Frequency Control in Variable Speed Drives: How, Why, and Applications
Introduction
Variable Speed Drives (VSDs) are electronic devices used to control the speed of electric motors. By altering the frequency of the electrical supply to the motor, VSDs enable precise control over motor speed, leading to enhanced energy efficiency and operational flexibility in various applications. In this article, we will explore how frequency changes in a VSD, the components involved, and the control methods used to achieve precise control.
How Frequency Changes in a VSD
The process of changing frequency in a VSD involves several steps, each serving a distinct purpose in modifying the electrical supply to the motor. The process roughly can be broken down into rectification, DC bus, and inversion.
1. Rectification
During the rectification stage, the VSD converts incoming alternating current (AC) power from the grid into direct current (DC) using a rectifier. This rectification process does not involve altering the frequency; instead, it transforms the AC voltage into a steady DC voltage.
2. DC Bus
The DC bus stage smooths and stores the DC voltage, maintaining a stable voltage level ready for the next step.
3. Inversion
The inversion process is where the key frequency changes take place. The VSD uses an inverter to convert the smoothed DC back into AC. This inverter uses power electronics such as transistors to create a variable frequency AC output. The inverter controls the output frequency by regulating the timing of the power switches.
Frequency Control in Motors
The frequency of the output AC voltage has a direct impact on the speed of the motor. By adjusting the frequency, the VSD can control the motor's speed:
Lower Frequency: Reduces the motor speed Higher Frequency: Increases the motor speedTypical Frequency Range and Control Methods
Typical Frequency Range: Most VSDs can vary the output frequency from 0 Hz (stop) to typically around 60 Hz or 120 Hz, depending on the motor and application.
Control Methods: Frequency changes can be controlled using different methods such as:
Open-loop Control: The VSD adjusts the frequency based on a fixed parameter without feedback. Closed-loop Control: The VSD adjusts the frequency based on feedback from the motor using sensors for speed and torque.Current VFD Designs and Pulse-width Modulation
Modern VFD designs use transistors to control both frequency and voltage output. They function as high-speed switches, pulsing DC on and off to the motor terminals in a pattern that produces a relatively smooth sinusoidal AC current. The switching pattern is known as Pulse-width Modulation (PWM).
Understanding PWM Through an Example
To better understand how PWM works, consider the inverter section of the VFD, which consists of six transistors. These transistors control the frequency and voltage output by managing the order and timing of switch closures:
At 0°: Close switch 1 and switch 5. Current flows into the motor windings at point B and out at point A, resulting in a voltage of 1V across each winding (AN V, BN V, CN V). At 60°: Leave switches 1 and 5 closed, and open switch 3, close switch 6. Current flows in B to C, resulting in a voltage of 1V across winding B and -1V across windings A and C. At 120°: Open switch 5 and close switch 2. Current flows in through coils C and out through coils A and B, resulting in voltages of V, V, and -1V. At 180°: Open switch 1 and close switch 4. Current flows out through coil B and in through coils A and C, resulting in voltages of -V, V, and V. At 240°: Open switch 2 and close switch 5. Current flows in through coils A and B and out through coil C, resulting in voltages of -V, 1V, and -V. At 360°: We are back at the starting point, with voltages of 1V, -V, and -V.If we plot the voltages across each winding to the neutral point in relation to the angle, we get a six-step waveform. Each cycle could take different durations depending on the switching rate, making the output frequency dependent on this switching rate.
Conclusion
The frequency in a variable speed drive is changed during the inversion process, allowing for precise control of motor speed. By adjusting the frequency of the AC output, the VSD effectively manages the performance of electric motors, enhancing energy efficiency and operational flexibility in various applications.