Electrical drives to increase the fluid processing efficiency

Drives devoted to fluid processing (pumps, fans, etc.) do not require high performance controls strategy but, since the fluid flow depends on the motor speed, a motor speed regulation is mandatory. In a standard drive the speed command is obtained by directly imposing the output voltage and frequency through a PWM voltage source inverter (VSI); the current loops are present just as inverter protection and low dynamic performances, without management of acceleration transients, can be accepted. In this paper some methods to improve the general efficiency of these drives are experimentally tested and compared. A first method is based on separating the voltage and frequency regulation tasks: the proposed converter topology is based on a variable DC link voltage supply followed by a unitary modulation index (UMI) PWM inverter. Two types of variable DC supply are examined: an AC/DC controlled rectifier and an the use of an AC/DC diode bridge and of a DC/DC converter. A second efficiency improvement technique is considered based on the possibility to reduce the supply voltage at speed lower than the rated one, in order to reach the maximum motor and inverter efficiency. This condition can be attained following an appropriate control strategy based on a minimum current control (MCC). Experimental tests and comparisons of the proposed solutions with a standard drive are pointed out and single improvements in motor and inverter efficiency are evaluated and discussed. Furthermore noise measurements are carried out in order to complete the comparison between the different supply converters.