What is Plugging in Electrical Motor and Braking System?

Plugging Definition

Plugging is the method of interchanging power supply terminals to reverse the supply voltage across an electrical load such as the armature coil of the DC motor or stator coil of an induction motor. You may hear the term 'plugging' with electrical braking in motors. Yes, plugging is one type of braking of an electrical motor. As we know it is mostly done with three-phase induction motors and DC motors.

Plugging comes from the word 'Plugged' which means stop. When an electrical motor is running in a particular direction and we want to stop it instantly then we need to force the motor to rotate in the opposite direction. So before starting to rotate in the opposite direction, the motor will stop immediately. And this phenomenon is used with a plugging mechanism.

How does Plugging Work?

Plugging in DC Motor

We know that a DC motor has two terminals from the armature coil - Positive and Negative. So if we interchange armature coil terminals, the direction of the current flow through the armature coil will change as well as the magnetic field, so the motor will stop. But remember that the field terminal should not be interchanged it should be kept as before.

Also one more important thing is when we reverse the polarity of the armature voltage, Back EMF no longer opposes the supply voltage. In fact, both the supply voltage and back emf will work in the direction. So there is a chance of high current flow due to the combination of back emf and supply voltage so it is good to connect a resistance to limit the current.

Plugging in Three Phase Induction Motor

We all know that if we want to change the direction of rotation of a three-phase induction motor we need to interchange any two terminals of the stator coil. So when it is done with a running induction motor it can work as plugging. Because before rotating in the opposite direction it must be stopped. So, the plugging is done in an induction motor by interchanging any two terminals of the stator coil. 

Theoretically, when the supply terminal is interchanged magnetic field rotates in the opposite direction so the motor Slip(the difference between the speed of the stator magnetic field and rotor rotational speed) becomes greater than Unity(S>1). It means the rotor will rotate faster than the stator rotating magnetic field speed. So a torque will be developed in the opposite direction of the rotor rotation that will force to stop the motor as soon as possible.

Application and Uses of Plugging

1. The plugging method is used for the braking of the DC Motor.

2. Plugging is used for the braking of the Induction Motor.

3. In practical applications such as Elevators, Cranes, Press machines, and Mills, the plugging method is used.

4. Used in Machine Tools.

Advantages of Plugging

1. Instant Braking

2. No mechanical arrangement or extra cost is required.

3. Easy to operate.

4. Easy to install than a mechanical braking system.

Plugging Disadvantages

1. Not working properly for heavily loaded machines.

2. Execcesive power loss for DC Motors.

3. It is difficult to brake the motor at exactly zero speed.

4. It generates significant heat.

How Plugging is Installed Practically?

1. Plugging switches are designed with electrical circuits and components and they are added to the pilot circuits for the motors. This helps for quick and automatic stopping of machines.

2. Some plugging switches are designed to mechanically connect to the motor shaft or to a shaft on the driven machine. The rotating motion of the machine is transmitted to the plugging switch either by a centrifugal mechanism or magnetic induction arrangement such as an eddy current disc. This plugging switch is also connected to a reverse starter that can control the motor. Basically, it acts as a link between the motor and reversing starter. This plugging switch can be driven by a gear mechanism or a chain drive or a direct flexible coupling system.

 

3. Some plugging switches are designed to be equipped with a lock-out relay or a safety latch relay. This safety feature helps to ensure that if the motor shaft is turned accidentally, the switch cannot able to start the motor.

4. Difference between Plugging and Braking: Braking is the method or process to stop the rotation of any rotating machine here we consider an electrical motor. On the other hand, plugging is one type of process to achieve the braking of an electrical motor it may be a DC motor or an Induction Motor.

Read Also: 

• Actual Difference between Servo Motor and Stepper Motor
• Shaded Pole Induction Motor Working and Application Explained
• Speed Control of DC Shunt Motor - Easy Methods
• Under Voltage and Over Voltage Effect on Motor Explained
• Electric Motor Control Procedure, Methods, Devices, Circuits

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