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Motor loss types and measures to reduce losses

Date:2024-06-28   Author:XINDA MOTOR
Motor loss types

While the motor converts electrical energy into mechanical energy, it also loses some energy. Motor losses can generally be divided into three parts: variable losses, fixed losses and stray losses .

1. Variable losses vary with load and include stator resistance loss (copper loss), rotor resistance loss and brush resistance loss.

2. Fixed loss is independent of load, including core loss and mechanical loss. Iron loss is composed of hysteresis loss and eddy current loss , which is proportional to the square of voltage, and hysteresis loss is inversely proportional to frequency.

3. Other stray losses are mechanical losses and other losses, including friction losses in bearings and windage losses caused by the rotation of fans and rotors.

Measures to reduce motor losses

1. Stator losses
Stator I^2R loss is commonly known as stator copper loss. Stator copper loss is closely related to output power. The greater the output power, the greater the input current, the higher the temperature, the greater the stator copper loss. Taking the rated input and rated load as a reference, for motors with higher efficiency, the stator copper loss accounts for the largest proportion of the five major losses, generally greater than 30% of the total loss.

The main methods to reduce the I^2R loss of the motor stator are:
(1) Increasing the cross-sectional area of the stator slots. Under the same stator outer diameter, increasing the cross-sectional area of the stator slots will reduce the magnetic circuit area and increase the magnetic flux density of the teeth;
(2) Increase the stator slot full rate, which is more effective for low-voltage small motors. The stator slot full rate can be increased by using the best winding and insulation size and large wire cross-sectional area.
(3) Try to shorten the length of the stator winding end. The stator winding end loss accounts for 1/4 to 1/2 of the total winding loss. Reducing the length of the winding end can improve the efficiency of the motor. Experiments show that when the end length is reduced by 20%, the loss decreases by 10%.

2. Rotor losses
Rotor I^2R loss is commonly known as rotor copper loss, which is mainly related to rotor current and rotor resistance.

The main energy-saving methods corresponding to the I^2R loss of the motor rotor are:
(1) Reduce the rotor current, which can be considered from two aspects: improving the voltage and the motor power factor;
(2) Increase the cross-sectional area of the rotor slot;
(3) Reduce the resistance of the rotor winding, such as using thick wires and low-resistance materials. This is more meaningful for small motors because small motors generally have cast aluminum rotors. If cast copper rotors are used, the total loss of the motor can be reduced by 10% to 15%. However, the manufacturing temperature required for today's cast copper rotors is high and the technology is not yet popular. Its cost is 15% to 20% higher than that of cast aluminum rotors.

3. Core loss
The alternating magnetic field of the AC motor generates eddy current loss in the iron core. If the eddy current is too large, the overall temperature rise of the motor will be too high, the heat dissipation rate of the winding will be reduced, and the winding will overheat and the motor will burn out.

Methods to reduce motor iron loss are:
(1) Reduce the magnetic density and increase the length of the iron core to reduce the magnetic flux density , but the amount of iron used in the motor will increase accordingly;
(2) Reduce the thickness of the core sheet to reduce the loss of induced current. For example, using cold-rolled silicon steel sheets instead of hot-rolled silicon steel sheets can reduce the thickness of the silicon steel sheets, but thin core sheets will increase the number of core sheets and the manufacturing cost of the motor.
(3) Use cold-rolled silicon steel sheets with good magnetic conductivity to reduce hysteresis loss;
(4) Use high-performance iron core insulation coating;
(5) Heat treatment and manufacturing technology. The residual stress after iron core processing will seriously affect the loss of the motor. When processing silicon steel sheets, the cutting direction and punching shear stress have a greater impact on the core loss. Cutting along the rolling direction of silicon steel sheets

4. Stray losses
The total miscellaneous loss of the motor when it is running under load is composed of no-load miscellaneous loss and load miscellaneous loss. No-load miscellaneous loss refers to the sum of various losses except the basic iron loss caused by magnetic flux in the stator magnetic conductive part in the iron loss measured by the no-load test; load miscellaneous loss refers to the sum of various losses caused by the load current of the motor except iron loss, mechanical loss and stator and rotor copper loss.

 At present, the understanding of motor stray loss is still in the research stage. The main methods to reduce stray loss are:
(1) Heat treatment and finishing are used to reduce rotor surface short circuit;
(2) Insulation treatment of the inner surface of the rotor slot;
(3) Reduce harmonics by improving stator winding design;
(4) Improving the rotor slot matching design and reducing harmonics, increasing the number of stator and rotor tooth slots, designing the rotor slot shape into skew slots, and using series-connected sinusoidal windings, distributed windings, and short-distance windings can greatly reduce high-order harmonics; using magnetic slot mud or magnetic slot wedges to replace traditional insulating slot wedges and filling the stator core slots of the motor with magnetic slot mud are effective methods to reduce additional stray losses.

5. Wind friction loss
During the rotation of the motor, the outer surface of the rotor and the cooling fan produce friction with the air, and the air will produce resistance to the rotating parts. The work consumed to overcome this resistance is called wind loss friction.

Wind friction loss accounts for about 25% of the total motor loss and should be taken seriously. Friction loss is mainly caused by bearings and seals, and the following measures can be taken to reduce it:
(1) Minimize the shaft size while meeting the output torque and rotor dynamics requirements;
(2) Use high-efficiency bearings;
(3) Use efficient lubrication systems and lubricants;