dave mellis

Variable Frequency DrivesBetter process control and regulation.
Speeding up or slowing down a machine or process.
Inherent power-factor correction
Emergency bypass capability
Produce adequate torque to handle the load.
Use power efficiently to produce the necessary torque at a given speed.
Effectively monitor the application or process.
PSI Pump Systems Inc.
Electric & Power Basics
The typical waveform consists of the frequency portion (time based) of the wave and the amplitude portion (the magnitude). This wave is actually in sine-wave form, commonly referred to as the fundamental.
How a motor shaft rotates
Torque is produced as the induction motor generates flux in its rotating field.
This flux must remain constant to produce full-load torque.
AC Induction Motor Theory
As shaft torque load increases, the slip increases and more flux lines cut the rotor windings, which in turn increases rotor current, which increases the rotor magnetic field and consequently the rotor torque.
Typical speed versus torque curve for a NEMA design B motor.
% Synchronous Speed
Starting Induction Motors
The starting current is very high, between 3 to 8 times the full load current. Depending on the size of the motor, this can result in voltage sags in the power system.
The full torque is applied instantly at starting and the mechanical shock can eventually damage the drive system, particularly with materials handling equipment, such as conveyors.
In spite of the high starting current, for some applications the starting torque may be relatively low, only 1.0 to 2.5 times full load torque.
The speed of the rotating electric field within the induction motor.
Synchronous Speed = 120 x frequency
# of motor poles
AC motor speed change can be accomplished in three ways:
(1) Change the number of poles in the motor; this means separate windings;
Change the slip characteristics of the motor; this is done with varying resistors, such as is done with a wound-rotor motor or by varying the stator voltage; or
Change the frequency of the power supplied to the motor. This is the method of choice .
Components of VFD’s
Power electronics is that field of electronics which covers the conversion of electrical energy from one form to another for high power applications.
Thyristors (SCR’s)
provided the standard method for rectifying AC. Also referred to as a diode.
Transistors provide fast switching capability for a relatively low cost.
The general types of transistors are:
The bipolar transistor;
The field-effect transistor (FET);
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.
Rectifier is that special type of converter that converts AC to DC.
VFD Basics
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.
Simplified Circuit showing DC bus components:
The DC link is an important section of the drive as it provides much of the monitoring and protection for the drive & motor circuit. It contains the base-drive fusing and pre-charge capacitor network, which assures steady voltage DC voltage levels prior to the inverter bridge and allows a path for over-voltage dissipation.
VFD Basics
Input Waveforms
The voltage that is not stepped down is rectified through the diodes, and a DC bus voltage should be present.
The DC + and DC – terminals will typically read approximately 325 volts DC on a 230 volts AC supplied drive and 650 volts DC on a 460 volts AC supplied drive.
This waveform, when viewed, is straight DC, possibly with some rippling effect from the AC input.
Power-Module
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.
VFD Basics
By switching the inverter-transistor devices on and off many times per half cycle, a pseudosinusoidal current waveform is approximated.
VFD Basics
The DC waveform looks more like an AC waveform but the voltage waveform is much different.
The power semi-conductors in the inverter section act as switches, switches of the DC bus, and therefore, are pulsing the motor with some voltage.
VFD Basics
A typical square wave takes its shape on the square-wave look due to this switching function ( which explains the sharp rise to peak) rather than a rotational, changing state of amplitude.
This frequency and amplitude pattern is sometimes called the carrier frequency of a PWM drive.
VFD Basics
The VFD changes the DC energy into three channels of AC energy that an AC induction motor can use to function properly.
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.
Types of VFD’s
Solid-state AC VFD’s can be named for their use, by their DC bus/inverter voltage or current source, by their waveform (PWM or PAM), by the type of power device used in their inverter section, or by their performance characteristics.
VFD Basics
The main objective of the VFD is to vary the speed of the motor while providing the closest approximation to a sine wave for current (while pulsing DC voltage to the motor).
VFD Basics
Volts-per-Hertz Control
The area within each pulse is the power delivered to the motor in volt-microseconds.
How VFD’s Operate
The PWMs drive ability to maintain the AC levels through all types of load conditions at given speeds is the factor which separates one drive manufacturer from the other.
Advantages
Sometimes getting a little more speed out of the motor can increase the flow just enough to satisfy the demand.
When there is capacity in the motor and the VFD can be programmed to do this, a new, larger motor does not have to be purchased and installed.
The VFD can run 10 to 20% higher in speed and make up for lost capacity in a flow-and-demand type of system.
However, many motors – as built – are not balanced for these speeds.
Fault Logs and On-Board
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