Protecting a three-phase motor from burning out

Key points for selecting three-phase motor protection

Selecting correct motor protection is a process that requires consideration of several important factors. Neglecting any of them can lead to insufficient protection or unnecessary trips.

• Checking Motor Parameters: Always thoroughly check the motor’s nameplate before connecting. It contains crucial information such as the supply voltage (e.g., 400V), power, rated current, and connection diagram (star or delta).

• Motor nominal current (In): This is the fundamental parameter that can be found on the motor nameplate. Currently, the motor draws at full load and nominal voltage. The protection must be selected to withstand this current continuously.

• Motor starting current (In): Induction motors draw a much larger current than their rated current at start. Inrush current can be 4 to 8 times greater than I nominal, but usually lasts for a short time. Protection must be able to withstand this instantaneous overload without tripping.

• Load characteristics: The type of load the motor is driving is important. Constant torque loads (e.g., pumps, fans) require a different approach than variable torque loads (e.g., conveyors).

• Work conditions: Ambient temperature, humidity, and dust – all of these can affect the engine’s operation and protection. In difficult conditions, it is worth considering protection with a higher IP.

• Protection types: There are various types of motor protection available on the market, and the choice depends on the specific application and required level of protection:
  – Overcurrent protection (breakers): Simple and relatively cheap, but offer limited protection. Selected for a current slightly higher than the rated motor current, considering the starting current.
  – Motor protection switches: Designed specifically for electric motor protection. They have an adjustable tripping current, often with temperature compensation and phase cut protection. They are more precise and offer better protection than fuses.
  – Overload relays: Protect the motor from long-term overload. They work together with contractors. They have an adjustable trip current and often a reset function.
  – Electronic motor protection relays: Most advanced protection. Monitor many parameters, such as current, voltage, temperature, phase asymmetry, overload, short circuit, operation at too low voltage, and others. They allow for precise adjustment of protection parameters and often have diagnostic functions.

How effectively protect the engine from burning out?

Apart from the correct selection of overcurrent protection, there are several other key aspects to protecting the motor from burning out:

• Thermal protection:
  – Built-in temperature sensors (Thermistors, Thermals): Many modern motors have built-in temperature sensors in the windings. The signal from sensors can be used to shut down the motor in the event of overheating.
  – Thermal relays: They monitor the motor temperature and respond to its increase.

• Phase loss and voltage unbalance protection: Phase loss or significant voltage imbalance in the supply can lead to overload and motor burnout. Electronic motor protection relays often offer such protection.

• Short circuit protection: Fast power shutdown in the event of a short circuit is essential. Motor circuit breakers and electronic relays offer effective short-circuit protection.

• Regular inspection and maintenance: Regularly checking the condition of the motor, bearings, ventilation, and cleanliness helps to detect potential problems before they lead to failure.

• Proper ventilation: Ensuring adequate airflow around the motor is essential for cooling it and preventing overheating.

• Avoid overloading: Avoid operating motors with a load more than the rated parameters.

Summary

Proper protection of electric motors is not an expense, but a saving. Correct selection of protection, considering the specifics of the application and operating conditions, combined with the implementation of additional protection measures, significantly reduces the risk of failures and downtime. Remember that prevention is always better than repair. In case of any doubts regarding the selection of protection, it is always worth consulting experienced specialists in electrical engineering.

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