Power engineering of electric drive
Power drive performance
The process of transmission and conversion of energy is accompanied by its losses. The power losses are made up of losses in the electric motor (ED) and mechanical losses in the gears. The power losses in ED are divided into constants and variables
where To - permanent losses are not load-dependent: steel losses, mechanical friction losses, ventilation, in the excitation windings of the LED and DCT of independent excitation; V - Variable load-dependent loss.
where - the nominal power loss variables.
For three-phase BP
where - the nominal power loss variables
AD; and - the nominal and current magnitudes of the reduced rotor and stator current.
For SD, the variable power loss is
where - the nominal power loss variables of the LED.
If we denote the frequency of the motor current through X, i.e. for DPT; for blood pressure; for LEDs, the power loss variables for different motors will be determined by the expression
The total power loss in the engine is determined by the formula
where - loss factor
If the engine is running in the nominal mode (X = 1), the losses are determined by the passport data:
Persistent power losses are found as
Energy loss during operation
If the load cycles cyclically, the energy losses are determined
where and under load - the number of load values in individual sections cycle; - the cycle time.
The coefficient of efficiency of the electric motor is defined as the ratio of the useful power on the shaft to the power consumed from the network
in the nominal mode with img src="images/image1037.jpg"> has a maximum of
The dependence is shown in Fig. 8.6.
For and = 1, when the constant loss is equal to the variable, ,
With under load
If is greater than the nominal load
Therefore, the engine has the largest in the area of nominal load, so it is necessary to load the engines to the rated load and, therefore, to select the motor for the electric drive.
If the voltage and current do not coincide in phase and have a non-sinusoidal shape, it is necessary to take into account in the loss calculation the power factor
where P is the active power, is the distortion factor; U, /, - the effective values of voltage, current, the first alternating current; - the angle of shear between the first voltage and current variables.
With sinusoidal U and and compared with DC losses will increase
If the load of the drive changes over time, the efficiency of the cycle is determined by the ratio of the supplied and the useful energy
where - useful energy and energy losses per cycle.
If the of the engine changes, the permanent losses change. For example, when the excitation current of the DCT of independent excitation or SD changes, the losses in steel, mechanical and ventilation, change substantially. Additional power losses are created in power converters in TRP-D systems, and consumers of reactive power appear, which reduces the power factor cosip.
It is necessary to take into account losses with uneconomical methods of speed control, such as the inclusion of resistances in the anchor chain of the DPT, the rotor of the AD. Economical methods include regulation by means of TPN in DFT, IF-AD, in cascade schemes of blood pressure. Let's consider separately regulated ЭП:
1. Adjustable EP with DFT of independent excitation. Constant losses
Variable losses in anchor chain DPT
where - speed of ideal idling (xx) when the engine is running on an artificial characteristic. When adjusting the resistance in the chain of the DCT anchor, the total losses are:
When adjusting the DPT speed by changing the voltage with a controlled converter, the converter's permanent losses (losses in transformer steel and reactors) remain essentially constant
Variable converter loss
If the DPT speed regulation occurs under the condition that its operation time is equal for each speed in the range D, then the efficiency of the control cycle is equal to
2. Adjustable electric drive with AD Permanent loss
since and, considering the stator and rotor steel volumes at
are evenly equal, we get:
where - losses in stator steel at nominal and .
With the rheostatic control method (), when
So, with a decrease in speed (with S) , the loss of increases, which is compensated by a reduction in mechanical losses,
Loss Variables for Adjustable Blood Pressure
where - variable losses in the stator and rotor circuits.
For fan load
Losses in the rotor
This dependence has an extremum at , then
With the frequency control of the speed of the AD, the operating slip remains small throughout the entire speed control range, then the losses in the rotor steel can be neglected and only the losses in the stator steel
If the speed control is carried out in the frequency manner at , the speed difference and the total loss variables remain constant
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