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 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 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
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
(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 |
---|---|---|
A | 0 – 3.14 | 1.6 |
B | 3.15 – 3.55 | 3.3 |
C | 3.55 – 3.99 | 3.8 |
D | 4.0 – 4.49 | 4.3 |
E | 4.5 – 4.99 | 4.7 |
F | 5.0 – 5.59 | 5.3 |
G | 5.6 – 6.29 | 5.9 |
H | 6.3 – 7.09 | 6.7 |
J | 7.1 – 7.99 | 7.5 |
K | 8.0 – 8.99 | 8.5 |
L | 9.0 – 9.99 | 9.5 |
M | 10.0 – 11.19 | 10.6 |
N | 11.2 – 12.49 | 11.8 |
P | 12.5 – 13.99 | 13.2 |
R | 14.0 – 15.99 | 15.0 |
S | 16.0 – 17.99 | |
T | 18.0 – 19.99 | |
U | 20.0 – 22.39 | |
V | 22.4 – 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
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.
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.
Related Articles:
- Operation of a contactor
- Operation of an overload relay
- Difference between relay and contactor
- Difference between VFD and soft starter
For selection of thermal overload 70% for min and 120% for max is chosen.
Please provide the reference.
thanks for providing useful information on Sizing of contactor and overload relay for 3 Phase DOL starter