What is a Variable Frequency Drive?
How does a VFD Work?
Do you have ac motors in your facility?
Do these motors need a variable output?
Do you use mechanical means to vary the output?
If you anwsered yes to these questions, you need to install variable frequency
drives on these ac motors.
If you are not using VFD's you are wasting energy and throwing money out the
Many utilities are now offering rebates for the installation of VFD's or
retro-fitting existing equipment with variable frequency drives. Contact your
local utility or search VFD rebate or go to the DSIRE website,
a comprehensive source of information on state, local, utility and federal
incentives and policies that promote renewable energy and energy efficiency.
Joliet Technologies has partnered with Electrical
Solutions Network (ESN), in our effects to promote energy
efficiency through the use of variable speed drives.
energy reduction incentives.
What is a VFD?
By: Dave Polka
You can divide the
world of electronic motor drives into two categories: AC and DC. A
motor drive controls the speed, torque, direction and resulting
horsepower of a motor. A DC drive typically controls a shunt wound
DC motor, which has separate armature and field circuits. AC
drives control AC induction motors, and-like their DC
counterparts-control speed, torque, and horsepower.
Application As An
Let's take a brief
look at a drive application. In Fig. 1, you can see a simple
application with a fixed speed fan using a motor starter. You
could replace the 3-phase motor starter with Variable Frequency
Drive (VFD) to operate the fan at variable speed. Since you can
operate the fan at any speed below its maximum, you can vary airflow
by controlling the motor speed instead of the air outlet damper.
Figure 1, Fixed Speed Fan
A drive can control two
main elements of a 3-phase induction motor: speed and torque. To
understand how a drive controls these two elements, we will take a
short review of AC induction motors. Fig. 2 shows the construction
of an induction motor. The two basic parts of the motor, the rotor
and stator, work through magnetic interaction. A motor contains pole
pairs. These are iron pieces in the stator, wound in a specific
pattern to provide a north to south magnetic field.
Figure 2, Basic Induction Motor
Figure 3, Operating Principles of
With one pole pair
isolated in a motor, the rotor (shaft) rotates at a specific
speed: the base speed. The number of poles and the frequency
determine this speed (Fig. 4). This formula includes an effect
called "slip." Slip is the difference between the rotor
speed and the rotating magnetic field in the stator. When a magnetic
field passes through the conductors of the rotor, the rotor takes
on magnetic fields of its own. These rotor magnetic fields will
catch up to the rotating fields of the stator. However, it never
does -- this difference is slip. Think of slip as the distance
between the greyhounds and the hare they are chasing around the
track. As long as they don't catch up to the hare, they will
continue to revolve around the track. Slip is what allows a motor
Shaft Speed =
120 X F
for NEMA B Motor = 3 to 5% of Base Speed which is 1800 RPM at
Frequency applied to the motor
P = Number of motor poles
Shaft Speed =
120 X 60 Hz
Induction Motor Slip Calculation
We can conveniently
adjust the speed of a motor by changing the frequency applied
to the motor. You could adjust motor speed by adjusting the number
poles, but this is a physical change to the motor. It would require
rewinding, and result in a step change to the speed. So, for
convenience, cost-efficiency, and precision, we change the
frequency. Fig. 5 shows the torque-developing characteristic
of every motor: the Volts per Hertz ratio (V/Hz). We change this
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
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.
Figure 5, Volts/Hertz Ratio
Drive Changes Motor Speed (Part 2)
This information has
been provided by: ABB Inc. - Drives and Power Electronics