Sizing of contactor and overload relay for 3 Phase DOL starter

Data required for the selection of DOL starter

  • KW rating of the motor
  • Operating voltage
  • Application
  • KVA code if available

All the above data can be obtained from the nameplate details on the motor.

Parameters to be considered during the selection

  • The full load current of the motor
  • Operational voltage and coil supply voltage
  • Type of load and duty cycle
  • Making and breaking capacity

Calculation of full load current (FLC)

Input power to the motor P =√3 V.I.Cosφ kW

Where,
P is rated power of the motor in kW
V and I are voltage and current respectively

From this, we can calculate the full load current of the motor

Full load currrent

    \[I_f_l =  \frac{ P \times 1000 }{ \sqrt3. V.Cos\theta } \]

Operational voltage and coil supply voltage

  • Power contacts of the contactor must be rated not less than the operating voltage of motor(as per nameplate details).
  • Coil voltage/control voltage of contactor must be determined based on the control circuit design.

Type of load and duty cycle

  • IEC 60947-5-1 defines the requirements for electromechanical devices for control circuits.
  • It indicates the type of electrical load and duty cycle of the loads to ease the selection of contactors and relays.

Utilisation categories for contactors according to IEC/EN60947-4-1

Utilization category Typical application
AC-1 Non-inductive or low-inductive load, resistance furnaces
AC-2 Slipring motors: starting, switching off
AC-3 Squirrel cage motors: starting, switching off while running
AC-4 Squirrel-cage motors: starting, regenerative braking, reversing, jogging

Making and breaking capacity

Making/breaking capacity of a contactor is the maximum current that it can make or break safely. It must be equal to or greater than the locked rotor current.

Locked rotor currrent

    \[  =  \frac{ HP \times 1000 \times \frac{kVA}{ HP}  }{ \sqrt3. V.Cos\theta }  \]

(KVA/HP) is indicated on the nameplate details of the motor based NEMA code.

NEMA
Code Letter
kVA/HP
with locked rotor
Approximate Mid-Range Value
A0 – 3.141.6
B3.15 – 3.553.3
C3.55 – 3.993.8
D4.0 – 4.494.3
E4.5 – 4.994.7
F5.0 – 5.595.3
G5.6 – 6.295.9
H6.3 – 7.096.7
J7.1 – 7.997.5
K8.0 – 8.998.5
L9.0 – 9.999.5
M10.0 – 11.1910.6
N11.2 – 12.4911.8
P12.5 – 13.9913.2
R14.0 – 15.9915.0
S16.0 – 17.99 
T18.0 – 19.99 
U20.0 – 22.39 
V22.4 – and up 

Example DOL starter calculation

Let us calculate the contactor and overload rating for DOL starter of the following motor:

7.5KW(10HP), Three-phase squirrel cage induction motor, 415 volts, 50Hz, 1450rpm, the efficiency of the motor: 90%, pf: 0.85, KVA code: H

Full load currrent

    \[I_f_l =  \frac{ P \times 1000 }{ \sqrt3. V.Cos\theta } \]

    \[I_f_l =  \frac{ 7.5 \times 1000 }{ \sqrt3. \times 4 0.8515 \times } = 12.28A \]

Locked rotor currrent

    \[  =  \frac{ HP \times 1000 \times \frac{kVA}{ HP}  }{ \sqrt3. V.Cos\theta } = 102A \]

Hence, Contactor must rating must be AC-3 duty with a current rating more than 12.28A and making breaking current more than or equal to 102A.

Selection of thermal overload

Min. Thermal Overload  Relay setting = 70%x Full Load Current(Phase)
Min. Thermal Overload Relay setting = 70%x12.28 = 8.6A
Max. Thermal Overload  Relay setting = 120%x Full Load Current(Phase)
Max. Thermal Overload Relay setting = 120%x12.28 = 14.7A

Hence, Overload must be chosen closer to the range of 8.6-14.7.

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This Post Has 2 Comments

  1. Qasim

    For selection of thermal overload 70% for min and 120% for max is chosen.
    Please provide the reference.

  2. thanks for providing useful information on Sizing of contactor and overload relay for 3 Phase DOL starter

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