Variable Speed Drives

A VARIABLE SPEED DRIVE, EXPERT IN MOTION TECHNOLOGY 4. WHAT IS IT?

Industrial processes requiring precise speed regulation initially used direct current (DC) engines
controlled by solid-state electrical drives. This technique consisted of varying the speed in proportion
to the voltage. However, given the complexity of maintaining DC motors, recent applications rarely use
this system…

The high-speed electronic variable speed drive belongs to the family of drives whose electric motor is generally asynchronous and sometimes continuous, with a non intelligent drive of a power generally greater than 150 KW or 200 KW.

It consists of an electric motor, a user interface and a controller. In the case of incoming alternating voltage, the controller makes it possible to modulate the phase and the amplitude, as required. If the incoming voltage is continuous then the drive will only be equipped with an inverter.
In order to transmit the voltage or current setting, the most common method is pulse width modulation (PWM).

The PWM then operates an algorithm such as the spatial vector method in order to produce the AC voltage from a DC voltage. Performance is greatly improved by a control electronics using flux vector control. Most modern drives incorporate this function as standard. Knowing or being able to estimate machine parameters makes it possible to do without a speed sensor for most applications. In this case, a standard motor can be used, with the usual limitation of prolonged operation at low speed. However, all variable speed drives integrating switching devices (diodes, thyristors, IGBT, etc.) form a non-linear load which generates harmonic currents that cause wave distortion (voltage drop or disruption) in the electrical network. This wave distortion can disrupt both the customer’s electrical equipment and those of the electricity network if no immunity measures are taken. Harmonic resonances may also appear between the variable speed drives and the capacitor banks.