Everything manufacturing companies need to know about the star-delta starter – from correct configuration to troubleshooting.
When is a motor switched to star and when to delta in the star-delta starting process?
Ein star delta motor always starts first in star connection, to reduce the starting current. After reaching about 80-90% of the rated speed it is switched to the delta connection for normal operation.
What is the main advantage of the star-delta starting circuit?
The main advantage is the reduction of the starting current to about one third (approximately 33%) of the value during a direct start. This protects the star delta motor, the electrical network, and the mechanical components of the system.
What are the disadvantages of the star-delta starting circuit?
The main disadvantages are a reduced starting torque (only about one third of the direct start torque) and the risk of switching current spikes during the transition from star to delta if the circuit is not optimally designed.
How does the switching from star to delta work in a star delta motor?
After the motor has accelerated in star connection, the star relay opens. After a star delta motor in Sternschaltung hochgelaufen ist, öffnet das Sternschütz. Nach einer short, currentless pause (typically 50ms) , the delta relay closes, allowing the motor to continue running in delta connection for continuous operation.
Which motors are suitable for star-delta starting?
Suitable are three-phase motors designed for the network voltage for delta connection (e.g. a motor rated at 400VΔ/690VY for operation on a 400V network) and whose six winding ends are externally accessible. The driven machine must also be able to start safely with the reduced torque.
What happens in case of incorrect wiring of a star-delta circuit?
Incorrect wiring, especially ignoring the so-called “preference circuit”, can lead to significantly higher switching current peaks. These can trigger protective devices or damage components of the star delta motor and the circuit.
Are there modern alternatives to the star-delta circuit for a star delta motor?
Yes, modern alternatives are soft starters und frequency converters (VFD). These often offer a gentler start, better control options, and can completely avoid the typical switching current peaks. star delta motor How are the relays in a star-delta circuit for a star delta motor dimensioned?
Wie werden die Schütze in einer Stern-Dreieck-Schaltung für einen stern dreieck motor dimensioniert?
The network and delta relays as well as the overload relay are typically rated for about 58% of the motor rated current. des star delta motor The star relay is dimensioned to about 33% of the motor rated current. The star-delta starting circuit
reduces the starting current reduziert den Anlaufstrom to star delta motor to about 30-35% of the direct start value, thus protecting the network and the mechanics, but is primarily suitable for applications with low breaking torque suitable.
Critical to the star delta motor are possible switching current peaks;a correct “preference circuit” and a switching pause of at least 50ms are crucial to minimizing these and ensuring safe operation.
Modern alternatives such as soft starters or frequency converters often offer superior starting characteristics and control options for motors. Frequency converters can, for example, achieve energy savings of up to 70% in applications such as pumps and fans.Learn how to reduce the starting current of your motors with the star-delta starting circuit, what critical points to consider during installation, and what modern alternatives exist.
The star-delta starting circuit is a proven method to limit the starting current of three-phase motors. But when is it the right choice and what pitfalls lurk in detail? ATEK Drive Solutions shows you what matters. Do you need a customized drive solution? [contact](href=”/contact”) us!
Do you need assistance in optimizing your drive technology?
Get a non-binding consultation now!
Introduction to star-delta motor starters.
What is a star-delta start?
The star-delta starting circuit is an established method to reduce high starting currents of powerful three-phase motors. A star delta motor initially starts in star connection with reduced voltage before it is switched to delta connection for normal operation. A 30 kW motor configured as star delta motor lowers its starting current from about 300A to around 100A.
Why is the star-delta starting circuit used?
The main purpose of the star-delta starting circuit for a star delta motor is to reduce the starting current to about one third of the value that would occur during a direct start. This current reduction protects the star delta motor network, the electrical network, and the mechanical components of the system. Energy supply companies (e.g. EnBW) often require compliance with specific limits for starting currents.
Areas of application for the star-delta starting circuit
The star-delta starting circuit is well suited for machines powered by a star delta motor that do not immediately require the full starting torque. Typical areas of application for a star delta motor are drives with low breaking torque or those that can start without load, such as fans, pumps, or certain machine tools. An example of this would be a 15 kW fan that star delta motor starts gently. Our drive technology offers suitable solutions.Functionality and structure of the star-delta circuit for the star delta motor
The star (Y) and delta (Δ) configuration
The voltage reduction during the start of a star delta motor is achieved by a different wiring of the motor windings. In the star connection (Y), each winding of the star delta motor receives the so-called line voltage, which is the network voltage divided by √3. For a 400V network, this means that each winding of the motor receives about 230V, reducing the current flow. More information on the basic structure can be found under three-phase motor..
- In star connection (Y), each motor winding of a star delta motor receives a reduced voltage (line voltage = network voltage/√3).
- This causes the starting current and starting torque of the star delta motor to drop to about 1/3 of the values that would occur during a direct start.
- Ein star delta motor must be designed for the network voltage in delta connection and the √3-fold voltage in star connection (e.g. a motor rated at 400VΔ/690VY for operation on a 400V network).
- The circuit for a star delta motor typically requires three relays: one network relay (K1), one star relay (K3), and one delta relay (K2).
- A timer relay controls the switching sequence from the star starting phase (K1+K3 active) to the delta operation (K1+K2 active) of the star delta motor.
- The switching usually occurs when the star delta motor has reached 80-90% of its rated speed, after a short, currentless pause to avoid short circuits.
Voltage and current ratios in star and delta connection
The lower current flow during the star delta motor occurs only during the star starting phase. The current and torque of the motor during this phase are then approximately one third of the values that would occur during a direct start (DOL – Direct On Line). Ein star delta motor with the specification 400VΔ/690VY receives the correctly reduced voltage at its windings in star starting on a 400V network. The ability to calculate the current consumption of a three-phase motor, is very relevant in this context.
Structure of a typical star-delta circuit
For the construction of a typical star-delta circuit for a star delta motor , three relays (network relay K1, star relay K3, delta relay K2) and a timer relay are required. For the starting of the star delta motor, the network relay K1 and the star relay K3 are activated; after reaching a predefined speed, K3 opens, and after a short pause, the delta relay K2 closes for continuous operation of the motor. A motor protection switch and an overload relay (often set to about 0.58 times the motor rated current) complement the protection devices for the star delta motor. Components for this are offered, for example, by Siemens SIRIUS.
Function process of the star-delta starting circuit
Der star delta motor initially runs in star connection, during which it develops only about one third of its rated torque. Once the motor reaches about 80-90% of its rated speed, the timer relay controls the opening of the star relay K3 and after a short switching pause (typically 50ms), the closing of the delta relay K2. This pause is crucial to prevent a short circuit between the star and delta configuration. The star-delta switch, often realized by the relay combination, is the central element of this starting method for the star delta motor.Pros and cons of the star-delta starting circuit for the star delta motor
reduction of the starting current
A significant advantage of using a star delta motor with this starting circuit is the considerable starting relief. The starting current of the motor drops to about 30-35% of the value that would occur during a direct start (DOL), which reduces the mechanical and electrical load on the motor and the system. Ein star delta motor with, for example, a 50A rated current and a typical starting current factor of 6 (in direct start) would thus start with only about 100A instead of 300A.
Lower load on the network
The reduced current consumption of a star delta motor during starting is particularly advantageous in networks with limited capacity. Minimized voltage sag protects other consumers in the same network and often allows for the connection of larger star-delta motors than would be possible with direct start. Energy suppliers often set limits for starting currents (e.g. a maximum of 150A), which can be met more easily with the use of the star-delta circuit. star delta motor A potential disadvantage of operating a
switching current spikes
Ein potenzieller Nachteil beim Betrieb eines star delta motor are possible switching current spikes that may occur during the transition from star to delta connection, especially with an unfavorable phase position between rotor voltage and network voltage. These peaks can even exceed the starting current of a direct start under certain circumstances. The correct implementation of a so-called “preference circuit” and a sufficient switching pause (at least 50ms) are crucial to minimize this risk for the star delta motor Measurements have shown that without optimization, switching currents can reach up to 12 times the motor’s nominal current.
Reduced starting torque
The reduction of starting current during the star delta motor is inevitably accompanied by a decrease in starting torque. The starting torque in star connection is only about one third of the torque that the motor would develop in a direct start in delta connection. Therefore, this starting method is only suitable for applications with a star delta motor that can safely start up with this reduced torque (e.g., not suitable for a 75kW crusher that requires high breakaway torque). An appropriate gearbox for the three-phase motor can help in some cases to adjust the torque requirements.Practical aspects and troubleshooting for the star-delta motor
Correct motor connection assignment
Errors in the connection assignment at the terminal block of a star delta motor are particularly critical and can lead to significant issues. To minimize the switching current peak in the star-delta motor, adherence to the so-called “preference circuit” (Phase L1 to terminals U1 and V2, L2 to V1 and W2, L3 to W1 and U2 for forward operation) is crucial. An incorrect assignment can increase the switching current factor ℘ (ratio of switching current peak to starting current in direct start) from optimal values of 0.8 to over 1.4. The fundamental question to operate the motor in star or delta depends on the design and the application case.
- The correct “preference circuit” at the terminal block of the star delta motor is crucial to minimizing switching current peaks.
- For the star connection of a star delta motor , the terminals U2, V2, W2 are bridged; for the delta connection, the connections U1-W2, V1-U2, W1-V2 are made.
- It must be ensured that the star delta motor is correctly sized for the voltage supplied (e.g., a motor with the specification 400VΔ/690VY for operation on a 400V network).
- Common sources of error in the installation and operation of a star delta motor include wiring errors, defective contactors, or incorrectly set time relays.
- An early switch from star to delta leads to high current peaks during the star delta motor, while a late switch unnecessarily burdens the star winding for too long.
- A mechanical interlock between the star and delta contactor is essential for the safety of the star delta motor and the system.
Star and delta bridge configuration at the terminal block
At the terminal block of a star delta motor the configuration is as follows (according to standards such as IEC 60034-8): For the star connection, the winding ends U2, V2, and W2 are bridged. For the delta connection, terminals U1 are connected to W2, V1 to U2, and W1 to V2. A common and fatal error is to operate a motor designed for 400V in star connection and 230V in delta connection (thus a 230VΔ/400VY motor) on the 400V network in delta – this leads to the destruction of the motor. For the star-delta start of a motor on a 400V network, the star delta motor should, for example, have the nameplate specification 400VΔ/690VY.
Common causes of errors
Typical sources of errors during the operation of a star delta motor include: Errors in wiring, defective contactors (e.g., stuck or burned contacts), or a incorrectly set or defective time relay. A too short switching time (switching before the star delta motor has reached about 80% of its rated speed) leads to high current peaks and mechanical stress; a too long switching time unnecessarily burdens the windings of the star-connected motor for too long. A mechanical interlock of the contactors is essential for the safe operation of the star delta motor .Alternatives to star-delta starting circuits for motors and modern developments
soft starters
A modern electronic alternative to the classic star-delta circuit, also for motors that would otherwise be driven as star delta motor , is the soft starter. These devices continuously control the motor voltage during the startup, usually using phase angle control, thereby enabling a very smooth start without the typical current peaks of a star-delta motor. Products like the Siemens SIRIUS 3RW not only reduce the starting current but often also offer integrated motor protection functions. Many soft starters also allow for torque control during startup.
frequency converters (VFD)
Variable frequency drives (VFD) represent the most advanced solution for the starting and speed control of motors, also as an alternative to the star delta motor. Ein Variable frequency drive enables complete motor control, allowing for high starting torque with low starting current and precise speed control across the entire operating range. This often leads to significant energy savings (especially for applications like pumps and fans, up to 70%) and optimizes industrial processes. ATEK Drive Solutions offers suitable components for this.
Comparison of star-delta, soft starter, and variable frequency drive
The choice of the optimal starting method largely depends on the specific requirements of the application:
- Star-delta circuit for the star-delta motor: This is often the most cost-effective solution, but it only offers limited starting torque and presents the issue of current peaks.
- Soft starters: Represent a good compromise. They allow for a smooth start for the motor, often offer integrated protection functions, but are more expensive than the pure star-delta solution.
- Variable frequency drives: Provide maximum control over motor starting and operation, highest efficiency, and the ability to control speed, but are the most expensive option.
For example, a simple 10kW pump may be sufficient, while a complex conveying system with variable speed requirements may call for a variable frequency drive to be the better choice. star delta motor Design and sizing of a star-delta starting circuit for a star-delta motor
Auslegung und Dimensionierung einer Stern-Dreieck-Anlaufschaltung für einen stern dreieck motor
Considering motor power and starting torque
In designing a star-delta starting circuit for a star delta motor , the following requirements should be considered: The star-delta motor must be designed for the supply voltage in delta connection (e.g., a motor with the specification 400VΔ/690VY for a 400V network) and must have six externally accessible winding ends. In addition, the starting torque of the driven machine must be less than the starting torque that the star delta motor can develop in star connection (about 1/3 of the rated torque at direct start in delta). The correct determination of the power of the electric motor is an important first step.
- Der star delta motor must be rated for the network voltage in delta connection and have six accessible winding ends for external connection.
- The required starting torque of the driven machine must be less than the starting torque that the star delta motor provides in star connection.
- The network contactor and the delta contactor as well as the overload relay are typically rated at about 58% of the motor’s nominal current of the star delta motor The star-delta starting circuit
- The star contactor is rated at about 33% of the motor’s nominal current of the star delta motor .
- Switching from star to delta should occur when the star delta motor has reached about 80-90% of its rated speed.
- The local network conditions and the specific load characteristics must be taken into account when setting the time relay for the star delta motor .
Sizing of the contactors and overload relay
The correct sizing of the switching devices is crucial for the reliable operation of the star delta motor. As a rule of thumb: The network contactor, the delta contactor, and the overload relay are sized at about 58% of the motor’s nominal current. The star contactor is rated at about 33% of the motor’s nominal current. Example: A 30kW star delta motor with a nominal current of about 58A at 400V thus requires network and delta contactors for about 34A and a star contactor for about 19A.
Optimal switching time from star to delta
The timing of the switch from star to delta is critical for the star delta motor to minimize switching current peaks and operate the motor optimally. The switching should ideally occur when the motor in star connection reaches about 80-90% of its rated speed. An early switch can cause high current peaks, while a late switch unnecessarily thermally stresses the star winding of the motor for too long. The typical switching time is, depending on the load and motor size of the star delta motor, between 5 and 15 seconds.
Considering network conditions and load characteristics
The electrical environment of the star delta motor plays an important role. A so-called “soft” network (network with high impedance) can dampen the starting current of the motor, but at the same time intensify voltage drops during start-up. The load characteristics of the driven machine significantly influence the starting time of the star delta motor and thus the necessary adjustment of the time relay. Starting a 22kW star delta motor on a transformer with only 50kVA power can be borderline and must be checked carefully.
The star-delta circuit is a proven method to reduce starting currents in a star delta motor. A correct design and awareness of potential problems, such as switching current peaks, are crucial for successful application. For particularly demanding applications or when speed control is required, modern alternatives such as soft starters or variable frequency drives often represent the better solution, even if the motor could fundamentally be driven as star delta motor . A careful selection of the starting method protects your systems and optimizes your processes. Contact ATEK Drive Solutions for expert advice!
