Introduction to the internal structure of the Delta winding motor

There are two winding designs inside three-phase industrial motors: Wye and Delta. Although the motors and connections look similar on the outside, this article discusses the internal structure of a Delta-wound motor.

Industrial motors are typically powered by a three-phase voltage source. The three-phase line input is connected to windings inside the motor to create the magnetic field that drives the rotor. There are two winding designs inside these motors: Wye and Delta. Although the motors and connections look similar on the outside, the internal construction creates some unique differences that are worth noting.

Differentiate between Wye and Delta internal wiring systems

All three-phase motors found in industrial applications can be simplified to either the Wye or Delta internal wiring system. These terms refer to the schematic diagram of the shape of the internal coils. In reality, both wiring schemes have six separate coils arranged around the perimeter of the motor - the electrician chooses how the current is transferred between the coils.

There is no strict statement on when one wiring style is preferred over another. You will often see the Wye arrangement used for lower horsepower motors as the winding resistance is slightly higher due to the internal series configuration. Therefore, a Wye motor will have higher resistance and less energy conversion than a Delta motor. But there are many construction factors that can create a useful motor of either type for a wide range of horsepower outputs.

In both wiring schemes, the motor can be configured for low voltage input between 208-240 volts, or high voltage input which is usually around 440-480 volts. The exact voltage depends on tolerances, geographic standards, and supply transformer configuration. The exact voltage can change the horsepower output slightly, but most motors only have these two wiring options - low voltage or high voltage.

The Wye configuration is easier to think about in a mathematical sense because the arrangement of the coils is very simple. On the other hand, the triangular arrangement is a little harder to visualize when only a diagram of the terminal connections is provided.

Delta Internal Connections

The use of the word Delta refers to the Greek letter that resembles a triangle. The schematic diagram of a Delta motor is this triangle pattern, except that it is broken on each side to allow for the connection of either high or low voltage.

Figure 2. Schematic diagram of low, high, and delta motor windings.

Terminal connections and internal arrangement of a typical nine-lead delta winding motor.

As with any three-phase motor, the six windings (shown as straight lines in the diagram) are distributed around the perimeter of the motor to produce balanced rotation.

A previous article on Y-wound motors described the need to use a lower voltage supply to reduce resistance. This also applies to the Delta arrangement. In short, if the voltage is reduced from 480 volts to 240 volts (1/2 the voltage), the current must be doubled to produce the same horsepower. However, if the voltage has been halved, the resistance must be reduced to 1/4 of what it was before to produce twice the current. This must be true in the case of both motors - the resistance of the high voltage wire should be 4 times that of the low voltage wire.

High voltage wiring

This may be a bit backwards, but we will cover the high voltage wiring scheme first. The following diagram shows the lead connections required for the high voltage.

Figure 3. Required lead connections for delta motor windings.

When a high voltage connection is made, this diagram shows the resulting connection.

By the way, the high voltage terminal connections in the Wye and Delta motor configurations are exactly the same!

In a Wye motor, high voltage results in a large Wye connected with the series coils. In contrast, inside a Delta motor, high voltage results in a large Delta shape. However, the coils are no longer simply connected in series. The result is a series-parallel combination circuit.

Imagine a current going from line 1 to line 2. There are two different paths for the current to flow.

First, the current can go directly through T1-T4-T7-T2. These are two windings in series.

Secondly, it may go through the path T1-T9-T6-T3-T8-T5-T2. It consists of four windings connected in series.

If there are two parallel paths, one with two windings of resistance and another with four windings, the result is a parallel combination of 2R || 4R yields 4/3R, or 1.33 x the value of one winding.

Compare this value to that in the high voltage version of the Wye motor. In the Wye motor, it is four times the resistance of one winding alone. As you can imagine, this results in more current (and therefore power output) in the Delta motor if all other parameters are equal.

Low voltage wiring

Due to the mathematical complexity required to analyze the winding resistance, low voltage is saved until last.

Figure 4. Lead connections for low voltage delta wiring.

When making a low voltage connection, this diagram shows the resulting connection.

The diagram above has been rearranged to simplify the situation as much as possible. Each line input is connected to a small delta inside the motor. This setup only requires three connections, so if you open a motor case and find only three wire nuts, you have identified a low voltage delta motor.

In this scenario, imagine going from row 1 to row 2. In this case, there are three different possible current paths, so we can list each option.

T1-T4 pass through one winding.

T7-T2 through a winding

T6-T3 is connected in parallel with T1-T9, and then in parallel with T5-T2 through T3-T8. This creates a series of two sets of parallel windings - the overall result is one winding equivalent.

When these three paths are placed parallel to each other, the final resistance is equal to 1/3R or 0.33 x the value of one winding. As expected, this is exactly 1/4 of the value of the resistor in the high voltage configuration.

Summary

For most projects, it doesn't matter what the internal wiring setup might be. If you have a 5 HP motor for high voltage, that motor will work for any 480 volt application that requires 5 HP. You don't need to worry about how the manufacturer glued the wires inside. It's a good practice for any electrician to understand why something works, not just how to make it work - understanding not only why it works correctly, but why it fails, is a huge benefit.