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Magnetic Motor Starter Control Circuit

Magnetic motor starters are used to turn motors on and off and provide overload protection. A magnetic motor starter also provides additional control contacts (NO and NC auxiliary contacts) in addition to power contacts (L1/T1, L2/T2, L3/T3) used to switch the motor on and off.

Magnetic motor starters work well in applications that require basic ON and OFF motor control.

When using a magnetic motor starter to control a motor, the starter is wired following a standard line diagram. For example, a pump motor is controlled by a magnetic motor starter that uses a control circuit.

The control circuit includes a three-position selector switch (HAND/OFF/AUTO) and a liquid level switch to control the motor starter. See Figure 11.

When the selector switch is placed in the HAND position, the motor is ON. When the selector switch is placed in the AUTO position, the motor is ON only when the liquid level switch contacts are closed (liquid at or above switch level). In the OFF position, the motor is OFF regardless of the position of the liquid level switch contacts.

magnetic motor starter can be used to control a pump motor

Figure 11. A magnetic motor starter can be used to control a pump motor.

The magnetic motor starter can turn the motor on and off, but it cannot set the acceleration and deceleration times for the motor.

 Also, the motor starter cannot be used to set the motor speed, provide circuit condition readouts, or display circuit or motor faults.

A magnetic motor starter does provide overload protection, and some models may provide phase-loss detection that turns the motor off if one of the three phases is lost.

Inherent Motor Protectors

An inherent motor protector is an overload device located directly on or in a motor to provide overload protection. Certain inherent motor protectors base their sensing element on the amount of heat generated or the amount of current consumed by a motor.

Inherent motor protectors directly or indirectly (using contactors) trip a circuit that disconnects the motor from the power circuit based on what the motor protector senses. Bimetallic thermo-discs and thermistor overload devices are inherent motor protectors.

Bimetallic Thermo-Discs

 A bimetallic thermo-disc operates on the same principle as a bimetallic strip. The differences between these devices are the shapes of the devices and their locations.

A thermo-disc has the shape of a miniature dinner plate and is located within the frame of a motor. See Figure 12.

A bimetallic thermo-disc warp and opens the circuit when a motor is overloaded. Bimetallic thermo-discs are normally used on small-horsepower motors to disconnect the motor directly from the power circuit.

Bimetallic thermo-discs may be tied into the control circuit of a magnetic contactor coil where they can be used as indirect control devices.

It is important to always ensure power to the motor is turned off before resetting a manual-reset thermo-disc. This prevents a potential hazard when the motor restarts.

Thermistor Overload Devices

A thermistor-based overload is a sophisticated form of inherent motor protection. A thermistor overload device combines a thermistor, solid-state relay, and contactor into a custom- built overload protector. See Figure 13.

Bimetallic thermo-discs

Figure 12. Bimetallic thermo-discs are normally used on small-horsepower motors to directly disconnect the motor from the power circuit.

A thermistor overload device combines a thermistor, solid-state relay, and contactor into a custom-built overload protector.

Figure 13. A thermistor overload device combines a thermistor, solid-state relay, and contactor into a custom-built overload protector.

Tech Fact

Overcurrent protective devices are designed for fast operation when protecting a circuit from short circuits and overcurrent.

 Overload protective devices are designed to protect a circuit from an overcurrent condition that exists for a relatively long time, such as during motor acceleration.

A thermistor is similar to a resistor in that its resistance changes with the amount of heat applied to it. As the temperature increases, the resistance of the thermistor decreases and the amount of current passing through the thermistor increases. The changing signal must be amplified before it can do any work, such as triggering a relay because the thermistor is a low-power device (normally in the thousandths of an ampere range).

When a thermistor overload device is amplified, a relay may open a set of contacts in the control circuit of a magnetic motor starter, de-energizing the power circuit of the motor.

The major drawback to thermistor overload devices is that they require a close coordination between the user and the manufacturer to customize the design.

Custom-designed overload protectors are costlier than standard, off-the-shelf overload protectors. With the exception of special and high-priced motors requiring extensive protection, custom-designed overload protectors are uneconomical and are not recommended.

About Ahmed Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.

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