When making a decision to stop a motor or to run at no-load a number of factors must be considered; these include motor type, power rating, speed, starting frequency, restrictions on inrush current, power demand charges, and the extra winding stress imposed by repeated accelerations and associated reduction in life expectancy of the EIS.
Applications involving extended periods of light load operations
A number of methods have been proposed to reduce the voltage applied to the motor in response to the applied load, the purpose of this being to reduce the magnetizing losses during periods when the full torque capability of the motor is not required. Typical of these devices is the power factor controller. The power factor controller is a device that adjusts the voltage applied to the motor to approximate a preset power factor.
These power factor controllers may, for example, be beneficial for use with motors rated less than 3 kW operating for extended periods of light loads where the magnetization losses are a relatively high percentage of the total loss. Care must be exercised in the application of these controllers. Savings are achieved only when the controlled motor is operated for extended periods at light load.
Particular care must be taken when considering their use with other motors rated less than 3 kW. A typical 7,5 kW motor should have idle losses in the order of 4 or 5 percent of the rated output. In this size range the magnetization losses that can be saved may not be equal to the losses caused by the distorted voltage wave form introduced by the power-factor controller.
Applications involving overhauling loads
Overhauling loads typically result in energy waste if some form of dissipative braking is used. Examples of overhauling loads are: deceleration of high inertia loads, absorption test stands, unwind stands, web process stands, and downhill conveyors. In these cases energy can be saved by the use of regenerative devices.