Selection of contactor and overload relay for DOL starter

The below article can guide you through the selection of contactor and overload relay for a three-phase DOL starter and how to optimally size a DOL starter.

Data required for the selection of contactor and overload relay

  • 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.

Selection of contactor and overload relay for DOL starter

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 of contactor required.

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 Slip ring motors: starting, switching off
AC-3 Squirrel cage motors: starting, switching off while running
AC-4 Squirrel-cage motors: starting, regenerative braking, reversing, jogging
https://webstore.iec.ch/publication/26973

Making and breaking capacity

The 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. The locked rotor current can be calculated using the following formula:

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: Sizing of contactor and overload relay for DOL starter

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

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

    \[I_f_l =  \frac{ 7.5 \times 1000 }{ \sqrt3. \times 415 \times 0.85 } = 12.28A \]

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 relay

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.

The right selection of contactor and overload relay is necessary to start a motor safely. The above calculations can help you select the optimal rating of contactors and overload relays. Learn more about the operation of contactors, overload relays and star-delta starters from the following article. You can also use DOL starter selection tool and Star-Delta starter design tool | Wye-Delta starter design tool for the selection of contactor and overload relays.

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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