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Design of three point starter of DC Motor

In this article we will learn what is three point starter of DC motor?  Why DC motor needs starter? Step by step design of three point starter of DC motor.

Starting problem in DC motors.

In DC motors, the initial starting current is very large. According to rough estimate, starting current is 10 times the full load rated current.  This high current can damage the motor windings therefore dc motor starter is necessary which can limit the current during starting of motor. Once the motor attains speed, the starter can be removed.

In order for DC motors to function properly, they must have some special control and protection equipment associated with them.

The purposes of DC motor starter are:

  1. To protect protection to motor against the damages of short circuit current.
  2. To protect the motor against damage from long-term overloads;
  3. To prevent high starting inrush current in motor.
  4. To provide a convenient manner in which to control the operating speed of the motor.

Why to use DC motor starter?

At starting condition, the armature (rotor) is not rotating. Therefore the internal generated voltage is zero. The armature current equation is

VT = Terminal voltage                         EA = Internal generated voltage                       RA= Armature resistance

In starting internal generated voltage EA = 0, therefore the starting current IA is very high.

For instance, for a 50 HP, 250 V DC motor with armature resistance RA of 0.06 Ω and a full-load current about  200 A. To find starting current put values in equation (i)

It can be seen that the value of starting current is almost 20 times the full load current. Such high current can damage the motor.

Solution to Above Problem is using three point starter of DC motor

One solution to high starting current is to insert an external resistor in series with the armature. High resistance will prevent high inrush current. However there will be I2R losses due to external resistor. Also the resistor will also decline the motor’s torque characteristics. Therefore external starting resistor should be removed from the motor when the motor attain speed and develop back emf.

If one big resistor is used and removed at once, a spike in current and voltage can be originated which can surpass the maximum limit and can damage the motor. Therefore the starting resistance can be removed in small steps to ensure smooth transition.

three point starter of DC motor

Designing of a three point starter of DC motor

In practice, the starting resistor consists of a number of series resistors which are removed in steps when the motor attains speed. So designing of a starter consists of two parts

  • Determination of number and values resistors needed to limit the starting current to a safe value.
  • Designing a control circuit which can bypass the starting resistor in steps and at proper time.

Case study To understand the design process, consider a 100 Hp, 250 V 350 A shunt DC motor with an armature resistance of 0.05Ω. Design a starter which can limit the starting current to twice its rated value and remove the resistors when motor attains its rated speed.

  1. How many steps of starting resistance will be needed to limit the current to the stated range?
  2. What should be the value of every segment of the resistor?
  3. At what voltage should each stage of the starting resistance be cutout?

Design calculation of Three point starter of DC motor 

Three point DC motor starter is such starter which will remove the starting resistor in three steps.

Since the rated current of motor is 350A. According to given requirement, the starter should limit the starting current to 700A.  As the motor speeds up, armature will generate internal voltage EA  which will reduce the current to rated value. As rated current is achieved, remove the first segment of resistor. As a result the current shoots up again however such values of each segment of resistors are selected so that current only shoots up to twice of the rated current. As the motor gains more speed, the back emf will increase, which will decrease the armature current to rated current. At this stage, second resistor should be removed which results in increase of starting current to twice of rated current again. Similar process is repeated for 3rd segment of resistor.

Now to calculate the total starting resistance of DC motor including dc motor starter. Total resistance should be such that it should limit the starting current to twice of the total load current.


Rtotoal= Total resistance of starter and armature

Ea= Back emf developed by motor

VT= terminal voltage=250V

Imin= Irated= 350A

Imax= 2xIrated = 700A

Ra = armature resistance

Rototal1= Rtotal – R1

Rtotal2= Rtotal1-R2 = Rtotal –(R1 + R2)

R1, R2, R3 = three resistors of motor starters

For the given case the rated current is 350A, so maximum current to which it should be limited is 700A.

When the current reaches rated current that is 350A, one segment R1 should be removed.

After One segment of resistor R1 is removed, the current increases to

Since back emf depends on speed of rotor. At particular instant, Ea will be constant and VT is constant throughout therefore

For n resistors

Where I min = Irated and I max = 2x Irated

The armature circuit will contain the armature resistance RA and three starting resistors. Initially when motor is static and there is no back emf, Ea = 0, maximum allowable current is 2x Irated that is Imax = 700 A, and the total resistance is (Vt/Imax) = 0.357 Ω. The total resistance will be in the circuit until the current drops to 350 A. This occurs when

When armature current drops to 350A, the resistor R1 should be removed from the circuit and current climbs again to 700A so we can find total new resistance required at that moment is

$latex Rtotal\ 1=R\ total-R1=R2+R3+Ra=\ \frac{Vt-Ea\ 1}{Imax}\ =\ \ \frac{250\ -125\ }{700\ }=0.1786\$

This (new) total resistance Rtotal 1 will remain in the circuit until armature current drops again to 350 A. This occurs when

At this time, the starting resistor R2 will be taken out leaving

This total resistance will be in the circuit until the current drops again to 350 A. This occurs when

At this time, the starting resistor R3 will be taken out leaving only RA in the circuit. The motor’s current at that moment will increase to

which is less than the allowed value. Therefore, the resistances are

The resistors R1, R2, and R3 are cut out when EA reaches 125 V, 187.5 V and 218.75 V, respectively.

As it can be seen that with DC motor starter, the current is limited to safe limits.

To implement and verfity the design and calculation of three point starter of DC motor please refer to my article modelling of DC motor starter in MATLAB simulink.

About Syed Noman ud din

Syed Noman ud din is an Electrical Engineer and working in Industry from last 3 years. He writes technical articles for electrical and electronic engineers. He has also published several research publications in renowned international journals.

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