Motor loss types
While converting electrical energy into mechanical energy, electric motors also lose some energy. Electric motor losses can generally be divided into three parts: variable losses, fixed losses, and stray losses.
1. Variable losses vary with the load and include stator resistance loss (copper loss), rotor resistance loss, and brush resistance loss.
2. Fixed losses are independent of the load and include core losses and mechanical losses. Core losses consist of hysteresis losses and eddy current losses, which are proportional to the square of the voltage. Hysteresis losses are inversely proportional to the frequency.
3. Other stray losses include mechanical losses and other losses, such as frictional losses of bearings and wind resistance losses caused by the rotation of fans, rotors, etc.
Measures to reduce motor losses
1. Stator loss
Stator I^2R loss, commonly known as stator copper loss, is closely related to output power. The higher the output power, the greater the input current, and the higher the temperature, the greater the stator copper loss. Taking rated input and rated load as a reference, in high-efficiency motors, stator copper loss accounts for the largest proportion of the five major losses, generally exceeding 30% of the total losses.
The main methods to reduce the I^2R loss of the motor stator are:
(1) Increasing the cross-sectional area of the stator slots will reduce the magnetic circuit area and increase the magnetic flux density of the teeth, given the same stator outer diameter;
(2) Increasing the stator slot fill factor is effective for low-voltage small motors. Applying optimal winding and insulation dimensions and large conductor cross-sectional area can increase the stator slot fill factor.
(3) Minimize the length of the stator winding ends. The stator winding end losses account for 1/4 to 1/2 of the total winding losses. Reducing the winding end length can improve motor efficiency. Experiments show that a 20% reduction in end length leads to a 10% decrease in losses.
2. Rotor losses
Rotor I^2R loss, commonly known as rotor copper loss, is mainly related to rotor current and rotor resistance.
The main energy-saving methods corresponding to the rotor I^2R loss of the electric motor are:
(1) Reduce rotor current, which can be considered from two aspects: improving voltage and motor power factor;
(2) Increase the cross-sectional area of the rotor slots;
(3) Reduce the resistance of the rotor winding, such as by using thick wires and low-resistance materials. This is more meaningful for small motors because small motors are generally made of cast aluminum rotors. If cast copper rotors are used, the total loss of the motor can be reduced by 10% to 15%. However, the current cast copper rotors require high manufacturing temperatures and the technology is not yet widespread, so their cost is 15% to 20% higher than that of cast aluminum rotors.
3. Core loss
The eddy current loss generated by the alternating magnetic field of an AC motor in the iron core, and the excessive eddy current, cause the overall temperature rise of the motor to be too high, reduce the heat dissipation rate of the windings, and lead to overheating of the windings and burnout of the motor.
Methods to reduce iron loss in electric motors include:
(1) Decrease 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 iron chip 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 thinner iron chips will increase the number of iron chips and the motor manufacturing cost;
(3) Use cold-rolled silicon steel sheets with good magnetic permeability to reduce hysteresis loss;
(4) High-performance iron chip insulating coating is adopted;
(5) Heat treatment and manufacturing technology: The residual stress after processing iron core chips will seriously affect the loss of the motor. During the processing of silicon steel sheets, the cutting direction and punching stress have a significant impact on the loss of the iron core. This can be achieved by cutting along the rolling direction of the silicon steel sheets and heat treating the silicon steel sheets, which can reduce the loss by 10% to 20%.
4. Stray losses
The total stray losses of an electric motor under load consist of no-load stray losses and load stray losses. No-load stray losses refer to the sum of all losses except the basic iron loss generated by magnetic flux in the stator's magnetic conductor, as measured by no-load tests. Load stray losses refer to the sum of all losses caused by the motor's load current, excluding iron loss, mechanical loss, and stator/rotor copper loss.
Currently, the understanding of stray losses in electric motors is still in the research stage. The main methods to reduce stray losses include:
(1) Heat treatment and precision machining are used to reduce short circuits on the rotor surface;
(2) Insulation treatment of the inner surface of the rotor slot;
(3) Reduce harmonics by improving stator winding design;
(4) Improve the rotor slot fit design and fit to reduce harmonics. Increasing the number of stator and rotor slots, designing the rotor slot shape as a skewed slot, and using series sinusoidal windings, distributed windings and short-pitch windings can greatly reduce high-order harmonics. Replacing traditional insulating slot wedges with magnetic slot mud or magnetic slot wedges and filling the stator core slot openings 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 all rub against the air. The air will generate resistance to the rotating parts, and the work consumed to overcome these resistances is called air loss and friction.
Wind-induced friction loss accounts for approximately 25% of the total losses in a motor and should be taken seriously. Friction losses are mainly caused by bearings and seals, and the following measures can be taken to reduce them:
(1) Minimize the shaft size as much as possible, but still meet the requirements of output torque and rotor dynamics;
(2) Use high-efficiency bearings;
(3) Use high-efficiency lubrication systems and lubricants;
(4) Advanced sealing technology is adopted.
Measures to reduce motor losses
1. Stator loss
Stator I^2R loss, commonly known as stator copper loss, is closely related to output power. The higher the output power, the greater the input current, and the higher the temperature, the greater the stator copper loss. Taking rated input and rated load as a reference, in high-efficiency motors, stator copper loss accounts for the largest proportion of the five major losses, generally exceeding 30% of the total losses.
The main methods to reduce the I^2R loss of the motor stator are:
(1) Increasing the cross-sectional area of the stator slots will reduce the magnetic circuit area and increase the magnetic flux density of the teeth, given the same stator outer diameter;
(2) Increasing the stator slot fill factor is effective for low-voltage small motors. Applying optimal winding and insulation dimensions and large conductor cross-sectional area can increase the stator slot fill factor.
(3) Minimize the length of the stator winding ends. The stator winding end losses account for 1/4 to 1/2 of the total winding losses. Reducing the winding end length can improve motor efficiency. Experiments show that a 20% reduction in end length leads to a 10% decrease in losses.
2. Rotor losses
Rotor I^2R loss, commonly known as rotor copper loss, is mainly related to rotor current and rotor resistance.
The main energy-saving methods corresponding to the rotor I^2R loss of the electric motor are:
(1) Reduce rotor current, which can be considered from two aspects: improving voltage and motor power factor;
(2) Increase the cross-sectional area of the rotor slots;
(3) Reduce the resistance of the rotor winding, such as by using thick wires and low-resistance materials. This is more meaningful for small motors because small motors are generally made of cast aluminum rotors. If cast copper rotors are used, the total loss of the motor can be reduced by 10% to 15%. However, the current cast copper rotors require high manufacturing temperatures and the technology is not yet widespread, so their cost is 15% to 20% higher than that of cast aluminum rotors.
3. Core loss
The eddy current loss generated by the alternating magnetic field of an AC motor in the iron core, and the excessive eddy current, cause the overall temperature rise of the motor to be too high, reduce the heat dissipation rate of the windings, and lead to overheating of the windings and burnout of the motor.
Methods to reduce iron loss in electric motors include:
(1) Decrease 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 iron chip 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 thinner iron chips will increase the number of iron chips and the motor manufacturing cost;
(3) Use cold-rolled silicon steel sheets with good magnetic permeability to reduce hysteresis loss;
(4) High-performance iron chip insulating coating is adopted;
(5) Heat treatment and manufacturing technology: The residual stress after processing iron core chips will seriously affect the loss of the motor. During the processing of silicon steel sheets, the cutting direction and punching stress have a significant impact on the loss of the iron core. This can be achieved by cutting along the rolling direction of the silicon steel sheets and heat treating the silicon steel sheets, which can reduce the loss by 10% to 20%.
4. Stray losses
The total stray losses of an electric motor under load consist of no-load stray losses and load stray losses. No-load stray losses refer to the sum of all losses except the basic iron loss generated by magnetic flux in the stator's magnetic conductor, as measured by no-load tests. Load stray losses refer to the sum of all losses caused by the motor's load current, excluding iron loss, mechanical loss, and stator/rotor copper loss.
Currently, the understanding of stray losses in electric motors is still in the research stage. The main methods to reduce stray losses include:
(1) Heat treatment and precision machining are used to reduce short circuits on the rotor surface;
(2) Insulation treatment of the inner surface of the rotor slot;
(3) Reduce harmonics by improving stator winding design;
(4) Improve the rotor slot fit design and fit to reduce harmonics. Increasing the number of stator and rotor slots, designing the rotor slot shape as a skewed slot, and using series sinusoidal windings, distributed windings and short-pitch windings can greatly reduce high-order harmonics. Replacing traditional insulating slot wedges with magnetic slot mud or magnetic slot wedges and filling the stator core slot openings 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 all rub against the air. The air will generate resistance to the rotating parts, and the work consumed to overcome these resistances is called air loss and friction.
Wind-induced friction loss accounts for approximately 25% of the total losses in a motor and should be taken seriously. Friction losses are mainly caused by bearings and seals, and the following measures can be taken to reduce them:
(1) Minimize the shaft size as much as possible, but still meet the requirements of output torque and rotor dynamics;
(2) Use high-efficiency bearings;
(3) Use high-efficiency lubrication systems and lubricants;
(4) Advanced sealing technology is adopted.
