Standard and Specification of Electric Motor starting times

The standards for the number of motor starting times mainly depend on the type, design, and operating conditions of the electric motor. The common international and industry standards are as follows:

1.Main Standard References
* IEC 60034-1 (International Electrotechnical Commission Standard)
This standard specifies the thermal protection and operating characteristics of rotating electrical machines and clearly states:
Direct starting times: Generally, it is allowed to start up to 2 times per hour in a cold state (at normal temperature) and 1 time per hour in a hot state (at high temperature) to avoid overheating of the windings.
If the starting is frequent (for example, more than 3 times per hour), a special design (such as enhanced insulation, optimized heat dissipation structure, etc.) is required.
* NEMA MG-1 (National Electrical Manufacturers Association Standard in the United States)
It is stipulated that the number of starting times per hour for squirrel-cage motors should not exceed 3 times in a cold state and not exceed 2 times in a hot state, which specifically depends on the electric motor design (such as insulation class, load inertia).
* GB/T 755 (National Standard of China)
It is basically consistent with IEC 60034-1, requiring that the number of starting times should ensure that the temperature rise of the motor does not exceed the limit value of the insulation class (for example, the temperature rise of class B insulation ≤ 80K).

2.Key Factors Affecting the Number of Starting Times
Load type: High-inertia loads (such as fans, pumps) will prolong the acceleration time and increase heat generation, so the number of starting times needs to be reduced.
Insulation class: Higher-class insulation (such as class F, class H) allows for a higher temperature rise, and the number of starting times can be appropriately increased.
Cooling method: Motors with forced cooling (such as IC 416) are more suitable for frequent starting compared to those with natural cooling (IC 411).
Starting method: Direct starting, star-delta starting, or variable-frequency soft starting have different impacts on the motor, and the latter can relax the restrictions on the number of starting times.

3.Practical Application Suggestions
Conventional industrial electric motors: According to the standard, the number of cold-state starting times per hour should not exceed 2 to 3 times. After continuous operation, the motor needs to be cooled to the ambient temperature before restarting.
In occasions with frequent starting (such as cranes, elevators): Motors with S4/S5 duty systems (specially designed for frequent starting and stopping) should be selected, allowing dozens of times or even a higher frequency of starting per hour.
Variable-frequency drive control: By using soft starting to reduce the current impact, the allowable number of starting times can be significantly increased.

4.Precautions
Priority of the technical manual: There may be differences in the specific parameters of motors of different brands, and it is necessary to refer to the manufacturer's manual.
Monitor the temperature rise: If the frequent starting causes the shell temperature to exceed 90°C (for class B insulation), the motor needs to be shut down for cooling or replaced.
Customization for special needs: In scenarios such as rolling mills, stamping equipment, etc., it is necessary to negotiate with the motor manufacturer to customize models with a high number of starting times.

If you need the standard for the number of starting times of a specific model, it is recommended to provide the parameters on the motor nameplate or the application scenario for further analysis.