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Part 13 – How to Size DOL Starter for Three-Phase Motors?

Electrical Technology
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Sizing Direct-On-Line Starter with Contactor, Fuse, Circuit Breaker and Overload Relay for 3-Phase Motors – NEC & IEC

DOL Starter

A DOL starter (also known as Direct-On-Line or across the line starter) is a basic motor starting device that directly connects the motor to the power supply when the start button is pressed, applying full voltage across the motor terminals. Direct On-line starter consists of a main contactor, protective devices (like circuit breakers or fuses), and often an overload relay.

The contactor is an electromagnetic switch that energizes the motor when the start button is pressed, completing the circuit and allowing the motor to draw current. Overload relays protect the motor from damage by shutting down the power supply if the motor draws excessive current or if it overheats.

As the name suggests, DOL means directly connecting the motor terminals to the line voltage in one step through the starter. The single-step connection makes sure to safely ON and OFF the motor without intermediate accelerating positions.  (IEC 60947-4-1 and 60050-441).

In the previous part, we learned that why we use a starter and controller with a motor, it’s components selection , sizing calculation for protection and smooth operation. In this part, we will simply explain with a solved example showing how to size a Direct-On-Line starter (DOL) with it main components e.g. contactor, circuit breaker, fuse, thermal overload relay for three phase motors.

How to Size DOL Starter for Three-Phase Motors

Example 1 – Sizing DOL Starter for Synchronous Motor

How to determine the correct size of DOL starter including OL and OCPD devices for a 5 HP, 480V, 3-Phase synchronous motor having the nameplate FLA = 6A and service factor 1.15?

Solution:

Sizing Fuse & Breaker for Starter

overcurrent proactive devices (OCPD) such as fuse and circuit breaker in the starter are used to protect the motor from the high starting current in the initial stage as well as the main breaker protects the circuit conductors from short circuit and ground faults. This is because the starting current of inductive load such as squirrel cage induction motors is 6-8 times more than the normal running current. It is good to know that these devices won’t protect against overload conditions.

The value of full load current (FLC) of 3-phase motors is determined using NEC Table 430.50 which is 7.6A for 5HP motor.

FLC : 7.6A

As per NEC 430.52 and Table 430.52(C)(1), the OCPD (i.e. fuse and circuit breaker) rating for short circuit and ground fault protection is rated at 250% of the full load current (FLC) for inverse time circuit breaker, 175% for dual element time delay fuses and 800% for instantaneous circuit breakers . 

  • Fuse Size= 7.6A × 175% =  13.3A
  • Inverse Time Breaker Size = 7.6A × 250% = 19A
  • Instantaneous circuit Breaker Size = 7.6A × 800% = 60.8A

Good to Know:

  • In an inverse time circuit breaker, the tripping time is inversely related to the magnitude of the overcurrent. This means the breaker trips faster for higher overcurrents and slower for lower overcurrents. Essentially, it provides a delay that shortens as the current level increases.
  • Inverse time circuit breakers are designed to respond to overcurrents with a delay, but the delay decreases as the overcurrent increases. This is different from instantaneous circuit breakers, which trip immediately when the current exceeds a certain threshold.

Sizing Overload Relay for Starter

As we know that fuses and circuit breakers are used to protect against short circuit and ground faults, similarly, overload relay doesn’t provide protection against short circuit currents. In short, thermal OL relay only protects the motor wiring against overload conditions.

The overload relay are rated at 125% of the motor’s FLA with marked service factor of 1.15 or greater or temperature rise 40°C or less as per NEC 430.32(A)(1). The overload protective device is sized at 115% of the full load amperes for general motors.

Overload Setting = 6A × 125% = 7.5A

This way, the ampere rating of relay setting should not be more than 7.5A.

Sizing Contactor

Anything other than fuse and circuit breaker, the rating of contactor and controller should not less than the full load current of motor or HP rating.

Based on the NEMA ICS 2: Table 2-4-1 for sizing contactor and starter based on HP and RLA, the suitable contactor for 5 HP, 480V, 3-phase motor is NEMA Size 0.

  • NEMA Contactor: Size 0 – Min. 5 HP
  • Size of Main Contactor = 100% × FLA (Line)
  • Size of Main Contactor = 100% × 6A = 6A

Example 2 – Sizing Direct-On-Line Starter for Induction Motors

What is the suitable size of Direct On-line (DOL) starter with contactor, OCPD and overload relay for a 10 HP, 3-Phase squirrel cage Induction motor having 25kA perspective current and rated current (ln) is 13A?

Solution:

Overcurrent Protection:

To avoid the unwanted tripping, the rated current of short-circuit protective device (SCPD) should be 6 – 7 times greater than the rated current of the motor (ln). i.e.

  • 7 × ln
  • 7 × 13A
  • 91A

Hence, the next available size for short circuit protective device is 100A. Keep in mind that the SCPD only protects against short circuit and ground faults, and not against overload condition.

Overload Protection

As we know that OCPD does not provide protection against overload, we need to add a thermal overload relay in the starter for this purpose.

Trip Setting

The trip setting is calculated at 720% of the full load current (FLA) based on trip class. The trip classes are classified in 5, 10, 20 and 30. This shows the OL should trip when overcurrent starts to flow in the circuit for more than 5, 10, 20 and 30 seconds.

Typically, Class 5 is used for quick tripping while Class 10 are used with little thermal capacity i.e. submersible and water pumps. Class 10 and Class 20 are more common for general purpose application. Class 30 are used to eliminate the unwanted tripping for motors having load with considerable inertia.

Generally, the OL trip setting is calculated based on 720% times the full load current (FLA).

  • 720% × ln
  • 7.2 × 13A
  • 93.6A

The class type of OL relay is based on specific applications. For example, if we use a Class 20, it means the OL will trip when it detects overcurrent flowing in the circuit for more than 20 seconds.

Size of OL Relay

The size of thermal overload relay shall not more than 100% of the full load current

  • 100% × FLA
  • 100% × 13A
  • 13A

In this case, the rating of overload relay should not more than 13A.

  • Minimum OL Setting = 70% × 13A = 9.1A
  • Max OL Setting = 120% × 13A = 15.6A

Contactor Selection

As required, the Contactor Class is AC-3 for squirrel-cage induction motor. The making and breaking capacity is for AC-3 is 10.

Contactor Class: AC-3 – For Squirrel Cage Motor

Size of Main Contactor:

  • 100% × FLA
  • 100% × 13A
  • 13A

Making & Breaking Capacity of the Contactor:

  • 10 × FLA
  • 10 × 13A
  • 130A

Series Overview: Motor Circuit Calculations

Resources & Tutorials:

Wire Sizing Guides

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Electrical Technology
0 6 minutes read

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