There are two connection methods for three-phase asynchronous motors: star connection (hereinafter referred to as star connection) and delta connection (hereinafter referred to as delta connection). From a theoretical point of view, either connection method can be chosen arbitrarily, and the analysis and calculation results will not differ in the slightest.
There are two connection methods for three-phase asynchronous motors: star connection (hereinafter referred to as star connection) and delta connection (hereinafter referred to as delta connection). From a theoretical point of view, either connection method can be chosen arbitrarily, and the analysis and calculation results will not differ in the slightest.
In practical engineering applications, this seemingly simple choice between two options actually involves quite a bit of complexity. How to choose or determine the correct option often perplexes sales and marketing personnel. Today, Ms. Can will delve into the details of motor connection methods and related issues, revealing the intricacies that can be confusing and perplexing in resolving this seemingly simple problem.
In my country, general-purpose motors are classified according to power rating: motors with a power of 3kW and below are star-connected, while those above 3kW are delta-connected. YVF frequency converter series motors with a power of 45kW and below use star connection, while those above 45kW use delta connection. For YZ, YZR, and YZP series hoisting and metallurgical motors, star connection is used. All of these are the conventions for asynchronous motor stator connections, but customers and manufacturers can adjust them according to actual needs. The above regulations actually reflect that choosing between star and delta connection is quite important; different types and applications require different connection methods.
Due to grid capacity limitations, the starting current of general-purpose motors must be restricted. For motors with a power of 3kW and below, the base current is small, and the starting current is insufficient to impact the grid. Star connection helps to block the flow of unbalanced current and harmonic current, hence the requirement for star connection. For motors with a power of 4kW and above, the starting current begins to affect the grid. The larger the power, the greater the impact on the grid. It is necessary to adopt a reduced voltage starting method to reduce the starting current. Detachment motors can achieve star-delta conversion for reduced voltage starting. General-purpose motors with a 6-terminal output configuration can easily achieve this, which is simple and economical.
The starting process of a variable frequency drive (VFD) motor is essentially a soft start; the starting current will not exceed twice the rated current (due to VFD capacity limitations). Starting has no impact on the power grid; therefore, motors with power ratings of 45kW and below all use a star connection. For YZ, YZR, and YZP series hoisting and metallurgical motors, operational reliability is paramount, making a star connection the ideal choice. Permanent magnet motors must use a star connection to eliminate the current induced by harmonic magnetic fields in the magnetic field established by the permanent magnet. High-voltage motors mostly use a star connection, primarily for safety and reliability reasons.
Relationship between voltage and current parameters under star connection
Once the connection method selection issue is resolved, the phase voltage, line voltage, phase current, and line current under the corresponding connection method also have various confusing and intricate details.
Triangle connection
Star connection
Consequences of incorrect connection
For dual-voltage operating motors, the stator requires six terminals. A star connection is used for high voltage, and a delta connection is used for low voltage. Unless otherwise specified by the customer, it is recommended that motor manufacturers ship the motor with a star connection to prevent motor burnout due to improper customer voltage application.
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