An inverter, or variable speed drive (VSD), in principle is an electronic device that, supplied directly from the power line, through a sophisticated control, allows to change the frequency and the amplitude of the supply mains voltage of a three-phase (or single-phase) asynchronous electric motor, and consequently, allows to change its angular rotation.
The range of Inverters Electroil specific for Industrial motor is presented in power from 1.5kW to 22kW.
THE MAIN ADVANTAGES RESULTING FROM THE USE OF THE INVERTER :
- Optimization of the industrial processes: the components that can better take advantages of the energetic benefifi ts of frequency converters are the ones having a resisting torque which varies with quadratic law by varying of the speed and, among these, can be mentioned the ventilators, compressors and centrifugal pumps. The energetic consumption reduction resulting from the utilisation of a VSD in these cases can be substantial, up to 50%.
- Replacement of choking mechanical systems : as far as the pumping and ventilation systems are concerned, generally the prevalence provided by the pump, or by the ventilator, is much higher than the one required by the downstream circuit, whose load can also be variable during the time. Therefore it’s necessary to adjust the load of the circuit, and this is usually done through the partial closing of the discharge valve (vent, lock-gate or restriction valve). From the energetic point of view, it involves a waste proportional to the additional load generated by the valve, since the motor keeps on turning to a constant speed. The installation of a variable speed drive on the pump or ventilator drive motor allows to adapt the number of revolutions of the motor (and thus of the impeller), reducing the absorbed power in case of low loads.
- Reduction of the noise: the noise generated by an electric motor jointed to its own load (ventilator, pump, conveyor belt, etc..) is principally connected to the aerodynamic shape of the load and to its rotation velocity.
- Soft start and a more signififififi cant reduction of the wear and tear of the components: the start-up through inverter is the only one type of start-up that allows to contain the starting current of the motor. This allows to avoid high starting currents and high starting torques and therefore the resultant mechanicals stress. In addition, peaks of demands from the power mains may involve generally increases on electricity supply costs.
- An integrated moto-inverter consists in the union of an inverter along with its correspondent three-phase asynchronous motor, and its peculiarity is the one to eliminate the costs for installation, wiring, programming and testing of the motor + inverter system as well as the risks coming from possible mistakes related to such an operations.
WORKING PRINCIPLE OF THE INVERTERS FOR INDUSTRIAL MOTORS :
The synchronous speed (n1 in rpm) of a three-phase asynchronous electric motor depends to the frequency of the supply voltage (f1 in Hz) and to the number of poles couple (p) according to the relation:
The rotor, with speed n2, pursues the synchronous speed n1, without ever reaching it; the slip, that substantially defines the difference between the rotor speed and that one of the rotating magnetic fififi eld (synchronous speed) depends on the applied load :
Being the synchronous speed directly proportional to the supply voltage frequency, it will be enough to vary the supply voltage frequency to be able to modify the motor rotation speed.
The effective power Pu delivered by an electric motor is obtained by the product of the motive torque (Cm measured in Nm) multiplied by the rotation velocity (n2 in rpm), or rather having to be the motive torque equal to the applied resisting torque (Cr measured in Nm):
Therefore, both the effective power and the power absorbed by the motor (P absorbed = Pu/effififi ciency) depend on the resistant applied torque and on the rotation speed: reducing the rotation speed, the power reduces in accordance of the variation of the applied torque.
The resisting torque depends on the type of machine that it’s considered and, by varying the rotation speed, it can show constant or quadratic run.
In the case of constant torque, the power is proportional to the rotation speed and therefore decreases linearly by decreasing the number of revolutions: a run of constant torque is present on applications such as belt conveyer, screw compressors, alternative compressors, mixers.
At the extreme opposite the case of quadratic torque: by decreasing the number of revolutions the power decreases with the cube of the speed.
The quadratic torque is present in applications such as centrifugal pumps and ventilators.
The installation of an inverter in manufacturing processes driven by electromechanical systems may depend to specifics production requirements or of plant engineering or energetic optimization (this last one especially in presence of highly variable loads).
MAIN APPLICATIONS :
- Replacement of single-phase motor through three-phase motor with Inverter supplied at 230V single phase.
- Speed regulation with start and stop ramps on the gearmotor as a replacement of the mechanical speed-variator with knob.
- Speed regulation by buttons, potentiometer, voltage or current signal and remote control on asynchronous motor for belt conveyer.
- Speed regulation on general industrial machines with the possibility of serial communication between multiple inverters.
- Speed regulation and torque control on cutting-off machines with braking resistances in presence of inertial loads.
- Speed regulation on oenologyc pumps with control of revolutions done through encoder or proximity.
- Control of other motors in general, ensuring a signifi cant energy saving.