Understanding variable frequency drives
VFD use is increasing because users say VFDs improve manufacturing
processes and reduce cost. Understanding how they do their job can help
as you consider deployments.
If you’re involved with trying to save energy in your plant, at
some point you have probably looked into variable frequency drives
(VFDs) for ac motors. Reports from all directions say they can help save
energy, reduce maintenance, and cut utility costs. The questions are:
are they as good as they sound, and how do they work?
A VFD controls the speed of an ac motor, which provides flexibility
to the process since speed can be changed easily for process
optimization. It takes the fixed power supplied to it and converts it
into a variable frequency and variable voltage source which then feeds a
motor. This allows the drive to control the speed and torque the motor
produces.
A VFD may enhance the user’s profitability by improving the process,
which in turn produces a fast return on investment (ROI). Process
improvements may come from better:
- Speed control;
- Flow control;
- Pressure control;
- Temperature control;
- Tension control;
- Torque control;
- Monitoring quality; and
- Acceleration/deceleration control.
Many applications that use ac motors would benefit from the use
of such drives because they can also reduce operating costs while
improving the process. Reduced costs come from:
- Increased system reliability;
- Reduced downtime;
- Reduced equipment setup time;
- Energy savings;
- Lower maintenance;
- Smoother operation—less wear and tear; and
- Power factor control.
The net result of these improvements is increased profitability.
Evaluating applications
Different drive applications have different criteria that should be
evaluated individually. For example, when used in a centrifugal pump or
fan application, an ac drive saves energy by allowing the user to adjust
the speed of the motor to the most efficient level. This can often be
as much as a 60% energy saving over fixed-speed motors with valve
controls. This is usually enough to pay for the drive within a short
period of time.
When discussing VFDs and energy savings, the attention often focuses
on this type of centrifugal fan or pump application. However, there are
other applications that also have large potential energy savings and/or
recovery based on easily applicable concepts that should not be
overlooked. These applications are power factor correction,
regeneration, common bus applications, or a combination of all three.
Let’s examine all these and see how using the right drive generates its
benefit.
Centrifugal applications: laws of affinity
The cost savings in a centrifugal fan or pump application are primarily derived from two components:
- The laws of affinity, which shows an operating range that produces the most flow or pressure per horsepower. (See Figure 1.)
-
Removal of any mechanical flow device that limits the flow of a fan or
pump while the motor turns the application at a fixed speed. (See
Figures 2 and 3.)
The ac drive installation continues to
save energy for many years after the initial payback period, as well as
lowers the maintenance costs and provides a more consistent flow of
product. When a centrifugal fan or pump is used with mechanical flow
control, converting the application to an adjustable-speed ac drive will
save from 10% to 60% of energy cost if the fan or pump system is
designed to operate between 40% to 80% percent of full speed. This
application typically produces a return on investment in the 6- to
24-month time frame.
The laws of affinity state:
- Flow is proportional to shaft speed;
- Head (pressure) is proportional to the square of shaft speed; and
- Power is proportional to the cube of shaft speed.
When
comparing the different methods of mechanical flow control, the graphs
clearly show that the only one that gets close to the maximum efficiency
of the theoretical fan curve or pump flow is a VFD.
Power factor
What is power factor (PF)? AC power has two basic components: voltage
and current. When these two components are not in sync, power is wasted
due to inefficiency. This is called power factor displacement. To make
matters worse, when the ac power has a high level of harmonic content
called power factor distortion, the displacement and distortion are
multiplied by each other, which further decreases efficiency.
If you have ever received a bill from your electric utility company
penalizing your plant there is a good chance that a power factor
displacement issue exists. Even if the power company does not charge an
extra penalty, you are still paying for the excess energy that is used.
Therefore, getting the power factor displacement close to unity is very
important.
Here is a graphic example of total power factor, power factor displacement and power factor distortion (harmonics):
Power factor penalties
While each utility may charge differently, two common ways that
utilities charge are by KVA (Lower PF = higher Amps) or by kW with a PF
penalty.
If the power factor is less than 90%, the measured kW demand will be
multiplied by the ratio of 90% divided by the actual power factor:
100 kW motor with 0.85 power factor: (100 kW*0.9) / (0.85 PF) = 105 kW
In this case the increase in cost is 5% of your bill in addition to the wasted kvar as in the previous example.
The second method is an adjustment of demand for power, where demands
will be adjusted to correct for average power factors lower than 95%.
Such adjustments will be made by increasing the measured demand 1% for
each 1% or major fraction thereof by which the average power factor is
less than 95% lagging.
In this case the penalty increase is 1% for every 1% that the power factor is below 0.95 in addition to the wasted kvar.
While there are power factor correction devices available, such as
capacitors and filters, an ac drive is often overlooked as a method for
correcting power factor displacement while at the same time having a low
distortion level. A VFD with an active front end (AFE) has the ability
to adjust its power factor operating point as well as limit harmonics to
less than 4%. As a comparison, when using a standard six-pulse ac drive
with a diode rectifier that converts input ac voltage to dc bus
voltage, the typical harmonics level is 30% to 40%. There is at least
one AFE ac drive available today that has the ability to adjust its
power factor from 0.8 leading to 0.8 lagging and that meets IEEE 519
harmonic standards for low power factor distortion. This means the drive
can improve the present power factor displacement in a facility.
Distortion power factor describes the decrease in average power
transferred due to harmonics and to phase shift between current and
voltage.
How much this costs and how much can be saved depends on the amount of displacement and distortion that currently exists.
Regeneration
An ac motor may act either as a motor that turns electrical power
into mechanical power or as a generator that converts mechanical power
into electricity. It depends on whether the motor is turning a machine
that requires power to turn the machine or whether the load of the
machine will at times overhaul the motor. (Overhauling is a condition
where the mechanics or physics of the load mechanically cause the motor
to attempt to turn faster than the motor speed the drive is commanding
and the drive is used to slow down the motor speed.) This overhauling
condition may exist in several types of applications.
1. Constant deceleration: When a load such as a decline
conveyor operating under the influence of gravity will overhaul the
motor’s speed and the drive is used to control the conveyor speed to a
slower level than what the natural physics of the application would
produce.
2. Periodic deceleration: When a load is stopped quickly and
the inertia of the load wants to keep turning, such as a large drum. In
this case the cycle time, or how many times the load is stopped over
time, as well as the magnitude of the stopping power required,
determines how much energy can be saved.
3. System tension/holding torque: When two sections of a
machine are used to create tension on the material between them, such as
on the metal strip in a strip mill. The two sections may be running at
the same speed, but the process requires a certain amount of tension on
the strip to run properly. This means the lead section will run in the
forward direction and pull the strip, and the following section will
also run in the forward direction and at the same time provide the
needed torque in the reverse direction of the strip, thus creating the
proper tension.
In each of these examples the motor and drive combination has the
ability to recover the electrical power produced by the motor when it is
acting as a generator, and sends that power to the utility company. How
much energy is saved is dependent on the application, but it can be
significant. One such application where significant savings can be
recovered is a gearbox test stand. When the gearbox is tested, one drive
and motor are used to turn the gearbox while another drive and motor
are used on the other end of the gearbox to simulate the load. Done
correctly, this application will operate with a very low amount of total
energy, as the amount of energy used to turn the gearbox is the same
amount of energy that is recovered from the simulated load on the
gearbox, less the losses in the system. The one question you should ask
when trying to determine if the application is regenerative is, “Does
the load, at any time, try to turn the motor (regenerative recovery), or
is the motor being used to turn the load?”
Common bus
When there are multiple ac drives in one location, a common bus
system is usually the most efficient way to operate. It can incorporate
the energy savings and recovery concepts that have just been discussed.
If there is a regenerative ac drive and motor section in the system, it
is ideally suited for maximizing energy recovery and cost savings. The
reason for this is that losses are generated when power is converted
from the ac supply to the dc bus or from the dc bus to the ac supply.
When you have multiple stand-alone drives, the power must go through two
or more ac-to-dc conversions, and two dc-to-ac conversions. (See
Figures 8 and 9.) In a common bus configuration, power goes through only
one ac-to-dc conversion in the motoring direction, and when an inverter
section of the drive regenerates power to the dc bus, the power goes
straight to another inverter via the common dc bus link, which is
motoring and does not have to travel through a converter at all. This
method eliminates two conversion points where energy would be lost. This
increases efficiency by 2% to 4% for each regenerative section. If you
have more sections that are regenerative, you will accumulate more
energy savings. In addition to the savings of a common bus solution, if
you have an AFE, the system will have the ability to do power factor
correction, which increases the savings of a common bus system. The
gearbox test stand is a great example of a common bus solution. Here
there is one forward motoring drive motor section and one regenerative
drive motor section. In this specific case the two drive and motor
sections were rated at 1,000 A at 690 Vac each. Yet the incoming ac line
and input modules were able to be sized at less than 1,000 A at 690
Vac. This was possible because one of the two sections required 1,000 A
in the motoring or torque producing direction, while the other section
that provided the load was able to recover through regeneration close to
1,000 A, less the losses in the system. Therefore, the amps generated
from the recovery section almost canceled out the 1,000 A from the
section providing torque to turn the gearbox, and the input ac could be
sized at slightly larger than the losses of the system, which in this
case was roughly 200 A at 690 Vac. This resulted in a lower installation
cost due to the smaller ac-to-dc section. The application recovered
$75,000 per year in energy costs, which translated to a four-year
payback.
This application combined the efficiency of a regenerative system in a
common bus configuration. If the plant would have had a power factor
correction issue, the common bus solution would have been able to
accumulate those savings into this total as well.
In conclusion, when using an ac drive and motor combination, there
are many different applications and methods for which the energy savings
and energy recovery can be significant. While there is typically much
focus on the drive’s initial cost, each application should be reviewed
to determine the maximum amount of increased productivity and decreased
operating cost due to energy savings and energy recovery. In many cases
energy savings and operating cost are much higher than the cost of
installing the drive.
kVA vs kW
Some
common terms you will find associated with LCD displays are TFT and
IPS. TFT stands for Thin Film Transistor, which makes the wiring of LCD
screens more efficient by reducing the number of electrodes per pixel.
One benefit of TFT displays is an improved image quality over standard
LCD screens. Another popular LCD technology is In-Plane Switching, or
IPS, which improves upon TFT by offering much wider viewing angles and
color reproduction on LCD screens. IPS screens are able to achieve this
by keeping all the liquid crystals parallel to the screen. IPS is
generally preferable to standard TFT.
