01-01-2013, 04:30 PM
Selection of Copper vs. Aluminum Rotors for Induction Motors
1Selection of Copper.pdf (Size: 636.25 KB / Downloads: 175)
Abstract:
On squirrel cage induction motors, there is an
important choice between utilizing a lower cost die cast or
fabricated aluminum rotor versus the more expensive copper
bar rotor. Utilizing the wrong rotor construction for the
application can either increase costs unnecessarily or lead to
catastrophic failure. This paper will provide the background
necessary to assist in making the proper choice. The
fundamentals of rotor construction and basic information on
how the induction motor works will be discussed.
Additionally, the effects of various materials and types of
rotor construction on motor performance will be analyzed.
INTRODUCTION
Many induction motors in service today are running critical
processes in which failure at any time can be very costly.
Many times these critical processes will not have a back up.
As a result a large portion of this process will shut down until
the motor is repaired and put back into service. Those aware
of this situation will strive to purchase a motor that will
maximize reliability. As a result of the strong push for
maximum reliability, it is easy to over specify costly
components not required for the specific application. These
decisions will result in an unnecessarily high motor purchase
price. To maximize reliability without overspending, large
induction motors need to be properly matched to the specific
application. With this consideration the choice between
various rotor constructions needs to be evaluated. It is
generally assumed that copper bar rotors are the most
reliable. In certain applications this may be true, and they
can at times out perform aluminum rotors. But many times,
the applications are such, that there will not be any
appreciable performance benefit from the use of copper bar
rotors. It will be shown later in this paper that the percentage
motor cost increase can be very large on smaller machines
but may not be as significant on larger machines.
ROTOR CONSTRUCTION
Four types of rotor construction exist today: aluminum die
cast (ADC), copper die cast (CuDC), fabricated aluminum
bars (AlBar), and fabricated copper bar (CuBar). In general,
only the aluminum die-cast, fabricated aluminum, and copper
bar rotors are in common use today. At this time this paper
will discuss how these are manufactured.
HOW TO ACHIEVE TIGHT ROTOR BARS
Loose rotor bars is the number one cause of CuBar rotor
failures. At starting, the rotor bars oscillate at:
Rotor Bar Vibration Freq. = 2 X % Slip X Line Freq.
The rotor bars vibrate as a consequence of high current
forces [6]. If the bars aren’t firmly seated, they will break
over time. There are many different methods to achieve tight
rotor bars, some methods may be better than others, but all
can work reasonably well if properly performed.
In one method bars can be driven into the slot then
swaged. Swaging is performed by pushing down at the
center of the top of the rotor bar as show in Fig. 5. It must be
pressed down deep enough to bulge the bars out on the side
and fill in the gap between the bars and the core.
ROTOR STRESSES
Now that the basic construction features are understood,
stresses resulting from manufacturing processes, and
operation can be discussed. Much attention is paid to the
design process to make sure that stresses are within
acceptable levels. It is not the intention of the authors to
describe the stress analysis in great detail here, but rather
discuss these stresses from a qualitative perspective.
Rotational Stresses:
Rotational stresses occur as a consequence of centrifugal
force. These stresses occur in any rotating component,
however, the larger the radius of rotation, the larger the
stress will be. The rotational stresses are insignificant in the
bars. However, the same is not true for lamination stresses.
The area above the bars is particularly affected as the
laminations not only are stressed as a consequence of their
own mass, but also have to provide the retaining force to
keep the rotor bars in place. This retaining force exerts
further stresses on the laminations. Copper bars are
approximately three times heavier than aluminum, so there
must be sufficient strength in the laminations above the bars.
This in turn forces designers to locate the bars somewhat
deeper radially into the core than aluminum bars.
Electrical Performance:
Normally aluminum die cast rotors are limited on rotor bar
conductivity due to the logistics of having multiple die-casting
machines or aluminum storage tanks for the molten
aluminum. On smaller NEMA size machines copper or
aluminum fabricated rotors are not available as standard
primarily due to higher labor cost associated fabricated
constructions. Copper bar rotors come in many different
alloys with relative resistively ranging from 1.0 to10.
Fabricated aluminum rotors come in a few alloys ranging
from 1.8 to 3.0.
Rotor Bar Heat Capacity:
Aluminum rotor bars have approximately 1/3 the density,
weight and 2.5 times the specific heat of copper bar rotors.
The coefficient of thermal expansion for a given temperature
change is 35% greater on aluminum than copper and at the
same time aluminum has lower strength as shown in the
Table II. As a result of the material density and specific
heat, aluminum bars will get much hotter, expand further and
generate much higher stresses while accelerating the same
WK2 . The bars will expand more and what could be a more
serious issue is the end ring thermal expansion that will
cause stress on bar where it exits the core and connects with
the end connector. On ADC rotors and some fabricated
aluminum rotors the end connectors are up against the core.
There may only be a slight transition coming out of the slot
and there is minimal allowance for movement in that location.
As a result there is little room for the bars to bend and the
stresses could be high at that joint. On fabricated copper bar
rotors that have copper or copper alloy end connectors
besides having 2.5 times the thermal capacity they are also
located greater than a ½ inch away from the core distributing
the bending stress along the extension. This rotor will have a
point of high stress either at core or at the braze joint on the
end connector. The end connector is normally the point of
highest stress. In general, all types of rotor construction
have stresses that are beyond the material yield point in this
area. However, this area is not subject to high frequency
cyclical reversing stress. The number of thermal cycles that
the rotor bars and end connectors will see due to this
phenomenon will be equal to the number of starts. For
reference, API 541 recommends capability for a minimum of
5000 starts.
STARTING
During starting there are basically four motor components
which are adversely affected by the heating and mechanical
affects of starting. These would include the end connectors,
the rotor bars, the stator winding, and the shaft extension
from the core as a result of the transient torque transmitted
through the shaft. For the purposes of this paper we will limit
discussions to the rotor bars and end connectors. For a
more comprehensive analysis of rotor shaft failure, Ref. [9]
should be consulted.
It has been proven by experience that the most damage is
typically done to the rotor bars and end connector during
starting. A stalled condition can be even more severe but
this is an abnormal condition and should be avoided since it
can lead to rapid catastrophic failure. Each motor is
designed for a limited number of starts. For example API
541 recommends a minimum of 5000 starts while starting a
load and inertia as defined by NEMA. A NEMA square load
curve is defined as a torque vs. speed curve where the load
torque varies as the square of the speed up to 100% load at
100% speed.
ROTOR BAR REPAIR
If it becomes necessary to repair a failed rotor in the field, it
is certainly easier to accomplish this on a fabricated rotor
than an aluminum die cast rotor. If an aluminum die cast rotor
fails it is virtually impossible to get access to the failed area.
It will either be buried in the core area or at the end
connector up against the core. In either case it will be hard to
access. Most fabricated aluminum rotor designs also have
their end connector up against the rotor core and would have
some difficulty but there is a chance that the end connectors
could be replaced. There is considerable technology
involved in manufacturing aluminum-fabricated rotors and if
the repair facility doesn’t understand the technology reliability
would be very questionable. Most service shops have more
experience repairing CuBar rotors than AlBar.
ROTOR COST COMPARISON
Although four rotor constructions were presented in this
paper, the two most common constructions are aluminum die
cast and copper bar. As such, a cost comparison will be
shown for those constructions only. The cost comparison
was performed on standard machines – standard starting
duty, NEMA inertia, no special slip or efficiency requirements,
etc. Additionally, the cost comparison was conducted taking
advantage of superior cooling of the CuBar motor, and the
subsequent reduction in core length. The cost comparison is
shown as a percentage increase in cost of the CuBar rotor
over ADC, with all other features being identical (voltage,
service factor, etc.). For example, if a 700 Hp, 2300V, 1.0 SF
TEFC motor costs $20,000 with a ADC rotor and $24,000
with a CuBar rotor, the motor with the CuBar rotor has a 20%
cost increase.
CONCLUSION
Many topics were addressed in the area of rotor
construction. A chart was put together to help summarize the
many different facets of rotor construction. The chart below
summarizes a comparison between the various rotor
constructions. The comparison is a relative one, with a
lower number indicating an advantage.