Sizing The DOL Motor Starter Parts (Contactor, Fuse, Circuit Breaker and Thermal Overload Relay)

Basic DOL Starter Calculations

In this technical article we’ll calculate the size of each part of DOL motor starter for the system voltage 415V, 5HP three phase household application induction motor, code A, motor efficiency 80%, motor RPM 750, power factor 0.8 and overload relay of starter is put before motor.

Direct-on-line starting is a method of starting a motor that involves connecting it directly to the supply at the rated voltage. This method is referred to by its name. Direct-on-line motor starting, often known as DOL, is an approach that is appropriate for steady supplies as well as shaft systems that are mechanically rigid and well-dimensioned.

Pumps are examples of such systems.

Direct on-line starting of a motor (DOL motor starting) is the most straightforward method, wherein the stator is immediately connected to the mains supply. The motor starts with its inherent characteristics. Upon energization, the motor operates like to a transformer, with its secondary, constituted by the low-resistance rotor cage, in a short-circuit condition.

Despite its benefits (simple devices, higher starting torque, swift start, economical cost), direct on-line starting is only appropriate when:

• the motor’s power is minimal relative to that of the mains, thereby mitigating the impact of inrush current,
• the machine to be driven does not require gradual acceleration or has a damping mechanism to mitigate starting shock,
• the starting torque can be substantial without compromising machine functionality or the driven load.

Basic Calculation of Motor Torque and Current

• Motor Rated Torque (Full Load Torque) = 5252 × HP x RPM
• Motor Rated Torque (Full Load Torque) = 5252 × 5×750 = 35 lb-ft.
• Motor Rated Torque (Full Load Torque) = 9500 × KW × RPM
• Motor Rated Torque (Full Load Torque) = 9500 × (5 × 0.746) × 750 = 47 Nm
• If Motor Capacity is less than 30 KW than Motor Starting Torque is 3× Motor Full Load Current or 2× Motor Full Load Current.
• Motor Starting Torque = 3 × Motor Full Load Current.
• Motor Starting Torque = 3 × 47 = 142Nm.
• Motor Lock Rotor Current = 1000 × HPx figure from below Chart / 1.732 × 415

Locked Rotor Current

• As per above chart Minimum Locked Rotor Current  = 1000 × 5 × 1 / 1.732 × 415 = 7 Amp
• Maximum Locked Rotor Current = 1000 × 5 × 3.14 / 1.732 × 415 = 22 Amp.
• Motor Full Load Current (Line) = KW × 1000 / 1.732 × 415
• Motor Full Load Current (Line) = (5 × 0.746) × 1000 / 1.732 × 415 = 6 Amp.
• Motor Full Load Current (Phase) = Motor Full Load Current (Line) / 1.732
• Motor Full Load Current (Phase) = 6 / 1.732 = 4 Amp
• Motor Starting Current = 6 to 7 × Full Load Current.
• Motor Starting Current (Line) = 7 × 6 = 45 Amp

1. Size of Fuse

The purpose of a fuse is to prevent damage to a circuit in the event of a short circuit – very simple and straightforward. The fuse’s design offers protection by rapidly disconnecting the electricity supplied to a system.

Inductive loads, like squirrel-cage motors, will draw between 6 to 10 times the full-load current upon initial startup. A 200-volt, 10-horsepower motor will use 193.2 amps momentarily before achieving its full-load amperage current (refer to Figure 1). In this instance, it draws six times its full-load current.

Figure 1 – 200 V, 10 HP motor pulls 193.2 A for a short duration before it reaches its full-load amperage current

When this circuit is protected by fuses against short circuits and ground faults (see to Figure 2), the fuse must permit the occurrence of this overcurrent condition without interrupting or disconnecting power to the motor circuit.

Figure 2 – Motor circuit protected with fuses against short circuits and ground faults

Fuse as per NEC 430-52

• Maximum Size of Time Delay Fuse = 300% × Full Load Line Current.
• Maximum Size of Time Delay Fuse = 300% × 6 = 19 Amp.
• Maximum Size of Non Time Delay Fuse = 1.75% × Full Load Line Current.
• Maximum Size of Non Time Delay Fuse = 1.75% × 6 = 11 Amp.

2. Size of Circuit Breaker

According to NEMA, a circuit breaker is a device that, when used as a disconnect, opens and closes a circuit in a non-automatic manner. A circuit breaker automatically interrupts the circuit due to specified overcurrents resulting from an overload or a short circuit. A circuit breaker triggers a mechanism that disconnects the circuit from the overcurrent state.

Circuit breakers employ two varieties of tripping mechanisms: a bimetallic or thermal tripping element, and a magnetic tripping element. Circuit breakers sometimes combine both varieties of tripping mechanisms. Such devices can trip instantaneously in an occurrence known as inverse time.

A bimetallic breaker operates with an inverse time characteristic. The current causes the bimetal breaker to flex, resulting in the circuit tripping (see to Figure 3).

Figure 3 – Bimetal breaker makes the circuit trip

A magnetic breaker contains an electromagnet component that activates in response to increased current generated by a short circuit. The activation of a magnetic breaker occurs instantaneously (refer to Figure 4).

Figure 4 – The activation in a magnetic breaker is instantaneous

Circuit Breaker as per NEC 430-52

• Maximum Size of Instantaneous Trip Circuit Breaker = 800% × Full Load Line Current.
• Maximum Size of Instantaneous Trip Circuit Breaker = 800% × 6 = 52 Amp.
• Maximum Size of Inverse Trip Circuit Breaker = 250% × Full Load Line Current.
• Maximum Size of Inverse Trip Circuit Breaker = 250% × 6 = 16 Amp.

3. Thermal Overload Relay

Overload protection for an electric motor is essential for preventing burnout and to guarantee optimal operational longevity. Whenever there is an overload, a motor will draw an excessive amount of current, which will result in the motor overheating. Motor winding insulation degrades from excessive heat, necessitating defined temperature limitations to prevent the motor from overheating.

Overload relays are utilized in motor control systems to restrict the current consumption.

An overload relay consists of:

• A current sensing unit (connected in the line to the motor).
• A mechanism to break the circuit, either directly or indirectly.

To meet motor protection needs, overload relays have a time delay to allow harmless temporary overloads without breaking the circuit. They also have a trip capability to open the control circuit if mildly dangerous currents (that could result in motor damage) continue over a period of time. All overload relays also have some means of resetting the circuit once the overload is removed.

Figure 5 – Schneider Electric’s Nema magnetic motor starter 240vac coil volts; overload relay with amp setting 45A-135A

It’s important to epmpasis that the overload relay DOES NOT PROVIDE short circuit protection. The purpose of overcurrent protective devices, such as fuses and circuit breakers, is typically found within the disconnecting switch casing.

Thermal Overload Relay (Phase):

• Min. Thermal Overload  Relay setting = 70% × Full Load Current (Phase)
• Min. Thermal Overload Relay setting = 70% × 4 = 3 Amp
• Max. Thermal Overload  Relay setting = 120% × Full Load Current (Phase)
• Max. Thermal Overload Relay setting = 120% × 4 = 4 Amp

Thermal Overload Relay (Phase):

• Thermal Overload Relay setting = 100% × Full Load Current (Line).
• Thermal Overload Relay setting = 100% × 6 = 6 Amp

Working principle of thermal motor protection relay

4. Size and Type of Contactor

A contactor, as depicted in Figure 5, would consist of the following parts: an armature, a coil, a spring, an electromagnet (E-frame), and two sets of contacts—one stationary and one moveable.

Figure 6 – Components of the magnetic contactor

What is the mechanism by which the contactor operates to open and close? The E-Frame, once energized by the coil, transforms into an electromagnet. The armature, a part to the E-frame, is connected to a series of contacts. The armature is movable yet restrained by a spring. When the coil is energized, the movable contacts are drawn toward the stationary contacts as the armature is attracted to the E-frame.

Upon the meeting of the two sets of contacts, electrical power can be transmitted through the contactor to the load. Upon de-energization of the coil, the magnetic field dissipates, causing the spring to separate the two sets of contacts.

In summary, contactors work electromechanically, utilizing a minimal control current to open and close the circuit. The electromechanics perform the function, rather than the human hand, as seen in a knife blade switch or a manual controller.

A primary client issue is the lifespan of a contactor. It has been asserted that, “The most harmful action one can take regarding a car is to start its engine.” The same applies to contacts. The greater the frequency of contact openings and closures, the shorter the lifespan of the contactor.

An electrical arc is generated between contacts as they open and close. The arcs generate excess heat, which, if sustained, can harm the contact surfaces.

Contactors are mostly utilized for controlling machinery that employs electric motors. It comprises a coil that is connected to a voltage source. Typically, single-phase motors utilize 230V coils, whereas three-phase motors need 415V coils. The contactor has three primary normally open (NO) contacts and additional lower power-rated contacts referred to as auxiliary contacts (both normally open and normally closed).

These are utilized for the control circuit.

As per above chart:

• Type of Contactor = AC7b
• Size of Main Contactor = 100% × Full Load Current (Line).
• Size of Main Contactor = 100% × 6 = 6 Amp.
• Making/Breaking Capacity of Contactor = Value above Chart × Full Load Current (Line).
• Making/Breaking Capacity of Contactor = 8 × 6 = 52 Amp.

Video Lesson – Simple Motor Start Stop Circuit, Direct Online DOL

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