ac Motor Control and Protection: Starting, Running Protection, and Surge Protection

Abstract

Starting methods for ac motors range across a broad spectrum from across-the-line starting to various types of reduced voltage starters; more recently, electronically controlled starting is being used. When rated voltage is applied to a motor at standstill, the inrush current is several times the rated load current. For motors in the medium ac size, the starting torque accompanying this high inrush current generally exceeds 100% of full-load torque, and for smaller horsepower sizes, starting torques of 200% or more are common. For most applications, these starting torques are adequate to start the load and, if the power system supplying the motor is not adversely affected by the inrush current, across-the-line starting can be used. However, if the power supply suffers a severe voltage dip when required to supply the motor’s inrush current, then an alternate type of motor starting control may be needed. Light flicker is a common symptom of voltage reductions that occur during motor starting. This is often tolerable, but if the voltage reduces to such a low level that the motor will not start the load, then some corrective action needs to be taken. Computers and production equipment may also be susceptible to voltage dips and may malfunction. If any of these problems create unacceptable conditions, then reduced voltage starting is indicated.