It is often believed that the start current of a motor under full voltage conditions depends on the driven load, but this is incorrect.
The start current of the motor depends on the motor design, rotor speed and stator voltage from zero speed until full speed is reached. The load only influences the time taken for the motor to reach full speed. The current/speed curve of the motor is independent of all external influences, other than stator voltage.
Full voltage starting: DOL starters
The DOL starter causes a current transition from zero to locked rotor current (LRC) at the instant of contactor closure. It also causes a torque transient from zero to locked rotor torque (LRT) at the instant of contactor closure, resulting in a severe mechanical shock to the motor drive system and the machine.
Reduced voltage starters
The following four starter types are examples of reduced voltage starting which is used extensively and, in many areas, is mandatory. However, many installations would suffer fewer disturbances if a full voltage starter replaced a poorly applied reduced voltage starting system. The reduced voltage starter must be able to accelerate the motor to almost full speed before stepping to full voltage, or it risks a current step that can be very close to LRC. Therefore, start time and start voltage must be correctly set.
Primary resistance starters
Using resistors, connected in series with each phase, between the isolation contactor and the motor, limits the start current and torque. If the resistors are too high for the start voltage, there will be insufficient torque to accelerate the motor to full speed. In this case, the step to full voltage will result in a high current and high torque step. Several stages of resistance can be used to control the current and torque more accurately, minimising the magnitude of the current and torque steps.
An autotransformer reduces the voltage during the start period. If the start voltage is too low, or the start time incorrectly set, the transition to full voltage will occur with the motor at less than full speed, resulting in a high current and high torque step. Autotransformer starters are usually rated for infrequent starting duties.
The star-delta starter is the most common reduced voltage starter used in industry because of its low cost. The motor is initially connected in star configuration and then, after a pre-set time, the motor is disconnected from the supply and reconnected in delta configuration. If there is insufficient torque available in star configuration to accelerate the load to full speed, a high starting torque motor such as a double cage motor should be used. If the motor does not reach full speed in star, the transition from star to delta configuration will result in a high current and high torque step, defeating the purpose of reduced voltage starting.
Solid-state soft starters
The solid-state soft starter controls the voltage applied to the motor through impedance in series with each phase connected to the motor. It uses solid-state AC switches with a very low power dissipation compared to traditional primary resistors, and is controllable to give a motor voltage from zero to full line voltage without any steps or transients.
It is possible to correctly apply and engineer electromechanical reduced voltage starters so the torque and current magnitudes and transients are reduced to acceptable levels. Reduced voltage starting reduces the electrical interference caused by motor starting and reduces mechanical damage caused by torque transients and steps. To achieve these objectives, the motor and starter must be correctly selected and commissioned for optimum performance.
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