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Importance of Motor Starting Studies in Power System

Motors are the fundamental loads in an electrical power system, both at commercial and industrial level. Some applications of motors include fans, vehicles, power tools, turbines, blowers, ships and compressors. Along with the ease, there are issues that are needed to be addressed to ensure the reliable operation of a power system. The following problems occur when a motor starts:

We will be discussing the problems that arise with motor starting and complications associated with them. Then we will discuss about the motor starting studies, its importance, and outcomes, along with techniques to mitigate the problems which arise due to motor starting techniques.

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Starter Motor Problems

At first, we should discuss about the problems we face in case of motor starting. 

High Inrush Current

Inrush current is the maximum instantaneous current which the motor draws at the time of its starting. To have a better understanding, first we should look at the model of motor which is given below:


As we can see from the model, motor is like a transformer with its secondary side as short circuited.

Stator of Motor ➜ Primary of Transformer

Rotor of Motor ➜ Shorted Secondary of Transformer

Like transformers, mutual induction also occurs in motors. The emf induced in rotor of the motor depends on the slip of rotor. Slip is the difference between rotor and stator speed given by:

s = 
Ns − NRNs


NS : Speed at with the magnetic field of stator rotates

NR : Speed at which rotor rotates

The rotor is stationary when the motor starts, so

NR = 0

Putting this in above equation we get

s = 1

It means that the slip is maximum in case the motor is starting. We also know that

ER = sEs


ER = Induced emf in rotor

Es = Stator voltage

s = slip

We get,

ER = Es

The above equation shows that the emf induced is maximum when the motor starts. In order to allow the flow of current in rotor, whether it is induced current or field current, the rotor is short circuited. Due to short circuit, the rotor offers minimum resistance which results in high current value.

The high current value implies high primary current due to motor’s similarity with transformer. This high value of current in primary is the Inrush Current An inrush current can cause complications such as:

    1. Every equipment has a certain value of current that they can withstand. In case of inrush current, the value of current is so high that it may cause overheating and damage the equipment.
    2. Similarly, insulations present in a system also have a certain withstand value after which the insulation will fail, leading to more complications.

We can see a high starting current (inrush current) in the graph above, which gradually decreases to nominal value of current. But we can limit the inrush current to protect our system from any damage.

Low Power Factor

At the time of motor starting, the magnetic field must be developed. For this, motor draws high magnetization current to overcome the high reluctance offered by air present between stator and rotor. As magnetization current is inductive in nature, it causes the power factor to decrease.

Power factor is the measure of how much of the power is usefully utilized. A low power indicates that the power being transmitted is not being utilized properly. It also indicated a high-power loss.

  1. An extended exposure to such condition would also harm the other equipment present in the system. Also, the power loss can cause over heating of the equipment, resulting in equipment damage.
  2. Low power factor indicates high current flow to accommodate the decreased power transmission.

After the magnetic field is developed, the motor starts to function and draws load current which is resistive in nature. This balances the decrease in power factor and its value increases. Quantitatively speaking, starting power factor of motor varies from 0.3 to 0.5 and increases towards unity as the motor accelerates and its kVA demand drops.

Voltage Drop

Inrush current is the reason behind the voltage drop at the starting of motor. According to ohms law

V = IR

When R is constant, increasing the current would increase the voltage.

In case of a power system, the high inrush current causes a voltage drop on the cable resistance and any other component present in between, before reaching the motor. This causes a decrease in voltage which reaches the motor or voltage drop.

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The term voltage drop indicates that the motor receives a decreased voltage during the time of its starting. This transient state of voltage affects the motor, as well as other loads present in the system. The severity of this affect depends on the value and duration of voltage dip.

The extent of this affect can be failure of the whole system. This is because every system, just like load, is functional in a specific range of voltage and current. If it is supplied a voltage that is below or above the range, the system would be harmed.

As discussed above, starting of motor causes many problems. These problems cannot be lefts as it is and need to be addressed. This is because the damage due to the stating of motor can be fatal for our system.

Importance of Motor Starting Studies

The need to avoid problems caused by motor starting gives rise to the necessity of a procedure that can detect small changes or signs leading to complications. Getting to know about them or the behavior of our system when faced with such complications can help us in understanding the methods for preventing such issues.

This understanding can help us in perceiving the effect off voltage dip, high currents, voltage drop, fluctuations in voltage, nuisance tripping and number of such things which are caused by the starting of motor. This knowledge prior to the operation of system can protect our system from any harm by applying mitigation techniques.

There are many things that need to be considered when starting a motor to ensure the safety of the system. These considerations will mitigate the risk and problems associated when a motor starts.

Motor starting study is used to find the cause of voltage dips and then provide solutions to avoid voltage drops and voltage flickering along with controlled current flow.

Along with understanding of effects of motor starting on the system, motor starting studies can help us in following ways:

  • Performing motor starting studies can help us in designing a protection scheme for our system
  • Motor starting studies help us to predict the uncertainties that may arise in the system When a motor starts.

How to Perform Motor Starting Studies?

The methodology to perform motor starting studies and to decide which mitigation techniques we should use depending upon the outcomes of studies is very crucial.

"We at AllumiaX also perform motor starting studies by following proper procedures and in accordance with some specified standards. We have a team of highly talented and professional engineers who help us in carrying out these procedures."

Firstly, a data collection procedure is carried out in accordance with NEC Standard. Then a comprehensive model of the system is developed on a software that helps us in performing motor starting studies. Some of these software are ETAP, SKM and CYME. The engineers at AllumiaX use ETAP and SKM. After this, calculations are performed for calculating voltage drop on feeder branches and buses, and the projected power flow is also calculated. Finally, when all these calculations are performed, the results are analyzed to check the values according to NFPA 70E, IEEE 3002.7-2018 and IEEE 493-1997 standards. The values which lie outside the specified limits are then marked and recommendations are provided to resolve those issues.

Techniques To Mitigate Starter Motor Problems

Problems causes by motor starting can be solved by using many techniques, some of them are given below

  1. Auto Transformer
    An auto transformer is connected to the motor to limit the inrush current. Transformers allow current flow under a specific range because transformer’s core tend to saturate. When the transformer core is saturated, a large increase in current on primary will only cause a slight increase in current on secondary side. This prevents the flow of inrush current and any further complications caused by it.
  2. Soft Starters
    Soft starters control the motor current and torque by using voltage. They ensure a gradual increase of voltage, causing a reduced starting current and torque, this avoids inrush current and voltage drop issues.

    Image Source

  3. Star Delta Starters
    When the motor is started, it has star connection which causes a low phase voltage in starting. Then gradually when the motor starts, it is connected in delta that increases the phase voltage but decreases the current providing a proper start to motor.

    Image Source

  4. Direct On-Line Starters
    In DOL starters, the motor is connected directly to the supply line. They are used in system which are not affected by voltage drop. A DOL has circuit breaker/fuse, contactor and overload relay for protection. It has two switches to start and stop the motor.

Comparison of Different Mitigation Techniques


Outcomes of Motor Starting Studies

There are certain results of motor starting studies that we can utilize in designing our system in a way that would reduce the problems that are introduced in the system due to motor starting. The outcomes of performing motor starting studies are as follows:

  1. We can find out the points of voltage dip and minimize their effect on our system loads and motor.
  2. Areas of nuisance tripping can be highlighted, and precautionary measures can be taken.
  3. We can also find out the effects of changes in motor load on the system and rectify it.
  4. These studies help us in increasing the reliability of our system.
  5. It also helps us in decreasing the value of inrush current through different protection schemes.
  6. We can significantly reduce motor stalling through these studies.

To perform motor starting studies, you can contact us through allumiax.com/motor-starting-study. Our team of certified professionals will help you in performing motor starting studies and deliver a solution to design a reliable protection scheme for your systems.


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