laws while allowing for static pressure, specify the drives and their side effects on the mill's power quality, and write a business case with a measurement and verification plan. No site visit is needed: all the readings you need are in the task instructions, and the modelling is done in Python or a spreadsheet.

Done looks like a ranked list of retrofits with savings in kWh and rupees, a VFD specification an electrical contractor can price, and a two-page business case. Be honest where a retrofit does not pay back. Motors that are already lightly loaded or that serve a mostly static head can disappoint.