The harm of "starting the motor frequently" is far more than "consting electricity". It is actually a very destructive operation to the motor, which will seriously shorten the life of the motor in many aspects such as electrical, mechanical and thermal effects.

The following are the main dangers of starting the motor frequently, which are explained in detail from the inside to the outside:

1.  Huge starting current (electrical shock)

this is the most direct and deadly danger.

Normal operation: the motor runs at the rated speed, and the current is stable at the rated current.

Start-up moment: The motor rotor starts to rotate from rest. At this time, a huge anti-electric potential will be generated, resulting in a huge current in the stator winding, which is called Inrush Current.

Current multiple: The starting current is usually 5-8 times the rated current of the motor, or even higher. For example, a motor with a rated current of 10A may have a current as high as 50-80A at the moment of start-up.

Consequences

Overheating of the winding: The huge current will cause the motor winding to produce a large amount of joule heat in a very short time (the heat is directly proportional to the square of the current, Q = I²Rt). Although the time is short, the cumulative thermal effect will gradually destroy the insulation of the winding.

Insulation aging and breakdown: Winding insulation paint and insulation materials will accelerate aging, brittleness and cracking under repeated violent thermal expansion and cold shrinkage, which will eventually lead to insulation failure, causing inter-turn short circuit or ground short circuit, and motor burning.

Impact the power grid: The huge starting current will cause the voltage of the power grid to drop instantly, just like turning on multiple high-power electrical appliances at the same time, causing the lights to dim, which will affect the normal operation of other equipment on the same line.

2.  Violent mechanical impact (physical damage)

When the motor is started, the torque instantly rises from 0 to the maximum, and this huge torque will be transmitted to the load through the coupling or belt.

Accelerated wear: The transmission mechanism (such as gears, belts, bearings) will bear a strong impact load every time it is started, which accelerates its wear and fatigue.

Shaft stress: The motor shaft itself will also be subjected to huge torsional stress, and frequent stress cycles may lead to metal fatigue, and even shaft breakage accidents at weak points.

Concentricity destruction: Violent vibrations and shocks may destroy the installation concentricity between the motor and the load, resulting in the intensification of subsequent operation vibrations, forming a vicious circle.

3.  The cumulative effect of thermal stress

At the time of design, the insulation level (such as B and F) determines the continuous operating temperature it can withstand.

Starting temperature rise: Every time it starts, the temperature of the winding will rise sharply due to the huge starting current.

Insufficient cooling: In the interval of frequent start and stop, the motor may not have enough time to cool down to the ambient temperature. Heat will continue to accumulate, causing the motor to be at an excessive working temperature for a long time.

10-degree rule: Experience shows that every time the working temperature of the motor exceeds the rated temperature of 10°C, the life of its insulating material will be halved. Overheating caused by frequent start-up is the "number one killer" of motor life.

4.  Damage to start-up and protection equipment.

Frequent start-up will also harm the electrical equipment supporting the motor

contactor/circuit breaker: the control electrical appliance (contactor) responsible for connecting the circuit and the circuit breaker that provides protection will generate a strong arc when the huge starting current is broken. Frequent operation will burn the contact point, resulting in increased contact resistance, heat, and even adhesion failure.

Soft starter/frequency converter: Even if it is a device specially designed for smooth start, its internal power semiconductors (such as IGBT) will generate a large amount of heat when it is frequently turned on and off, which is a great test for the heat dissipation system and shortens its service life.

In summary and analogy

you can imagine it as driving a car with manual transmission:

normal driving (rated operation): 5 gears, smooth refueling, stable engine speed, economical fuel consumption, and very little wear.

Frequent start: It is equivalent to stopping the engine in the congested section - stepping on the accelerator clutch to start - brake and stop - turn off the engine again. Every time the engine starts, it roars to three or four thousand rpms, the clutch plate rubs violently, the fuel consumption soars, and the whole transmission system (engine, gearbox, clutch, half-axle) is under great impact. If you drive like this, the car will be scrapped soon.

How to avoid it? What is the solution?

If the application scenario really requires frequent changes in the state of the motor (such as automated production lines, fisting equipment), the following measures must be taken:

1.  Choose a suitable motor: choose a high-turn differential motor or a deep-slot rotor motor, which have a small starting current and a large starting torque.

2.  Adopt advanced start-up method

frequency converter (VFD): the best solution. It can realize the real "soft start" of the motor. By changing the frequency and voltage, the motor can accelerate slowly and steadily from 0 to the predetermined speed. The starting current can be controlled within 1.5 times the rated current, eliminating almost all electrical and mechanical shocks.

Soft starter: by controlling the conduction angle of the thyristor, the voltage is gradually increased, and the starting current and torque are effectively limited.

3.  Star-triangle (Y-Δ) start-up: The traditional method is to connect to a star shape to reduce the voltage and current when starting, and then switch to triangular operation after starting. It can effectively reduce the starting current, but there is still a switching impact.

4.  Ensure good maintenance: regularly check the lubrication, middle condition, insulation resistance, etc. of the motor bearing to prevent trouble before it happens.

Conclusion: The core reason why the frequent start-up of the motor is prohibited is that it will bring electrical impact, mechanical stress and thermal stress to the motor several times that of normal operation, thus sharply accelerating the aging and damage of the motor and its supporting equipment, and ultimately leading to unexpected downtime and higher maintenance costs.