Energy savings with variable speed drives

This paper focuses on the economic benefits that can be obtained by replacing mechanical flow control of pumps and fans with variable speed drives. Firstly, different topologies of variable speed drives are discussed: rectifier topologies (thyristor, IGBT) as well as the control of inverters with pulse width modulation. A short overview of the advantages for the different topologies is given. The most important restrictions are also mentioned. The core of the paper consists of three case studies, which are taken from our energy consulting experience in Belgian industry. Case study 1 discusses VSD installation on the circulation pumps of the electrocoat bath in a car assembly factory. Before, flow was controlled by valve regulation. Installing a variable speed drive instead generated a yearly saving of 176400 kWh for one pump. The payback time amounts to approximately 14 months. Case study 2 explains the economic benefits of VSD on lubricated screw compressors. Depending on the load profile, energy savings up to 41% can be obtained by applying frequency regulation instead of load/unload regulation. Case study 3 shows the case of a convertor in a stainless steel factory. Two 800 kW fans blow air into the convertor. The air flow is controlled by a valve. Replacing the valve with a VSD implies a yearly saving of 4188277 kWh