INVERTERS FOR AC MOTORS
...provide a waveform very close to sinusoidal, but this increases the losses in the drive. Thus the switching frequency is limited to 3 or 4 kHz. The devices with low switching frequency (like SCRs) also cause harmonics in the wave, thus forcing a deration , leading to a machine of a higher frame size. There is also a possibility of cogging/ non-continuous rotation due to a pulsating torque at low frequency of operation. The latest device technologies like IGBT facilitate a higher switching frequency (as high as 20 kHz) and a near sinusoidal wave is possible, with no requirement of deration, working to its full capacity. The device efficiency is as high as 97-98%. The harmonic content is less than 2.5% as is the noise due to a higher frequency of switching. On the contrary, there is a possibility of larger voltage spikes (dV/dt) due to high switching frequency and a relatively higher voltage, which is nearly 3 times the DC bus. There is also a likelihood of over voltage due to long cable lengths. This is because a high dV/dt, associated with a low rise time could result in over voltage (double), if the full reflection of the pulse waveform is allowed. The voltage spikes cause corona discharge, affecting the insulation system, especially degrading the organic material. This leads to a shortened life of the motor, due to inter turn (usually first and last) short, earth fault etc. Non hygroscopic, inverter grade insulation process, use of high pulse endurance wire, high grade electro-magnetic steel laminations for low core loss and high efficiency, reinforced end turn lacing, lead separators etc. (2) Unbalanced input current (3) Input voltage waveform distortion When the power supply system is connected to a large capacity thyristor power converter etc. its voltage ay cause high waveform distortion. The inverter connected with this type of power supply system may be damaged by a large harmonic current.The effet can reduced by using the AC reactor. (4) Vibration When a motor is used with an inverter ,two conditions must be taken into consideration. !. harmonics are included in the inverter output voltage and current 2.Operation is performed at any rotating speed between low and high speeds Torsional vibration is given to the shaft by the pulsation torque (six times the fundamental frequency) generated by low harmonic components included in the inverter output wave form A large vibration may be caused when the torsional vibration matches the natural vibration of the shaft. Vibration problems can be reduced by a larger extent by reducing V/F ,increasing the rigidity of components like the installation area , shaft etc,using a flexible coupling or inserting an AC reactor in the inverter output circuit. (5) Beat (6) Noise The noise of the motor driven by the inverter differs in volume and quality due to harmonic components included in the inverter output or the motor current as compared to the motor driven by the commercial power supply. Further ,radio wave noise is generated by the inverter input and output . This may cause interference with communications equipment etc. This noise can be reduced by connecting a filter to the inverter input and output.and bundling the inverter output cables in the grounded steel conduit. (7) Torque pulsation 7a. Shaft voltage The shaft voltage is more often generated in the motor driven by the inverter due to harmonic components .The shaft voltage may be measured across the shaft ends and across the shaft and ground (8) Effect on bearings There is also a likelihood of bearing currents due to high dV/dt, which causes a capacitively coupled voltage to ground across the motor bearings. Grease having partial insulating effect gives rise to accumulation of electric charge. Grooves may occur on the bearing raceway surface when the discharge takes place due to breakdown of insulating property of grease and bearing might become noisy. Hence special conductive grease and a shaft grounding brush have to be used. (9) Effect on losses For high speed operation (at constant power), the air gap flux decreases with increase in frequency above the rated frequency, with a constant supply voltage. Iron losses can become excessive for a frequency above 75 Hz for a 50 Hz designed motor, necessitating a low loss or thin lamination material. The blower/ fan should take care of increased noise and windage loss. A suitable bearing should be used. Motor requirements: Deration for low speed operation, forced ventilation for very low speed operation, reinforced insulation system, adequate thermal margin to account for harmonics, pr...