Why are H.T. motors utilized in star connection and L.T. motors utilized in delta connection?

Within industrial settings or electrical systems, what specific electrical, mechanical, or operational considerations drive the selection for high-tension (H.T.) motors in the star connection & low-tension (L.T.) motors in the delta connection? In their individual applications, how do these connections optimize efficiency, motor performance, or operational aspects?

Since there are many different reasons for this, high-tension (H.T.) motors are commonly connected in a star configuration. During the initial starting phase, this configuration allows for a reduced starting current, which in turn minimizes the amount of stress placed on the motor windings. Furthermore, the star connection offers better insulation. This is because the phase voltage along each winding is lower than in other connections.

On the other end, low-tension (L.T.) motors typically connected in a delta configuration since this arrangement provides a larger starting torque, that is beneficial for a wide range of low-voltage applications. A larger voltage is applied across each winding as a result of the delta connection, which leads to an increase in the amount of torque that is produced during startup.

The voltage & current requirements of the motors are taken into consideration when selecting these configurations. This allows for the motors’ performance characteristics to be optimized for a variety of applications and their operational requirements.

Why are H.T. Motors Utilized in Star Connection and L.T. Motors Utilized in Delta Connection?

  • The selection of connecting motors in either star or delta structure is a significant decision in manufacturing environments, affecting both technical efficiency and cost concerns.
  • Low tension (L.T.) motors are usually built in a delta connection since they normally operate at lower voltages, such as three-phase 415 V.
  • High Tension (H.T.) motors, on the other hand, are frequently coupled in a star configuration and are suitable for higher voltages such as 6.6 kV. This decision was made for technical reasons that ensured both profitability and optimum motor performance.

Reasons for High Tension Motors in Star Connection:

Insulation Requirement:

  • When H.T. motor stators are connected in a star configuration, the phase voltage is reduced to Vline/Square root of 3, resulting in lower insulation requirements. This is critical for efficiently managing high voltages.

Starting Current and Power:

  • Motor starting currents are significantly larger, often 6 to 7 times the full load current.
  • The phase current remains the same as the line current in a star connection, lowering starting power and reducing the possibility of voltage dips in the power system.

Copper Utilization:

  • Because the star arrangement causes lower current in the windings, the amount of copper (Cu) required for the winding is reduced.
  • This not only makes the motor more efficient, but it also makes it more inexpensive.

Reduced current in the star arrangement adds to decreased I2R losses, resulting in more energy-efficient motor operation.

Reasons for Low Voltage Motors in Delta Connection:

Insulation and Voltage Level:

  • Lower voltage L.T. motors with delta connection experience fewer insulation problems. Because of the lower voltage levels, it is possible to properly regulate insulating requirements.

Starting Current and Power:

  • Starting power is fundamentally lower in L.T. motors, reducing the likelihood of voltage dips during motor startup.
  • In systems with limited power capacity, delta-connected L.T. motors provide reliable performance.

Starting Torque:

  • Connecting the stator in a delta configuration ensures a larger current, resulting in increased starting torque. This is especially important in small-capacity motors.

In order to improve the effectiveness of maintenance, the delta connection makes it possible to easily identify and rectify errors in particular phases.


  • For H.T. and L.T. motors, the strategic decision between star and delta connections is based on technological considerations that enhance motor performance and efficiency.
  • Understanding these factors is critical for businesses looking to improve the dependability, efficiency, and profitability of their electrical systems.
  • This knowledge becomes increasingly valuable as industries grow for engineers and decision-makers managing the complicated environment of motor configurations.