Adjustable speed drives (ASD) are devices comprised of electronic components that regulate the current or frequency of power to the motor. They convert the AC main’s fixed voltage and frequency to a variable output. Capable of providing different motor speeds and torques to suit a range of application needs, the ASD offers many benefits.
The frequency determines the speed of the motor, while the combined voltage and frequency determines the torque. The adjustable control feature allows the matching of the motor speed or torque to the process requirements.
Even though there are both fixed and variable drives, the adjustable version is preferred for a majority of applications. This is because it has a higher degree of motor control. Also, the ability to accurately and smoothly vary motor speeds from zero RPM to the maximum rated speed or higher.
Some of the major benefits of Adjustable Speed Drives include energy savings, efficient motor starting, reversing, and speed and position control. Typical applications include fans, pumps, blowers, conveyors, machine tools, film lines, extenders, hoists, cranes and more. Basic ASDs are used for pumps, fans, and other simple applications, while more advanced drives are used for complex applications. This is a good solution when precise speeds and torque are needed.
This adjustable speed drive is also known as a variable speed drive or variable frequency driver.
Why electrical drives are required
Controlling the speed of a motor using traditional methods involves running the motor at full speed and then using mechanical means such as gears, hydraulic couplings or pulleys. This is not only expensive, but also consumes a lot of energy. On the other hand, variable speed drives help in saving energy while providing more operational control in terms of speed, starting and stopping. A consumer will consider using a motor driver when there is a need to:
- Improve efficiency
- An application requires different speeds of operation
- Have more control and precision to accurately stop, reverse, or brake the motor
- Control the motor startup current
- Improve protection
- Control the motor based on parameters such as pressure, temperature, oxygen levels and other process parameters
Components and operation of adjustable speed drives
The drive consists of three basic sections: the rectifier, the inverter and a controller. The rectifier converts the AC mains voltage to a direct current voltage. The inverter then changes the DC voltage to an adjustable frequency and voltage based on a signal from the controller.
The controller keeps on monitoring the application requirement in a continuous control loop. It then commands the inverter to produce the required voltage and frequency to meet the application requirements. To do this, the controller, comprised of a microprocessor, takes the input signal from the sensor and compares this with a reference signal. It then generates the appropriate control signal for the inverter to vary its output power.
In a typical application, the controller will monitor process parameters such as pressure, flow rate, temperature or liquid level. For example, an oxygen sensor in a boiler provides a signal to adjust the speed of the fan and control the amount of oxygen in the boiler system.
Today’s drives, using microprocessors, semiconductor power switches, and SMDs are now smaller, compact, cheaper, more reliable and efficient. A typical drive will also include a filter to suppress harmonics generated when converting the DC to AC voltage.