Large Motor Starting 101: Discover the Constraints

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Motor starting is a hot topic for many of us, and I had an opportunity to verify this at the last PCIC Oil & Gas conference during the presentation of the tutorial for large motor starting. So, in a series of 3 blog posts I will discuss the constraints for large motor starting, the various motor applications, and their relation to motor starting methods, and discuss how it influences the motor feeder, according to the 5 steps for efficient motor management. As you can guess, our topic not only concerns the oil and gas industry but also mining and water industries, where large motors are widely used.

A failure in large motor starting can take down an entire electrical installation. It can lead to motor overheating, excessive mechanical stress, disconnection of parallel loads, and even loss of the synchronism of generators. Of course, this is a rare occurrence and motor starting is usually problem free.

What makes motor starting so special?

First, an important reason is that the current, hence the power drawn from the system during the start, can be 5-7 times the motor rated power. This means that compared to other “normal” loads, starting a motor will multiply its power by a factor that may put the installation in a critical situation if not considered early in the design. This can also have an economic consequence because excessive reactive power demand and low power factor can lead to penalties from electric utilities.

The next figure presents a typical case of critical motor starting where the motor leads to an almost 20% voltage drop. Due to the voltage drop, the starting current of the motor is also reduced by % of its expected value and is ~4.2 times the rated motor current.

In this case parallel motors suffer a reduced torque, which in some cases may lead to instability.

In practice, the maximum allowed voltage drop is around 15%, and undervoltage protection relays are usually set to trip for values above 20%.

But not only the electrical system is affected during the start.

A second consequence of motor starting is rotor heating. During the starting, the thermal exchange of the rotor with the stator and cooling system is very low. The thermal losses in the rotor are not evacuated and there is an accumulation of heating that can causes the rotor to reach temperatures as high as 500°C. The consequences of repetitive thermal stress are the deterioration of the electrical insulation of the rotor, which later will lead to internal faults, or even an unbalance of the rotor magnetic field and vibrations. Therefore, protection relays evaluate the prospective heating of the motor at finished start and motor starting is not allowed if the motor will get overheated. Usually the objective is to maintain the motor below 90% heating. Depending on the starting method and the motor and application parameters, heating during start can be 40-90%.

Mechanical stress is the third consequence of the motor start. It is generated by the torque applied to the motor shaft at the beginning of the starting. The higher the value and the higher is this stress. Depending on the starting solution there could be oscillations increasing the overall impact of the start.

For that reason, frequently started motors will typically use progressive starting solutions where the oscillations will be reduced or eliminated and the torque will be controlled and smoothly increased.

Don’t miss the second post of this blog series to learn more. I will discuss motor applications, their characteristics, and their relationship with motor starting methods.

You can find more information about motor starting in the 2017 PCIC Conference tutorial here.

Learn more about motor protection and management here.