What are different types of motor starting methods?

Three phase induction motors of large power rating draw high starting current which can affect the grid voltage and cause a voltage drop to other electrical loads on the same grid.

That’s why we use motor starting methods. A motor starting method, is a method that is used for safe and smooth starting of an electric motor.

That’s exactly what happened with us in my work, when trying to start a large induction motor, the large voltage drop value caused the motor under-voltage protection to stop its starting. We used Soft Starter to run the motor.

So, the main functions of motor starter includes safe start and protect from the high starting current impact.

Why is it important to use motor starting method?

Not all motors types need a starting method. Only large motors which draw a lot of electrical current at the starting need starting method.

This starting method is a protection against the large current and voltage drop which happen at starting, the high starting current decreases the motor lifespan, while the voltage drop could cause other loads on the same power supply to stop.

Motor starting can be divided into two types i.e. manual starters and magnetic starters. Let’s discuss different types of motor starting methods.

Stator Resistance starter

Stator resistance starter uses reduced voltage starter technique for motor starting.

External resistance is added in series with the stator of each phase of the Three Phase Induction Motor. The resistor reduces the line voltage that is applied to the stator.

When the variable resistor is at the maximum position. It offers maximum resistance and the voltage drop across the motor is a minimum and at a safe level.

The low stator voltage limits the starting inrush current. The current can damage the windings of the motor.

When the motor picks up the speed, the resistance decreases & the stator phase connects directly to the power lines.

Advantages of Stator Resistance starter

  • It offers flexibility in motor starting
  • The variable voltage supply ensures smooth acceleration of the motor.
  • We can connect it in both the star and delta connected motor.

Disadvantages of Stator Resistance starter

  • Power dissipation occurs due to resistors.
  • Due to voltage reduction, the value of starting torque is very low.
  • High Price of resistors for large motors.

Slip Ring Motor Starting (Rotor Resistance Starter)

The starting technique for this type of motor starter is based on a full voltage. Only a slip ring induction motor is used for operation. Therefore it is also called a slip ring motor starter.

In this type of starter, the external resistances are connected in a star combination with a rotor via slip rings. The function of the resistor is to increase torque and limit the rotor current.

This process ultimately reduces the stator current. Once the motor picks up speed the resistances are then removed.


  • This type of motor starter offers a low starting current using full voltage.
  • The motor can start with a connected load due to high torque.
  • Slip Ring Motor or Rotor Resistance Starter improves power factor and offers a wide range of speed control.


  • The rotor is heavier &expensive.
  • It is applicable on slip ring induction motor only.

Auto transformer Starter

In this type an auto transformer is used as a step-down transformer. During this process, the voltage we apply to the stator during the starting stage reduces.

It can be connected via both star and delta combinations. While the secondary is connected with each phase of the motor.

During the starting process, the relay at the starting position provides a reduced voltage.

To increase the speed of the motor the relay switches between the tap points. Until it connects with the full rated voltage.


  • Autotransformer Starter offers flexibility in starting characteristics
  • It provides manual speed control and better starting torque.
  • We can use it for starting large motors with a significant load.


  • It occupies too much space due to the large size of the auto transformer.
  • The circuit of the Auto transformer starter is complex & expensive as compared to other starters.

Motor Soft Starter

motor soft starter
motor soft starter

These types of motor starters use semiconductor switches to control the voltage & the starting current of the induction motor.

The switches may include TRIAC etc. You can vary the voltage by changing the firing angle of the TRIAC.

Soft starters are programmable based on the motor parameters and operation conditions.

The one in the image is in my work, we use it for low voltage induction motor starting. We set the motor and network parameters for the first time.

The cool thing about this soft starter is that it protects the motor against many faults like, over voltage, under voltage, over current and phase loss.

Advantages of soft starter

  • It controls overcurrent and voltages.
  • No jerks occur during the switching
  • The size is small and occupies low space.
  • Programmable according to the motor power rating.
  • Protects the motor against many faults.

Disadvantage of soft starter

  • It dissipates energy in the form of heat.
  • Higher cost.

Motor Star Delta Starter

Star Delta Starter is the most common way to start large motors in which the motor starts in a star connection, then the connecting switches to Delta, to reduce the voltage disturbance at the supply due  to large starting current of the motor.

Why Star Delta Starter Is Used?

When a motor is stopped, it needs larger current (up to 8 times full load current) to get started.

For small and single phase motors, this starting current is acceptable.

This is why no starting method is needed for small motors, and (Direct On Line) DOL is the common starting way.

Starting large 3 phase motors directly online, the motor draws large starting current, and causes a disturbance of voltage on the supply lines, which affect other loads, also it affects the motor itself, as the voltage drops the motor will not be able to start.

Star Delta is used to start these motors at voltage reduction then, the motor have full supply voltage reconnected.

How Does A Star Delta Connection Work?

Star and delta connection
Star and delta connection

At the motor starting, the motor windings are re-configured to be connected as star, then after that configured to be delta connected.

This re-configuration process is done by the help of a control and a power circuit using 3 contactors, one for delta connection the other one is for star connection and the third is the main contactor.

Later in this article you will find the control and power start delta diagram.

Now, let’s find out How much star delta connection reduce starting current.

At star connection line Current is equal to phase Current, While Voltage is not the same,

  • IL = Iph
  • Vph = VL/√3

Starting the motor in the Star connection reduces voltage at starting to be just VL/√3, which is 58% of the motor rated delta voltage.

How does the line current reduce? During star connection, the current is,  ILY = IphY = VL/(Z√3)

When the motor is switched in to delta:

  • VL = Vph
  • IL∆    = √3 Iph =  √3 VL/ Z

Now let’s compare line current in the star and delta connections of the same motor.

ILY / IL∆ =(VL / √3 Z) / (√3 VL/ Z)

ILY = 1/3 IL∆

This answers the important question of How much star delta connection reduce starting current?

The answer is, star delta starter reduces the motor starting current to the third of the starting current of delta connection ILY = 1/3 IL∆

N.B: Reducing the voltage also reduces the starting torque by 1/3 of motor torque, because the torque is proportional to square the voltage.

Which means,

TY / T = (VL/(√3))2 / (VL)2 = 1/3

Circuit Components of Y/∆ Starter

The star delta starter circuit consists of:

  • Three contactors (Main contactor, Star connection and delta connection contactors)
  • One timer
  • Thermal overload for motor protection
  • Start green colored push button.
  • Stop red colored push button.
  • Indicator lamps, Red, Yellow, green for three phase power. And one more orange for overload protection.
  • Three phase voltage volt-meter, and current-meter.

Star Delta Wiring and circuit Diagram

Star delta starter diagram divided in to power and control wiring. Both are in the following wiring diagram.star delta starter diagram

Contactors Selection

Star Delta Starter Contactor Selection is important because, the currents are not the same as the motor full current, and also the wiring diagram has three contactors with three tasks.

Contactors size are smaller than the Direct on Line single contactor, because the fact that they are controlling winding currents only, not the line currents.

I’ve written a detailed article about Star Delta vs DOL Starter, read it now for more information.

Main and Delta Contactors

The currents through the motor winding are (58%, where Iph  = IL∆   /√3) of the line current.

So, main and delta contactors of star delta starter circuit are, AC3 with 58% of the motor current rating.

Star Contactor

While, In case of star connection, ILY = 1/3 IL∆,  So, sizing of the star contactor is 1/3 of full load current of the motor.

Because the star contactor carries star current only, current in the star configuration is 1/3 of the full voltage (delta) current of the motor.

Y/∆ starter applications

This starting method is not applicable for loads which require high starting torque because, the starting torque of the motor is reduced to the third of its full rated torque.

Centrifugal compressors and pumps are an example for this motor starting method.

DOL starter for motors

DOL starter or Direct online starter is a the simplest method used for starting induction motors.

As we know that an induction motor needs a large amount of current during the staring process. But for smaller sized motors and when the starting current will not affect the grid voltage and the other loads we connect the motor directly on line.

DOL Starter circuit components

Direct online starter is one of the simplest type of motor starters for the induction motor.

The device consists mainly of protective devices such as a circuit breaker, main contactor & an overload relay.

The electromagnetic contactor is operated using a thermal overload relay normally closed contact.

The Contactor is coil operated and is controlled by start and stop push button.

DOL Starter Circuit Diagram and Working Principle

DOL starter circuit

The circuit diagram for Direct Online Starter is given above. Let’s describe the working principle of the circuit diagram.

When we press the START button S1, the contactor C energizes from two-line conductors L1 and L2.

The auxiliary contact and three main contacts M closes. The terminals “a” and “b” become short-circuits and the motor connects to the supply mains.

Under the spring action as the pressure releases the button S1 moves back. As a result, coil C remains energized through ab.

Thus, the main contact M remains closed during this operation. While the supply to the motor continues.

During this whole process, The Stop button “S2” is held by the spring. When we press the Stop button the contact disconnects.

Coil C de-energize and the auxiliary contact A and the main contacts M are opened. The Voltage supply to the motor disconnects, and as a result, the motor is stopped.

DOL Starter circuit Protection

  • Overcurrent: DOL circuit has protective devices such as circuit breakers, relays, and fuses. And it limits the chances of overcurrent flow.
  • Overload: If the load connected to the motor is more than the rated load of the motor, it causes motor overload. During the overload period, the load draws more current through the motor. The overload relay trips during the condition to disconnect power supply. And the system is protected from overheating.

Direct online starter advantages

The advantages of this motor starting method are:

  1. Simple in design and no complicated wiring needed.
  2. The most economical starting method.
  3. The motor starts with almost its full starting torque.
  4. Easy troubleshooting process.
  5. Easy to perform periodic maintenance and electrical inspection.


  1. The motor starts with high starting current, which means you should be sure that the motor starting current won’t affect the circuit voltage.
  2. Causes voltage dip which affect other motors on the same power source, it should be within accepted limits.
  3. DOL Starter reduces the motor expected life time.

Applications of DOL starter

The DOL starting method is mainly applicable for small sizing motors because, its starting current is not very high and not effecting the power supply voltage.

It is very common in applications like small water pumps, fans, air conditioners and compressors.

DOL starter circuit maintenance

As you can see above that the direct online circuit is very simple and has only one contactor, one overload, and some basic components like bush buttons and fuses.

This simple construction makes it simple to perform periodic maintenance to keep it working perfectly and to prevent unplanned shutdown.

Take the following points into account while preparing the maintenance program for DOL circuit.

  • Measuring voltage monthly for both power and control circuits.
  • Measure motor current.
  • Record voltage and current values in a recording sheet.
  • Ensure that the motor and its control panel are well grounded.
  • Measure the value of grounding resistance and record it quarterly (R should be less than 5 Ohms).
  • Check DOL panel for any abnormal noise, dirt, dust and unused openings.
  • Thermal imaging the motor body helps in overheating detection.
  • Megger the motor annually and check the results with the motor data sheet.

Which type of motor starting method should I use?

To choose a motor starting method you should make a balance between cost and work needs.

For example, if you are choosing a starting method for a large motor which drives important load, a soft starter with advanced protection features is the best choice for you.

While a less important motor load with lower power a Star Delta starter is a good choice for it.

In some cases we can run the motor direct online DOL, in this case we should make sure that the motor starting current will not affect other loads voltage by the caused voltage drop.

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