# 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

## 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.

## 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

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

## 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

Locked rotor currrent

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.

Min. Thermal Overload Relay setting = 70%x12.28 = 8.6A
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.