In the world of electrical machines, motors are classified based on their winding configurations, with delta and star being the two primary connection types.
This article will explore the key characteristics of delta and star motors, providing insights into their unique properties and how to identify them in practical applications.
Visual Inspection of Terminals
The connection type of a three-phase motor, either star or delta, can often be determined through a visual inspection of the terminal box. The arrangement of the wires connecting the motor windings provides clear indications of the internal configuration.
Star Connection
In a star-connected motor, also known as a wye connection, one end of each of the three windings is joined together at a common point called the neutral or star point. This connection forms a “Y” shape when viewed schematically. The other end of each winding is connected to one of the three power supply terminals, typically labeled U, V, and W.
A star-connected motor results in a four-wire system, consisting of three phase wires and one neutral wire. However, in some cases, the neutral point may not be brought out of the motor, resulting in only three terminals being visible.
When examining the terminal box of a star-connected motor, you will typically see four wires (three phases and one neutral) connected to four terminals. If the neutral point is not used, there will be three wires connected to three terminals, with the common point of the windings hidden inside the motor.
Delta Connection
In a delta-connected motor, the windings are arranged in a closed loop, forming a triangular shape that resembles the Greek letter delta (Δ). Each phase winding is connected to two adjacent phase windings, with no neutral point.
A delta-connected motor operates on a three-wire system, as there is no neutral connection. The three power supply terminals, typically labeled U, V, and W, are directly connected to the three junctions of the delta-connected windings.
When inspecting the terminal box of a delta-connected motor, you will observe three wires connected to three terminals, forming a triangular pattern. There will be no fourth wire or terminal for a neutral connection.
Use of Multimeter for Testing
If visual inspection is inconclusive, a multimeter can be used to definitively identify the motor’s winding configuration. Set the multimeter to measure resistance (ohms). With the motor disconnected from power, measure the resistance between each pair of terminals:
- For a delta-connected motor, the resistance between any two terminals should be the same, as each represents one winding.
- In a star (wye) configuration, the resistance between two terminals will be twice that of a single winding, as it includes two windings in series. The resistance between each terminal and the neutral point would equal the resistance of one winding.
If a neutral wire is present, measuring from each power wire to neutral should yield identical resistances for a balanced star connection. An open circuit indicates a delta configuration.
Nameplate Information
The motor nameplate often specifies the connection type. Look for “Delta,” “Δ,” “Wye,” “Star,” or “Y” on the nameplate. If both configurations are listed, the motor is dual-voltage capable and can be wired for either configuration, with the supply voltage determining the appropriate connection.
The nameplate also indicates the motor’s voltage and current ratings for each configuration. For a dual-voltage motor, the delta configuration rating is typically the lower voltage and higher current, while the star connection is used for the higher voltage and lower current operation.
FAQs
What happens if you connect a delta motor in Star?
Connecting a delta motor in star configuration reduces the voltage applied to each winding by a factor of √3 (approximately 1.732). This results in lower torque and speed compared to the delta configuration. Star connection is often used for starting the motor to limit the starting current and torque.
Can any motor be wired in star-delta?
Not all motors can be wired in star-delta configuration. The motor must be designed with six terminals (three pairs) to allow for both star and delta connections. Most commonly, three-phase induction motors with six terminals are suitable for star-delta starting. Single-phase motors and motors with only three terminals cannot be wired in star-delta.
