Key to longevity and safety of induction motors

KEY TO LONGEVITY AND SAFETY OF INDUCTION MOTORS

Discover the principles of proper motor switch selection, minimizing the risk of failure and optimizing machine operation.

Key tips for selecting motor protection

Rated current (In) is fundamental: always start by reading the motor’s rated current from its nameplate, as this is the basic parameter determining the protection setting.

Safety margin for thermal setting: set the thermal trip unit of the motor switch in the range of 100-125% of the rated current to ensure overload protection while avoiding unnecessary tripping during startup.

Comprehensive protection: remember that a motor switch should provide both thermal protection against overload and magnetic protection against short circuits, adapted to the motor’s current-time characteristic.

Selecting the appropriate motor protection is extremely important to ensure its reliable operation, long lifespan, and protection against potential damage resulting from overloads or short circuits. Improperly selected protection can lead to unplanned downtime, damage to expensive equipment, and even fire hazards. This article provides a comprehensive guide to the process of selecting motor protection, considering key parameters and practical tips.

Fundamentals of motor protection selection

Introduction

The selection of motor protection is a process that requires careful consideration of the motor’s specific characteristics and its operating conditions. The main goal is to protect the motor windings from overheating caused by prolonged overload and to protect against the effects of short circuits. A properly selected motor switch or fuse should react quickly to dangerous conditions, while allowing for normal operation, including short duration starting currents, which can be significantly higher than the rated current.
By using precisely selected protection, such as that offered by Tripus, the risk of failure can be minimized, operational safety of machines can be increased, and their lifespan extended. It is always advisable to refer to the motor manufacturer’s technical documentation and protection manufacturers’ catalogs for the most accurate data.

Key motor parameters essential for protection selection

To select appropriate protection, it is necessary to thoroughly understand the motor parameters, which are usually found on its nameplate or in the technical documentation. The most important ones are listed below:

Rated current (In): the rated current (In) is the most important parameter. It specifies the current that the motor draws under rated conditions, i.e., at full power and normal operation. It is around this value that the thermal setting of the protection is calibrated.

Motor power (kW or HP): motor power, expressed in kilowatts (kW) or horsepower (HP), is closely related to the rated current and is used for general motor sizing. Although In is crucial, power is often used for initial estimation and comparison of different models.

Supply voltage (V): the voltage for which the motor was designed must match the supply network voltage. The motor switch must also be adapted to the given voltage.

The thermal tripping class: the thermal tripping class (e.g., 10, 10A, 20, 30) specifies the time after which the thermal protection will trip in case of an overload. Lower numbers mean faster tripping, which is important for motors with shorter permissible heating times. Class 10A is often used, providing a balance between protection and tolerance to starting currents.

Current-time characteristic: the protection should have a current-time characteristic matched to the motor’s thermal curve. This means that it should disconnect the motor before its windings reach a critical temperature, both in the case of prolonged overload and short-circuit.

Methodology for motor protection selection step by step

Correct selection of motor protection is a process that requires considering both the motor’s rated data and the switch characteristics. Below is a systematic approach to this task:

Step 1: Verification of motor rated data: always start by reading the rated current (In) from the motor’s nameplate. This is the base value for all further calculations and settings. The motor manufacturer provides detailed parameter tables that can be helpful, especially in the absence of a physical nameplate or for data verification.

Step 2: Selection of a motor switch with appropriate thermal setting range: the thermal trip current setting of the motor switch should be adapted to the motor’s rated current. Generally, it is accepted that the setting value should be in the range of 100% to 125% of the rated current (In). For example, for a motor with In = 10 A, the setting should be between 10 A and 12.5 A. Such a safety margin allows for safe motor starting and tolerance of short-duration, normal overloads without unnecessary tripping. Tripus motor protection switches are designed to offer precise setting ranges, which facilitates ideal matching to various motor models.

Step 3: Analysis of the current-time characteristic: it is necessary to ensure that the selected motor switch is capable of disconnecting the motor in a time shorter than the permissible winding heating time at starting current. This is crucial for protecting the motor from overheating during the starting phase, when the current can be several times higher than the rated current. The thermal tripping class of the switch (e.g., 10A) plays a decisive role here.

Step 4: Ensuring short-circuit protection: in addition to thermal protection, the motor switch must also provide effective short-circuit protection. This is achieved by magnetic protection, which quickly cuts off the power supply in the event of a short circuit, protecting the motor and the installation from serious damage.

Step 5: Considering environmental conditions and operating specifics: ambient temperature, humidity, frequency of switching on/off, and type of operation (continuous, intermittent, heavy starting) can influence the selection of protection. In the case of heavy starting or operation in variable conditions, it may be necessary to use more precise or specialized solutions, such as a switch with a higher thermal tripping class or with additional monitoring functions.

Auxiliary table for motor protection selection

The table below presents exemplary rated current values for selected electric motors and suggested protection settings. This table is indicative and serves as a tool to support the selection process. The final selection should always be confirmed with the technical documentation of the motor and the manufacturer of the switch, e.g., Tripus.

Rated output (kW)	Full-load amps at 400V *	Suggested protection setting (A)	Suggested Tripus’ switches
0.55	1,41	1.4 – 1,75	207P263.06 I 207P263.07 I 207P262.06 I 207P262.07
0.75	1,9	1,9 – 2.37	207P263.07 I 207P262.07
1.1	2,6	2,6 – 3,25	207P263.08 I 207P262.08
1.5	3,5	3.5 – 4,37	207P263.08 I 207P263.09 I 207P262.08 I 207P262.09
2.2	4,8	4,8 – 6,0	207P263.09 I 207P262.09
3.0	6,5	6.5 – 8,12	207P263.10 I 207P262.10
4.0	8.3	8.3 – 10,37	207P263.10 I 207P263.11 I 207P262.10 I 207P262.11 I 207P262.12
5.5	11.0	11.0 – 13,75	207P263.11 I 207P291.11 I 207P262.11 I 207P262.12
7.5	14,6	14,6 – 18,25	207P291.11 I 207P291.12 I 207P262.12 I 207P290.12 I 207P290.13
11	21	21 – 26,25	207P291.13 I 207P291.14 I 207P290.14 I 207P290.15
15	27,8	27,8 – 34,75	207P291.14 I 207P290.15

*Table developed based on data obtained from one of the motor manufacturers – always check the rated current value of the motor in your application!

Watch video

To better understand the practical aspects of motor switches, we recommend watching the video below. It presents the process of selecting an electromagnetic switch, which is a key element in many industrial and workshop machines. The film simply explains what to pay attention to during selection, which is invaluable knowledge for every engineer or technician.

Summary

The correct selection of motor protection value is crucial for its reliable operation, longevity, and the safety of the entire electrical installation. This process is based on analyzing the motor’s rated current, adjusting the motor switches’ thermal setting in the range of 100-125% of the rated current, and considering the current-time characteristic and operating conditions. Using reliable sources, such as motor nameplates and switch manufacturers’ catalogs, is essential for optimal protection. Through conscious selection of protection, the risk of failures and unplanned downtime can be effectively minimized, ensuring stable and safe machine operation.

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Key to longevity and safety of induction motors