Monitor the operation of electrical, electronic and control systems (continued)

 

 

An electrical and electronics device which operates and controls the speed, torque and direction of movement of electric motor and is generally employed for speed or motion control applications such as machine tools, transportation, robots, fans, etc. is known as electrical drive.

 

Electrical drives 

 

are integral part of "industrial and automation processes", particularly where precise control of speed of the motor is the prime requirement. In addition, all modern electric trains or locomotive systems have been powered by electrical drives. Robotics is another major area where adjustable speed drives offer precise speed and position control.

The drives can be of constant or variable type. The constant speed drives are inefficient for variable speed operations; in such cases variable speed drives are used to operate the loads at any one of a wide range of speeds.

 

A question arises why Electrical Drives are needed?

 

The adjustable speed drives are necessary for precise and continuous control of speed, position, or torque of different loads. Along with this major function, there are many reasons to use adjustable speed drives. Some of these are as:

• To achieve high efficiency: Electrical drives enable to use wide range of power, from milliwatts to megawatts for various speeds and hence the overall cost of operating the system is reduced
• To increase the speed of accuracy of stopping or reversing operations of motor
• To control the starting current
• To provide the protection
• To establish advanced control with variation of parameters like temperature, pressure, level, etc.

The advancement of power electronic devices, microprocessors and digital electronics led to the development of modern electric drives which are more compact, efficient, cheaper and have higher performance than bulky, inflexible and expensive conventional electric drive system that employs multi-machine system for producing the variable speed.

 

Block Diagram of an AC Electric Drive

 

The components of a modern electric drive system are illustrated in below figure.

 

 

STARTERS FOR INDUCTION MOTOR:

 

An induction motor

 

can self start owing to the interaction between the rotating magnetic field flux and the rotor winding flux, causing a high rotor current as torque is increased. As a result, the stator draws high current and by the time the motor reaches to full speed, a large amount of current (greater than the rated current) is drawn and this can cause heating up of the motor, eventually damaging it. To prevent this, motor starters are needed. 

Motor starting can be done in 3 ways as:

• Applying full load voltage at intervals of time: Direct On Line Starting (DOL)
• Applying reduced voltage gradually: Star Delta Starter and Soft starter
• Applying part winding starting: Autotransformer starter

 

DIRECT ON LINE STARTER (DOL):

 

1. It is simple and cheap starter for a 3-phase induction motor.
2. The contacts close against spring action.
3. This method is normally limited to smaller cage induction motors, because starting current can be as high as eight times the full load current of the motor. Use of a double –cage rotor requires lower staring current (approximately four times) and use of quick acting A.V.R. enables motors of 75 kW and above to be started direct on line.
4. An isolator is required to isolate the starter from the supply for maintenance.
5. Protection must be provided for the motor. Some of the safety protections are over-current protection, under-voltage protection, short circuit protection, etc. Control circuit voltage is sometimes stepped down through an autotransformer.

 

Below is the circuit diagram of DOL Starter:

 

 

 

STAR-DELTA STARTER:

 

A three phase motor will give three times the power output when the stator windings are connected in delta than if connected in star, but will take 1/3 of the current from the supply when connected in star than when connected in delta. The starting torque developed in star is ½ that when starting in delta.

1. A two-position switch (manual or automatic) is provided through a timing relay.
2. Starting in star reduces the starting current.
3. When the motor has accelerated up to speed and the current is reduced to its normal value, the starter is moved to run position with the windings now connected in delta.
4. More complicated than the DOL starter, a motor with a star-delta starter may not produce sufficient torque to start against full load, so output is reduced in the start position. The motors are thus normally started under a light load condition.
5. Switching causes a transient current which may have peak values in excess of those with DOL.

 

Below is the circuit diagram of STAR-DELTA Starter:

 

AUTO TRANSFORMER MOTOR STARTER:

 

1. Operated by a two position switch i.e. manually / automatically using a timer to change over from start to run position.
2. In starting position supply is connected to stator windings through an auto-transformer which reduces applied voltage to 50, 60, and 70% of normal value depending on tapping used.
3. Reduced voltage reduces current in motor windings with 50% tapping used motor current is halved and supply current will be half of the motor current. Thus starting current taken from supply will only be 25% of the taken by DOL starter.
4. For an induction motor, torque T is developed by V2, thus on 50% tapping, torque at starting is only (0.5V)2 of the obtained by DOL starting. Hence 25% torque is produced.
5. Starters used in lager industries, it is larger in size and expensive.
6. Switching from start to run positions causing transient current, which can be greater in value than those obtained by DOL starting.

 

Below is the circuit diagram of AUTO TRANSFORMER Starter:

 

Rotor Resistance Starter:

 

1. This starter is used with a wound rotor induction motor. It uses an external resistance/phase in the rotor circuit so that rotor will develop a high value of torque.
2. High torque is produced at low speeds, when the external resistance is at its higher value.
3. At start, supply power is connected to stator through a three pole contactor and, at a same time, an external rotor resistance is added.
4. The high resistance limits staring current and allows the motor to start safely against high load.
5. Resistors are normally of the wire-wound type, connected through brushes and slip rings to each rotor phase. They are tapped with points brought out to fixed contactors.
6. As the motor starts, the external rotor resistance is gradually cut out of circuit ; the handle or starter is turned and moves the three contacts simultaneously from one fixed contact to the next.
7. The three moving contacts are interconnected to form a start point for the resistors.
8. To ensure that the motor cannot be started until all rotor resistance is in circuit, an interlock is fitted which prevents the contactors from being closed until this condition is fulfilled.

 

Below is the circuit diagram of ROTOR RESISTANCE Starter:

SOFT STARTERS:

 

A Soft Starter is a device that starts motors with reduced power supplied at start-up. Reducing the power reduces potentially damaging electrical and mechanical shocks on the system.
As the name implies, starters "start" motors. They can also stop, reverse, accelerate and protect them. Whether it's a small fan, or piece of mining equipment, electric motor are often the driving force behind them.

In technical terms, a soft starter is any device that reduces the torque applied to the electric motor. It generally consists of solid-state devices like thyristors to control the application of supply voltage to the motor. The starter works on the fact that the torque is proportional to the square of the starting current, which in turn is proportional to the applied voltage. Thus the torque and the current can be adjusted by reducing the voltage at the time of starting the motor.

 

There can be two types of control using soft starter:

 

1. Open Control: A start voltage is applied with time, irrespective of the current drawn or the speed of the motor. For each phase, two SCRs are connected back to back and the SCRs are conducted initially at a delay of 180 degrees during the respective half-wave cycles (for which each SCR conducts). This delay is reduced gradually with time until the applied voltage ramps up to the full supply voltage. This is also known as Time Voltage Ramp System. This method is not relevant as it doesn’t control the motor acceleration.
2. Closed-Loop Control: Any of the motor output characteristics like the current drawn or the speed is monitored and the starting voltage is modified accordingly to get the required response.  The current in each phase is monitored and if it exceeds a certain set point, the time voltage ramp is halted.

Thus the basic principle of the soft starter is by controlling the conduction angle of the SCRs the application of supply voltage can be controlled.

 

BLOCK DIAGRAM OF SOFT STARTER (below):