What is it VFD  and VFD Cable ?



Do you know about this? VFD cable is only screened cable or other?

and test standards.

thanks for help.

BASIC PRINCIPLE OF A VFD

In theory the basic idea is simple, the process of transforming the line frequency to a variable frequency
is basically made in two steps.

1. Rectify the sine voltage to a DC-voltage.
2. Artificially recreate an AC-voltage with desired frequency. This is done by chopping the DC-voltage
into small pulses approximating an ideal sine wave. A VFD consists basically of three blocks: the rectifier,
the DC-link and the inverter.

There are three different types of VFDs:
• VSI - Voltage Source Inverter, e.g. PWM.
• CSI - Current Source Inverter.
• Flux vector control.
The CSI has a rough and simple design and is considered to be very reliable, but the output signal
means a lot of noise. Furthermore the CSI induces high-voltage transients in the motor. The flux vector
control is a more sophisticated type of VFD which is used in applications where the speed
should be controlled very precisely, e.g. paper mills. This type is expensive and pump applications
cannot take advantage of its benefits. The ended by Flygt, is the PWM – Pulse Width Modulation.

VFDs (Variable Frequency Drives) seem to be ever present in applications ranging from motion control to commercial flow/pumping. VFDs, also known as Adjustable Speed Drives or Variable Speed Drives require special considerations for the proper installation and operation of the drive system as well as the proper operation of nearby or adjacent systems. The nature of their operation impacts both longevity and reliability of these systems. This paper examines the motor-supply cable's impact on VFDs and surrounding equipment. Included are some fundamental guidelines for their installation and design.

Background

VFD is the acronym for Variable Frequency Drive, which has become a very popular method to
adjust a motors speed. Frequency can be defined as an electrical term meaning power pulses of
voltage and current. In the United States 60 Hertz, or 60-power pulse cycles per second, are
conducted through most wires and cables throughout the country. As the power in the United
States is alternating current (AC) each of these frequency cycles contain one positive and one
negative power pulse of voltage and current. Increasing the number of power pulses per second
will make the motor turn faster. Likewise, decreasing the number of power pulses per second will
make the motor turn slower. Therefore by increasing and decreasing the frequency of the power
pulses, the drive can adjust the speed of the motor.

The components of the drive system are broken into four major categories: Source power, VFD,
Cable and the Motor. Other ancillary components exist such as resolver and encoder feedback
devices, tachometers, sensors, relays and others that help supplement the system.
The local electrical company provides the source power for all electrical equipment and apparatus
for operation. The source power may go through a transformer to either increase or lower the
voltage but the frequency will remain constant at 60 Hertz.
The function of the drive is to send all power pulses that control the motor's start-up, operating
speed, and stopping. The three major tasks that a VFD drive has to accomplish prior to adjusting a
motor's speed are:
First, the source power must be converted from Alternating Current (AC) to Direct Current (DC).
This conversion is accomplished by means of a rectifier, a diode is used for simple rectifying and a
Silicone Controlled Rectifier (SCR) is used for more intelligent rectification. The power source that
was 460 Volts AC, 60 Hertz now is converted to 650 Volts DC. This AC to DC conversion is
necessary before the power can be changed back to AC at a variable frequency. In short the
power goes from AC to DC then back to AC again so it can be used for VFD applications

Second is a large capacitor, which is known as the DC bus. A capacitor can be simply defined as
an electronic component that stores energy. The DC bus acts like a storage battery to supply DC
power to the third part of the drive.
The third part of the drive is known as the inverter. An inverter converts DC back to AC by utilizing
an electronic component known as a Bi-polar transistor. The inverter can be controlled to vary the
frequency so that the motor receives the correct flow of power pulses. This is the advantage of
utilizing a Variable Frequency Drive. The pulse width modulation (PWM) frequency is
approximately 20,000 Hertz and offers finer control by only varying a few cycles. In contrast at the
power source frequency of 60 Hertz, changing a few cycles only offers a much coarser change and
will not allow for as close control.
The VFD drive output is basically a flow of AC power pulses at a certain frequency that provides or
maintains the desired speed of a running motor. The motor through means of power supply cables
receives these power pulses. Cables and their functions will be discussed later.
The motor consists of two major parts the armature (rotor) physically turns and the field (stator)
that remains stationary. The cables are connected to the stator and upon the application of power
cause an electromagnetic field to rotate. This rotating field causes the rotor to follow the moving
electromagnetism and turn. The stator of the motor is constructed of insulated wire that is wound
through slots a specific number of times in a required pattern. This is the defenseless part of the
motor. The wire insulation is extremely thin and can get nicked during the winding process. These
nicks are the bare spots in the wire that causes arcing and can lead to motor failure.

What Happens

The VFD drive transition from AC to DC then back to variable AC is not done cleanly. Power
distortions are created by the first part of the drive (rectifier) and then sent back through the source
power system. Power distortions are also created by the third part of the drive (inverter) and sent
on to the motor. Therefore, power distortions are created at both ends of the drive causing nonlinear
spikes, wave reflections and inrush currents. The following will explain these types of
phenomena:
Harmonics, non-linear spikes, and reflections are common power distortions. In rush currents is an
additional problem that the motor and cable are subjected to. Harmonics are multiples of a
fundamental frequency. As an example if the fundamental frequency is 60 Hertz, the 5th harmonic
is 300 Hertz (5x60). Power distortions are caused by harmonic frequencies. Some harmonic
frequencies are "in-phase" with the fundamental frequency and rotate in the same direction as the
field, except more quickly due to the higher harmonic frequency. This condition causes the power,
both voltage and current of each harmonic to add to the voltage and current of the fundamental
frequency. Keep in mind that the power in the fundamental frequency is all that is needed or
wanted. The additional power from the harmonic frequencies cause overheating, high voltage
stress and also confuse electronic functions that depend on the fundamental frequency for clock or
timing functions. The added harmonic power also affects the motor and power supply cables.

As previously mentioned, a higher flow of power pulses will cause a motor to run faster. Harmonic
frequencies cause a motor to fine-tune its adjustments with braking and other heat generating
reactions. The 7th and 13th harmonics rotate with the field, supply additional power pulses and
require the brake to regulate motor speed. The 5th and 11th harmonics oppose the field's rotation
and require additional current (heat) to regulate motor speed. The 3rd, 9th, & 15th are triple
harmonics which do not rotate but are the most in-phase and additive to neutral current (heat). The
harmonics that are commonly generated by a VFD drive are the 5th, 7th 11th, and 13th.
The definition of non-linear power is that a change in voltage does not generate the same change
in current. The motor expects a power pulse to be regulated. When the frequency is increased,
the motor expects the right amount of current to be included in the power pulse to sustain the
increase in speed. In the case of non-linear power, the current does not properly support the
motors requirements. This distorted current either fights or overdoes the change, resulting in high
voltage stress and heat.



A spike is a very quick increase in voltage that occurs for a short time. The inverter in the drive
which is a fast switching transistor must rise from zero to 650 volts (rectified 460 volt system) and
then go back to zero 20000 times a second. This is a very fast rise time. Several things can
happen as the inverter is switching and conducting the power pulses through the cable to the
motor. The 650-volt normal voltage can overshoot higher to 2000 volts or more. The cable that
connects the VFD drive to the motor looks electrically different to the power pulse as the length
increases. The longer the cable length the greater the increase in inductance, which in turn affects
the overshoot of the voltage spike. Therefore a long power supply cable will have greater and
more intense voltage spikes than a shorter cable. Voltage spikes are very quick lasting for only a
few millionths of a second. The inverter in the drive is called an Insulated Gate Bipolar Transistor
(IGBT) and is one of the fastest switching inverters in the Pulse Width Modulation (PWM) type of
variable frequency drives.

Frequency waves that seem to be standing still are reflections and are often called standing
waves. As a comparison this would be similar to seeing a spinning wheel that appears to be frozen
but actually is in motion. At the cable attachment to the motor the standing wave sees a large
difference in impedance and can be reflected back to the drive. A long power supply cable allows
for more opportunity for a reflected standing wave to get in phase with itself at a certain point in the
cable and double the voltage and current there. The 650-volt normal voltage now becomes 1300
volts and the current also doubles. At this spot the insulation is severely stressed and will in time
overheat and puncture causing the cable to fail.
During motor start-up inrush currents occur. The motor and power supply cable act as a large
capacitor that must be charged up to the normal operating power level. When a motor is first
energized, it can draw up to six times its full load running power requirements. The cable must be
of adequate conductor AWG size so there will not be any significant voltage drop.
When the current in power pulses from a source circuit end up in the power pulses of another
circuit, capacitive coupling takes place. This is caused from a changing electric field in the source
circuit. The change in voltage, from zero to full and back to zero, causes the current to flow and
couple from one copper conductor to another. In short, power pulses from one circuit are added to
compatible pulses of another circuit that result in heat and stress.

Arve i added picture your topic.

Thanks for information Ali..

i add some type of VFD cable ..

Why to match the impedance of the VFD cable 

Engineers try or should try to match the impedance of the VFD cable to the impedance of the motor as closely as possible. Thi sis done for two main reasons.

1- First, when the impedances match, maximum power is transferred from the VFD to the motor to do useful work, with only a small amount of energy lost. However, if there is a substantial mismatch bwetween the impednaces of the cable and motor a lot of power is wasted. So, this is a "green " consideration

2- Secondly, when there is an impedance mismatch, a substantial amplification of voltage will appear at the motor terminals and some of the power is reflected back toword the drive from the motor terminals. This problem is even more likely to occur when the cable run is longer. The over voltage condition at the motor terminals can cause premature insulation breakdown in the motor windings.