18-05-2010, 10:13 PM
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Variable Voltage And Variable Frequency drives
Presented By:
P.NATARAJU S.E.V.S.RAGHU VARMA
3rd year E.E.E. 3rd year E.E.E.
Abstract:-
Our varying industrial needs demand more precise control of the outputs of our basic electrical prime movers i.e, THE MOTORS. Basically dc motors are easy to control compared to their ac counter parts, but they have their own limitations with increase in capacity. Conversely ac motors in particular squirrel cage induction motors are very economical but their speed control is comparatively difficult because it requires alteration of supply frequencies.
Due to technological advancements some drives which can control ac motors are available which are economical, easy to use and which can provide wide range of speed control both below and above base speeds.
These drives fundamentally alter the voltage and frequency being fed to motor according to the requirements using a technique called pulse width modulation (PWM).
These are increasingly becoming popular due their reasonable cost and other user friendly features. Since they use embedded systems they can be interfaced to the computers and can be programmed for automatic control reducing manual intervention.
Variable Voltage Variable frequency drive
A variable-frequency drive (VVVFD) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive. Variable-frequency drives are also known as adjustable-frequency drives (AFD), variable-speed drives (VSD), AC drives or inverter drives.
CONTENTS
Introduction
Operating principle
Example
VVVFD system description
VVVFD controller
VVVFD operator interface
VVVFD Operation
Available VVVFD power ratings
Shrinking cost and size
INTRODUCTION
Induction motors have been used in the past mainly in applications requiring a constant speed
Because conventional methods of speed control have been either expensive or inefficient.
Variable speed applications have been dominated by dc drives. Availability of Thyristors, Power Transistors, IGBT have allowed that development of variable speed induction motor drives
The main drawback of dc motors is a presence of commutator and brushes, which require frequent maintenance and make them unsuitable for explosive and dirty environment.
On the other hand, I.Ms, particularly squirrel-cage are rugged, cheaper, lighter, smaller, more effcient, require lower maintenance and can operate in dirty and explosive environments.
Although variable speed induction motor drives are generally expensive than dc drives,
They are used in number of applications like cranes, conveyers etc. because of the advantages of Induction motors.
SPEED CONTROL
Following methods are employed for speed control of Induction motors:-
1. Pole changing 2. Stator voltage control
3. Supply frequency control 4. Eddy current coupling
5. Rotor resistance control 6. Slip power recovery
We are going to study supply frequency control method. we can conveniently adjust the speed of motor by changing the frequency applied to the motor. We could adjust motor speed by adjusting the number of poles, but this physical change to the motor, would require rewinding, and result in a step change to the speed. So for convenience, cost efficiency and precision, we change the frequency.
OPERATING PRINCIPLE
Variable frequency drives operate under the principle that the synchronous speed of an AC motor is determined by the frequency of the AC supply and the number of poles in the stator winding, according to the relation:
Where,
RPM = Revolutions per minute
f = AC power frequency (Hertz)
p = Number of poles (an even number)
Synchronous motors operate at the synchronous speed determined by the above equation. The speed of an induction motor is slightly less than the synchronous speed.
Example
A 4-pole motor that is connected directly to 60 Hz utility (mains) power would have a synchronous speed of 1800 RPM:
120 x 60 = 1800 rpm
4
If the motor is an induction motor, the operating speed at full load will be about 1750 RPM.
If the motor is connected to a speed controller that provides power at 40 Hz, the synchronous speed would be 1200 RPM:
120 x 40 = 1200 rpm
4
Voltage induced in stator is proportional to the product of supply frequencies and area flux, if stator drop is neglected , terminal voltage can be considered proportional to product of frequency and flux .
Any reduction in supply frequencies without change in terminal voltage causes an increase air gap flux. The increase in flux will saturate the motor. While an increase in flux beyond rated value is undesirable from the consideration of saturation effects. A decrease in flux is avoided to retain the torque capability of the motor
Therefore the variable frequency control below the rated frequency is generally carried out at rated air-gap flux by varying the terminal voltage with frequency so as to maintain (V/F) ratio constant at rated value.
Tmax = +
Where K is constant ,
This equation suggests that with a constant (v/f) ratio, motor develops a maximum torque, except at low speeds. Motor therefore operates in constant torque mode. When it is required that same maximum torque retained at low speeds also in monitoring operation, (v/f) ratio is increased at low frequencies.
The variable frequency control provides a good running and transient performance because of following features:-
1. Speed control and braking operations are available from zero to above base speed operations.
2. During transient (starting, braking and speed reversal) the operation can be carried out at t the max. torque while reduced current giving good dynamic response.
3. Cu. Losses are low, the efficiency and power factor are high as the operation is restricted b/w syn. Speed and max. Torque point at all frequencies.
4. Drop in speed from no lad to full load is small.
Fig. Shows the torque-developing characteristic of every motor: the Volts per Hertz ratio (V/F). We change this ratio to change motor torque. An induction motor connected to a 460V, 60 Hz source has a ratio of 7.67. As long as this ratio stays in proportion, the motor will develop rated torque. A drive provides many different frequency outputs. At any given frequency output of the drive, you get a new torque curve.
Hertz
V/F Ratio
VVVFD system description
VVVFD system
A variable frequency drive system generally consists of an AC motor, a controller and an operator interface.
VVVFD controller
Just how does a drive provide the frequency and voltage output necessary to change the speed of a motor That's what we'll look at next. Fig. shows a basic PWM drive. All PWM drives contain these main parts, with subtle differences in hardware and software components.
The input section of the drive is the converter. It contains six diodes, arranged in an electrical bridge. These diodes convert AC power to DC power. The next section-the DC bus section-sees a fixed DC voltage.
The DC Bus section filters and smoothes out the waveform. The diodes actually reconstruct the negative halves of the waveform onto the positive half. In a 460V unit, you'd measure an average DC bus voltage of about 650V to 680V. You can calculate this as line voltage times 1.414. The inductor (L) and the capacitor © work together to filter out any AC component of the DC waveform. The smoother the DC waveform, the cleaner the output waveform from the drive.
The DC bus feeds the final section of the drive: the inverter. As the name implies, this section inverts the DC voltage back to AC. But, it does so in a variable voltage and frequency output. How does it do this That depends on what kind of power devices your drive uses. If you have many SCR (Silicon Controlled Rectifier)-based drives in your facility, see the Sidebar. Bipolar Transistor technology began super ceding SCRs in drives in the mid-1970s. In the early 1990s, those gave way to using Insulated Gate Bipolar Transistor (IGBT) technology, which will form the basis for our discussion.
Switching Bus with IGBTS
Today's inverters use Insulated Gate Bipolar Transistors (IGBTs) to switch the DC bus on and off at specific intervals. In doing so, the inverter actually creates a variable AC voltage and frequency output. As shown in Fig., the output of the drive doesn't provide an exact replica of the AC input sine waveform. Instead, it provides voltage pulses that are at a constant magnitude.
The drive's control board signals the power device's control circuits to turn "on" the waveform positive half or negative half of the power device. This alternating of positive and negative switches recreates the 3 phase output. The longer the power device remains on, the higher the output voltage. The less time the power device is on, the lower the output voltage shown in Fig. Conversely, the longer the power device is off, the lower the output frequency.
The speed at which power devices switch on and off is the carrier frequency, also known as the switch frequency. The higher the switch frequency, the more resolution each PWM pulse contains. Typical switch frequencies are 3,000 to 4,000 times per second (3 KHz to 4 KHz). (With an older, SCR-based drive, switch frequencies are 250 to 500 times per second). As you can imagine, the higher the switch frequency, the smoother the output waveform and the higher the resolution. However, higher switch frequencies decrease the efficiency of the drive because of increased heat in the power devices.
An embedded microprocessor governs the overall operation of the VVVFD controller. The main microprocessor programming is in firmware that is inaccessible to the VVVFD user. However, some degree of configuration programming and parameter adjustment is usually provided so that the user can customize the VVVFD controller to suit specific motor and driven equipment requirements.
VVVFD operator interface
The operator interface provides a means for an operator to start and stop the motor and adjust the operating speed. Additional operator control functions might include reversing and switching between manual speed adjustment and automatic control from an external process control signal. The operator interface often includes an alphanumeric display and/or indication lights and meters to provide information about the operation of the drive. An operator interface keypad and display unit is often provided on the front of the VVVFD controller as shown in the photograph above. The keypad display can often be cable-connected and mounted a short distance from the VVVFD controller. Most are also provided with input and output (I/O) terminals for connecting pushbuttons, switches and other operator interface devices or control signals. A serial communications port is also often available to allow the VVVFD to be configured, adjusted, monitored and controlled using a computer.
VVVFD Operation
When a VVVFD starts a motor, it initially applies a low frequency and voltage to the motor. The starting frequency is typically 2 Hz or less. Starting at such a low frequency avoids the high inrush current that occurs when a motor is started by simply applying the utility (mains) voltage by turning on a switch. When a VVVFD starts, the applied frequency and voltage are increased at a controlled rate or ramped up to accelerate the load without drawing excessive current. This starting method typically allows a motor to develop 150% of its rated torque while drawing only 150% of its rated current. When a motor is simply switched on at full voltage, it initially draws at least 300% of its rated current while producing less than 150% of its rated torque. As the load accelerates, the available torque usually drops a little and then rises to a peak while the current remains very high until the motor approaches full speed. A VVVFD can be adjusted to produce a steady 150% starting torque from standstill right up to full speed while drawing only 150% current.
With a VVVFD, the stopping sequence is just the opposite as the starting sequence. The frequency and voltage applied to the motor are ramped down at a controlled rate. When the frequency approaches zero, the motor is shut off. A small amount of braking torque is available to help decelerate the load a little faster than it would stop if the motor were simply switched off and allowed to coast. Additional braking torque can be obtained by adding a braking circuit to dissipate the braking energy or return it to the power source.
Available VVVFD power ratings
Variable frequency drives are available with voltage and current ratings to match the majority of 3-phase motors that are manufactured for operation from utility (mains) power. VVVFD controllers designed to operate at 110 volts to 690 volts are often classified as low voltage units. Low voltage units are typically designed for use with motors rated to deliver 0.2kW or 1/4 horsepower (Hp) up to at least 750kW or 1000Hp. Medium voltage VVVFD controllers are designed to operate at 2400/4160 volts (60Hz), 3300 volts (50Hz) or up to 10kV. In some applications a step up Transformer is placed between a low voltage drive and a medium voltage load. Medium voltage units are typically designed for use with motors rated to deliver 375kW or 500Hp and above. Medium voltage drives rated above 7kV and 5000 or 10,000Hp should probably be considered to be one-of-a-kind (one-off) designs.
Shrinking cost and size
Drives vary in the complexity of their designs, but the designs continue to improve. Drives come in smaller packages with each generation. The trend is similar to that of the personal computer. More features, better performance, and lower cost with successive generations. Unlike computers, however, drives have dramatically improved in their reliability and ease of use. And also unlike computers, the typical drive of today doesn't spew gratuitous harmonics into your distribution system-nor does it affect your power factor. Drives are increasingly becoming "plug and play." As electronic power components improve in reliability and decrease in size, the cost and size of VVVFDs will continue to decrease. While all that is going on, their performance and ease of use will only get better.
Conclusion
Due to many advantages offered by ac drives like automatic control, closed loop control, economical cost etc. ac motors are being replaced in fields which where totally capitalized by dc motors like traction, some industrial applications etc.
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