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Why do you need to start the motor with a reduced voltage?

Starting a motor with reduced voltage helps to reduce the inrush current and prevent damage to the motor and other electrical equipment. It also helps to prolong the life of the motor by reducing the stress on the motor's windings during start-up.

In the process of starting, the starting current of the motor may be several times higher than the rated current, generally 4 to 7 times of the rated current, which not only affects the motor but also affects the normal operation of other electric equipment on the same line. For motors with large capacity and need to start and stop frequently, in order to limit the starting current, it is necessary to take a reduced voltage start. Otherwise, the strong current brought by frequent starting may bring many problems.

What is reduced voltage start?

The motor starting current is proportional to the voltage of the stator, so we should use the method of reducing the stator voltage to limit the starting current. In other words, when the motor starts, the voltage on the stator winding is reduced, and when the motor turns up, the voltage on the stator winding is restored to the normal value.

Problems caused by direct start

Direct motor start may bring a series of problems, such as voltage dips, thermal stress, and equipment damage due to high speed. Voltage dips may affect the normal operation of equipment in the entire line. Thermal stress causes damage to motor windings and insulation, shortening their service life. At the same time, direct starting of the motor can also bring excessive starting speed, which can easily lead to wear of the shaft, coupling, gears and other parts.

Which motors are suitable for reduced voltage starting?

Generally speaking, motors with power greater than 10Kw need to be started at reduced voltage. However, you may have seen other motors with hundreds of kilowatts using full voltage starting in your life. On this issue, you need to consider the purpose of voltage reduction starting. The main function of voltage reduction starting in real life is to avoid affecting the normal work of other equipment. If there is no other equipment on the same line, only the transformer and the motor, then the motor power can be close to the transformer capacity, taking into account the power factor, efficiency and other factors, generally the motor power is 80% of the transformer capacity, both can be started directly, or the transformer and the motor can be started at the same time. Take the fan for example, when the fan of 10Kw, the current can reach 7-9 times of the rated current, the configuration of the thermal relay can not be used properly, it is necessary to use the reduced voltage start.


The reduced voltage starting method can reduce the starting current of the motor and also avoid the impact on other electric equipment on the same line, reduce the negative impact of the powerful current on the motor and help the normal operation of the electric equipment.

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