Three-Phase Motor Starting Auto-Type Transformers

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Three-phase motor starting auto-type transformers are special transformers used to reduce the high inrush current generated when electric motors start. The high current drawn by motors during initial operation can cause strain on fuses, thermal magnetic circuit breakers, contactors, and other switching equipment.

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Motor starting transformers limit the starting current by applying a lower voltage to the motor during startup. This helps the motor to start in a more controlled manner and contributes to reducing sudden current surges in the electrical installation.​

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Omsan three-phase motor starting auto-type transformers are designed for the safe and controlled operation of electric motors used in industrial facilities. Products can be manufactured specifically for each project based on motor power, operating voltage, starting time, and application requirements.

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What is a Motor Starting Transformer?

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A motor starting transformer is an automotive-type transformer used to reduce the high current drawn by an electric motor from the mains during startup. During startup, a voltage lower than the mains voltage, usually by a certain percentage, is applied to the motor terminals.

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The output voltage is typically around 70% of the mains voltage in standard applications. Different output voltage ratios can be considered depending on demand and application requirements.

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When the motor approaches its nominal speed, the starting transformer is disconnected, and the motor continues to operate at its normal operating voltage.

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Why Use Motor Starting Transformers?​

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Electric motors can draw currents exceeding their nominal current during startup. This can have negative effects on the power grid, switchboard equipment, and protection elements, especially in high-power motors.

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Motor starting transformers are used for the following purposes:

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• Reducing motor starting current

• Preventing unnecessary tripping of switchgear

• Reducing sudden current surges on the power grid

• Ensuring more controlled motor starting

• Reducing stress on the switchboard and installation equipment

• Providing a safer start in industrial systems

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Thanks to these features, motor-starting transformers offer an effective solution for industrial applications that use high-power three-phase motors.

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Operating Principle​

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Three-phase motor starting auto-type transformers are activated at the moment the motor starts. At this stage, a reduced voltage is applied to the motor, limiting the starting current.

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After the motor has completed its start, the transformer is deactivated. Since these products are not designed to remain continuously in operation, it is important to set the on-time correctly. In standard applications, the motor start-up time is considered to be an average of 6–8 seconds.

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The start-up time should be determined according to the motor's power, load type, mechanical inertia, and application conditions.

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Areas of Application

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Three-phase motor-starting autotransformers can be used in many industrial applications where high-power motors need to be started in a controlled manner.

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Main application areas:

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• Pump systems

• Fan and aspirator systems

• Compressors

• Conveyor systems

• Crushing and grinding machines

• Industrial production lines

• Water and wastewater treatment plants

• Ventilation systems

• Crane and lifting systems

• Heavy industry applications

• Machinery and control panel systems

 

​General Features

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Omsan three-phase motor starting auto-type transformers are designed for industrial operating conditions and can be manufactured according to application requirements.

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General technical specifications:

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• Nominal operating voltage: Up to 1100 V

• Output voltage: Approximately 70% as standard or different percentages according to demand

• Motor start-up time: Average 6–8 seconds

• Thermal protection: 120 °C, 1NK direct contac

• Operating cycle: Maximum 3 consecutive starting cycles

• Number of starting cycles per hour: Average 10–12 times

• The operating cycle frequency can be increased in response to demand.

• Standards: EN 61558-2-13, IEC 60076

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Duty Cycle and On-Time​

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Motor starting transformers are designed for short-duration starting duty rather than continuous operation. Therefore, the transformer’s duty cycle, on-time, and starting frequency must be properly specified.

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In standard applications, starting transformers can operate a maximum of 3 times consecutively and can be used for starting an average of 10-12 times per hour. In applications requiring more intensive operation, the duty cycle and thermal design can be specifically evaluated according to project requirements.

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Incorrect time setting or excessive starting frequency can cause the transformer to overheat and reduce its lifespan. Therefore, the motor power, load type, and operating scenario must be analyzed correctly.

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Information Needed for Correct Product Selection

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The following information is important for determining the appropriate motor starting transformer for your project:

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• Motor power

• Mains voltage

• Desired output voltage ratio

• Motor starting time

• Number of starts per hour

• Requirement for consecutive starts

• Load type

• Operating environment

• Requirement for thermal protection

• Panel or mounting dimensions requirements

• Special production requests

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Based on this information, a motor starting transformer suitable for your application can be designed.

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Omsan Three-Phase Motor Starting Transformer Solutions​

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Omsan offers reliable, project-specific three-phase motor-starting autotype transformer solutions for industrial motor applications. Products are designed considering the motor's starting characteristics and the plant's operating conditions.

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Omsan motor starting transformers;

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• Help reduce motor starting current.

• Contribute to the protection of switchgear.

• Provide controlled motor starting.

• Offer safe use in industrial facilities.

• Can be manufactured with project-specific voltage and duty cycle options.

• Offer solutions suitable for panel and machine applications.

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Technical Proposal and Order

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Three-phase motor-starting autotransformers are an effective solution for the safe, controlled commissioning of high-power electric motors. A correctly selected transformer limits the motor's starting current, helps protect installation equipment, and supports more stable system operation.

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To determine the appropriate motor starting transformer for your project or to receive a technical proposal, you can share information about motor power, operating voltage, starting time, and starting frequency with us.