Are you curious about how a synchronous motor starts? If you’re like most people, you don’t think about the inner workings of machinery too much unless something goes wrong. But understanding the basics of how this type of motor works can help you make informed decisions when you’re selecting machinery for a project or trying to troubleshoot an issue later on.
So, how does a synchronous motor start? To put it simply, this type of motor requires a magnetic field to be created in order to start rotating. Once the magnetic field is established, the motor stays in sync with the AC power source that’s providing the electrical current. This means that the motor will continue to rotate at the same speed even if there is a change in the load it’s moving. This makes synchronous motors ideal for applications where consistent speed is crucial, such as powering precision equipment or keeping trains running smoothly.
In this article, we’ll dive into the mechanics of how synchronous motors work, including the different types of motors available and the key components that make them function. Whether you’re a curious hobbyist or a seasoned professional looking to expand your knowledge, this guide will give you a solid foundation to build on as you explore the world of synchronous motors. So grab a cup of coffee and settle in – it’s time to learn something new.
Synchronous Motor Overview
A synchronous motor is a type of AC motor that operates at a constant speed determined by the frequency of the power supply. In contrast to an asynchronous motor, the rotor and stator magnetic fields in a synchronous motor rotate at the same speed, which is called synchronous speed. This characteristic allows synchronous motors to be used in applications that require precise speed control, such as large industrial machines and power plants.
How does a synchronous motor start?
- Initially, the rotor of the synchronous motor is stationary and the stator winding is energized with three-phase AC power.
- As the stator’s magnetic field rotates, it induces an electromagnetic field in the rotor, causing it to rotate at synchronous speed.
- However, during startup, the rotor does not rotate at synchronous speed because of the high inertia of the motor and the large amount of torque required to overcome the static friction of the rotor.
- To overcome this problem, synchronous motors are equipped with additional devices like a damper winding or starting capacitor to generate an auxiliary magnetic field and provide the initial impulse for the rotor to rotate at synchronous speed.
Synchronous Motor Starting Methods
There are two common starting methods for synchronous motors:
- Direct on line (DOL) starting – In this method, the motor is directly connected to the power supply, causing a large inrush of current. This method is suitable for small motors with low inertia.
- Soft starter – This method uses a variable frequency drive (VFD) to gradually increase the current to the motor and reduce the starting torque. This method is more suitable for large motors with high inertia.
Damper Winding and Starting Capacitor
As mentioned earlier, synchronous motors may require an auxiliary magnetic field during startup. The two common methods to achieve this are:
- Damper winding – This method involves adding a winding in the rotor that’s connected to a resistor. The resistor creates an auxiliary magnetic field that helps the rotor reach synchronous speed.
- Starting capacitor – This method involves adding a capacitor in series with the main winding during startup. The capacitor creates an auxiliary magnetic field that helps the rotor reach synchronous speed. Once the motor is running at synchronous speed, the capacitor is disconnected from the circuit.
| Advantages of synchronous motors | Disadvantages of synchronous motors |
|---|---|
| – Precise speed control | – More expensive than asynchronous motors |
| – High efficiency and power factor | – Require external devices to achieve starting |
| – Better suited for high power applications | – Not suitable for applications with frequent changes in load |
Overall, synchronous motors are a reliable and efficient choice for industrial applications that require precise speed control. Proper selection of the starting method and additional devices like a damper winding or starting capacitor can ensure successful startup and operation of the motor.
Synchronous Motor Working Principle
A synchronous motor is a highly efficient type of motor that operates with a fixed speed corresponding to the frequency of the AC voltage applied to its terminals. In contrast with an asynchronous motor, which operates at a speed slightly lower than the synchronous speed, a synchronous motor runs constantly at a synchronous speed
How does a Synchronous Motor start?
- A synchronous motor has a pair of windings on its rotor. One winding is excited by a continuous DC current source, while the other winding is used as an AC excitation supply.
- The stator winding is supplied with a three-phase AC voltage source.
- In order to start a synchronous motor, its rotor windings must be initially magnetized.
The magnetizing current flows through the DC winding and creates a strong magnetic field, which generates a torque in the rotor. Once the rotor reaches a certain speed, the excitation power is switched on, and the rotor is pulled into synchronism with the rotating magnetic field of the stator.
When the rotor reaches synchronous speed, the stator magnetic field pulls it into phase with itself and the rotor keeps rotating in synchronism with the stator. This speed can vary up to a certain extent in synchronous motors, but the synchronous speed will remain constant.
Advantages of a Synchronous Motor
A synchronous motor has a few advantages over other types of motors, such as:
- Highly efficient at constant speeds and large power levels.
- Can be used for high-power applications such as compressors, heavy-duty motors, and generators.
- More reliable than other types of motors because of their simpler construction, lack of slip resistance, and absence of brushes.
- Provide smooth, constant speed even under varying load conditions.
Synchronous Motor Applications
Synchronous motors are ideal for high-power applications that require a constant speed, including:
| Application | Examples |
|---|---|
| Compressors | Air compressors used for industrial applications. |
| Heavy-duty motors | Motors used in heavy machinery such as cranes, elevators, and fans. |
| Generators | Electric power generators used for both industrial and residential applications. |
In summary, a synchronous motor is a highly efficient motor that operates at a fixed speed corresponding to the frequency of the AC voltage applied to its terminals. It is ideal for high-power applications such as compressors, heavy-duty motors, and generators because of its reliability, constant speed, and lack of brushes. Once the rotor is magnetized, the synchronous motor starts by being pulled into phase with the stator magnetic field.
Starting Mechanisms for Synchronous Motor
Synchronous motors are electric motors that operate synchronously with the frequency of the supply current. They operate at a constant speed with a stable output, making them ideal for many industrial applications. But how does a synchronous motor start? There are several starting mechanisms used to initiate synchronous motor rotation. Understanding these starting mechanisms can help you choose the best application suited to your needs.
- AC Synchronous Motor Starting: One common method of starting a synchronous motor is to connect it to a small induction motor. This induction motor is used to rotate the synchronous motor up to its synchronous speed, at which point it becomes self-sustaining. The process of the induction motor starting the synchronous motor is called the slip ring method.
- Damper Winding Starting: Another way to start a synchronous motor is through the use of a damper winding. This starting mechanism includes a winding of heavy copper bars that are placed around the perimeter of the rotor. The rotor rotates during the start-up phase, and by inducing opposite magnetic fields, the motor starts. This method of starting is useful in large, high-powered machines that need to start up slowly.
- Electronic Starting: Electronic starting mechanisms are becoming more popular due to their increasing efficiency and precision. These starting mechanisms use power electronics to control the frequency and voltage supplied to the motor, allowing for a soft start with adjustable torque. This reduces the impact on the motor and the mechanical equipment it drives, lowering maintenance costs and increasing the lifespan of the equipment.
Each starting mechanism has its own advantages and disadvantages. The best starting method for your synchronous motor will depend on the motor’s size, power requirements, and the application it is needed for. Consideration of the starting mechanisms can help you choose the most cost-effective and efficient starting mechanism for your business.
Starting mechanisms for synchronous motors can be summarized in the following table:
| Starting Mechanism | Advantages | Disadvantages |
|---|---|---|
| Slip Ring | Easy to install, accessible, low cost | Requires extra power source, can lead to premature brush wear, torque can be difficult to control |
| Damper Winding | High-torque, rugged, reliable | Time-consuming to install, difficult to access, expensive |
| Electronic Starting | Precise control, soft starting, adjustable torque | Expensive, requires complex electronics, can be difficult to install |
With a proper understanding of the synchronous motor starting mechanisms, you can choose the right mechanism that will serve your industrial application needs. Proper selection of the starting mechanisms will help improve its performance, reduce repair and maintenance costs of the motor in the long run, and serve the application purpose efficiently.
Direct Starting Method
The direct starting method is one of the simplest ways to start a synchronous motor, where the motor is directly connected to the power source. This may seem like a straightforward solution, but it requires a large amount of energy and can put a lot of stress on both the motor and the power grid.
- Advantages:
- Simple to implement and configure.
- The cost of the equipment required is low, since there are no additional components.
- Disadvantages:
- The starting current is very high and can damage the motor and the power grid.
- The high starting torque can produce mechanical stress on the motor and the driven load.
- The voltage drop on the power supply line during starting could cause voltage fluctuations in the grid.
- This method cannot be used for larger synchronous motors, where the starting current would exceed the capacity of the power grid.
Direct starting a synchronous motor is appropriate for smaller loads that do not require much starting torque and current. It is common in applications such as fans, pumps, and small compressors. However, for larger synchronous motors, where the starting torque and current can be very high, other methods such as soft starters, variable frequency drives, and autotransformers are preferred.
It is essential to select the appropriate starting method to ensure the safe and efficient operation of the synchronous motor. In doing so, you will reduce the likelihood of equipment damage, extend the lifespan of the motor, and improve the reliability of the system.
| Advantages | Disadvantages |
|---|---|
| Simple to implement and configure. | The starting current is very high and can damage the motor and the power grid. |
| The cost of the equipment required is low, since there are no additional components. | The high starting torque can produce mechanical stress on the motor and the driven load. |
| The voltage drop on the power supply line during starting could cause voltage fluctuations in the grid. | |
| This method cannot be used for larger synchronous motors, where the starting current would exceed the capacity of the power grid. |
Autotransformer Starting Method
When it comes to starting up a synchronous motor, there are several methods that engineers can choose from. One of the most popular methods is the autotransformer starting method.
- Reduced Voltage Starting Method – This method involves using an autotransformer to reduce the voltage applied to the motor during start-up. The reduced voltage helps to limit the inrush current that occurs when the motor starts up. This method is particularly useful for larger synchronous motors that require a lot of power to start up.
- Advantages of Autotransformer Starting – One of the biggest advantages of the autotransformer starting method is that it helps to reduce the starting current of the motor. This is important because starting current can be several times higher than the motor’s full load current, which can cause problems for the electrical system. By limiting the starting current, the autotransformer starting method can help to protect the motor and the electrical system from damage.
- Disadvantages of Autotransformer Starting – While autotransformer starting is an effective method for reducing the inrush current of synchronous motors, it does have some drawbacks. One of the biggest disadvantages is that it can be quite expensive. Autotransformers are typically larger and more expensive than other types of transformers, which can drive up the cost of the electrical system. Additionally, autotransformers can be less efficient than other types of transformers, which can lead to higher energy costs over time.
Overall, the autotransformer starting method is an effective way to reduce the inrush current of synchronous motors during start-up. While it may be more expensive than other types of starting methods, the benefits of reduced starting current can help to protect the motor and electrical system from damage in the long run.
| Advantages | Disadvantages |
|---|---|
| – Reduces inrush current – Protects motor and electrical system |
– Expensive – Less efficient than other transformers |
As with any type of starting method, engineers should carefully consider the needs of their specific application when choosing between different starting methods for synchronous motors.
Resistance Starting Method
One of the most commonly used methods for starting synchronous motors is the resistance starting method. In this method, a resistance is added to the stator circuit of the motor in order to reduce the starting current and increase the starting torque. The resistance is gradually reduced as the motor comes up to speed until it is finally removed from the circuit completely. This method is especially useful for high torque loads and applications where a high starting current could damage the motor or the driven load.
- The resistance starting method can be applied to both wound rotor and squirrel cage induction motors.
- The amount of resistance added to the circuit is determined by the motor rating and the starting torque requirements of the driven load.
- This method provides a smooth acceleration of the motor, thereby reducing mechanical stresses on the shaft and bearings.
A typical resistance starting circuit consists of a set of resistors connected in series with the stator winding during the starting period. The series combination of the resistors and the stator winding forms a voltage divider circuit, which reduces the applied voltage to the motor during starting. This results in a corresponding reduction in the stator current and starting torque. As the motor reaches sufficient speed, the resistors are gradually removed from the circuit by means of switches or contactors, until the motor is running at normal speed with no resistance in the circuit.
The following table shows a typical sequence of resistance steps for a 3-phase wound rotor synchronous motor with a rating of 200 kW and a starting torque requirement of 400% of full load torque:
| Resistance Step | Starting Resistance (per phase) | Starting Current (per phase) | Starting Torque (per phase) |
|---|---|---|---|
| 1 | 0.5 Ω | 290 A | 1.87 x Full Load Torque |
| 2 | 0.3 Ω | 245 A | 2.11 x Full Load Torque |
| 3 | 0.2 Ω | 212 A | 2.34 x Full Load Torque |
| 4 | 0.1 Ω | 182 A | 2.55 x Full Load Torque |
| 5 | 0.05 Ω | 163 A | 2.68 x Full Load Torque |
As can be seen from the table, the starting resistance is gradually reduced in steps, resulting in a corresponding increase in starting torque. This enables the motor to overcome the inertia of the driven load and accelerate smoothly until it reaches normal speed.
Soft Starting Method
Starting a synchronous motor can be challenging due to its inherent characteristic to synchronize with alternating current (AC) frequency. The soft starting method is one of the effective ways to overcome this challenge and start the motor smoothly.
- What is Soft Starting?
- How Does Soft Starting Work?
- Advantages of Soft Starting:
- Reduced voltage and current peaks during motor starting, which minimizes the mechanical and electrical stress on the motor and related equipment.
- Lower starting torque, which eliminates torque surges, vibrations, and other mechanical stresses on the motor and connected load.
- Improved motor efficiency and power factor during operation, which results in energy savings and reduced operating costs.
- Soft Starting vs. DOL Starting Method:
The soft starting method is an electrical technique to start the motor at a slow speed and gradually increase its speed to the full-rated speed. This is achieved by controlling the voltage and frequency applied to the motor through a variable frequency drive (VFD).
When a motor is started using the soft starting method, the voltage and frequency applied to the motor are slowly increased from zero to the rated values, in a controlled manner. This allows the motor to gradually accelerate and avoids the sudden inrush of current that typically occurs during a traditional direct-on-line (DOL) starting method.
A soft starting method offers several advantages, including:
The traditional direct-on-line (DOL) starting method involves connecting the motor directly to the power grid, which results in a high inrush current that can damage the motor and related equipment. On the other hand, the soft starting method controls the voltage and frequency supplied to the motor, which allows it to start smoothly and reduce mechanical and electrical stress on the motor and equipment.
The table below summarizes the differences between the two starting methods.
| Parameter | DOL | Soft Starting Method |
|---|---|---|
| Starting Current | High and uncontrolled | Low and controlled |
| Starting Torque | High and uncontrolled | Low and controlled |
| Electrical Stress on Motor and Equipment | High | Low |
In conclusion, the soft starting method is an effective way to start a synchronous motor smoothly and reduce the mechanical and electrical stress on the motor and related equipment. It offers several advantages over the traditional direct-on-line (DOL) starting method and results in energy savings and reduced operating costs.
Frequently Asked Questions About How Does a Synchronous Motor Start
1. How does a synchronous motor differ from an induction motor?
A synchronous motor is designed to operate at a constant speed, while an induction motor has a variable speed.
2. Why is synchronous motor starting difficult?
Synchronous motor starting is difficult because it requires a precise electrical match between the motor and the power system.
3. What is a synchronous motor starting method?
One common method is to start the motor as an induction motor, and then transition to synchronous operation once the motor has achieved full speed and synchronization.
4. What is synchronous motor torque?
Synchronous motor torque is directly proportional to the excitation current and the sine of the angle between the stator voltage and the rotor current.
5. What is the starting current of a synchronous motor?
The starting current of a synchronous motor is usually higher than that of an induction motor due to the need for a larger excitation current.
6. How is synchronous motor excitation maintained during starting?
The excitation of a synchronous motor can be maintained through the use of an auxiliary power source or by connecting the motor to an external power system.
7. How long does it take for a synchronous motor to start?
The starting time for a synchronous motor varies depending on the size and design of the motor, but it can take several seconds to several minutes.
Closing: Thanks for Reading!
We hope you found this article on how synchronous motors start informative and helpful. If you have further questions, be sure to leave a comment below. Don’t forget to visit us again for more interesting topics.