Variable-speed drives can also run a motor in specialized patterns to further minimize mechanical and electrical stress. This reduction is in accordance with affinity laws that define the relationship between various centrifugal load variables. Certain applications involve two-quadrant loads operating in quadrant I and II where the speed is positive but the torque changes polarity as in case of a fan decelerating faster than natural mechanical losses. Retrieved Feb 15,
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Poor grounding of motor stators can lead to shaft-to-ground bearing currents. Now, going one step further, if the magnetising voltage is close to the stator supply voltage, then the stator resistance can be neglected from the torque equations:. In starting a motor, a VFD initially applies a low frequency and voltage, thus avoiding high inrush current associated with direct-on-line starting.
The term VVVF is often used in the mining vvf — it refers to a variable voltage variable frequency drive. In large motors, the stray capacitance of the windings provides paths for high-frequency currents that pass through the motor shaft ends, leading to a circulating type of bearing current.
The frequency and voltage applied to the motor are ramped down at a controlled rate. When the power company's voltage becomes distorted due to harmonics, losses in other loads such as normal fixed-speed AC motors are increased. VFDs are available with voltage and current ratings covering a wide deives of single-phase and multi-phase AC motors.
This filtered DC voltage is converted to quasi- sinusoidal AC voltage output using the inverter's active switching elements. A variable voltage, variable frequency drive is capable of operating over a wide speed range. Retrieved Feb 3, Most applications involve single-quadrant loads operating in quadrant I, such as in variable-torque e.
The capital cost of regenerative drives is, however, relatively high.
So which should I use? Passive filters involve the design of one or more low-pass LC filter traps, each trap being tuned as required to a harmonic frequency 5th, 7th, 11th, 13th.
This distributes the acoustic noise over a range of frequencies to lower the peak noise intensity. Legend for Tables 1 to 3. Electrical Control Systems in Industry. Fixed-speed loads subject the motor to a high starting torque and to current surges that are up to eight times the full-load current. The electrical world is full of acronyms.
AC vs DC The difference in the terms revolves around the type of motors they can control. DC is direct current — mostly used in extra low voltage circuits, batteries or solar applications.
Variable-frequency drive - Wikipedia
Retrieved from " https: Additional operator control functions might include reversing, and switching between manual speed adjustment and automatic control from an external process control signal. Cycloconverter, Scherbius, matrix, CSI, and LCI drives inherently allow return of energy from the load to the line, while voltage-source inverters require an additional converter to return energy to the supply. Thus, no braking resistor is needed, and the efficiency of the drive is vgvf if the drive is frequently drivea to brake the motor.
A higher carrier frequency produces a better sine wave approximation but incurs higher switching losses in the IGBT, decreasing the overall power conversion efficiency.
Proceedings of the IEEE. Retrieved Feb 14, Operation of the motors above rated nameplate speed base speed is possible, but is limited to conditions that do not require more power than the nameplate rating of the motor. Elevated-voltage stresses imposed on induction motors that are supplied drivess VFDs require that such motors be designed for definite-purpose inverter-fed duty in dfives with such requirements as Part 31 of NEMA Standard MG Dynamic braking wastes braking energy by transforming it to heat.
VFDs are used in applications ranging from small appliances to large compressors. Hardware - Part 2: Variable-frequency drives are also categorized by the following load torque and power characteristics:.