The characteristics of DC motors for rolling mills, especially intermediate mills and large roughing mills, blooming mill machines, seamless steel tube rolling mills, hot and cold rolling mills, etc. are summarized as follows:
(1) The motor capacity is large;
(2) often start, accelerate, decelerate and brake;
(3) Forward and reverse (reversible operation);
(4) The load changes drastically, frequently, and the inrush current - the overloaded motor can reach 2.75 times the rated current;
(5) The motor speed and armature current are often in a changing state, and the motor is often in the dynamic commutation process;
(6) The motor is often operated under no-load conditions;
(7) The ambient temperature is high, dry, and low in humidity, making it difficult for the commutator to form an oxide film.
There are two kinds of power supply methods for DC motors for rolling mills: one is the power supply mode of the early converter unit; the other is the thyristor rectification to the armature and the excitation power supply method which are widely used at present. Due to the characteristics of the DC motor itself used in the rolling mills, the commutation of the DC motor for the rolling mills powered by the converter unit is very difficult. The commutation condition is worse after switching to the thyristor rectification power supply. This is because the thyristor supply and excitation have two prominent effects on the DC motor.
First, the current through the thyristor rectification contains the pulsating component, which will cause the commutating pole flux to pulsate, the commutation state is deteriorated, and the width of the non-sparking commutation region is narrowed with the increase of the load current pulsation rate, which is prone to occur spark.
Second, the current value of the pulsating power source fluctuates at each instant. Therefore, the spark generated when using the pulsating power source is different at each instant. Since the pulsation of the current is periodic, the change in the spark size can also be periodic. Therefore, the system reflects the speed, because the time constant of the thyristor system is much smaller than that of the generator set, so the output voltage changes rapidly, and the rate of change of the armature current di/dt is larger than that of the converter unit. 5-8 times, so that the dynamic commutation is deteriorated. When the load suddenly increases or decreases, the motor generates a hysteretic commutation or a forward reversal, so that the sliding surface of the contact surface of the brush and the commutator slip or slips out. The above two points are the main reasons why the thyristor-powered and excited DC motor is more difficult to commutate than the converter unit. In addition, the characteristics of the operating conditions of the motor for the rolling mills make the commutation performance of the motor more than normal. The DC motor used is much worse.