This article is about the starting of motors and the effect of motor loads on an electrical system, it also contains a description of the control of those motors. Although this article is intended to be purely descriptive, it has by necessity to include some of the formulae associated with motors and electrical networks.
Any load placed on an electrical system will affect the supply; with most loads the rated current of the load will be relatively constant from the instant that the load is switched on to the instant it is switched off. When designing an electrical supply system the maximum load that can be applied to the system can be calculated to prevent excessive voltage fluctuations in the electrical system during load application and removal, excessive fluctuations can effect other loads connected to the system; either on the same consumers system or other consumers systems.
This is not the case with motors, when a motor is switched on the load presented to the electrical system is much greater than the load when the motor is running. To maximise the size of motor that can be supplied by the electrical system the ‘switch on load’ of the motor can be controlled by varying the method employed to start the motor.
This article examines the various starting methods and the determination of the most appropriate starting method for the electricity system to which it is to be connected.
This article also examines the control systems that may be employed to control a motor and the standardisation of controls, to enable the early determination of the electrical system requirements and the placement of contracts for the capital equipment of a project.
There is a trend to integrate systems more and more; with starters this involves ‘bolt together’ modular components or completely integrated starters, the downside of this integration is survivability. With integrated starters the failure of one component can lead to the total failure of the starter with the requirement to replace the entire unit, with separate components it is easy to replace a faulty component or to by-pass it so that the starter will still operate.