Efficient motors, as the name suggests, are more efficient and meet the required energy standards. To achieve this, controlling and reducing motor losses is key, and the mechanism of loss must be clarified.
The importance of motor loss
The consideration of motor loss is important for three reasons:
(1) Loss determines motor efficiency and greatly affects 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 factor associated with these losses must be reasonably considered in the motor design.
● Ohmic loss
●Mechanical loss
Mechanical losses include friction losses on brushes and bearings, and losses due to wind resistance. If there is ventilation, whether it is a self-contained fan or an external fan, it should also include the power required to circulate air in the motor and ventilation system (required to remove forced airflow through long or narrow pipes outside the motor when the duct is ventilated) Power), friction and windage losses can be determined by measuring the input power of the motor, at which point the motor operates at the appropriate speed but is unloaded and does not excite. Typically, friction and windage losses are combined with core loss and determined simultaneously.
●Open circuit or no-load core loss
The open circuit or no-load core loss envelope hysteresis and eddy current loss is the loss caused by the time-varying flux density in the motor core only in the case of the main field winding excitation.
In DC motors and synchronous motors, although the flux change caused by the slotting causes loss in the pole core, especially in the pole piece and pole face of the pole core, the core loss is mainly limited to the armature core. , or only calculate the armature core loss.
Generally, near the rated voltage, the curve data of the no-load core loss as a function of the armature voltage is measured. It can be considered that the voltage drop of the armature resistance under load is corrected for the rated voltage (the phasor is corrected for the AC motor), and the loss of the iron core 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 core losses at rated voltage are generally used. If only to determine efficiency, it is not necessary to separate the open core loss from the friction and windage losses, and the sum of the two is collectively referred to as the no-load rotation loss.
● Load stray loss
Load stray losses include losses due to non-uniform current distribution in the copper conductor, additional core losses due to load current distortion of the magnetic field, and the like. This type of loss is difficult to determine accurately. According to the convention, the DC motor generally takes 1.0% of the output power. For synchronous and induction motors, load stray losses are typically determined by multiple standard tests.
When the motor is loaded, the magnetic potential generated by the load current will seriously affect the spatial distribution of the magnetic flux density, and the actual core loss may increase significantly. For example, harmonic magnetic potential can cause considerable loss in the core near the air gap. The increased total iron loss is usually classified as part of the load stray loss.
