The Impact of Rewinding
on Motor Efficiency
Jim Custodio, GE Motors
When a motor fails, users can (1) rewind, possibly for high
efficiency; (2) replace the failed motor with a new motor; or
(3) invest in a premium efficiency product. Here are some
advantages and disadvantages of each approach and the
precautions to take to achieve the best investment.
W
hen a motor fails, the user must decide whether to
repair or replace it. To make a proper decision, one
must consider the cost of the repair, the availability
of a replacement, the age of the motor, the electrical design
required for the application, any special mechanical features,
and the urgency of returning the failed motor to service.
Placing the driven equipment back into service is frequently the highest priority, and users often make their decision based on this criterion alone. Plant managers tend to be
less concerned if the rewound motor is less efficient when
their operation’s downtime is costing thousands of dollars a
minute.
Increased Profits by Energy Saving
U.S. industry will continue to feel the squeeze on profits, and
manufacturers who fail to implement energy reduction programs will find themselves at a competitive disadvantage. In
some industries, motor operation costs may even exceed those
of labor cost.
It is no longer practical to view the power bill as a fixed
base cost, not worth the effort it would take to reduce. The
need to minimize power usage should be as important to the
CFO as it is to the plant manager. Due to the improvements
that have occurred in motor technology, even companies that
already had energy programs ten years ago should now reevaluate their criteria.
Future Power Costs
In 1970, electricity cost the average industrial user about one
cent per kilowatt-hour. By 1980 that had jumped to four
cents: a 300 percent increase. Power costs in some areas are
PUMPS & SYSTEMS
Motor operation costs may even exceed those of labor cost in some
industries. Even companies that had energy programs 10 years ago
should reevaluate their criteria due to improvements in current
motor technology.
over seven cents. The cost for electricity may fluctuate with
economic cycles, but long-term it will continue to increase.
The nature of today’s power bill has also changed. The
contract rate of the past now only covers 60 percent to 75 percent of the actual amount paid by users. In addition to taxes,
today’s bill can include such additions as “fuel adjustment” and
“demand charges.” The bottom line is that electric power can
be a major cost element in your product.
First Cost vs. Operating Cost
Manufacturers who understand their business look at the total
cost of a motor, rather than simply making a decision based on
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JUNE 2007
33
Getting the Most from Motors
initial purchase price.
Let’s consider the example of a pre-EPAct 75-hp, 1800rpm TEFC motor that originally sold for approximately
$2700, with an efficiency of 91.7 percent. This motor, operating continuously and using power costing $.07/kW-hr, will in
just one year cost $37,414 to operate – or 1,386 percent of the
original purchase price! In fact, operating costs will overtake
the purchase price after the first 26 days of operation. Even if
the motor is only used for two shifts (assuming 4,160 hours
per year), this will still occur after 55 days.
For this reason, any expenditure related to repair or
replacement of a failed motor should be evaluated based on a
total cost of operation calculation.
An Incorrect Decision Costs Money
Building on the example above, consider the impact on the
operating cost of the same 75-hp motor using a more efficient
motor. Let’s say the motor efficiency improves to that of an
EPAct design with a nominal efficiency of 94.1 percent. The
new EPAct motor would do the same amount of useful work,
but use $979 less energy.
In mid-2001 motor manufacturers, in cooperation with
conservation groups and the DOE, introduced products with
even higher efficiencies: NEMA Premium. With this new
specification, the same motor rating would have a nominal
efficiency of 95.4 percent and save $1,510 in the first year
compared to an EPAct level efficiency motor. These savings
continue to accrue as the cost of power goes up. Even without increasing the cost of a kilowatt-hour, a NEMA Premium
motor would generate $10,563 in savings over seven years,
compared to $6,854 for an EPAct level efficiency motor.
Factors in the Rewind Decision
Hundreds of thousands of older T-frame motors were manufactured well before there were any government standards.
Many of these still operate in U.S. industry. Each time one
fails, an opportunity is created to improve the user’s bottom
line.
Power costs will certainly continue to rise and further
escalate motor operating expense. So the question of how
repair affects motor efficiency is an important one. Some claim
a rewound motor is never as efficient as the original; others say
a well-executed rewind can be better than the original design.
These differences in perception suggest there may be several
factors involved.
Armed with the right information, understanding the factors that affect rewind performance does not need to be complicated. Let’s examine the various types of motor losses and
how they are influenced by engineering decisions.
Keep in mind that actual motor losses may differ between
two motors of the same design, depending on how the motor
is used. Figure 1 shows how motor losses vary with load.
As a motor approaches 100 percent of rated load, losses
increase dramatically, with most of the increase found in the
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JUNE 2007
Figure 1. How motor losses vary with load.
Hp
7 1/2
15
25
50
75
100
1944
Design
1955
U-Frame
84.5%
87.0
89.5
90.5
91.0
91.5
87.0%
89.5
90.5
91.0
90.5
92.0
T-Frame
1965
NEMA
Normal Efficiency EPAct Premium
84.0%
88.0
89.0
91.5
91.5
92.0
88.5%
91.0
91.7
93.0
94.1
94.1
91.0%
93.0
93.6
94.5
95.0
95.4
Figure 2. The history of motor efficiency, TEFC, 1800-rpm.
Standard Efficiency Designs
3600 rpm
1800 rpm
Motor Loss Component
• No Load Losses:
- Windage & Friction
- Core Loss
30%
18
11%
22
Total No Load Losses
48%
33%
• Load Losses:
- Stator I 2R
- Rotor I 2R
- Stray Load Losses
34%
8
10
47%
9
11
Total Load Losses
52%
67%
100%
100%
Total Motor Losses
Figure 3. Representative losses as a percentage of total losses.
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Friction & Windage
340 Watts
Core Loss
750 Watts
Stray Load
520 Watts
Motor Loss
Improvement Possibilities
Stator Loss - I 2R
Increase amount of copper wire in slot
Decrease length of coil endturns
Decrease turns in stator
Rotor Loss
Decrease turns in stator
Figure 5. Possible methods to reduce losses through rewinding.
Stator & Rotor I 2R
1600 Watts
Figure 4. Losses in a 50-hp, 1800-rpm, TEFC Pre-EPAct motor.
form of rotor and stator losses. The age of the motor is also
a factor. Figure 2 shows the progression of motor efficiencies
through the years, driven by improvements in engineering
design and material technologies. (Note that these ratings are
for typical GE motors from 1944 up to EPAct; actual efficiencies will vary from manufacturer to manufacturer.)
The distribution of losses will also be different for various
motor designs. Variations in speed, design and enclosure will
all affect loss distribution, as shown in Figure 3.
The ability of the repair shop to analyze and replace those
parts which most influence losses, such as the stator core, the
windings and the rotor, will affect the outcome of a rewind.
With all that in mind, let’s take a look at losses in a typical 50-hp, 1800-rpm, TEFC standard efficiency design. The
distribution of losses is shown in Figure 4.
The table in Figure 5 shows how these losses can be
reduced. Following that are detailed explanations of the
techniques.
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Getting the Most from Motors
Stator Losses
2
Stator losses are primarily I R losses, released in the form of heat
as current passes through the stator windings.
When rewinding a motor, smaller diameter wire will
increase the resistance and therefore I2R losses; larger diameter
will have the opposite effect. If the original wire was aluminum, changing to the same size copper wire will also reduce
resistance and loss. Obviously, using a larger diameter copper
wire will affect the best reduction.
Another option for reducing stator losses is to reduce the
number of wire turns. Use this method with caution. While
full load efficiency may be increased, starting current will go
up and power factor will be reduced. Both starting and maximum torques will be increased. A change from ten turns to
nine turns would increase starting current by as much as 23 percent.
Core Loss
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Core loss is the sum of the eddy current
and hysteresis losses that occur while
energizing the motor’s magnetic field.
Motors are insulated between the
core laminations to minimize eddy currents, but the process of stripping can
destroy this insulation. When stripping a motor for rewinding, insulation
burnout must be done at carefully controlled temperatures. Otherwise it’s easy
to overheat the laminations, breaking
down the core insulation and actually
increasing core loss. Not all repair shops
use the same insulation burnout techniques; investigate them thoroughly
before deciding where to have the motor
rewound.
Another item that is often ignored
is the condition of the core after motor
failure. Failures caused by excessive loading, extended stall conditions, single
phasing, or bearing failure leading to
rotor striking can all cause increases in
the core loss.
It is very unlikely that the original
core loss data would be available from
a ten year old T-frame motor. Repair
shops may have equipment to evaluate
the core in its failed condition, but are
unable to relate the results to original
factory core loss specifications.
Applying even the best techniques
to improve the efficiency may be inappropriate without the original core loss
information. The repair shop would
have to conduct a full efficiency test
using a dynamometer, which takes two
to three hours depending on frame size,
in order to validate the finished motor.
Manufacturers do this on every
motor they design and have programs
registered with the DOE to assure
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JUNE 2007
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PUMPS & SYSTEMS
that design efficiencies are maintained
throughout production. The DOE
requires that testing laboratories be
third-party certified to assure compliance with the testing procedure defined
in IEEE 112 B. This process was written into the Federal Energy Act to
prevent overly optimistic efficiency
claims. Repair shops have no such
requirement.
No single aspect of the rewind process
is as important as preserving the
electrical integrity of the stator core.
The intelligent choice
Rotor Loss
2
Rotor losses are I R losses, released as heat
through the rotor slots and endrings. It
is unlikely that a repair shop will be able
to improve rotor losses.
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Efficiency vs. Time
You have undoubtedly heard or read
more than once that motor efficiency
naturally decreases through motor life
as a result of “heat aging.” This argument says that as the motor starts and
stops the core temperature increases
and decreases, causing deterioration in
the core steel’s electrical properties and a
resulting increase in internal losses.
In fact, this is only a problem if an
aging type of steel is used in the core.
Most manufacturers use non-aging steel
that does not lose its electrical properties over time.
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Rewind vs. a New Motor
Now that you know some of the pitfalls of rewinding, let’s reexamine our
options in the face of a motor failure.
Provided that downtime isn’t the critical
factor, a user now has these choices:
1. Rewind the motor to the original
efficiency.
2. Rewind the motor to a higher
efficiency.
3. Replace with a new motor of same
efficiency.
4. Replace with a premium-efficiency
motor.
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A fifth option that no one should
knowingly choose is to rewind the
motor to a lower efficiency – but many
users unwittingly make this decision.
As explained above, it is very easy to
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(630) 859-7000 FAX: (630) 859-7060
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JUNE 2007
37
Getting the Most from Motors
damage the stator core insulation while stripping out
High-Fill Rewind
the old winding and so increase core loss by three times
PremiumPoor
1/2
Full
Efficiency
or more.
Hp Quality Rewind
Wire Size
Wire Size
Motor
Figure 6 compares the typical results of each of the
choices, with assumptions for the “poor quality rewind”
10
-6.0
+0.6
+1.1
+4.2
figured at three times the original core loss.
50
-3.7
+0.5
+0.8
+2.8
100
-2.1
+0.4
+0.7
+4.5
The high-fill rewind produces some efficiency gains
200
-1.9
+0.3
+0.6
+2.1
when a larger wire size is used. As would be expected,
the greatest efficiencies are realized by retrofitting with
new premium-efficiency motors.
Figure 6. Efficiency Gain (Losses): Rewind vs. Premium-Efficiency Motor
Figure 7 shows the annual operating cost difference for each of the options listed above. Note that
motor that operated well for years before initial failure, but
operating cost can be increased by a poor rewind just as much
failed again shortly after being rewound. The failure is more
as it can be decreased by a new, premium-efficiency motor.
often the result of temperature rise than of defective materials
The conclusion is obvious: either replace failed motors
or faulty workmanship in the new windings.
with new, premium-efficiency motors, or else exercise extraorThe obvious question is “what is a safe insulation burnout
dinary care in the rewinding process.
temperature?”
Unfortunately, there is no simple answer. Manufacturers
Protecting the Stator Core in a Rewind
use a wide variety of materials for the core. Steel may be supNo single aspect of the rewind process is as important as preplied with either organic or inorganic insulation coatings, or
serving the electrical integrity of the stator core.
with no coating at all. If uncoated steel is used, the motor manNot only can insulation damage increase core loss, but
ufacturer will add an oxide insulation coating while annealing.
the resulting rise in motor temperature could also then cause
Each of these lamination insulations has a different limit in
the motor to fail prematurely. You’ve probably had at least one
temperature that it can withstand before deteriorating, so it’s
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PUMPS & SYSTEMS
High-Fill Rewind
Hp
10
50
100
200
Poor
Quality Rewind
+$404
+ 1050
+1148
+2241
1/2
Wire Size
-$37
-135
-215
-312
Full
Wire Size
PremiumEfficiency
Motor
-$68
-215
-375
-621
-$239
-739
-2334
-2122
the winding insulation.
Inorganic insulation can withstand temperatures up to 700-deg F, allowing the old winding to be
burned out safely if the oven temperature is carefully
controlled.
Uncoated semi-processed steel laminations will
stick together if they get too hot, increasing eddy current losses dramatically. Motors built with this type of
steel can be stripped at oven temperatures of 700-deg
F or below.
Figure 7. Annual Operating Cost Difference: Rewind vs.
Premium-Efficiency Motor Continuous Operation @ $.07/kW-h
impossible to name a temperature that is safe for all motors.
But there are some guidelines.
In all cases, the stripping operation must control the core
temperature to prevent damage to the interlamination insulation. Damage can occur even in a low temperature oven when
several cores are stacked, and fire from the burning organic
materials results in increased temperature beyond the oven
setting.
If the motor has an organic lamination insulation, it will
begin to deteriorate rapidly at around 500-deg F and may
actually change its chemistry at higher temperatures. Organic
insulation can be damaged in any oven hot enough to burn out
Stripping Methods
In the past few years, awareness has grown among users
that poor quality motor rewinds can cause an increase in losses.
Users have demanded an end to the practice of burning out the
old windings at uncontrolled temperatures.
Motor repair shops that have kept pace with technology
have switched to temperature-controlled ovens and have discontinued the practice of softening varnish with a handheld
torch. Ask your rewind shop if they can perform any of the
following non-injurious stripping techniques:
• Mechanical stripping
• Chemical stripping
• High-pressure water jets
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• Freezing process
• Ultrasonic stripping
Regardless of the process used, make sure your rewinder
can follow the motor manufacturer’s recommended safe
burnout temperature limits. Some do and some don’t – which
is probably the main reason for one shop’s reputation over
another for better quality rewinds.
There is really only one way to make sure that losses have
not been increased in the process, and that’s to perform a
qualitative core loss test before and after rewinding (assuming the core is okay to begin with). This can also help you
screen the motor population to determine if any given motor
is even repairable. More repair shops now offer this service,
so ask yours.
In all cases, the stripping operation
must control the core temperature
to prevent damage to the
interlamination insulation.
Establish a Repair/Replace Policy
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In response to the rising cost of electrical power, every company should establish a repair/replace policy to help make
intelligent decisions.
Give every motor-driven machine in your plant a repair/
replace priority, and consider investing in spare motors for
any continuous process machines that are critical to plant
operation. These “critical” machines are excellent candidates
for retrofitting with premium-efficiency motors; the existing
standard-efficiency motor should not be kept as a spare.
There is a good deal more to comparing relative inuse costs between motors than simple energy usage. The
local power company may offer rebates on new premiumefficiency motors. Extended warranties may be available to
reduce MRO costs. A new motor could be installed with a
variable speed drive to maximize the process productivity (the
drive may also be covered under a rebate). Inventory costs can
be reduced by not storing repaired motors that could become
obsolete before they are needed again.
These are only a few of the possibilities. Many motor
distributors are willing to assist you in evaluating a program.
Check the Internet for the one nearest you.
P&S
Jim Custodio is the product support manager for GE
Motors, 1635 Broadway 19-5, Fort Wayne, IN 46802,
260-439-3245, 260-439-3338, [email protected],
www.gemotors.com.
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