Variable Frequency Motor

Variable Frequency Motor focuses on its performance parameters of overload capacity, starting performance, efficiency and power factors. However, since its critical slip ratio is inversely proportional to power frequency, a variable frequency motor is capable of starting operation directly even when the critical slip ratio is around 1. Hence, the overload capacity and starting performance are unnecessary to be taken into consideration during its design. Commonly adopted solutions to said issue include:1) Reduce the electric resistance in stator and rotor to the minimum amountReducing electric resistance in stator helps to reduce the basic copper consumption so as to offset the increase in copper consumption caused by higher harmonic waves.2) In order to restrain the higher harmonic waves in electric currents, there is a need for properly increased electric induction in the motor. However, skin effect becomes greater when the rotor slot leakage resistance becomes larger, resulting in higher copper consumption caused by higher harmonic waves. Therefore, the amount of slot leakage resistance in the motor shall be controlled to provide a rational electric resistance match within the entire range of speed regulation.3) Generally the main magnetic circuit of a variable frequency motor is designed to be unsaturated, due to two reasons: firstly, higher harmonic waves increase the saturation of magnetic circuits; secondly, the output voltage of frequency convertor is to be increased accordingly in order to increase the output torque during low frequency operations. Features : During the structural design, the effect nonsinusoidal power places upon the insulation structure, vibration, noise, cooling approach and other aspects in a variable frequency motor shall be taken into consideration as well, with regard to the following issues:1) Insulation level. Normally Level F or higher is preferred, with enhanced insulation to ground and coils. One also need to take into consideration the capacity of insulation impulse to withstand voltage. 2) With regards to the vibrations and noises of the motor, the rigidity of motor components and the entire motor thoroughly reviewed, and their inherent frequency shall be maximized so as to prevent resonance with force waves.3) Cooling: compulsory ventilation cooling is normally applied, in which the main motor cooling fan is driven by an independent electric motor.4) Shaft current prevention: bearing/shaft insulation shall be applied to electric motors with a capacity higher than 160KW. In such motors unsymmetrical magnetic circuits and shaft currents are likely to occur; when combined with currents produced by other high frequency energy, the shaft current becomes greater, resulting in possible damage to the motor shaft. Therefore, insulation measures are necessary on such occasions.5) For constant-power variable frequency motors, special high-temperature resistant grease lubricant shall be used in case that the rotary speed exceeds 3000/min, so as to offset the temperature rise in motor shafts.