Working of Variable Frequency Drive (VFD) or VFD for motor control

What is VFD?

VFDs (Variable Frequency Drive) are drivers which regulate the electric current to instruments. Instruments such as motors and fans work only at one speed if they are directly connected to the 3 phase power supply, that is these motors typically only have an on/off state of operation.

VFD is needed if multiple operation speeds are desired. Also, apart from selectable speeds, the efficiency of the overall system is increased due to the fact that the motor only sees the necessary amount of input power to achieve desired output power. Also, the motor can be slowly brought up to speed, eliminating huge start-up current spikes.

Working of VFD for a motor:

To understand the working of the VFD, internal diagram is shown below:

The circuit consist of three main blocks:

• Input converter/ input rectifier
• DC bus
• Output inverter

Two rectifiers are used for each phase, one is to convert the AC signal to DC signal and other will allow power to pass through only when the voltage is negative. Thus the AC voltage is converted to DC at the rectifier.

Then the DC voltage is passed through DC bus circuit, which contains capacitors and inductor or resistors, thereby smoothing the incoming power supply to the dc bus.

The final section of the VFD is referred to as an “inverter.” The inverter contains transistors that deliver power to the motor. The “Insulated Gate Bipolar Transistor” (IGBT) 26 is a common choice in modern VFDs. The IGBT can switch on and off several thousand times per second and precisely control the power delivered to the motor.

The IGBT uses Pulse Width Modulation (PWD) 19 to simulate a current sine wave at the desired frequency to the motor.

Pulse Width Modulation means amplitude of the analoge current signal increases as the width of the DC pulse increases. The voltage will be in pulse and current signal will be converted into sinusoidal. The amplitude of the sinusoidal wave is dependent on the width of the pulse and the delay of the pulse, which is adjusted to the desired current value.

Motor speed (rpm) is dependent upon frequency. Varying the frequency output of the VFD controls motor speed:

Speed (rpm) = frequency (hertz) x 120 / no. of poles

The frequency of the signal is increased by the VFD for increase the speed of the motor. As frequency increases the motor torque decreases and speed increases.

Advantages:

• Limit torque
• Save energy
• Maintenance cost can be lowered
• Control of process temperature and pressure

Industrial applications:

• Conveyor application
• Exhauste fan application
• Compressor application
• Water supply pump application

What are Control relays?

Basic working principle:

An electromechanical relay is an electrical switch actuated by an electromagnet coil. As for switching devices, they exhibit simple “on” and “off” behaviour with no intermediate states. Relays are very useful electrical switches through the coil of relay creates an electromagnetic field which attracts a lever and changes the switch contacts.

Electromagnetic induction 17 is the operating principle of the relay switches. which is the production of an electromotive force (i.e., voltage) across an electrical conductor in a changing magnetic field and vice-versa.

When a voltage is applied to the electromagnetic coil system in the relay, current flow in the coil produces a magnetic energy which pulls the armature down. When pulls down the circuit gets closed as the armature touches the No contact lead.

Types of Relay switching contacts:

SPST (NO contact) :

SPST, that is ‘single-pole, single-throw’ relay which is a normally open contact ( NO contact) which means the switch will be in the open (off) state when the relay coil is de-energized. When the coil is energized the magnetism generated pulls down the armature and close the switch. This type of switch is synonymously know as Form-A.

There is also SPST normally closed type switches

SPST (NC contact):

The armature is arranged as shown in above picture so that the switch is normally closed and when an electric voltage is applied to the coil, the switches open and the circuit brakes. This type of switches are also known as Form-B in SPST switches.

SPDT:

The single-pole, double-throw (SPDT) relay contact, otherwise known as a “Form-C” contact. This design of switch provides both a normally-open and normallyclosed contact set in one unit, actuated by the electromagnet coil.

DPDT :

DPDT relays are some of the most common found in industry, due to their versatility. There are two SPDT switches arranged in the DPDT switch circuit and both are electrically isolated from each other. Each contact set offers the normally open and normally closed contacts.

There are 4 pin and 5 pin relays available:

5 pin switch most commonly contain normally open and normally pen contacts.

4 pin relay contain either normally open or normally closed contacts.

Advantages:

• Relays can operate in DC or AC.
• Relays can switch many contacts at once.
• Relays can switch higher voltages.

##Disadvantages:

• The contacting becoming worn, resulting in a limited lifespan
• Would perform well when it comes to switch high inrush currents.

What are the essential features of a switchgear and how a switchgear can be used for the protection of power system

What is a switchgear

Switchgear is a device which is used for the switching controlling and for the protection of the electrical circuits and equipment, the importance of electricity in our modern civilization is very high so in order to get a continuous power we must secure the power system from large faults and we must protect machinery and devices and thus we can ensure maximum continuity of the power supply

What are the essential features which are needed for switchgear

Complete reliability

Due to the heavy requirement of electricity in our modern world the number of generating stations is also high and so the fault occurring in the power station will be high too and to prevent these faults we need to depend on switchgear so when a fault occurs switchgear can isolate the faulty system and the continuous power will be generated and supplied without any interruption

Absolutely certain discrimination

If any fault occurs in a power system the switchgear must have the ability to discriminate between the faulty section and the healthy section, it must isolate 1 the faulty section without any damage to the health system so that it won’t affect the continuity of the power supply

Quick operation

When a fault occurs in a power system the switchgear must act quickly so that it won’t damage any equipment like the generators and transformers, at the time of fault occurrence the switchgear must act quick and isolate the faulted system from the healthy, if the fault is not cleared it will affect the healthy parts and thus endangering of the complete shutdown will happen

Manual control provision

In some cases, there will be electrical or electronic control fail so in order to protect the power system the necessary operation must be carried out through the manual control, so switchgear must have provision for manual control

Provision for instruments

Instruments provision are needed, it may be in the form of ammeter or voltmeter in the unit itself or necessary current and voltage transformers for connecting to the main switchboard or a separate panel

What are the roles that switchgear plays for the protection of a power system

The major role of switchgear in a power system is to ensure the safe operation of the power system, and by that, we can ensure the people safety too. So by ensuring the safe operation to the power system, it will also ensure the equipment protection and the personnel who works in there.

Many dangerous situations will arise from overcurrents and overvoltages, by the occurrence of earth faults it could cause high touch voltages and it will endanger people, so in order to continue the safe operation of the power system the overcurrent and overvoltage must be prevented

Reliability and stability

The protection system must take care for the reliable operation of the relays so that the reliability can be improved, reliable functions can be referred to as backup protection, reliability can be reached by combining different protection principles example distance and differential protection of transmission lines

Stability of the system is the quality of the system by which the system remains stable and inoperative and stable under certain conditions

Requirement adequateness

There could be many faults and disturbances in a power system so it is impossible to provide protection against each and every fault or abnormal condition due to economic reasons. Instead of that, the system must provide adequate protection

Simplicity

A protection system should be simple so that it could be easily operated and maintained

What is a Transformer? Types and Transformer protection

The transformer is a device that transfers electrical energy from one circuit to another through the magnetic field without change of frequency.

Based on service

1. Power Tr
2. Distribution Tr.

What is the use of transformer?

In this age of modern transmission system with optimal loss it is necessary to transmit the power at very high level.it is not economical to produce electrical power at such a high voltage.Here comes the vital role of transformer, which enhances the voltage level from generating voltage to such a high level where the loss is minimal.

It is also used as booster in distribution system to give better voltage regulation.

General Fault In Transformer

General Fault in transformer are

1. Open circuit Fault
2. Over heating
3. Winding short circuit fault
4. faults due to lightening

Protection For Transformer

1. Buchholz Relay
2. Earth Fault Relay
3. Over current relay
4. Differential relay
5. Digital relay

DC COMPOUND MOTOR AND ITS TYPES

DC COMPOUND MOTOR


A compound wound DC motor is a self-excited motor it is a combination of Dc series motor and shunt Dc motor it has both shunt and series field windings with the help of field coils the required amount of magnetic flux which helps the armature coil to rotate at desired speed with the necessary flux

The compound wound DC motor is created by the combination of the shunt wound DC motor and series wound DC motor so that it could achieve both of their properties a shunt wound Dc motor has better speed regulation characteristics while the DC series motor has high starting torque so that the compound wound Dc motor has achieved both of these characteristics so it has higher starting torque and better speed regulation ability so it can be used for industrial applications such as rolling mills, freight elevators, stamping presses, mixers, and metal shears

The compound wound Dc motor can be classified into two types according to their field winding connection with respect to the armature winding

• Long shunt compound motor

In this case, the shunt field winding is connected in parallel across both the armature and to the series field coil’s

Short shunt compound wound DC motor

In the case of short shunt compound wound of DC motor, the field winding is connected across the armature winding only in parallel and the field coil which is series is exposed to the supply current

Cumulative compound DC motors

In this type the polarity of the shunt winding adds to the series fields it can be either short shunt in which the shunt is parallel only to the armature or long shunt in which the shunt would be parallel to both armature and series field and thus it will create a cumulative effect

• Differential compound motor

In this type the armature polarity and shunt field oppose each other and the negative terminal of the shunt field is connected to the positive terminal of the armature here an opposite effect is created, if the load is reduced differential compound motor will act as a series motor and it will over speed if we increase the load its speed will reduce

Motor control circuits – Types

The function of the motor control circuit is to control the AC motors 89. A higher voltage is required to start the motor, where a voltage lower than the starting voltage is required to operate. Changing the motor from a higher voltage to a lower voltage is the main function of a motor control circuit.

There are mainly there are three types of motor control circuits:

• Direct On Line Starter (DOL starter)
• Star Delta Starter 145
• Auto Transformer Starter

DOL Starter:

Direct Online Starter (DOL) are used in small motors with has lower power under 10kW.

In overcurrent protection circuits, fuses and circuit breakers are commonly used. Fuses and circuit breakers are configured to interrupt the connection if the current in the line exceeds the rated motor current.

Star Delta Starter:

To decrease the starting current cage motors of medium and larger sizes are started at a reduced supply voltage. The reduced supply voltage starting is applied in the Star Delta methods. This is applicable to motors designed for delta connection in normal running conditions.