German industry alone could save DM2.8bn ($1.5bn) a year by using more electronic speed controls and energy-efficient optimised electric motors. So claims a report from the German electrical and electronic manufacturers association ZVEI (Zentralverband der Elektrotechnik und Elektronikindustrie). And carbon dioxide emissions would be cut by 11m tonnes as a result of reduced energy consumption. This potential is even higher in other countries. An additional benefit is that less investment would be required in new power generation facilities.
In 1997 more than 18% (462bn kWh) of Germany’s total energy consumption was electricity and 42.5% of it was used by industry. Electric drive systems account for more than two thirds (133bn kWh) of this industrial usage. ZVEI estimates that German industry has as many as 30m low voltage motors, and more than 3.2m were sold there in 1997. Around 180m fractional horsepower motors and 464,000 variable speed drives (VSDs) were also sold in the same year. Sales of frequency converter variable speed drives increased to 544,000 in 1998. Energy is about 97% of the total purchase and running cost of a motor during its lifetime.
Electronic speed controllers operate about 5% of the motor capacity in German industry but an increase to 35% could sensibly be achieved, the report says. An average 40% energy saving per drive unit, equates to potential electricity savings of about 8%, or 16bn kWh. Similarly, efficiency of standard motors could be raised by 8% on 1kW models and 1.5% on 100kW units, for example, by using more electroactive material in their construction. If half the motors in use were ‘energy optimised’ in this way, and overall efficiency was improved by 4%, energy cost savings of DM400m ($211m) could be expected.
Some machines need electronically controlled motors for process technology reasons but others, such as pumps and fans, are often operated using energy-wasting mechanical throttling devices. Electronic control has the most potential here; it can save 20% to 70% of energy costs and pay for itself in a few months. Too much priority is given to initial purchasing costs and not enough to longer term running costs.
Typical uses for frequency-controlled VSDs in the lifting industry include hoisting and travel motions on EOTs and tower cranes and pumps. An electronically controlled pump motor saves energy because a governor valve (with its attendant losses) is no longer needed to reduce pressure. Directly switched three-phase motors have high rotor losses during acceleration which result from a difference between rotor speed and the mains supply frequency. Stored kinetic energy under braking is usually dissipated and lost in the form of heat. But a variable speed drive increases frequency in line with rotor speed, and under braking returns the kinetic energy to the power supply. Motors have a longer working life as they are not subjected to such high thermal loading.
An efficiency of 90% can be expected from a typical 11kW standard motor and as much as 94% for a 100kW unit. Losses come from current flow in the windings, demagnetisation and eddy currents, mechanical friction, and parasitic effects. Design can influence any combination of these losses and produce improvements, the added cost of which can be justified according to application. Three phase motors generally run at between 750 and 3,600min-1 but for many applications output speeds of between 15 and 300min-1 are typical. A variable speed drive to achieve this speed reduction is uneconomic which is why gears, ideally spur or bevel, are used. The electronic motor control then operates on the range already reduced by the gears. Permanent magnet synchronous motors can also be used and are a highly efficient design without rotor or exciter losses.
