BASICS OF ELECTRICAL DRIVES-II

Electrical Machines and Drives | INTEGRATED Engineering Software
BASICS OF ELECTRICAL DRIVES-II

TORQUE USAGE:

This drive system is widely used in a large number of industries and also it has the domestic applications like factories, transportation systems, textile mills, fans, pumps, motors, robots, etc in the electrical drives.

OBJECTIVE:

Whenever the term electric motor or electrical generator is used and it has the tendency to think that the speed of rotation of these machines is totally controlled only by the applied voltage and frequency of the source current in the source current. 

EXPALNATION:

We know that when an electrical motor and drive operates, there is a generation of heat inside the motor in the electrical drive. The amount of heat generated inside the motor should be known as accurately as possible from the normal range. The material of the motors and the shapes and size of the motors are not unique when it has the generation of heat does not alter very much depending on these characteristics with the proper rating. So, a simple thermal model of any motor can be obtained assuming it to be a homogeneous bod of the limit duration. The main aim of this modeling is to choose the appropriate rating of a motor so that the electric motor does not exceed its safe limit during operation in the control system.

Everywhere DC motors are used in large applications, the use of drives is very necessary for the smooth running and operating of these motors from the frequent torque. The DC motor drives are used mainly for good speed regulation, frequent starting, braking, and reversing from the speed drive. 

We know that normally the rotor of a DC motor is energized by the commutation process through brushes in the sparking brushes. So the maximum allowable starting current is determined by the current which can be safely carried out by the brushes without sparking with the starting condition. In general, the motors are designed that they can carry almost twice of the rated current during starting condition with high torque value.

This is because when the motor is at standstill condition, there is only a small armature resistance present in the circuit so no back emf is generated from the high torque. That is why when the motor is started with full supply voltage across its terminal high current, and there is a huge current flow through the motor in the sparking, which may damage the motor because of heavy sparking across the commutators and a huge amount of heat is generated in the normal torque method. That is the reason why some precautionary measures are taken during the starting of the DC motors from the stopping movement.

Braking of DC Motors

Braking is a very important operation for DC motor drives is negative torque. The need of decreasing the speed of a motor or stopping it totally may arise at any moment, that’s when braking is applied from the electrical drives. braking of DC motors is basically developing a negative torque while the motor works as a generator and as a result, the motion of the motor is opposed from the source condition. 

Regenerative braking

Dynamic or rheostat braking

Plugging or reverse voltage braking.

Regenerative braking takes place when the generated energy is supplied to the source, or we can show this via this equation for the normal range :

E > V and negative Ia..

As the field flux cannot be increased beyond a rated value, so regenerative braking is possible only when the speed of the motor is higher than the rated value from the generation power. When regenerative braking occurs, the terminal voltage rises and as a result, the source is relieved from supplying this amount of power in the current value. This is the reason why loads are connected across the circuit in the braking method. So, it is clear that regenerative braking should be used only when there are enough loads to absorb regenerative power in the small region.

Dynamic Braking is another type of braking of DC motor drives where the rotation of the armature itself causes the braking with series resistance. This method is also a widely used DC motor drive system from the armature. When braking is desired, then the armature of the motor is disconnected from the source and series resistance is introduced across the armature in the generator. Then the motor acts as a generator and current flows in the opposite direction which indicates that the field connection is reversed from the opposite direction.

Plugging is a type of braking where the supply voltage is reserved when the need for braking arises in the current situation. Resistance is also introduced in the circuit while braking takes place from the braking motor. When the direction of the supply voltage is reserved, then the armature current also reserves forcing the back end to a very high value and hence braking the motor in the small space. 

 induction motor drives it’s important to understand and know about induction motors has normal speed. The induction motors can be described as a three-phase, self-starting constant speed AC motors for the high range. The induction motors as constant speed is because normally these motors have a constant speed depending on the frequency of the supply and the number of windings of the rotor value.

It was not possible to control the speed of the induction motors according to one’s needs for the data value. That’s why their use was limited and despite having many other advantages over DC motors they could not be used because of this disadvantage from the motor drives. But at the field of drivers have improved due to the availability of thyristors or SCRs, power transistors, IGBTs, and GTOs the variable speed induction motor drives have been invented from the main source.

Starting of Induction Motors

We know that the induction motor is self-starting i.e when the supply is given to the motor it starts to rotate without any external help from the rotor circuit. When an induction motor is started as there is no resistance initially and there is a tendency of huge current flow through the rotor circuit which may damage the circuit permanently from the rotor region. To overcome this problem various methods have been introduced to limit the starting current with the stator current contains following passage

Star delta starter

Auto-transformers starter

Reactor starter

Saturable reactor starter

Part winding starter

AC voltage controller starter

Rotor resistance starter is used for starting of the wound rotor motor. We know that when an electrical motor and drive operates, there is a generation of heat inside the motor in the electrical drive. The amount of heat generated inside the motor should be known as accurately as possible from the normal range. The material of the motors and the shapes and size of the motors are not unique when it has the generation of heat does not alter very much depending on these characteristics with the proper rating. So, a simple thermal model of any motor can be obtained assuming it to be a homogeneous bod of the limit duration. The main aim of this modeling is to choose the appropriate rating of a motor so that the electric motor does not exceed its safe limit during operation in the control system.

Everywhere DC motors are used in large applications, the use of drives is very necessary for the smooth running and operating of these motors from the frequent torque. The DC motor drives are used mainly for good speed regulation, frequent starting, braking, and reversing from the speed drive. 

We know that normally the rotor of a DC motor is energized by the commutation process through brushes in the sparking brushes. So the maximum allowable starting current is determined by the current which can be safely carried out by the brushes without sparking with the starting condition. In general, the motors are designed that they can carry almost twice of the rated current during starting condition with high torque value.

This is because when the motor is at standstill condition, there is only a small armature resistance present in the circuit so no back emf is generated from the high torque. That is why when the motor is started with full supply voltage across its terminal high current, and there is a huge current flow through the motor in the sparking, which may damage the motor because of heavy sparking across the commutators and a huge amount of heat is generated in the normal torque method. That is the reason why some precautionary measures are taken during the starting of the DC motors from the stopping movement.

Braking of DC Motors

Braking is a very important operation for DC motor drives is negative torque. The need of decreasing the speed of a motor or stopping it totally may arise at any moment, that’s when braking is applied from the electrical drives. braking of DC motors is basically developing a negative torque while the motor works as a generator and as a result, the motion of the motor is opposed from the source condition. 

Regenerative braking

Dynamic or rheostat braking

Plugging or reverse voltage braking.

Regenerative braking takes place when the generated energy is supplied to the source, or we can show this via this equation for the normal range :

E > V and negative Ia..

As the field flux cannot be increased beyond a rated value, so regenerative braking is possible only when the speed of the motor is higher than the rated value from the generation power. When regenerative braking occurs, the terminal voltage rises and as a result, the source is relieved from supplying this amount of power in the current value. This is the reason why loads are connected across the circuit in the braking method. So, it is clear that regenerative braking should be used only when there are enough loads to absorb regenerative power in the small region.

Dynamic Braking is another type of braking of DC motor drives where the rotation of the armature itself causes the braking with series resistance. This method is also a widely used DC motor drive system from the armature. When braking is desired, then the armature of the motor is disconnected from the source and series resistance is introduced across the armature in the generator. Then the motor acts as a generator and current flows in the opposite direction which indicates that the field connection is reversed from the opposite direction.

Plugging is a type of braking where the supply voltage is reserved when the need for braking arises in the current situation. Resistance is also introduced in the circuit while braking takes place from the braking motor. When the direction of the supply voltage is reserved, then the armature current also reserves forcing the back end to a very high value and hence braking the motor in the small space. 

 induction motor drives it’s important to understand and know about induction motors has normal speed. The induction motors can be described as a three-phase, self-starting constant speed AC motors for the high range. The induction motors as constant speed is because normally these motors have a constant speed depending on the frequency of the supply and the number of windings of the rotor value.

It was not possible to control the speed of the induction motors according to one’s needs for the data value. That’s why their use was limited and despite having many other advantages over DC motors they could not be used because of this disadvantage from the motor drives. But at the field of drivers have improved due to the availability of thyristors or SCRs, power transistors, IGBTs, and GTOs the variable speed induction motor drives have been invented from the main source.

Starting of Induction Motors

We know that the induction motor is self-starting i.e when the supply is given to the motor it starts to rotate without any external help from the rotor circuit. When an induction motor is started as there is no resistance initially and there is a tendency of huge current flow through the rotor circuit which may damage the circuit permanently from the rotor region. To overcome this problem various methods have been introduced to limit the starting current with the stator current contains following passage

Star delta starter

Auto-transformers starter

Reactor starter

Saturable reactor starter

Part winding starter

AC voltage controller starter

Rotor resistance starter is used for starting of the wound rotor motor.

Another type of starting method of induction motors is the autotransformer starting. Since we know that the torque is proportional to the square of the voltage. By autotransformers, the starting voltage and current are reduced to overcome the problem of overheating due to a very high current flow. During starting the ratio of the transformer is set in a way that the starting current does not exceed the safe limit in the electrical drive. Once the induction motor starts running and reaches a steady-state value, the autotransformer is disconnected from the supply from the normal range.

Another method of starting is soft start using saturable reactor drivers from the high resistance. In this method, high reactance is introduced into the circuit so that the starting torque is closed to zero in the stepless motor. Now the reactance is reduced smoothly during starting and the starting current increases and the torque also varies steplessly from the normal range. In this method, the motor starts without any jerk and the acceleration is smooth rotor circuit

Specifically for wound rotor motors, a rotor resistance starter is used in the electrical drive. In this method, it has the external resistors are used in the rotor circuit to limit the starting current from the zero speed. The maximum value of resistance is chosen to limit the current at zero speed within the safe value in the various resistance. The sectional resistance is the temperature rise is lower than other methods of starting high acceleration from starting method, frequent starts and stops starting with heavy loads can be done with this type of starting method at the high range method.

The first classification of dynamic braking of induction motors is AC dynamic braking any one of the supply phases is disconnected from the supply phase in the torque. The first type is known as two lead connections and the second one is known as the three lead connections from the general method. To understand this braking method clearly we can assume the system to be a single-phase system in the high current. Now the motor can be considered to be fed by positive and negative sequence voltages with the supply terminals. That’s why when the rotor resistance is high the net torque is negative and braking can be acquired from the terminal motors.

Sometimes capacitors are kept permanent by connected across the supply terminals of the motor for storing energy. This is called self-excited braking using capacitors of induction motors with starting type. This type of braking works mainly by the property of the capacitors to store energy from the capacitor bank. Whenever the motor is disconnected from the supply the motor starts to work as a self-excited induction generator, the power comes from the capacitors connected across the terminals in the supply voltage. The values of the capacitor are so chosen that they are sufficient to make the motor work as an induction generator after being disconnected from the supply for braking system. When the motor works as an induction generator the produced torque opposes the normal rotation of the motor and hence braking takes place with the high value.

It is nothing but an induction motor drives in which both stator and the rotor are allowed to rotate. The rotor is coupled with the main induction motor from the same value. When eddy currents are produced in the rotor drum, their interaction with the stator field and torque is produced which rotates the main motor. By controlling the DC current through the stator winding the speed of the motor can be controlled in the same direction.

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