CS261258B1 - Device for continuous indication of DC isolation state with rectifier - Google Patents

Abstract

The solution concerns the continuous indication of the insulation state of a DC drive powered by a rectifier. The device allows the insulation state of the drive to be indicated both during operation and at rest when the motor is powered by a rectifier. The device consists of a rectifier bridge, two diodes, a resistor, an ammeter, potentiometers, a transformer, a capacitor and a choke.

Description

The invention relates to a device for continuous indication of the insulation state of a DC drive with a rectifier. The device allows indicating the insulation state of the drive during operation and at rest of the motor when it is powered by a rectifier, both diode and thyristor.

For large DC drives, it is important to check their insulation status, i.e. the value of the insulation resistance to ground, because in the case of low insulation resistance of the motor or power rectifier, a short circuit may occur in the event of an accidental ground connection, resulting in a serious failure of the drive.

Previously known indication systems, operating with an AC relay powered via a capacitor by a small transformer, which are derived from the known rotor earth fault protection of synchronous machines, fail in cases where the motor is powered by a thyristor rectifier exhibiting large voltage ripple caused by strongly developed higher harmonic currents.

The above-mentioned disadvantage is eliminated by a device for continuous indication of the insulation state of a DC drive with a rectifier according to the invention, consisting of a rectifier bridge, two diodes, an ammeter, a potentiometer, a resistor, a transformer, a capacitor and a choke. The essence of the invention lies in the fact that the positive terminal of the rectifier bridge is connected via the first diode and via the resistor to one terminal of an ammeter, the second terminal of which is simultaneously connected to the ground terminal and to the negative terminal of the rectifier bridge. The positive terminal of the rectifier bridge is connected via the second diode to the center of a potentiometer connected in parallel to the armature of the DC drive motor with a rectifier, while the alternating side of the rectifier bridge is connected to the secondary winding of a transformer, the primary winding of which is bridged by a capacitor and connected via a choke to the alternating power supply network.

The advantage of the device according to the invention is that it is simple and therefore reliable. It works continuously while the drive is running, so that it is not necessary to switch off the drive to check the insulation state. The ammeter is practically at ground potential, so it is safe for the operator.

The attached drawing shows an example of a device according to the invention. A rectifier 2 of a DC drive is connected in parallel to an armature 2 of a DC drive motor, which has an insulation resistance 3A and 3B to ground and an insulation capacitance 4A and 4B, connected in parallel to the insulation resistance 3A and 3B.

A potentiometer 10 is connected in parallel to the armature 2 of the DC drive motor. The rectifier bridge 5 is connected by its positive terminal + through the first diode 6 and through the resistor 2 ⁸ to one terminal of the ammeter 8, which has its second terminal connected simultaneously to the ground terminal and to the negative terminal - of the rectifier bridge 5. Furthermore, the positive terminal + of the rectifier bridge 5 is connected through the second diode 9 to the center of the potentiometer 10.

The alternating side of the rectifier bridge Q is connected in parallel to the secondary winding of the transformer 11, the primary winding of which is bridged by a capacitor 12 and connected via a choke 13 to the alternating supply network with voltage U _g .

In the following description of the function, the following symbols indicate:

2 _A - ammeter current 8.

2^ - total direct current in insulation resistances 3A, 3B

I. - rectifier bridge current 5 —u . —

1^ - primary current of transformer 21

2 ₂ - secondary current of transformer 21

R _A - ohmic value of resistor 2 including ammeter 2

2R^ - ohmic value of the first and second insulation resistance 3A, 3B

2Rp - ohmic value of each half of the potentiometer 22

U _g - voltage of the alternating supply network

X - capacitor reactance 12

-c —

Xi - choke reactance 12

- impedance of the primary side of the transformer 11

By using direct current from the rectifier bridge 2 3 ^, it is achieved that it is not necessary to respect the influence of the insulation capacitance 4A or 4B, which does not allow direct current to pass through.

Diode 2 is oriented to allow direct current 2 _A to enter resistor 7 and ammeter JB, but prevents the entry of higher harmonics coming from rectifier 2 into ammeter 2 ⁱⁿ those cases when their positive half-wave is at the ground terminal.

Diode 2 is oriented in such a way that it allows direct current to enter the insulation resistors 3A and 3B, respectively, but prevents the entry of higher harmonics coming from rectifier 2 into ammeter 2 in those cases when their positive half-wave is at the center of potentiometer 10.

For the current 1^ entering the primary side of the transformer 11, which together with the capacitor 12 and the choke 13 forms a Boucherot circuit, the following relation holds:

X jX _L + Z (1 - )

In the event that X^ ⁼ «Xq is intentionally made, it will be

D _with

I _η = - ⁵ — = CONST.

With a constant thread ratio of the transformer 11, the current 2 ₂ ⁱⁿ the secondary side of the transformer 11 will also be constant, and with a constant current ratio of the rectifier bridge 2, its direct current 2 _U will also be constant.

If the resistance of each half of potentiometer 10 is designated as 2 and the insulation resistances 3A and 3B as 2 R^, then the resulting resistance Rp + fL· is placed between the center of potentiometer 10 and the ground terminal. Then the voltage between the ground terminal and the positive terminal of the rectifier bridge is:

(R _p + RJ . I. = R _ft . I _A

Of which _T ^R A · ^X A i R_ + R,

Furthermore, it applies

I =1.+1.= const. u A i or ^R A · ^I A

1=1+ ——-2 = I . (1 + u ¹ AR„ + R. ¹

P ¹

Rp+R£

The current measured by the ammeter is ^R p ^{+ R} i

Rp + R. + R _a +

^R p ^{+ R} i

If the insulation resistance R^ were infinite, i.e. R^ =· , the ammeter would show a current

R + ·« — ^E - . I =

Sp + ** + R _a • ^T u = V

If the insulation resistance were zero, i.e. R^ = 0, the ammeter would show a current of ^R p + ^{0 R} P ⁺ ? ^{+ R} A

R + R. P A . I..

If II is negligible against R _& , i.e. if R_^JR _A , or R = 0, then I _A = 0.

Based on the deflection of ammeter J3, it is possible to determine the insulation state of the directional drive.

Claims (1)

. Device for continuous indication of rectifier bridge DC isolation state, consisting of rectifier bridge, two diodes, ammeter, potentiometer, resistors, transformer, capacitor and choke, characterized in that the positive terminal (+) of rectifier bridge (5) is connected via the first diode (6) and through resistor (7) to one terminal of ammeter (8), the other terminal of which is connected both to ground terminal and negative terminal (-) of rectifier bridge (5), positive terminal (+) of rectifier bridge ( 5) is connected via a second diode (9) to the center of the potentiometer (10) connected in parallel to the armature (2) of the DC drive with the rectifier (1), the alternating side of the rectifier bridge (5) being connected to the secondary winding of the transformer (11) the primary winding of which is bridged by a capacitor (12) and connected via an inductor (13) to an AC power supply.