Energy relations in an electric machine amplifier of adjustable frequency accounting for the circuit of a controlled semiconductor switch
Abstract Generated abstract
The paper analyzes energy relations in an adjustable-frequency electric-machine amplifier while explicitly accounting for the circuit of a controlled semiconductor commutator. It compares contactless and contact implementations, considering operation of the additional stage in motor and asynchronous frequency-converter modes, and derives power-balance relations and energy diagrams for rated, no-load, and partial-load conditions. The analysis indicates that the motor mode of the additional stage is the most favorable for the contactless amplifier, while the contact version may operate in asynchronous-generator, motor, or converter modes. The presence of return diodes in the commutator circuit is shown to reduce input control power and avoid additional control-winding losses, with calculated no-load values reported as close to experimental measurements.
Full Text
ENERGY RELATIONS IN AN ADJUSTABLE-FREQUENCY ELECTRIC-MACHINE AMPLIFIER TAKING INTO ACCOUNT THE CIRCUIT OF A CONTROLLED SEMICONDUCTOR COMMUTATOR
Sh. S. Roiz, A. I. Skorospeleshkin, E. F. Obergan
(Recommended by the scientific seminar of the Departments of Electrical Machines and General Electrical Engineering)
The analysis of energy relations in an adjustable-frequency electric-machine amplifier should be carried out taking into account the operation of the controlled semiconductor commutator (UPK). In [1] a circuit of an adjustable-frequency electric-machine amplifier is presented (the UPK circuit is represented only by semiconductor cells and freewheeling diodes). As was pointed out in [1], the presence of freewheeling diodes has a favorable effect on the operation of the UPK, reducing commutation overvoltages on the elements and unloading the switches.
In the absence of freewheeling diodes \(D_1, D_2, \ldots, D_6\) in the UPK circuit, part of the rotor power, in no-load modes or under incomplete load of the amplifier, cannot be transferred to the DC supply network, since the transistor switches \(T_1, T_2, \ldots, T_6\) do not pass active power in this direction. Moreover, the power transmitted to the amplifier input is dissipated in the form of losses, additionally loading the control winding.
The presence of freewheeling diodes in the UPK circuit makes it possible to transfer power to the DC network (in no-load mode or under incomplete amplifier load), which leads to a decrease in input power and an increase in the amplification factor.
The energy relations in the contactless version of the amplifier depend to a considerable extent on the operating mode of the additional stage. Let us consider several typical operating modes of the amplifier.
Mode 1. Contactless version of the amplifier. The additional stage operates in motor mode. Freewheeling diodes in the UPK circuit are absent.
The electromagnetic output power of the amplifier at rated load is equal to
\[ P_{\psi 2} = P_{m1} - P_{\psi 3}, \]
where
\[
P_{\psi 2}
\]
— electromagnetic power of winding \(W_2\),
\[
P_{m1}
\]
— mechanical power on the shaft of the main stage,
\[
P_{\psi 3}
\]
— electromagnetic power of winding \(W_3\).
For the additional stage operating in motor mode, the expression is valid
\[ P_{\psi 3} = P_{\psi 4} = P_{m2} + P_{\psi 5}, \tag{2} \]
where
\(P_{\psi4}\) is the electromagnetic power of winding \(W_4\),
\(P_{m2}\) is the mechanical power on the shaft of the additional stage,
\(P_{\psi5}\) is the electromagnetic power of winding \(W_5\).
Taking (2) into account, the output electromagnetic power of the winding is equal to
\[ P_{\psi2}=P_{m1}-P_{m2}-P_{\psi5}. \tag{3} \]
The energy diagram for this mode is shown in Fig. 1, a. In the figure, the power of the control winding \(P_{oy}\) denotes the power required to create the magnetic flux in the air gap of the main stage, and also to compensate the reactive powers of the rotor and the load. The reactive power \(Q_{oy}\) is the transformed power \(P_{uy}\), consumed from the direct-current network.
When the amplifier operates in no-load mode or under partial load, the power of the control winding is
\[ P_y=P_{oy}+P', \tag{4} \]
where \(P'\) is the part of the rotor power going to the amplifier input.
The energy diagram corresponding to this mode is presented in Fig. 1, b. The powers \(P'\) and \(P'_{\psi2}\) in the energy diagram are components of the output power \(P_{\psi2}\).
Their magnitude is determined by the parameters of the amplifier windings, the load, and the control frequency.
Fig. 1. Energy diagrams of a contactless amplifier. The additional stage operates in motor mode
Thus, considering the energy relations in a contactless adjustable-frequency EMA and the circuit of a CSC without feedback diodes, one may conclude:
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The additional stage, when operating in motor mode, returns mechanical power to the shaft of the drive motor, which substantially increases the efficiency of the amplifier.
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The absence of feedback diodes in the CSC circuit at no-load operation of the amplifier or at partial load causes additional losses in the control winding.
Mode 2. Contactless design of the amplifier. The additional stage operates in motor mode. Feedback diodes are present in the CSC circuit.
The energy relations in the amplifier at rated load remain the same as in mode 1 (Fig. 1, a).
When the amplifier operates at no load or at partial load, the control power is equal to
\[ P_y = P_{oy} - P'. \tag{5} \]
The energy diagram of this mode of operation is shown in Fig. 1, c. Analysis of the energy relations shows that the presence of feedback diodes in the CSC circuit makes it possible to reduce the input power (no-load or partial-load mode) and to eliminate additional losses in the control winding.
Mode 3. Contactless design of the amplifier.
The additional stage operates in the mode of an asynchronous frequency converter. Feedback diodes are absent from the CSC circuit.
The electromagnetic output power at rated load is equal to
\[ P_{\psi 2}=P_{m1}-P_{\psi 3}=P_{m1}+P_{m2}-P_{\psi 5}, \tag{6} \]
where
\[ P_{\psi 3}=P_{\psi 4}=P_{\psi 5}-P_{m2}. \]
The energy diagram corresponding to this mode is shown in Fig. 2, a. The control power in this mode is equal to \(P_{oy}\). When the amplifier operates at no load or at partial load, the control power is calculated from expression (4). The energy diagram of this mode is presented in Fig. 2, b.
Thus, considering the operation of the amplifier in this mode, the following conclusions may be drawn:
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When the additional stage operates in the mode of an asynchronous frequency converter, additional losses arise in the windings of the amplifier, which reduces the efficiency of the amplifier.
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The absence of feedback diodes in this mode produces an effect analogous to that in mode 1.
Mode 4. Contactless design of the amplifier. The additional stage operates in the mode of an asynchronous frequency converter. Feedback diodes are present in the CSC circuit.
The energy relations at rated load remain the same as in mode 3 (Fig. 2, a).
When the amplifier operates at no load or at partial load, the control power is determined from expression (5). The energy diagram is presented in Fig. 2, c.
Thus, analysis of the energy relations in the contactless design of the amplifier shows that the most acceptable mode of the additional stage is the motor mode. As for the CSC circuit, preference is given to a CSC with feedback diodes.
For this mode of operation of the additional stage (motor-
Fig. 2. Energy diagrams of the contactless amplifier. The additional stage operates in the mode of an asynchronous frequency converter.
...the control power was calculated with and without flyback diodes in the controlled semiconductor commutator circuit in the no-load mode of the amplifier. The results of the calculation are given in Table 1.
Table 1.
| Calculation | Experiment | Note |
|---|---|---|
| 320 | 300 | \(U_y = 15\ \text{V},\quad f_y = 140\ \text{Hz},\) flyback diodes are absent in the UPC circuit |
| 54 | 45 | \(U_y = 15\ \text{V},\quad f_y = 140\ \text{Hz},\) flyback diodes are present in the UPC circuit |
As can be seen from the table, the experimental values are close to the calculated ones. The control powers with and without flyback diodes differ significantly.
In the contact version (the additional stage is absent), the most characteristic operating mode of the main stage is the asynchronous-generator mode.
The electromagnetic output power at rated load is determined by expression (1), and the energy diagram is given in Fig. 3, a. The effect of the feedback diodes on the energy relations in the amplifier is taken into account in a manner analogous to that used for the contactless version.
In addition to the asynchronous-generator mode of the main stage, the amplifier can operate in the motor mode (winding \(W_2\) is open, there is no torque on the shaft of the driving motor) and in the converter mode (winding \(W_3\) is open).
The torque on the amplifier shaft when operating in the motor mode is produced by the CSC. The energy diagram corresponding to the motor mode is given in Fig. 3, b.
The power \(P\) in the figure denotes the required power for producing torque on the amplifier shaft and for covering losses in the rotor winding \(W_3\).
The output power in the converter mode is likewise produced by the CSC. The energy diagram corresponding to the converter mode is shown in Fig. 3, c.
Fig. 3. Energy diagrams of a contact amplifier
The power \(P\) in the figure denotes the required power needed to produce the output power \(P_{\mathrm{u}2}\).
As a result of the work, the following conclusions may be formulated:
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An analysis has been carried out of the energy relations in the contactless implementation of the amplifier. The most expedient operating mode of the additional stage (motor mode) has been established.
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An analysis has been carried out of the energy relations in the contact implementation of the amplifier. The possible modes of its operation (asynchronous-generator, motor, converter modes) have been established.
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The influence of the CSC circuit on the energy relations in the amplifier has been shown. It has been established that the most acceptable CSC circuit is the CSC with return diodes.
REFERENCES
- Sh. S. Roiz, A. I. Skorospeshkin. Theoretical study of an electric-machine amplifier of adjustable frequency taking into account a controlled semiconductor commutator. Izvestiya TPI, vol. 212, in the present collection.