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 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

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

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
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

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

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

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

Qasim 6 Feb 2020 Reply
For selection of thermal overload 70% for min and 120% for max is chosen.
Please provide the reference.
nexa electrical supplies 24 Feb 2020 Reply
thanks for providing useful information on Sizing of contactor and overload relay for 3 Phase DOL starter