Napędy elektryczne wielobiegowe

Multi-speed electric drives are usually used in these cases, where work cycle of a given drive requires two or more different rotational speeds. Examples of such drives are pumps, fans, centrifuges, lifts etc. Another reason for using multi-speed drives is decreasing the energy consumption. Low energy consumption is ecological and economical criterion of the drive. If energy is not spent unnecessarily, then it need not be generated. About 98 of electrical energy in Poland is produced by burning coal and therefore, if less coal is burnt, the emission of carbon dioxide, sulphur dioxide and nitrogen oxides goes down. Lower speed of the drive causes diminished wear of driven mechanical devices and decreases generated noise; a good instance of such drive is belt conveyor. Energy-saving drive is the type of drive drawing minimum energy from the network necessary from the viewpoint of the engineering process - Fig.1. Energy required for engineering process during time t (e.g. day, month, year) is equal to. The driven mechanical system imposes load torque T(ob) and angular speed omega(m) on the motor. Equation (1) shows that the minimum energy consumption is achieved, when the drive operates at minimum allowable angular speed (omega (m min)). Usually the load torque is also minimum under such conditions. This type of working cycle algorithm in modern electric drives can be obtained by using cage induction motor supplied from power electronics converter (inverter). If we assume that, for a chosen drive, e.g. belt conveyor, the energy consumption per time unit (e.g. t = 24 h): - is equal to 100 per cent, when angular speed is kept constant (omega(m) = const) - drops down to 50 per cent in accordance with omega(m min) criterion (for a belt conveyor this speed corresponds to 100 per cent loading of the belt), then when two-speed drive is used, the energy consumption will go down to c. 70 percent, and with three-speed drive to c. 60 per cent. Two-or three-speed drive is cheaper and as easy to design as one-speed drive. Multi-speed drives can utilize the following motors: - multi-speed cage induction motors, - system of two induction slip-ring motors, supplied either individually or as a cascade system, - synchronous motors of special design, stator winding with switchable number of poles, - induction or synchronous motors supplied either from the network or from power-generator set (lower frequency, also lower voltage).