Common VFD Terms
There are several terms used to describe devices that
control speed. While the acronyms are often used
interchangeably, the terms have different meanings.
Variable Frequency Drive (VFD)
This device uses power electronics to vary the fre-quency of input power to the motor, thereby con-trolling motor speed.
Variable Speed Drive (VSD)
This more generic term applies to devices that con-trol the speed of either the motor or the equipment
driven by the motor (fan, pump, compressor, etc.).
This device can be either electronic or mechanical.
Adjustable Speed Drive (ASD)
Again, a more generic term applying to both
mechanical and electrical means of controlling
This paper will discuss only VFDs.
Understanding the basic principles behind VFD
operation requires understanding the three basic
sections of the VFD: the rectifier, dc bus, and
The voltage on an alternating current (ac) power
supply rises and falls in the pattern of a sine wave
(see Figure 1). When the voltage is positive, current
flows in one direction; when the voltage is negative,
the current flows in the opposite direction. This type
of power system enables large amounts of energy to
be efficiently transmitted over great distances.
AC sine wave
The rectifier in a VFD is used to convert incoming
ac power into direct current (dc) power. One rectifi-er will allow power to pass through only when the
voltage is positive. A second rectifier will allow
power to pass through only when the voltage is neg-ative. Two rectifiers are required for each phase of
power. Since most large power supplies are three
phase, there will be a minimum of 6 rectifiers used
(see Figure 2). Appropriately, the term “6 pulse” is
used to describe a drive with 6 rectifiers. A VFD
may have multiple rectifier sections, with 6 recti-fiers per section, enabling a VFD to be “12 pulse,”
“18 pulse,” or “24 pulse.” The benefit of “multi-pulse” VFDs will be described later in the harmon-ics section.
Rectifiers may utilize diodes, silicon controlled rec-tifiers (SCR), or transistors to rectify power. Diodes
are the simplest device and allow power to flow any
time voltage is of the proper polarity. Silicon con-trolled rectifiers include a gate circuit that enables a
microprocessor to control when the power may
begin to flow, making this type of rectifier useful for
solid-state starters as well. Transistors include a gate
circuit that enables a microprocessor to open or
close at any time, making the transistor the most
useful device of the three. A VFD using transistors
in the rectifier section is said to have an “active
After the power flows through the rectifiers it is
stored on a dc bus. The dc bus contains capacitors
to accept power from the rectifier, store it, and later
deliver that power through the inverter section. The
dc bus may also contain inductors, dc links, chokes,
or similar items that add inductance, 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) is a common choice in
modern VFDs. The IGBT can switch on and off sev-eral thousand times per second and precisely control
the power delivered to the motor. The IGBT uses a
method named “pulse width modulation” (PWM)
to simulate a current sine wave at the desired fre-quency to the motor.
Motor speed (rpm) is dependent upon frequency.
Varying the frequency output of the VFD controls
Speed (rpm) = frequency (hertz) x 120 / no. of poles
2-pole motor at different frequencies
3600 rpm = 60 hertz x 120 / 2 = 3600 rpm
3000 rpm = 50 hertz x 120 / 2 = 3000 rpm
2400 rpm = 40 hertz x 120 / 2 = 2400 rpm
BENEFITS OF VFD
As VFD usage in HVAC applications has increased,
fans, pumps, air handlers, and chillers can benefit
from speed control. Variable frequency drives pro-vide the following advantages:
• energy savings
• low motor starting current
• reduction of thermal and mechanical
stresses on motors and belts during starts
• simple installation
• high power factor
• lower KVA
Understanding the basis for these benefits will allow
engineers and operators to apply VFDs with confi-dence and achieve the greatest operational savings.
VFD Capacity Control Saves Energy
Most applications do not require a constant flow of
a fluid. Equipment is sized for a peak load that may
account for only 1% of the hours of operation. The
remaining hours of operation need only a fraction of
the flow. Traditionally, devices that throttle output
have been employed to reduce the flow. However,
when compared with speed control, these methods
are significantly less efficient.
Mechanical Capacity Control
Throttling valves, vanes, or dampers may be
employed to control capacity of a constant speed
pump or fan. These devices increase the head, there-
by forcing the fan or pump to ride the curve to a
point where it produces less flow (Figure 3). Power
consumption is the product of head and flow.
Throttling the output increases head, but reduces
flow, and provides some energy savings.
Variable Speed Capacity Control
For centrifugal pumps, fans and compressors, the
ideal fan (affinity) laws describe how speed affects
flow, head and power consumption (Table A).
When using speed to reduce capacity, both the head
and flow are reduced, maximizing the energy sav-
ings. A comparison of mechanical and speed control
for capacity reduction (Figure 4) shows that variable
speed is the most efficient means of capacity
Low Inrush Motor Starting
Motor manufacturers face difficult design choices.
Designs optimized for low starting current often
sacrifice efficiency, power factor, size, and cost.
With these considerations in mind, it is common for
AC induction motors to draw 6 to 8 times their full
load amps when they are started across the line.
When large amounts of current are drawn on the
transformers, a voltage drop can occur2
affecting other equipment on the same electrical
system. Some voltage sensitive applications may
even trip off line. For this reason, many engineers
specify a means of reducing the starting current of
large AC induction motors.
Wye-delta, part winding, autotransformer, and solid-state starters are often used to reduce inrush during
motor starting. All of these starters deliver power to
the motor at a constant frequency and therefore must
limit the current by controlling the voltage supplied
to the motor. Wye delta, part winding, and auto-transformer starters use special electrical connec-tions to reduce the voltage. Solid-state starters use
SCRs to reduce the voltage. The amount of voltage
reduction possible is limited because the motor
needs enough voltage to generate torque to acceler-ate. With maximum allowable voltage reduction, the
motor will still draw two to four times the full load
amps (FLA) during starting. Additionally, rapid
acceleration associated with wye-delta starters can
wear belts and other power transmission
VFDs as Starters
A VFD is the ideal soft starter since it provides the
lowest inrush of any starter type as shown in
Table B. Unlike all other types of starters, the VFD
can use frequency to limit the power and current
delivered to the motor. The VFD will start the motor
by delivering power at a low frequency. At this low
frequency, the motor does not require a high level of
current. The VFD incrementally increases the
frequency and motor speed until the desired speed is
met. The current level of the motor never exceeds
the full load amp rating of the motor at any time
during its start or operation. In addition to the bene-
fit of low starting current, motor designs can now be
optimized for high efficiency.
Comparison of Starter Types Based on Inrush
Many pieces of equipment are factory shipped with
unit mounted VFDs that arrive pre-programmed and
factory wired. Motor leads, control power for auxil-iaries, and communication lines are all factory
wired. The VFD cooling lines on unit-mounted
chiller VFDs are also factory installed. The
installing contractor needs only to connect the line
power supply to the VFD.
High Power Factor
Power converted to motion, heat, sound, etc. is
called real power and is measured in kilowatts (kW).
Power that charges capacitors or builds magnetic
fields is called reactive power and is measured in
Kilovolts Amps Reactive (kVAR). The vector sum
of the kW and the kVAR is the Total Power (energy)
and is measured in Kilovolt Amperes (KVA)
(Figure 5). Power factor is the ratio of kW/KVA.
Motors draw reactive current to support their mag-netic fields in order to cause rotation. Excessive
reactive current is undesirable because it creates
additional resistance losses and can require the use
of larger transformers and wires. In addition, utilities
often penalize owners for low power factor.
Decreasing reactive current will increase power
Typical AC motors may have a full load power fac-
tor ranging from 0.84 to 0.88. As the motor load is
reduced, the power factor becomes lower. Utilities
may require site power factor values ranging from
0.85 to 0.95 and impose penalties to enforce this
requirement. Power factor correction capacitors can
be added to reduce the reactive current measured
upstream of the capacitors and increase the meas-
ured power factor. To prevent damage to the motor,
power factor correction capacitors should not exceed
the motor manufacturer’s recommendations. In most
cases, this results in maximum corrected values of
0.90 to 0.95.
The VFDs include capacitors in the DC Bus that per-
form the same function and maintain high power
factor on the line side of the VFD. This eliminates
the need to add power factor correction equipment to
the motor or use expensive capacitor banks. In addi-
tion, VFDs often result in higher line side power fac-
tor values than constant speed motors equipped with
Low Full Load KVA
Total Power (KVA) is often the limiting factor in the
amount of energy that can be transmitted through an
electrical device or system. If the KVA required by
equipment can be reduced during periods of peak
demand, it will help alleviate voltage sags, brown
outs, and power outages. The unit efficiency and
power factor are equally weighted when calculating
KVA. Therefore, equipment that may be equal or
worse in efficiency, but higher in power factor has
significantly lower KVA (Table C).
In this example, equipment with a higher power fac-
tor uses 15% less KVA while performing the same
job. This can lower electrical system cost on new
projects and free up KVA capacity on existing sys-
Power Factors and Energy Usage
Backup generators are typically sized to closely
match the load. Lowering KVA can reduce the size
of the generator required. When VFDs with active
front ends are used, the generator size can approach
an ideal 1:1 ratio of kW/KVA because the power
factor is near unity (1.0) and the harmonics pro-duced by the VFD are extremely low.
Lower KVA also benefits utilities. When the power
factor is higher, more power (kW) can be delivered
through the same transmission equipment.
HARMONIC DISTORTION AND INDUSTRY
A discussion of the benefits of VFDs often leads to
a question regarding harmonics. When evaluating
VFDs, it is important to understand how harmonics
are provided and the circumstances under which
harmonics are harmful.
In the United States, three-phase AC power typical-ly operates at 60 hertz (60 cycles in one second).
This is called the fundamental frequency
A harmonic is any current form at an integral mul-
tiple of the fundamental frequency. For example, for
60-hertz power supplies, harmonics would be at
120 hertz (2 x fundamental), 180 hertz, 240 hertz,
300 hertz, etc.
What Causes Harmonics?
VFDs draw current from the line only when the line
voltage is greater than the DC Bus voltage inside the
drive. This occurs only near the peaks of the sine
wave. As a result, all of the current is drawn in short
intervals (i.e., at higher frequencies). Variation in
VFD design affects the harmonics produced. For
example, VFDs equipped with DC link inductors
produce different levels of harmonics than similar
VFDs without DC link inductors. The VFDs with
active front ends utilizing transistors in the rectifier
section have much lower harmonic levels than
VFDs using diodes or silicon controlled rectifiers
Electronic lighting ballasts, uninterruptible power
supplies, computers, office equipment, ozone gener-
ators, and other high intensity lighting are also
sources of harmonics.
Rocks and Ponds
Obviously, the magnitude of the contributing wave
forms has an effect on the shape of the resultant
wave form. If the fundamental wave form (60 Hz)
has a very large magnitude (5,000 amps) and the
harmonic wave forms are very low (10 amps), then
the resultant wave form will not be very distorted
and total harmonic distortion will be low. If the har-
monic wave form current value is high relative to
the fundamental, the effect will be more dramatic.
In nature, we see this effect with waves in water. If
you continually throw baseball size rocks into the
ocean, you would not expect to change the shape of
the waves crashing onto the beach. However, if you
threw those same size rocks into a bathtub, you
would definitely observe the effects. It is similar
with electrical waves and harmonics.
When you calculate harmonics you are calculating
the effect of the harmonics on the fundamental cur-rent wave form in a particular distribution system.
There are several programs that can perform esti-mated calculations. All of them take into account the
amount of linear loads (loads drawing power
through out the entire sine wave) relative to non-lin
ear loads (loads drawing power during only a frac-tion of the sine wave). The higher the ratio of linear
loads to non-linear loads, the less effect the non-lin-ear loads will have on the current wave form.
Are Harmonics Harmful?
Harmonics that are multiples of 2 are not harmful
because they cancel out. The same is true for 3rd
order harmonics (3rd
etc.). Because the power
supply is 3 phase, the third order harmonics cancel
each other out in each phase 3
. This leaves only the
etc. to discuss. The magnitude of the
harmonics produced by a VFD is greatest for the
lower order harmonics (5th
, 7thand 11th) and drops quickly as you move into the higher order harmon-ics (13th
and greater).Harmonics can cause some disturbances in electrical
systems. Higher order harmonics can interfere with
sensitive electronics and communications systems,
while lower order harmonics can cause overheating
of motors, transformers, and conductors. The oppor-tunity for harmonics to be harmful, however, is
dependent upon the electrical system in which they
are present and whether or not any harmonic sensi-tive equipment is located on that same electrical
Understanding IEEE 519
IEEE (Institute of Electrical and Electronics
Engineers) created a recommendation for evaluating
harmonics. The IEEE-519 standard provides recom-mended limits for harmonic distortion measured at
the point of common coupling. The point of com-mon coupling is the point at which the customer’s
electrical system is connected to the utility.
Although the IEEE standard recommends limits for
both voltage distortion and current distortion, speci-fications that reference a 5% harmonic limitation are
generally referring to current distortion. In most
cases, if the current distortion falls within IEEE-519
requirements, the voltage distortion will also be
Determining compliance with IEEE-519 requires an
actual measurement of the system during operation.
Predicting compliance in advance often requires a
system study that accounts for all electrical equip-ment (transformers, wires, motors, VFDs, etc.) in
Introduction To Harmonic Terms
Total Harmonic Voltage Distortion – THD (V)
As harmonic currents flow through devices with
reactance or resistance, a voltage drop is developed.
These harmonic voltages cause voltage distortion of
the fundamental voltage wave form. The total mag-nitude of the voltage distortion is the THD (V). The
IEEE-519 standard recommends less than 5% THD
(V) at the point of common coupling for general
systems 69 kV and under.
Total Harmonic Current Distortion – THD (I)
This value (sometimes written as THID) represents
the total harmonic current distortion of the wave
form at the particular moment when the measure-ment is taken. It is the ratio of the harmonic current
to the fundamental (non-harmonic) current meas-ured for that load point. Note that the denominator
used in this ratio changes with load.
Total Demand Distortion – TDD
Total Demand Distortion (TDD) is the ratio of the
measured harmonic current to the full load funda-mental current. The full load fundamental current is
the total amount of non-harmonic current consumed
by all of the loads on the system when the system is
at peak demand. The denominator used in this ratio
does not change with load. Although TDD can be
measured at any operating point (full or part load),
the worst case TDD will occur at full load. If the full
load TDD is acceptable, then the TDD measured a
part load values will also be acceptable. To use ou
rock analogy, the full load fundamental current is th
size of our pond and the harmonic current is the siz
of our rock. (See Table D.)
short Circuit Ratio
Short circuit ratio is the short circuit current value of
the electrical system divided by its maximum load
current. Standard IEEE-519 Table 10.3 defines dif-ferent acceptance levels of TDD depending on the
short circuit ratio in the system. Systems with small
short circuit ratios have lower TDD requirements
than systems with larger short circuit ratios. This
difference accounts for the fact that electrical sys-tems with low short circuit ratios tend to have high
impedances, creating larger voltage distortion for
equivalent harmonic current levels. (See Table E.)
Some utilities now impose penalties for introducing
harmonics onto their grid, providing incentives for
owners to reduce harmonics. In addition, reducing
harmonic levels can prevent potential damage to
sensitive equipment residing on the same system.
There are many approaches to mitigating harmonics.
Several commonly used methods are discussed here.
Line reactors add reactance and impedance to the
circuit. Reactance and impedance act to lower the
current magnitude of harmonics in the system and
thereby lower the TDD. Line reactors also protect
devices from large current spikes with short rise
times. A line reactor placed between the VFD and
the motor would help protect the motor from current
spikes. A line reactor placed between the supply and
VFD would help protect the supply from current
spikes. Line reactors are typically only used
between the VFD and the motor when a freestand-
ing VFD is mounted more than fifty feet from the
motor. This is done to protect the motor windings
from voltage peaks with extremely quick rise times.
Trap Filters are devices that include an electrical cir
cuit consisting of inductors, reactors, and capacitor
designed to provide a low impedance path to ground
at the targeted frequency. Since current will trave
through the lowest impedance path, this prevents the
harmonic current at the targeted frequency from
propagating through the system. Filters can be
mounted inside the drive cabinet or as free standing
devices. Trap filters are typically quoted to meet a
THD(I) value that would result in compliance with
IEEE-519 requirements if the system were other
wise already in compliance.
Some devices measure harmonic currents and
quickly create opposite current harmonic wave
forms. The two wave forms then cancel out, pre
venting harmonic currents from being observed
upstream of the filter. These types of filters general
ly have excellent harmonic mitigation characteris
tics. Active filters may reduce generator size
VFDs Using Active Front End Technology (AFE)
Some VFDs are manufactured with IGBT rectifiers.
The unique attributes of IGBTs allow the VFD to
actively control the power input, thereby lowering
harmonics, increasing power factor and making the
VFD far more tolerant of supply side disturbances.
The AFE VFDs have ultra low harmonics capable of
meeting IEEE-519 standards without any external
filters or line reactors. This significantly reduces
installation cost and generator size requirements.
An AFE drive provides the best way to take advan-
tage of VFD benefits and minimize harmonics.
Multi-Pulse VFDs (Cancellation)
There are a minimum of six rectifiers for a three-
phase AC VFD. There can be more, however.
Manufacturers offer 12, 18, 24, and 30 pulse drives.
A standard six-pulse drive has six rectifiers, a
12-pulse drive has two sets of six rectifiers, an
18-pulse drive has three sets of six rectifiers and so
on. If the power connected to each set of rectifiers is
phase shifted, then some of the harmonics produced
by one set of rectifiers will be opposite in polarity
from the harmonics produced by the other set of rec-
tifiers. The two wave forms effectively cancel each
other out. In order to use phase shifting, a special
transformer with multiple secondary windings must
be used. For example, with a 12-pulse VFD, a
Delta/Delta-Wye transformer with each of the sec-
ondary phases shifted by 30 degrees would be used.