Overview of ..
Many of the energy conservation measures in the Energy Efficiency
Manual involve variable-speed output of motors. This Note covers
all the contemporary methods of getting variable-speed output from
larger electric motors. It will guide you to the best choice of
a variable-speed or adjustable-speed motor drive for your application.
The main types covered are multi-speed AC motors, multiple-motor
drives, variable-frequency drives, direct-current (DC) motors and
drives, variable-pulley drives, eddy current clutches, and wound-rotor
Multi-speed AC motors change speed by switching their internal
windings. This is one of the most efficient methods of changing
speed. Multi-speed AC motors are relatively inexpensive, simple
to install, and free of the problems that plague some modulating
types of variable-speed drives. Their main limitation is the stepwise
nature of the speed change, which usually imposes an efficiency
penalty in the overall application. The Note explains the speed
options, efficiency, torque, and relative prices of these motors.
Another method of achieving multi-speed motor operation is to use
two or more motors to drive a common piece of equipment. One motor
runs while the other motor or motors idle. For example, two-motor
arrangements are commonly used to drive fans in air handling units
and cooling towers. The Note explains how you can achieve virtually
any fixed combination of two speeds. The motors are conventional,
so you can select them for high efficiency without making other
compromises. Equipment cost is modest because the motors are conventional
and because the smaller motor is usually a fraction of the size
of the larger motor.
The variable-frequency electronic drive (VFD) has become the preferred
type of adjustable-speed drive for many applications. The VFD changes
the speed of the motor itself. The drive, which is totally electronic,
changes motor speed by changing the frequency of the current that
is supplied to the motor. They are used in combination with ordinary
types of AC induction motors or synchronous motors, although it
may be prudent to use special models that are designed to operate
with electronic drives.
There are three main types of electronic variable-frequency drives:
pulse width modulation (PWM), current source input (CSI), and variable
voltage input (VVI). The characteristics of three types are compared.
You have a wide range of options in selecting VFD’s. The
Note explains them, including, torque and current limits, adjustments
for voltage-speed ratio, soft starting, acceleration control, ability
to drive multiple motors, full-load efficiency, part-load efficiency,
turndown ratio, speed increase capability, fixed speed settings,
speed range lockout, reversing, ability to start with the motor
turning, automatic restart, braking, regeneration, emergency bypass,
power factor, electrical fault protection, diagnostics, and selection
for altitude, temperature, and humidity.
VFD’s can be reliable and trouble-free if they are selected
and installed properly. Their advantages include high efficiency,
large turndown ratio, ease of installation, even in retrofit, and
low maintenance. However, VFD’s have operating limitations.
In applications requiring high torque, it may be impossible to provide
adequate motor cooling with conventional motors.
VFD’s can cause damage the motors they drive and cause a
wide range problems in the facility’s electrical system if
they are not selected and installed properly. Most of the problems
arise from distortion of the voltage and current waveforms in the
input and output of the drive, commonly called harmonic distortion.
The Note explains the causes of the distortion, the problems it
causes, and solutions.
Direct current (DC) motors and drives where the dominant type of
adjustable-speed drive before variable-frequency drives became practical.
There are still applications where a combination of a DC motor and
an electronic DC drive is the best choice. Old motor-generators
have been replaced by electronic sources of direct current for the
motors. The Note explains the advantages and disadvantages of DC
drives. Modern electronic DC current sources share some of the problems
of variable-frequency drives, but the problems are generally less
Variable-pulley drives are belt drives in which the effective diameter
of one of the sheaves can be changed as the equipment is operating.
Commercially available models are equipped to accept standard control
signals. Variable-pulley drives are efficient and free of electrical
problems. However, they have substantial limitations. The Note explains
the advantages and disadvantages of these units.
An eddy current clutch drive is an electromagnetic slip clutch.
The motor drives a rotating electromagnet that induces eddy currents
in a metal disk or drum connected to the output shaft. The magnetic
field of the driven element interacts with the induced eddy currents
to create torque in the driven element. The speed of the driven
element is controlled by varying the current that creates the magnetic
field. Eddy current drives are less efficient than the types covered
previously. The Note explains the other advantages and disadvantages
of this type.
Wound-rotor motors are an older type of motor that was designed
for variable speed. The Note describes this type of motor, its advantages,
and its shortcomings.