## Wednesday, February 24, 2021

Star or delta coupling: asynchronous motor

Asynchronous motor coupling: star or delta

The coupling of an asynchronous motor can be delta, star, or impossible depending on the mains voltage and the nominal voltage of the motor.

Asynchronous motor coupling: star or delta

.

Nominal motor voltage and mains voltage

In delta coupling, each winding of the asynchronous motor directly sees the applied voltage. In star coupling, each winding sees a reduced voltage (divided by the root of 3).

Note: the neutral is never used.

Star or triangle coupling: the right choice
For star or delta connections of the asynchronous motor, the different cases are possible.

Equal voltages: star coupling
When the network and nominal voltage of the motor are equal, the star connection is chosen.

example: 230V / 400V motor on 230V / 400V network: star coupling

Here, the voltage is 400V between phases. However, each winding needs 230V to operate. We therefore choose the star coupling. If we used delta coupling, each winding would undergo 400V at its terminals and would be destroyed.

Smaller motor voltage = greater line voltage
When the "smallest" nominal voltage of the asynchronous motor is equal to the "largest" network voltage (therefore the voltage between phases), delta coupling is chosen.

example: 230V / 400V motor on 127V / 230V network: triangle coupling

Here, the voltage is 230V between phases. However, each winding needs 230V to operate. We therefore choose the triangle coupling.

Unable to pair
- higher motor voltage = lower line voltage

When the "largest" nominal voltage of the asynchronous motor is equal to the "smallest" line voltage (therefore the voltage between phases), no coupling is possible. We cannot use this asynchronous motor.

example: 230V / 400V motor on a 400V / 690V network: coupling impossible

- no common voltage

When no voltage is common, no coupling is possible.

example: 127V / 230V motor on a 400V / 690V network: coupling impossible (motor destroyed!).

In addition, a 400V / 690V asynchronous motor will never have enough voltage at its terminals with a 127V / 230V network, regardless of the coupling.

example: 400V / 690V motor on a 127V / 230V network: coupling impossible

Coupling strips: star or delta
Star or delta coupling of an asynchronous motor is made using bars on the terminal board:

.

Star-delta couplings for asynchronous motor: position of the bars

2 bars are required for star coupling. 3 bars are necessary for the triangle coupling.

Idea trick to retain the coupling bars:

Star coupling: horizontal bars like the branches of the capital E of "Star"

Triangle coupling: 3 bars as tri-angle.

Coupling of an asynchronous motor in practice
In practice, the star-delta coupling can look like this:

.

Delta coupling strips for asynchronous motor

.

Star coupling strips for asynchronous motor

Example of asynchronous motor
The asynchronous motor does not only exist in electrical engineering lessons! Here is a nameplate of an asynchronous motor of 740W power (= 1 horsepower):

.

Asynchronous motor 220V / 380V

.

Asynchronous motor nameplate

We recognize the old network voltages 220V and 380V (today: 230V and 400V), the cos φ, the couplings and the power. This motor can be connected (star coupling) to a current network, the voltage increase being only very slight.

Note:

"Y" = "star coupling"

"Δ" (uppercase delta) = "triangle coupling"

Star or delta coupling: asynchronous motor

Asynchronous motor coupling: star or delta

The coupling of an asynchronous motor can be delta, star, or impossible depending on the mains voltage and the nominal voltage of the motor.

Asynchronous motor coupling: star or delta

.

Nominal motor voltage and mains voltage

In delta coupling, each winding of the asynchronous motor directly sees the applied voltage. In star coupling, each winding sees a reduced voltage (divided by the root of 3).

Note: the neutral is never used.

Star or triangle coupling: the right choice
For star or delta connections of the asynchronous motor, the different cases are possible.

Equal voltages: star coupling
When the network and nominal voltage of the motor are equal, the star connection is chosen.

example: 230V / 400V motor on 230V / 400V network: star coupling

Here, the voltage is 400V between phases. However, each winding needs 230V to operate. We therefore choose the star coupling. If we used delta coupling, each winding would undergo 400V at its terminals and would be destroyed.

Smaller motor voltage = greater line voltage
When the "smallest" nominal voltage of the asynchronous motor is equal to the "largest" network voltage (therefore the voltage between phases), delta coupling is chosen.

example: 230V / 400V motor on 127V / 230V network: triangle coupling

Here, the voltage is 230V between phases. However, each winding needs 230V to operate. We therefore choose the triangle coupling.

Unable to pair
- higher motor voltage = lower line voltage

When the "largest" nominal voltage of the asynchronous motor is equal to the "smallest" line voltage (therefore the voltage between phases), no coupling is possible. We cannot use this asynchronous motor.

example: 230V / 400V motor on a 400V / 690V network: coupling impossible

- no common voltage

When no voltage is common, no coupling is possible.

example: 127V / 230V motor on a 400V / 690V network: coupling impossible (motor destroyed!).

In addition, a 400V / 690V asynchronous motor will never have enough voltage at its terminals with a 127V / 230V network, regardless of the coupling.

example: 400V / 690V motor on a 127V / 230V network: coupling impossible

Coupling strips: star or delta
Star or delta coupling of an asynchronous motor is made using bars on the terminal board:

.

Star-delta couplings for asynchronous motor: position of the bars

2 bars are required for star coupling. 3 bars are necessary for the triangle coupling.

Idea trick to retain the coupling bars:

Star coupling: horizontal bars like the branches of the capital E of "Star"

Triangle coupling: 3 bars as tri-angle.

Coupling of an asynchronous motor in practice
In practice, the star-delta coupling can look like this:

.

Delta coupling strips for asynchronous motor

.

Star coupling strips for asynchronous motor

Example of asynchronous motor
The asynchronous motor does not only exist in electrical engineering lessons! Here is a nameplate of an asynchronous motor of 740W power (= 1 horsepower):

.

Asynchronous motor 220V / 380V

.

Asynchronous motor nameplate

We recognize the old network voltages 220V and 380V (today: 230V and 400V), the cos φ, the couplings and the power. This motor can be connected (star coupling) to a current network, the voltage increase being only very slight.

Note:

"Y" = "star coupling"

"Δ" (uppercase delta) = "triangle coupling"