03-07-2012, 12:19 PM
Considerations for the Use of AC Induction Motors on Variable Frequency Controllers in High Performance Applications
Considerations for the Use of AC Induction Motors.pdf (Size: 688.16 KB / Downloads: 41)
ABSTRACT
Until recently the majority of AC variable speed drives have been
applied to variable torque, pump and fan applications. Advances
in drive technology have led to the use of induction motors in
high performance applications that exceed the capability of
motors designed for operation on sine wave power. These applications,
which have traditionally been served by DC systems,
have created the need for definite purpose AC induction motors
designed specifically for operation on adjustable frequency controllers.
INTRODUCTION
The reasons for operating industrial motors over a range of speeds are
as varied as the industries served. The need for variable speed prime
movers is widespread - energy savings on fan drives, constant surface
speed cutting on machine tool spindles, wind and unwind operations
of a bridle drive, etc. Improved performance of these variable-speed
drive systems has always been a key means for achieving increased
factory productivity. While various methods have historically been
used to achieve these speed ranges, advances in technology are making
one of the options more attractive than ever.
HIGH PERFORMANCE DRIVES
When the “drive” (motor and control) performance requirements are
minimal, a standard industrial AC induction motor can often be successfully
applied to adjustable-frequency power, variable-speed applications.
Indeed, some applications can be converted from constant
speed to variable speed while utilizing an existing induction motor.
However, when the performance level required is more demanding, a
definite-purpose motor design is appropriate. This is usually the case
when maximum process productivity is the goal.
DEFINITE PURPOSE MOTORS FOR HIGH PERFORMRNCE DRIVES
GENERAL CONSIDERATIONS
The first task is to design a basic motor configuration which is
matched to the general needs of adjustable frequency power and variable
speed operation. Second, the design must be adaptable to match
the specific needs of many different drive applications. Third, by
relaxing inappropriate constraints associated with fixed frequency,
fixed voltage, fixed speed applications the design can be tailored to
meet the performance objectives by making typical design trade-offs
as outlined in Table 1. Also, when the controller design is known,
more subtle techniques which include the controller can be used. An
example is the use of a lower than usual voltage at the low speed end
of a region of constant horsepower, so that the flux level.
PEAK CURRENTS
In addition to the RMS current level, an important rating point for a
transistor (typically used in adjustable frequency controllers) is the
peak current capability. The high frequency transient current which
results from the electronic switching of the control output voltage is
inversely proportional to the leakage inductance of the motor. As
noted in Table 1 the leakage inductances can be increased by altering
the design of the windings and the magnetic cores in the motor. The
use of an electromagnetic design specifically for adjustable frequency
power can significantly reduce the peak current required for a given
level of power output (see Figure 3).
MOTOR HEATING
One of the more obvious sources of increased stress on an induction
motor insulation system is higher operating temperature when run on
variable frequency controllers. The higher operating temperatures are
the result of increased motor losses and often reduced heat transfer as
well. As a result, many standard efficient, fixed frequency design
motors will not achieve their nameplate rating when operated on an
adjustable frequency control at 60 Hz while remaining within temperature
limits. While these elevated temperatures may not lead to an
immediate insulation failure they will result in a significantly shorter
life. In most modern insulation systems, a 10 degree Celsius increase
in operating temperature will result in a 50% reduction in expected
life. This is one of the reasons why “High Efficient” designs, which
have inherently greater thermal reserves, are often recommended for
operation on adjustable frequency controls.
MOTOR COOLING
As has been well documented in the literature, when AC motors are
run across a wide speed range their heat transfer effectiveness will
vary a great deal. Cooling fans whose rotation is directly supplied by
the motor are subject to high windage losses and noise at high speeds.
Modern AC controllers are capable of operating across a very wide
frequency range, often up to several hundred hertz. While this provides
great flexibility in the control, it places the motor cooling fan
well above its fixed frequency design operating point which often
leads to inefficient air flow and objectionable noise. In low speed
operation the fan’s effectiveness falls off with the motor’s speed. Figure
5 shows typical cooling curves for a family of totally enclosed fan
cooled motors.
DISADVANTAGES OF OVERSIZING (DERATING) MOTORS
In applying variable frequency controllers attempts are often made to
use either “inplace” AC motors, or standard sinewave power designs.
To do this, and operate across a speed range the motor is often oversized
relative to the rating required by the application. This can sometimes
be done successfully, but there are a number of potential
pitfalls. These can range from something as basic as a motor insulation
system which is fine on sinewave power, but inadequate for the
voltage and current waveshapes on the controller, to drive system
instability due to a lack of damping. The oversized motor will have
correspondingly higher rotor inertia, which could lengthen acceleration
and deceleration times and reduce process productivity. Also,
since no load current tends to be a fairly constant percentage of full
load current within a motor product line.