Hengli Motor's Knowledge Answer on Motor Efficiency
There are three reasons why considering motor losses is important:
(1) Losses determine motor efficiency and greatly affect motor operating costs.
(2) The loss causes the motor to heat up, and the corresponding temperature rise level determines the maximum power output that can be obtained.
(3) The voltage drop or current factors related to these losses must be reasonably considered in the motor design scheme.
Motor efficiency calculation
The efficiency of the motor is given by the following equation:
Efficiency=Output/Input (1)
Or expressed as equations (2) and (3)
Efficiency=(Input Loss)/Input=1-Loss/Input...... (2)
Efficiency=Output/(Output+Loss)...... (3)
Types of losses and their generation mechanisms
Directly measuring the input and output power under load conditions using equation (1) to determine motor efficiency is limited or constrained by many factors. Usually, by measuring losses, equations (2) and (3) are used to calculate motor efficiency. If identical measurement and calculation methods are used, the efficiency determined by measuring losses can be used to compare competing motor products. Several losses that usually need to be considered include Ohmic losses, mechanical losses, open circuit or no-load iron core losses, and load stray losses.
Ohmic loss
Ohmic loss, also known as I2R loss, exists in all windings of the motor. Although the calculation is usually corrected by measuring the winding temperature at each specific operating point, it is agreed that the DC resistance of the winding at 75 ℃ is used to calculate these losses. In addition, the I2R loss of the winding under communication depends on the effective (AC) resistance of the winding, which is related to the operating frequency and motor magnetic flux. The loss deviation caused by the difference between DC resistance and effective resistance is included in the load stray loss.
For the excitation systems of synchronous motors and DC motors, only the losses in the excitation winding are included in the motor efficiency calculation; The losses within the external power supply that provides excitation are included in the efficiency of the power plant by considering the motor as a part of the power plant.
Closely related to I2R losses are brush contact losses on slip rings and commutators. Traditionally, this loss is usually ignored in induction motors and synchronous motors. In industrial DC motors, when carbon and graphite brushes with leads (wire segments) are used, the total contact voltage drop of the brushes is considered to be a constant value of 2V.
● Mechanical wear and tear
Mechanical losses include friction losses on electric brushes and bearings, as well as losses caused by wind resistance. If there is a ventilation device, whether it is a built-in fan or an external fan, it should also include the power required to circulate air in the motor and ventilation system (excluding the power required to force airflow through long or narrow ducts outside the motor during duct ventilation). Friction and wind resistance losses can be determined by measuring the input power of the motor, which is running at an appropriate speed but without load or excitation. Usually, friction and wind resistance losses are combined with iron core losses and determined simultaneously.
● Open circuit or no-load iron core loss
Open circuit or no-load iron core losses, including hysteresis and eddy current losses, are losses caused by the time-varying magnetic flux density in the motor iron core only when the main excitation winding is excited.
In DC motors and synchronous motors, although the magnetic flux changes caused by slotting can also cause losses in the magnetic pole core, especially in the pole shoes and pole faces of the magnetic pole core, the core losses are mainly limited to the armature core, or only the armature core losses are calculated.
In induction motors, the core loss is mainly limited to the stator core, while the core loss in the rotor is usually ignored due to the very low slip frequency of the magnetic field alternating frequency. By operating the motor without load at rated speed or frequency and under appropriate magnetic flux or voltage conditions, the input power of the motor is measured, and then friction and wind resistance losses are deducted. If the motor is self driven during the test, the no-load armature I2R loss (induced stator I2R loss of the motor) is also added to obtain the open circuit core loss.
Usually, the function curve data of the no-load iron core loss as a function of the armature voltage is measured near the rated voltage. It can be considered that the armature resistance voltage drop under load is used to correct the rated voltage (phasor correction is required for AC motors), and the iron core loss under load is taken as the loss value measured when the voltage is equal to the correction value. However, for induction motors, this correction is usually omitted and the iron core loss at rated voltage is generally used. If only to determine efficiency, there is no need to separate open circuit shovel core loss from friction and wind resistance loss, and the sum of these two is collectively referred to as no-load rotation loss.
● Load stray losses
Load stray losses include losses caused by uneven current distribution in copper conductors, additional core losses caused by magnetic field distortion due to load currents, and so on. This type of loss is difficult to accurately determine. According to convention, the output power of a DC motor is generally taken as 1.0%. For synchronous motors and induction motors, load stray losses are generally determined through multiple standard tests.
Calculation of eddy current and hysteresis losses
Eddy current loss varies with the square of magnetic density, frequency, and stack thickness. Under normal operating conditions, eddy current losses can be accurately approximated as
Pe=Ke(Bmax fδ)2……………（4）
In the formula, is the thickness of the laminate; Bmax is the maximum magnetic density value; F is the frequency; Ke is the proportionality coefficient. The coefficient Ke depends on the unit used, core volume, and core resistivity.
The variation law of hysteresis loss can only be represented by empirical formulas. The most commonly used relationship is shown in equation (5)
In equation (5), Ks is the proportionality coefficient, which depends on the characteristics and volume of the iron core, as well as the units used; The index n is between 1.5 and 2.5, and the value often taken for estimation in motors is 2.0. In equations (4) and (5), frequency can be replaced by velocity, and magnetic flux density can be replaced by voltage, but the proportionality coefficient needs to be changed accordingly.
When the motor is loaded, the magnetic potential generated by the load current will seriously affect the spatial distribution of magnetic flux density, and the actual iron core loss may significantly increase. For example, harmonic magnetic potential can cause significant losses in the iron core near the air gap. The increased total iron loss is usually classified as part of the load stray loss.
