Data required for selection of DOL starter
- KW rating of motor
- Operating voltage
- KVA code if available
All the above data can be obtained from the name plate details on the motor.
Parameters to be considered during selection
- Full load current of 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
P is rated power of motor in KW
V and I are voltage and current respectively
From this, we can calculate the full load current of motor
Ifl =(P × 1000)/(√3 V.Cosφ) A
Operational voltage and coil supply voltage
- Power contacts of the contactor must be rated not less than the operating voltage of motor(as per name plate details).
- Coil voltage/control voltage of contactor must be determined based on control circuit design.
Type of load and duty cycle
- IEC 60947-5-1 defines the requirements for electromechanical devices for control circuits.
- It indicate the type of electrical load and duty cycle of the loads to ease selection of contactors and relays.
Utilisation categories for contactors according to IEC/EN60947-4-1
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.
(KVA/HP) is indicated on the nameplate details of the motor based NEMA code.
Let us calculate the contactor and overload rating of DOL starter for the following motor:
7.5KW(10HP), Three phase squirrel cage induction motor, 415 volts, 50Hz, 1450RPM, efficiency: 90%, pf: 0.85, KVA code: H
Full load current Ifl =(P × 1000)/(√3 V.Cosφ) A
= (7.5 × 1000)/(√3 x415x0.85) = 12.28A
Locked rotor current = (HP × 1000 x (KVA/HP))/(√3 V.Cosφ) = 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.