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

Data required for the choice of contactor and overload relay

  • KW rating of the motor
  • Operating voltage
  • Awarding
  • KVA code if bachelor

All the to a higher place 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.

Adding of full load current (FLC)

Input power to the motor P =√3 V.I.CosĪ† 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

  • Ability contacts of the contactor must be rated not less than the operating voltage of motor(every bit per nameplate details).
  • Roll voltage/control voltage of contactor must be determined based on the control circuit design.

Type of load and duty wheel

  • IEC 60947-five-i defines the requirements for electromechanical devices for command circuits.
  • It indicates the type of electrical load and duty cycle of the loads to ease the selection of contactors and relays.
Utilization category Typical application
AC-1 Non-anterior or low-inductive load, resistance furnaces
Air-conditioning-2 Slip ring motors: starting, switching off
AC-3 Squirrel muzzle 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 brand or break safely. It must be equal to or greater than the locked rotor current. The locked rotor current can exist 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 lawmaking.

NEMA
Lawmaking Alphabetic character 		kVA/HP
with locked rotor 		Gauge Mid-Range Value
A 0 – 3.xiv 1.6
B iii.fifteen – iii.55 iii.3
C 3.55 – 3.99 3.8
D 4.0 – 4.49 4.3
E 4.5 – 4.99 iv.7
F v.0 – five.59 v.3
G v.half dozen – 6.29 5.9
H 6.3 – seven.09 6.7
J 7.1 – 7.99 7.5
Chiliad viii.0 – 8.99 8.5
L 9.0 – ix.99 ix.five
M 10.0 – xi.19 10.6
N 11.2 – 12.49 xi.viii
P 12.5 – 13.99 13.ii
R 14.0 – fifteen.99 15.0
S sixteen.0 – 17.99
T eighteen.0 – 19.99
U twenty.0 – 22.39
V 22.four – and upwardly

Let u.s. calculate the contactor and overload rating for DOL starter of the following motor:

7.5KW(10HP), Iii-stage squirrel cage induction motor, 415 volts, 50Hz, 1450rpm, the efficiency of the motor: 90%, pf: 0.85, KVA lawmaking: 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 exist Air-conditioning-3 duty with a current rating more than 12.28A and making breaking electric current more or equal to 102A.

Selection of thermal overload relay

Min. Thermal Overload  Relay setting = seventy%x Full Load Electric current(Phase)
Min. Thermal Overload Relay setting = seventy%x12.28 = eight.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.vii.

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

  • Operation of a contactor
  • Functioning of an overload relay
  • Operation of star-delta starter
  • Difference between relay and contactor
  • Difference betwixt VFD and soft starter
  • A comprehensive DOL starter choice tool
  • A comprehensive Star-delta starter selection tool