What’s Motor Efficiency?
Motors convert electrical power(input power) into mechanical power(output power). All motors have stator and rotor. When an electric current flows through, the magnetic field produced by the stator rotates, causing the rotor to rotate. Since the rotor is fixed to the shaft, the shaft also rotates. For an electric motor, efficiency is the ratio of mechanical power output to the electrical power input.
Stator and Rotor Conductors
For a motor, either the stator or the rotor can be permanent magnet. When the stator is an electromagnent instead of a permanent magnent, it will be a field coil or field winding, made of either copper or aluminum.
For an induction motor, the rotor can be either wound type or squirrel-cage type. A wound-rotor motor has a rotor winding made of insulated wire. A squirrel-cage rotor is composed of conductive bars and end rings commonly made of copper or die cast aluminum. Almost 95% of the induction motors used is of squirrel cage type.
Electric motors experience various losses during power conversion, including magnetic core losses(iron losses), friction and windage losses(mechanical losses), stator resistance losses, rotor resistance losses, and stray load losses. Losses reduce the efficiency of the machine and usually result in unwanted heat.
Stator and rotor I2R losses, also called copper losses, are major losses in motors. They are caused by current flowing through stator and rotor conductors. Reducing stator and rotor losses can improve motor efficiency.
Importance of Improving Efficiency
* High efficiency cuts the operating costs. Even one-point rise in efficiency can result in massive savings in the motor’s service life.
* Improving efficiency contributes to energy saving and reduces gas emissions. By reducing energy loss, we can reduce the consumption of fossil fuels used to produce electricity, thus reducing emissions of CO2.
Use More Copper for Higher Efficiency
Since stator and rotor losses are I2R losses, losses reduction can be achieved by decreasing R(resistance).
The stator coil can be either copper or aluminum, while copper coils are the norm due to its high electrical conductivity. To reduce stator loss, one can increasing the cross-sectional area of the copper conductors to decrease resistance.
Aluminum is also used for stator coils for its lower cost. As aluminum has a much lower electrical conductivity, it requires a cross-section that is much larger.
Squirrel cage rotors are commonly made of copper or die cast aluminum. To reduce rotor losses, one can increase the size of rotor conductive bars and end rings.
Motor Stator Recycling Equipment
Companies can recover copper from motors that are no longer in use. While you may think it is labor intensive, there are machines that special designed for recycling copper windings from motor stator. The equipment involves a copper wire cutting machine and copper wire pulling machine. The former is used to cut one end of the copper windings in the stator, while the latter is used to pull the windings out.