Determination of Power Rating of Three-Phase Induction Motors which Work Frequently, Intermittently, and Periodically Based upon Losses

Abstract

This paper proposes a novel approach for determining the power rating of three-phase induction motors which work frequently, intermittently, and periodically based upon losses. The approach is also based on the following aspects: (1) there are several processes in a period; (2) the output power of a process varies with time; (3) the experienced time of a process, the switched-off time between two adjacent processes, and the cycle time are perhaps very short; (4) the rotational angle of the motor in a process is given; (5) the moment of inertia and the load torque of the motor drive system referred at the motor axis in a process are provided. The relationship between total losses and slip is formulated and the motor drive model differential equation presented. From the relationship and the differential equation, the heat due to losses of a process can be calculated, and then the total heat of a period can also be worked out. Accordingly, the average total losses of a period can be obtained by dividing the total heat of a period by the cycle time. Whether the rated power of the motor is fit for the application depends on the results from comparing average total losses with its rated value. To determine a motor's power rating, relevant parameters must be known. This paper introduces a mathematical model for extrapolating those figures from a manufacturer's catalog data. Data used in the estimation include rated levels for power, voltage, speed, current, efficiency, power factor, and the ratio relating the breakdown torque to the rated torque. Estimated parameters include equivalent circuit parameters and others. Among these are the magnetizing branch current of the per-phase " o " type equivalent circuit referred to the stator, rated core losses, rated mechanical losses, etc. The approach presented in this paper is used to determine the power rating of induction motors which operate in the interested application. Since the approach is based upon quantitative calculation of losses, overheating will not be caused. The approach is particularly effective when there is frequent starting, with brief experienced times and switched-off times. The method has successfully determined the power rating of the pusher drive motor in high-speed palletizers (the motor starts 24 times a minute).