JPS596788A - Control device for dc motor - Google Patents

Abstract

PURPOSE:To enable to operate a DC motor in good power factor by controlling to decrease a voltage command which is inputted to a field control circuit in the amount corresponding to the decrease when the received voltage of a converter connected to the motor decreases. CONSTITUTION:A power transformer 1 is connected to converter 2 which supplies power to a motor 3, the difference between the speed command 10 and the signal of a speed detector 4 is calculated, thereby controlling the thyristor of a converter 2. Further, the received voltage 19 and a rated voltage command 18 are inputted to a voltage setter 17, and when the received voltage decreases, the decreased amount is reduced from the command 13, inputted to a field control circuit 8 together with the voltage feedback signal 14, thereby controlling the motor speed. Accordingly, since the relationship between the received voltage and the terminal voltage of the motor 3 can be maintained constantly in the state that the received voltage is lower than the rated voltage, the secondary voltage of the transformer 1 can be decreased, thereby improving the power factor of the received voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a control device for a DC motor having a field winding.

FIG. 1 shows a conventional control device for a DC motor using a SIRISK converter. Power transformer 1, main sirisk converter 2, electric motor 3, speed detector 4, speed control circuit 5,
Field thyristor converter 6, field winding 7, field control circuit 8
, a power factor correction capacitor 9 is also configured. In this device, a speed control circuit 5 calculates the difference between a speed command 10 and a speed feedback signal 11 that is the output of a speed detector 4, and the output 12 controls the firing phase of the main thyristor converter 2, thereby controlling the motor. The voltage applied to 3 is controlled to control the speed of the motor 2. Further, the terminal voltage of the motor is controlled by the field control circuit 8 so as not to exceed the rated voltage. That is, the field control circuit 8 calculates the difference between the voltage command 13 (normal rated voltage command) and the voltage feedback signal 14, and the voltage feedback signal 14
is larger, and only then its output signal 15 has a value such that it reduces the current in the field winding 7 of the motor. This signal 15
is applied to the field thyristor converter 6, and the field current flowing through the field winding 7 is controlled. With this control, (voltage command 13)=(voltage feedback signal 14) in terms of electrical circuit. At normal motor operating speeds, the terminal voltage of the motor is at the rated voltage.

In the above operation, the voltage required on the secondary side of the power transformer 1 must be such that a sufficient commutation margin angle can be obtained in the inverter operation of the main thyristor converter 2.
Considering the fluctuation rate of power supply voltage for power reception (usually ±10%), it was necessary to use a high power supply voltage with a margin as shown in equation (3) below.

In a 3-phase power supply, as an example, if we substitute the numerical value, = 705v. If we take into account the power supply voltage fluctuation rate of 10%, E ac
=705vX1.1 =776v-(3) From this,
The power factor of the power source is found to be 71.6%. As mentioned above, the receiving power factor is poor at 71.6%, and the capacity of the power transformer is increased (proportional to the power supply voltage).
We had no choice but to increase the capacity of the power factor correction capacitor (inversely proportional to the power factor).

SUMMARY OF THE INVENTION An object of the present invention is to provide an electric motor control device with a good power factor in order to eliminate the drawbacks of the prior art described above.

In order to achieve this objective, the present invention controls the terminal voltage of the motor in proportion to the decrease when the received power voltage decreases, thereby enabling operation with a good power factor.

An embodiment of the present invention will be described below with reference to FIG.

FIG. 2 shows a configuration in which a receiving voltage detection transformer 16 and a voltage setting circuit 17 are added to FIG. 1.

'The voltage setting circuit 17 normally operates according to the rated voltage command 18 and voltage command 13, but only when the received power voltage is lower than the rated voltage, the voltage setting circuit 17 operates according to the voltage command 13 by a corresponding value.
It works to reduce the value of.

In this case, when the received voltage is below its rated voltage, the relationship between the received voltage and the terminal voltage of the motor can be kept constant, so the secondary voltage of the power transformer 1 is determined by the fluctuation rate of the received voltage. There is no need to take this into consideration, and as a result, the voltage can be lower than that of the conventional method.

In the equation L(1), the amount EM commensurate with the decrease in EAC
In order to reduce .

Here, when the received power voltage decreases, 'f! Since the terminal voltage of the LtIJJ machine is controlled to be low, it is possible that the power will be insufficient, but there will be no actual harm for the following reason.

(1) The power of the electric motor usually has a margin of about 10 inches.

(2) The drop in the received power voltage is temporary, and a short-time overload of about 10% is not a problem, as evaluated from the motor's thermal time constant (usually about 3 hours).

(3) This control applies only when the motor speed is in the field control region, and otherwise outputs the same power as conventional control.

According to the present invention, it is possible to lower the secondary voltage of the power transformer and improve the power reception rate compared to the conventional method. As a result, the capacitance of the power factor improvement capacitor can be reduced. Alternatively, there is an effect that the capacity of the power transformer can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of a conventional electric motor control device, and FIG. 2 is a diagram showing an embodiment of the present invention. 1...Power transformer, 2...Main thyristor converter,
3... Electric motor, 4... Speed detector, 5... Speed control circuit, 6... Field thyristor converter, 7... Field winding, 8... Field control circuit, 9... ...Capacitor for power factor improvement, 16th agent Patent attorney Akinetsu Takahashi!甚）D 9 4

Claims (1)

[Claims] 1.' A field power source that supplies field current to the field winding of the motor, such as a DC motor connected to the DC output end of a converter that converts current to DC, and controls the speed of the DC motor. A DC motor comprising: a speed control circuit; and a field control circuit that inputs a terminal voltage and a voltage command of the DC motor and controls the field power source to reduce the field current when the terminal voltage is large. In the control device, a DC voltage setting circuit is provided which reduces the voltage command input to the field control circuit by an amount commensurate with the drop when the received voltage of the converter is lower than the rated voltage command. Electric motor control device.