Engineering, Automation

1. Reduce starting voltage and current Series inductors (figure 1) or resistors (figure 2) reduce the voltage to the motor and therefore also the starting current. The starting torque is reduced by about the square of the current. From chokes At rest, the motor impedance is low. Most of the supply voltage drops across inductors connected in series. The engine take-off torque is therefore greatly reduced. With increasing speed, the voltage across the motor increases due to the decrease in current consumption and vector distribution of voltage between the motor and the reactance connected in series. Starting the motor via chokes connected in series Therefore, the engine torque also increases. After starting the engine, the throttles are short- circuited by the time-delayed main contactor K1M and the start contactor K2M is dropped. From resistances The basic circuit diagram is basically the same as described above, with the only difference that the inductors are replaced with resistors at a lower cost. Motor starting via resistors connected in series With this method, the starting current can only be reduced slightly, since the motor torque drops with the square of the voltage, and the voltage at the motor terminals, other than with starting via inductors, increases only slightly with the voltage. speed. It is more advantageous to reduce the series resistance when starting in stages. This reduces the voltage across the resistor and increases it across the motor. The expenditure on computer equipment is therefore much higher. A simpler solution is closed electrolytic resistors with a negative temperature coefficient. Their ohmic resistance automatically decreases during start-up due to heating by the starting current.

2. Reference: Low voltage switchgear and control switchgear application guide - A technical reference manual for electrical engineers by NHP In this regard, three points are worth mentioning: Insulation resistance, measurement and verification. 1.insulation resistance The minimum insulation resistance of new, cleaned, or repaired windings to the ground is 10 megohms or more. The minimum insulation resistance, R, is calculated by multiplying the nominal voltage U n, with the constant factor 0.5 Megohm / kV. For example: If the rated voltage is 690 V = 0.69 kV, the minimum insulation resistance is: 0.69 kV x 0.5 Megohm / kV = 0.35 Megohm 2.measurement The minimum insulation resistance of the winding to earth is measured with 500 V DC. The coiling temperature should be 25 ° C ± 15 ° C. The maximum insulation resistance should be measured with 500 V DC with the windings at an operating temperature of 80 - 120 ° C depending on the motor type and efficiency. 3. Verification If the insulation resistance of a new, cleaned, or repaired motor and stored for some time is less than 10 Mohm, the reason could be that the windings are wet and need to be dried. If the motor has been running for a long time, the minimum insulation resistance may drop to a critical level. As long as the measured value does not fall below the calculated value of the minimum insulation resistance, the motor can continue to operate.

3. However, if it falls below this limit, the engine should be stopped immediately, in order to avoid personal injury due to the high leakage voltage