What device is used to drive a motor at speeds that vary from zero to a maximum speed using digital or analog control signals?

The device used to drive a motor at speeds that vary from zero to a maximum speed using digital or analog control signals is the variable speed drive (VSD). A VSD allows for precise control of motor speed, enabling adjustments to be made based on specific requirements or applications. This is particularly beneficial in applications where speed modulation is crucial for efficiency and performance.

Variable speed drives can achieve this by utilizing methods like pulse width modulation (PWM) or frequency control. They convert the input power into variable voltage and frequency signals, which the motor interprets to adjust its speed accordingly. This capability is essential in processes that require changing speeds, such as in conveyor systems, fans, or pumps, allowing for enhanced productivity and energy savings.

In contrast, other devices mentioned, such as direct online starters or soft starters, primarily facilitate motor start-up and overload protection but do not offer variable speed control. A servo motor, while capable of precise position control and speed, typically operates with a dedicated controller and is not primarily meant for varying speeds in the same way a VSD is designed to do. Thus, the VSD stands out as the appropriate choice in this context.