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An Overview Of The Main Ways To Actuate Medium-Voltage Motors

An Overview Of The Main Ways To Actuate Medium-Voltage Motors

In our modern world economy, one of the many indispensable components that every industry relies on is the AC induction electric motor. This ever-present innovation helps boost productivity and efficiency in various applications. Thus, these types of machinery require much care and attention.

However, many organisations do not always know that their industrial activities may unintentionally put extreme stress on their motors during their start-up.

Below, we discuss some common motor-starting methods and how they work.

1. Direct online (DOL)

The DOL starter is the simplest form of starter and traditional way of starting MV motors, comprising of a motor overload protection device and isolation contactor. MV DOL control facilitates various industrial applications such as HVAC, oil and gas, pulp and paper, and water and wastewater. Full voltage starting results in a torque transition from zero to LRC or locked rotor current at the moment of contactor closure; LRC is generally between 5-10 times a motor’s FLC or Full Load Current. This instantaneous torque application imposes an extreme mechanical shock on the drive system, electric motor, and machine.

In the supply, the swift rising current transient generates a voltage transient and produces a voltage deflection 6-9 times greater than expected under full load conditions. Thus, the high inrush current is among the top disadvantages of classic DOL starting as it stresses the windings, causing them to move in the end turns of the stator and promoting the breakdown of insulation. Phase-to-phase shorts eventually occur and lead to motor failure.

2. Auto-transformer

As its name implies, these starters rely on an auto-transformer that helps to reduce voltage during start-up. It typically comes with various output voltage taps that set the start voltage and a timer that controls the start time. The start voltage reduction reduces the motor’s current, further brought down by the transformer action, resulting in a line current lower than the actual motor current. The initial line current is similar to the LRC, lessened by the square of the voltage reduction. Therefore, an electric motor started on an autotransformer’s 50% tap will, for example, both have one-quarter of the line start current of the LRC and a start torque of the LRT (Locked Rotor Torque).

If either the start voltage or start time is too low or short, there will be a transition to full voltage with the electric motor at less than full speed, causing a high torque and high current step. Simple auto-transformer starters are generally single step and typically only control two phases. The more sophisticated variants may step through two voltage steps or more while accelerating from the initial start tap up to the maximum voltage.

3. Primary resistance

Primary resistance starters employ resistors linked in series with each phase between the motor and isolation contactor, limiting the starting current and torque; these resistors may be cast, wound, or liquid in make. The motor current and line current will thus be equal while the torque is decreased by the square of the current reduction ratio. The reduction in the current is based on the ratio of the motor impedance to the sum of the motor impedance and added primary resistance.

The stator impedance increases as the motor accelerates, increasing the stator voltage with speed. When the motor reaches maximum speed, a second contactor bridges the resistors to supply the motor with full voltage. The initial start voltage is thus identified by the value of the resistors employed; too high a value, and there will not be enough torque to accelerate the electric motor to its max speed, causing the step to full voltage resulting in a high current and torque step.

4. Variable frequency drives (VFD)

VFDs are motor controls that convert an input source’s power frequencies into a variable frequency. To put it another way, they control the speed of electric motors at a certain frequency level. With extra refinements like revolvers and encoders, VFDs can control more than just power frequencies but also shaft positions. But despite enabling the smooth movement of electronics at a frequency, they do not provide the same surge protection possible on soft starters. VFD systems are ideal for electronics that need to control the speed of their AC motors, like pumps, conveyors, and blowers.


Choosing the appropriate method of starting a motor is essential to prevent subjecting it to unnecessary stress that could compromise its longevity. While there are several criteria for selecting a starting method, the primary considerations are restrictions concerning supply loading, available options with extra functions or higher optimisation, and the inclusion of speed control once the motor has started.

For organisations that want a second opinion on their starting method, consulting a firm specialising in mechanical and electrical engineering in Singapore is the best option. Experts would know the best practices in motor rewinding and can aid in expediting various maintenance tasks like electric motor rewinds and generator overhauling to minimise operation downtimes.