Power electronic control has many advantages compared to old fashion electromechanical motor starting. The starting and stopping operations are smooth and perfectly controlled, and have low inrush currents. When the machine is in operation, its speed can be adjusted to suit to production constraints.
Both speed and torque can be regulated. A stabilized speed or torque must be maintained regardless of load variation or any other disturbance such as flow rate, pressure, and traction regulation, to name just a few. The new generations of power electronic drives are more than simple speed or torque controllers.
There are many embedded application functions that allow automation control, such as mechanical braking for hoists, process regulation, lift control, and many others. In most of the devices, a large range of field bus protocols allow communication with many types of controller. Some functions also contribute to machine safety.
Why we must control the starting of asynchronous motors? When the supply voltage is connected directly to a stationary motor, the resulting starting current is very high. There is often a severe voltage drop at the motor and at any other electrical equipment connected to the same supply. Squirrel cage motors are the only type of motor that can be used with direct-on-line starters. The starting characteristics of standard squirrel cage motors are well known, and are the same whoever the manufacturer is.
Essential Guide of Motion & Drives: catalogue