Starting characteristics determine how the engine behaves at the moment of starting. Understanding them is important for the correct selection of protection and starting equipment. Let's look at the key concepts.
Starting current
At the moment of starting, the motor consumes 5–7 times the rated current. This happens because the stationary rotor does not yet create a back-EMF. As it accelerates, the current drops to the rated value.
A large starting current must be taken into account when choosing machines and cable cross-section, otherwise the protection will falsely trip at every start.
Starting torque
This is the torque that the engine develops when starting. It must exceed the moment of resistance of the load, otherwise the engine will not budge and will only hum.
How to reduce the starting current
| Method | Effect |
|---|---|
| Star-delta starting | Reduces the starting current by ~3 times |
| Soft start device | Smooth acceleration, minimum shocks |
| Frequency converter | Full start control |
Why is this important in practice
Frequent heavy starts overheat the winding and accelerate wear. If the engine operates with frequent starts (for example, a crane or pump), the use of a soft start or frequency generator will extend its life. If the engine has already suffered from heavy starts, rewinding will be required. The condition is assessed by diagnostics.
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