How do Variable Speed Drives (VSDs) Work?

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How do Variable Speed Drives (VSDs) Work?

AC induction motor speed depends on the number of motor poles and the frequency of the applied power. When operated from a constant frequency power source (typically 50Hz in South Africa), AC induction motors are fixed speed devices. AC motors can however have their speed changed by changing the frequency of the voltage used to power it up. If the voltage applied to an AC motor is 50 Hz (as in South Africa), the motor works at its rated speed. If the frequency of the supply voltage is increased, the motor will run faster than its rated speed, and if the frequency of the supply voltage is reduced, the motor will run slower than its rated speed. The innovative exploitation of these capabilities (of AC motors) is what brought Variable Speed Drives (VSDs) into prominence. VSDs connect to standard AC induction motors with control capabilities for adjusting speed, torque, and horsepower (similar to the principles of DC drives), making AC squirrel cage induction motors as controllable and efficient as their DC counterparts.

VSD technology continues to evolve, with each new generation of VSDs providing higher performance, smaller size, and more advanced control. The earliest VSDs had rather small solid-state components that limited the amount of current it could supply, limiting the size of the motor that could be controlled (and were therefore not commonly used). These early VSDs also used linear amplifiers to control all aspects of the VSD and jumpers and dip switches to provide ramp-up (acceleration) and ramp-down (deceleration) features by switching larger or smaller resistors into circuits with capacitors (to create different slopes).

With the arrival of advanced microprocessors VSDs evolved into extremely versatile devices that not only controls the speed of the motor but also protects against overcurrent during ramp-up and ramp-down conditions, provide methods of braking, power boost during ramp-up, and a variety of controls during ramp-down etc. These modern VSDs deliver high frequency source power during start-up to reduce current due to the increased inductive impedance which can be advantageous if control is desired after full speed is reached and allows for continuing control as the load on the motor is changing. Modern VSDs are efficient and can start motors under considerable loads while controlling the speed of the motor, the direction the motor shaft is turning, the torque the motor provides to a load and many other motor parameters that can be sensed. Not only do modern VSDs provide an extremely smooth and controlled start, but they also efficiently control motors to nearly any desired level of speed, torque, or position, including over-speed and full torque at zero speed. Special applications and safety features are also included, providing even greater benefit. In most cases, variable speed control greatly improves processes and yields impressive energy savings.

Modern VSD benefits include power-factor correction and low power consumption, along with full-speed control and not just start/stop control. Other benefits include dynamic braking, PID control, built-in I/O and logic, circulatory control mode, multi-motor control, communication interfaces etc. In the simplest VSDs or applications, the speed reference is simply a set-point; however, in more complex applications, the speed reference comes from a process controller such as a Programmable Logic Controller (PLC). Another capability that some VSDs have is the ability to run 3-Phase motors when only Single-Phase power is available (act as a power converter).

So how do VSDs work?

Modern VSDs contain three key components which can be used to describe its basic working principle:

  1. Rectifier: The first step in this process is to convert the AC supply voltage into DC using a rectifier.
  2. DC Filter (DC Link or DC Bus): The DC circuit contains the capacitor and inductor used for filtering (smoothing) the DC power output from the previous step which contains voltage ripples.
  3. Inverter: The basic working principle of an inverter is switching the DC on and off so rapidly that the motor receives a pulsating voltage similar to AC. This conversion is typically achieved using power electronic devices such as IGBT power transistors using a technique called Pulse Width Modulation (PWM). The output voltage is turned on and off at a high frequency, with the duration of on-time, or width of the pulse, controlled to approximate a sinusoidal waveform. The switching rate is controlled to vary the frequency of the simulated AC that is applied to the motor.

The VSD also regulates the output voltage in proportion to the output frequency to provide a relatively constant ratio of voltage to frequency (V/Hz), as required by the characteristics of the AC motor to produce adequate torque. E.g., if a motor is designed to operate at 460 Volts at 60 Hz, the applied voltage must be reduced to 230 Volts when the frequency is reduced to 30 Hz. Thus, the ratio of Volts per Hertz must be regulated to a constant value (460/60 = 7.67 in this case). To ensure operation of the motor within established parameters, this entire process is controlled by a microprocessor which monitors the:

  • Incoming Voltage supply
  • Speed set-point
  • DC link Voltage
  • Output Voltage and Current

In summary, with a standard AC across-the-line motor starter, line voltage and frequency are applied to the motor and the speed is solely dependent on the number of motor stator poles. In comparison, a VSD delivers a varying voltage and frequency to the motor, which determines its speed. The higher the frequency supplied to the motor, the faster it will run. Power applied to the motor through the VSD can make the motor working speed lower than the nameplate base speed or increase the speed to synchronous speed and higher (motor manufacturers list the maximum speed at which their motors can safely be worked).

How a VSD works

For more information regarding VSDs, please refer to the following related blog posts:

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