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The core reason why DC motors need to adopt reduced-voltage starting lies in the mismatch between their electrical characteristics and mechanical characteristics at the moment of starting — the back electromotive force (Ea) is zero at the initial stage of starting, which leads to a starting current far exceeding the rated value when starting with full voltage. This in turn causes a series of problems such as motor damage and circuit failures. Reduced-voltage starting suppresses excessive current by lowering the starting voltage, thus ensuring system safety. The details can be explained from three aspects: the generation mechanism of starting current, the hazards of full-voltage starting, and the principle of reduced-voltage starting.
First of all, the abnormal increase of the starting current of DC motors stems from the core characteristic of “lack of back electromotive force”. According to the voltage balance equation of the armature circuit of a DC motor: U = Ea + IaRa, where U is the applied voltage across the armature, Ea is the back electromotive force generated by the rotation of the armature, Ia is the armature current, and Ra is the armature winding resistance. At the moment the motor starts, the rotor is in a static state, and the armature conductor does not cut the magnetic field, so the back electromotive force Ea = 0. At this time, the circuit equation is simplified to Ia = U/Ra. Since the armature winding is made of copper wire, its resistance Ra is usually very small (the Ra of small DC motors is only a few ohms, and that of large motors is even less than 1 ohm). If the rated full voltage U is applied directly, the starting current Ia will increase sharply, usually reaching 10-20 times the rated current. For example, a DC motor with a rated voltage of 220V and an armature resistance of 1Ω can have an instantaneous current of 220A when starting with full voltage, while its rated current may only be 15A, and the current amplification factor is far beyond the safe range.
Secondly, such an ultra-large starting current will cause multiple fatal hazards to the motor itself and the power supply system. For the motor, on the one hand, the excessive current will make the armature winding bear a huge electric force. According to the Ampere force formula, the electric force is proportional to the square of the current. A current 10 times the rated current will generate an electric force 100 times the rated value, which is very easy to cause the winding to deform and the insulation layer to break, leading to inter-turn short circuit. On the other hand, the rapidly increasing current in a short time will generate a lot of Joule heat in the winding, causing the temperature to rise sharply, exceeding the heat resistance limit of the insulating material, resulting in insulation aging or even burning. For the power supply system, the ultra-large starting current will cause the grid voltage to drop suddenly, forming a “voltage shock”, which affects the normal operation of other equipment in the same grid. For example, it may make the lighting lamps dim and precision instruments out of control. At the same time, the large current will also generate a strong electric arc on the control components such as switches and contactors, accelerating the wear of the contacts and even causing short-circuit faults.
Reduced-voltage starting suppresses the starting current from the source by “artificially reducing the initial starting voltage”, and gradually restores the rated voltage after the motor speed increases, which perfectly matches the starting characteristics of the motor. Its core logic is: reduce U at the initial stage of starting. Even if Ea = 0, Ia = U/Ra can be controlled within a safe range (usually 1.5-2.5 times the rated current). As the motor speed n increases, Ea increases in proportion to n. At this time, U is gradually increased to maintain Ia at the current value corresponding to the appropriate starting torque. Until the motor reaches the rated speed, Ea stabilizes at the rated back electromotive force. At this time, U is increased to the rated value, and the motor enters normal operation.
In practical applications, there are various implementation methods of reduced-voltage starting. Small DC motors often use “series resistance voltage reduction”, which divides the voltage by connecting a variable resistor in the armature circuit and gradually cuts off the resistor after starting. Large DC motors mostly use “thyristor voltage regulation”, which accurately controls the output voltage by adjusting the conduction angle of the thyristor to achieve smooth starting. These schemes not only avoid the hazards of full-voltage starting, but also ensure the torque required for motor starting, making the application of DC motors in industrial production, transportation and other fields safer and more reliable.
To sum up, the reduced-voltage starting of DC motors is not an “unnecessary operation”, but a necessary protective measure based on its electrical principle. Its core is to balance the relationship between starting current and torque through voltage regulation, which not only protects the motor itself, but also ensures the stability of the power supply system. It is a key link for the safe operation of DC motors.
