1. Copper saves more
money in the long run
There is a capital investment that can repay
many times its original value over the next 20
years. And at the same time, improve equipment
reliability, reduce downtime and repair costs, and
result in lower releases of carbon dioxide to the
atmosphere.
The formula is straightforward: install high efficiency
motors having the highest electrical energy
efficiency commensurate with your needs.
High efficiency motors pile up savings worth many
times their purchase cost for as long as they remain
in service.
2. Understanding or its equivalent,
Efficiency, % = Watts (Input ) - Watts (Losses) x 100
Electrical Efficiency Watts (Input)
“Losses” stands for all the energy “fees” the motor
Electric motors are simply devices that convert charges in order to make its electrical-to-mechanical
electrical energy into mechanical energy. Like all energy conversion. Their magnitude varies from
electromechanical equipment, motors consume motor to motor and can even vary among motors of
some “extra” energy in order to make the conversion. the same make, type and size. In general, standard
Efficiency is a measure of how much total energy a motors have higher losses than HEMs.
motor uses in relation to the rated power delivered
to the shaft. Types of Losses
A motor’s nameplate rating is based on output Energy losses in electric motors fall into four
horsepower, which is fixed for continuous operation categories:
at full load. The amount of input power needed to • Power losses
produce rated horsepower will vary from motor to • Magnetic core losses
motor, with high-efficiency motors (HEM) requiring less • Friction and windage losses, and
input wattage than less-efficient models to produce • Stray load losses.
the same output. Electrical energy input is measured
in watts, while output is given in horsepower. (This Power losses and stray load losses appear only
convention applies in the USA; output power for when the motor is operating under load. They are
motors manufactured in other countries may be therefore more important than magnetic core losses
stated in watts or kilowatts.) One horsepower is and friction and windage losses, which are present
equivalent to 746 watts. even under no-load conditions.
There are several ways to express motor efficiency, Power losses, also called I²R losses, are the most
but the basic concept and the numerical results are important of the four categories and can account for
the same. For example: more than one-half of a motor’s total losses. Power
Efficiency, % = 746 x Horsepower (output) x 100 losses appear as heat generated by resistance
Watts (input) to current flowing in the stator windings and rotor
conductor bars and end rings.
Efficiency, % = Watts (output) x 100
Watts (input) Stator losses make up about 66% of power losses, and
it is here that motor manufacturers have achieved
The ratio describes efficiency in terms of what can
significant gains in efficiency. Since increasing
be observed from outside the motor, but it does
the mass of stator windings lowers their electrical
not say anything inside that makes one motor more
resistance (and therefore reduces I²R losses), HEMs
or less efficient than another. For example, we can
typically contain about 20% more copper than
rewrite the equation as:
standard models of equivalent size and rating.
Efficiency, % = Watts (output) x 100
Watts (output) + Watts (Losses)
3. Rotor losses, another form of power losses, are also High electrical conductivity
called slip losses because they are largely (but not
entirely) dependent on the degree of slip the motor Conductivity is an important characteristic of the
displays. Slip is the difference in rpm between the rotor. Conductor bars in large motors are normally
rotational speed of the magnetic field and the made from high-conductivity copper. Conductor
actual rpm of the rotor and shaft at a given load. bars in small-to-intermediate size motors, up to
S = Ns - N about 200 hp, depending on manufacturer, are in
Ns the form of a die-cast aluminum “squirrel cage” that
Where “S” is “slip”, “N” is “output speed under load” gives these motors their common name. Increasing
and “Ns” “synchronous (no-load) speed, rpm”. the mass of the die-cast bars requires changes in
the slots in the rotor laminations, through which the
Rotor losses are reduced by decreasing the degree bars are cast, and that changes the rotor’s magnetic
of slip. This is accomplished by increasing the mass structure. Lowering rotor I²R losses in what are typically
of rotor conductors (conductor bars and end-plates) aluminum alloy squirrel cage motors is therefore not
and/or increasing conductivity, and to a lesser a simple task.
extent by increasing the total flux across the air gap
between rotor and stator. Copper has higher electrical conductivity than
aluminum, and it would be an ideal conductor
Magnetic core losses arise from hysteresis effects, bar material except for the fact that it is difficult to
eddy currents and magnetic saturation, all of which die cast. A process to produce die-cast copper
take effect in the steel laminations. Magnetic losses rotors has recently been developed and, when fully
can account for up to 20% of total losses. With commercialized, it will enable the production of
proper design, use of better materials and stringent motors with even higher efficiencies than the best
quality control, these losses can be reduced models currently available.
considerably.
The fact that HEMs tend to have less slip (run faster)
The most effective means to reduce hysteresis and than standard motors must be taken into account in
saturation losses is to utilize steels containing up to certain applications.
4% silicon for the laminations in place of lower-cost
plain carbon steels. The better magnetic properties For example, energy consumption by centrifugal
offered by silicon steels can reduce core losses by loads such as fans and rotary compressors is
10 to 25%. proportional to the cube of rotational speed. If such
loads are driven at the higher speed of a low-slip,
Reducing the laminations’ thickness also helps: HEM directly replacing a standard motor, energy
substituting 26-ga or 29-ga steel for the 24-ga steel consumption can actually increase. This situation
found in standard motors lowers core losses by can sometimes be resolved by lowering rotational
between 15 and 25%. Lengthening the lamination speed with a variable-speed drive, gears or pulleys.
stack, which reduces the flux density within the stack, There are other parameters, such as torque or
also reduces core losses. Eddy current losses can be starting current, that can vary among motors of
reduced by ensuring adequate insulation between the same nominal horsepower. It is important to
laminations, thus minimizing the flow of current (and properly engineer the application of any motor to
I²R losses) through the stack. the intended task.