Variable frequency drive and variable frequency control

VARIABLE FREQUENCY DRIVE &

VARIABLE FREQUENCY CONTROL

CONTENTS

• What is a VFD?• Purpose of the VFD• VFD Basics• Variable Frequency Control

What is a VFD?– Variable Frequency Drive (VFD)– A VFD can be used to control both the speed and

torque of a standard induction AC electric motor. – It varies both the frequency of the AC waveform

being delivered to the motor saving money in electricity.

– Basic components of a VFD:

• Input section, draws AC electric power from the utility, Rectifier section, converts the AC into DC power.

Purpose of the VFD

• VFDs help to limit demand and electrical consumption of motors by reducing the amount of energy they consume.– Standard motors are constant speed and when they are

energized they run at a 100% no matter the load.– Soft Start– Only use energy you need

VFD Basics

Block diagram of VFD

converter

VFD Basics

All VFD’s need a power section that converts AC power into DC power.

This is called the converter bridge. Sometimes the front end of the VFD, the converter

is commonly a three-phase, full-wave-diode bridge.

DC Bus

The DC bus is the true link between the converter and inverter sections of the drive.

Any ripple must be smoothed out before any transistor switches “on”.

If not, this distortion will show up in the output to the motor.

The DC bus voltage and current can be viewed through the bus terminals.

VFD Basics

Inverter The inverter section is made up primarily of

modules that are each made up of a transistor and diode in combination with each other which inverts the DC energy back to AC.

The power semi-conductors in the inverter section act as switches.

Inverters are classified as voltage-source, current-source of variable-voltage types. This has to do with the form of DC that the inverter receives from the DC bus.

VFD Basics

RECTIFIER

PositiveDC Bus

NegativeDC Bus

+

-

INVERTER

VFD Fundamentals

RECTIFIER

PositiveDC Bus

NegativeDC Bus

+

-

INVERTER

RECTIFIER

PositiveDC Bus

NegativeDC Bus

+

-

INVERTER

Area Under The Square-Wave PulsesApproximates The Area Under A Sine Wave

Frequency

Volta

ge

RECTIFIER

PositiveDC Bus

NegativeDC Bus

+

-

INVERTER

How Often You Switch From PositivePulses To Negative Pulses DeterminesThe Frequency Of The Waveform

Frequency

Volta

ge

Frequency = 30Hz

Frequency = 60Hz

Variable Frequency Control• The block diagram for a voltage source inverter

drive for asynchronous motor is as shown in fig

• The voltage source inverter shown in the figure generates a 3 phase voltage waveform the frequency of which is variable right from 0 HZ.

• The output voltage of the inverter can be either six step or PWM in order to reduce the harmonic contents.

Modes of Operation• The synchronous motor can operate in one of the two

following modes of operation at a given time :1. Constant torque mode2. Constant power mode

1) Constant torque mode :• As shown in fig. , upto the base speed i.e. rated speed, the

synchronous motor operates in the constant torque mode.• The torque produced by the motor remains constant, as

airgap flux is maintained constant & the airgap flux remains constant due to the fact that the ratio of stator voltage to stator frequency (v/f) is constant upto the base speed.

• The (v/f) ratio is maintained constant in order to avoid the saturation of core. The core saturation may take place if the ratio (v/f) exceeds a particular value.

• At the low speeds the characteristics in fig is not linear. This boost in the stator voltage at low speeds is essential to overcome the effect of stator resistance at low frequency.

Torque/power

Constant torque operation

Constant power operation

Power

Torque

(v/f) constant

Base speed

Constant V Variable operation

Speed stator frequency

0

Torque–speed characteristics

Output voltage V

Constant torque Constant power

V boost

Constant (v/f)

Rated frequency Output frequency f

Stator voltage-stator frequency

2) Constant power mode :• Above the base speed i.e. rated speed, the synchronous

motor is operated in the constant power mode as shown in fig1 .

• At base speed, the output voltage of the inverter reaches the rated value of stator voltage, therefore it is not possible to increase this voltage further. Thus the (v/f) ratio does not remains constant after this point, infact it decreases with increase in the stator frequency.

• This will weaken the air gap flux and hence will decrease the torque producing capacity of the motor as shown in fig1.

• The mechanical power developed by the motor is equal to the product of Torque & angular velocity i.e. P=T.

• Therefore as torque reduces with increase in the speed, the power output remains constant.

•THANK YOU

Variable frequency drive and variable frequency control

Engineering

Transcript of Variable frequency drive and variable frequency control