10 Things to Consider When Buying Synchronous Motor Starting Methods

SYNCHRONOUS MOTOR WORKING PRINCIPLE

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Electrical motor, in general, is an electro-mechanical device that converts energy from the electrical domain to the mechanical domain. Based on the type of input we have classified it into single phase and 3 phase motors. Among 3 phase induction motors and synchronous motors are more widely used.

When a 3 phase electric conductors are placed in certain geometrical positions (In certain angles from one another) there is an electrical field generated. 

Now the rotating magnetic field rotates at a certain speed, that speed is called synchronous speed. Now if an electromagnet is present in this rotating magnetic field, the electromagnet is magnetically locked with this rotating magnetic field and rotates with the same speed of the rotating field.

Synchronous motors are called so because the speed of the rotor of this motor is the same as the rotating magnetic field. It is basically a fixed speed motor because it has only one speed, which is synchronous speed and therefore no intermediate speed is there or in other words, its in synchronism with the supply frequency.

Synchronous speed is given defined by the number of pole pairs and the supply frequency. i.e. a motor running at 50Hz and using only one pole pair per phase would run at 60 x 50 = rpm. A motor running at 60Hz and using 2 pole pairs per phase would run at (60 x 60) / 2 = rpm.

Normally construction is almost similar to that of a 3 phase induction motor, except the fact that the rotor is given dc supply, the reason for which is explained later.

Now, let us first go through the basic construction of this type of motor From the above picture, it is clear how this type of motors is designed. The stator is given three-phase supply and the rotor is given dc supply.

MAIN FEATURES OF SYNCHRONOUS MOTORS

Synchronous motors are inherently not self-starting. They require some external means to bring their speed close to synchronous speed before they are synchronized.

The speed of operation is in synchronism with the supply frequency and hence for constant supply frequency they behave as constant speed motor irrespective of load condition

This motor has the unique characteristics of operating under any electrical power factor. This makes it is used in electrical power factor improvement.

PRINCIPLE OF OPERATION OF SYNCHRONOUS MOTORS

Synchronous motor is a doubly excited machine i.e two electrical inputs are provided to it. Its stator winding which consists of a 3 phase winding is provided with 3 phase supply and the rotor is provided with DC supply. The 3 phase stator winding carrying 3 phase currents produces 3 phase rotating magnetic flux.

The rotor carrying DC supply also produces a constant flux. Considering the frequency to be 50 Hz, from the above relation we can see that the 3 phase rotating flux rotates about revolutions in 1 min or 50 revolutions in 1 sec. At a particular instant rotor and stator poles might be of the same polarity (N-N or S-S) causing repulsive force on rotor and the very next second it will be N-S causing attractive force.

But due to the inertia of the rotor, it is unable to rotate in any direction due to attractive or repulsive force and remain in standstill condition. Hence it is not self-starting.

To overcome this inertia, the rotor is initially fed some mechanical input which rotates it in the same direction as a magnetic field to a speed very close to synchronous speed. After some time magnetic locking occurs and the synchronous motor rotates in synchronism with the frequency.

METHODS OF STARTING OF SYNCHRONOUS MOTOR

1.EXTERNAL PRIME MOVER

Synchronous motors are mechanically coupled with another motor. It could be either 3 phase induction motor or DC shunt motor. DC excitation is not fed initially.

It is rotated at speed very close to its synchronous speed and after that DC excitation is given. After some time when magnetic locking takes place supply to the external motor is cut off.

2.DAMPER WINDINGS

In case, the synchronous motor is of salient pole type, the additional winding is placed in the rotor pole face. Initially, when the rotor is standstill, the relative speed between damper winding and rotating air gap flux in large and an emf is induced in it which produces the required starting torque.

As speed approaches synchronous speed, emf and torque are reduced and finally when magnetic locking takes place, the torque also reduces to zero.

Hence in this case synchronous is first run as three-phase induction motor using additional winding and finally, it is synchronized with the frequency.

3.SYNCHRONOUS INDUCTION STARTING

Here, the stator of the synchronous motor is excited by 3 phase voltage. The rotor is connected to slip rings and further to a TPDT (Triple Pole Double Throw) switch.

Initially, this switch is connected to the start position which has external resistance. This way of starting is similar to rotor resistance starting in Induction Motor. As soon as the motor starts rotating and attains a certain pre-determined speed, the switch position is changed to run.

The Run position has an external battery supply, which when connected excites the rotor with DC Current and poles are formed, and on the rotor and based on the principle of magnetic locking.

SYNCHRONOUS MOTOR EXCITATION

Prior to understanding this synchronous motor excitation, it should be remembered that any electromagnetic device must draw a magnetizing current from the ac source to produce the required working flux. This current lags by almost 90° to the supply voltage. In other words, the function of this magnetizing current or lagging VA drawn by the electromagnetic device is to set up the flux in the magnetic circuit of the device.

The synchronous motor is doubly-fed electrical motor i.e it converts electrical energy to mechanical energy via a magnetic circuit. Hence it comes under an electromagnetic device. It receives 3 phase ac electrical supply to its armature winding and DC supply is provided to the rotor winding.

Synchronous motor excitation refers to the DC supply given to the rotor which is used to produce the required magnetic flux. One of the major and unique characteristics of this motor is that it can be operated at any electrical power factor leading, lagging or unity and this feature is based on the excitation of the synchronous motor.

When the synchronous motor is working at a constant applied voltage V, the resultant air-gap flux as demanded by V remains substantially constant. This resultant air-gap flux is established by the cooperation of both AC supply of armature winding and DC supply of rotor winding.

CASE 1: When the field current is sufficient enough to produce the air-gap flux, as demanded by the constant supply voltage V, then the magnetizing current or lagging reactive VA required from ac source is zero and the motor operates at unity power factor. The field current, which causes this unity power factor is called normal excitation or normal field current.

CASE 2: If the field current is not sufficient enough to produce the required air gap flux as demanded by V, additional magnetizing current or lagging reactive VA is drawn from the AC source. This magnetizing current produces the deficient flux (constant flux- flux set up by dc supply rotor winding). Hence, in this case, the motor is said to operate under lagging power factor and the is said to be under excited.

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CASE 3: If the field current is more than the normal field current, the motor is said to be over-excited. This excess field current produces excess flux ( flux set up by DC supply rotor winding – resultant air-gap flux) that must be neutralized by the armature winding. Hence the armature winding draws leading reactive VA or demagnetizing current leading voltage by almost 90 degree from the AC source. Hence, in this case, the motor operates under a leading power factor.

APPLICATIONS OF SYNCHRONOUS MOTORS

usually, synchronous motors are used for applications where precise and constant speed is required. Low power applications of these motors include positioning machines. These are also applied in robot actuators. Ball mills, clocks, record player turntables, Electrical propulsion also make use of synchronous motors. Besides these motors are also used as servomotors and timing machines.

These motors are available in a fractional horseshoe size range to high power industrial size range. While used in high power industrial sizes, these motors perform two important functions. One is as an efficient means of converting AC energy into mechanical energy and the other is Power factor correction.

Conclusion: An overexcited synchronous motor operates at a leading power factor, under-excited synchronous motor operates at lagging power factor and normal excited synchronous motor operate at the unity power factor.

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Introduction:

Electric motors are ubiquitous in modern society, powering everything from household appliances to industrial machinery. As the world moves towards greater sustainability, the importance of selecting the right electric motor for specific applications has become more critical than ever.

There are many different types of applications that utilize electric motors, each with its own unique requirements. For example, electric motors are commonly used in manufacturing processes to power conveyor belts, robots, and other equipment. They are also used in transportation applications, such as electric vehicles and trains, as well as in renewable energy systems, such as wind turbines and solar tracking systems.

When it comes to electric motors, there are a variety of types to choose from, including AC motors, DC motors, and stepper motors, each with its own strengths and weaknesses. For example, AC motors are commonly used in high-power applications due to their efficiency, while DC motors are often used in low-voltage applications where precise speed control is required. Stepper motors, on the other hand, are ideal for applications that require precise positioning, such as in 3D printing or robotics.

Given the range of applications and motor types available, selecting the right electric motor for a specific application can be a complex task. In this article, we'll discuss the six key factors you should consider when choosing an electric motor for your project.

Performance Requirements:

The first step in selecting an electric motor is determining the Performance requirements (power, torque, speed, and acceleration requirements) of your application. Understanding these parameters is crucial because they influence the performance and efficiency of the motor. Power is the rate at which work is done and is typically measured in watts or horsepower. It is a fundamental requirement that determines how much work the motor can do in a given time. It is critical in selecting the right motor to match the application's power requirements. Torque is the rotational force applied to a shaft and is typically measured in Newton-meters (Nm) or pound-feet (lb-ft). Speed is essential for applications that require a specific rate of rotation or motion, such as conveyors, fans, and pumps and it is typically measured in revolutions per minute (RPM). Finally, acceleration is the rate of change of velocity and is typically measured in meters per second squared (m/s^2). It is crucial for applications that require a fast response time or quick start-up, such as robotics, medical equipment, and vehicles.

• To determine the power requirement, it is necessary to have information about the amount of work to be done and the time frame in which it needs to be completed. For example, if you need to lift a certain weight a certain height in a given amount of time, you can calculate the power requirement using the formula:

Power (W) = (Force x Distance) / Time.

• In order to calculate the torque requirement, you must have data on the amount of force that needs to be applied and the distance from the center of rotation where the force is applied. You can then use the formula:

Torque (Nm) = Force (N) x Distance from the center of rotation (m).

• To figure out the speed requirement, you need to know how fast the object or load needs to rotate. This information can be used to calculate the required speed. For example, if you need to rotate a conveyor belt at a certain speed, you can calculate the speed requirement using the formula:

Speed (RPM) = (60 x Velocity (m/s)) / (2 x π x Radius (m)).

• If you need to accelerate an object to a certain speed in a specific amount of time, you must know the starting velocity, final velocity, and the duration of acceleration. You can then use the formula:

Acceleration (m/s^2) = (Final Velocity (m/s) - Initial Velocity (m/s)) / Time (s).

Once you have determined the power, torque, speed, and acceleration requirements for your application, you can use the motor manufacturers' data sheets to choose the right motor that meets those requirements. These data sheets typically contain detailed technical specifications, including the motor's performance characteristics and operational curves. The performance characteristics typically include information such as the motor's rated power, torque, speed, and efficiency at various operating conditions. The operational curves, such as speed-torque, power-speed, and efficiency-speed, help you understand the motor's performance across its entire operating range.

Selecting proper drive:

When selecting a motor for an industrial application, it's important to consider the drive as an integral part of the system. The drive is responsible for controlling the speed and torque of the motor, and can have a significant impact on the overall efficiency and performance of the system.

There are several factors to consider when selecting a drive, including:

1. Type of Drive: There are several types of drives available, including variable frequency drives (VFDs), servo drives, and stepper drives. Each type of drive has its own unique features and benefits, and the selection will depend on the specific application requirements.
2. Compatibility with the Motor: The drive must be compatible with the motor being used, including the voltage, current, and frequency ratings. Choosing a drive that is not compatible with the motor can result in reduced performance and efficiency, as well as potential damage to the motor.
3. Control Capabilities: The drive should have the necessary control capabilities to meet the requirements of the application. This may include features such as speed control, torque control, and position control.
4. Energy Efficiency: The drive should be energy efficient and able to operate at high levels of efficiency across a range of loads. This can help to reduce energy consumption and operating costs over time.

In addition to these factors, it's important to ensure that the motor and drive are properly sized for the application. This involves considering factors such as the load requirements, duty cycle, and operating environment, and selecting a motor and drive combination that can meet these requirements while operating at maximum efficiency.

By considering the drive as an integral part of the system, engineers can help to ensure that the motor and drive combination selected for an industrial application provides optimal performance, efficiency, and cost-effectiveness over time.

Size and Mounting:

When selecting an electric motor, it's crucial to consider the physical space available for the motor, as well as any weight or vibration limitations. Electric motors come in a wide variety of sizes and mounting options, from compact designs to larger, heavier models. It's essential to choose a motor that fits within the available space and meets any weight or vibration requirements.

For example, in applications where there are severe physical space limitations, such as in-wheel electric traction systems for electric vehicles or electric bicycles, axial flux topologies may be more suitable compared to radial flux motors.

Additionally, you'll want to consider the mounting options available for your application, such as flanges or brackets. Choosing the right mounting option can help ensure that the motor is securely and safely installed.

Environmental Conditions:

The environment in which your motor will operate can have a significant impact on its performance and lifespan. For example, motors used in harsh or corrosive environments may require special coatings or materials to protect against damage. Similarly, motors used in high-temperature environments may require additional cooling mechanisms.

There are several standards that engineers could refer to regarding the environmental conditions of motors. Two commonly used standards are:

1. NEMA MG 1-- This standard provides guidelines for motor enclosures, which protect motors from the surrounding environment. The NEMA standard categorizes motor enclosures based on their ability to protect against dust, moisture, corrosion, and other environmental factors.
2. IEC -1 - This standard provides guidelines for motor performance and efficiency in various environmental conditions, including temperature, humidity, altitude, and vibration. The IEC standard also provides guidelines for motor enclosures and protection against environmental factors.

Efficiency:

Electric motors can vary widely in terms of efficiency, which can have a significant impact on both operating costs and environmental impact. To meet the required standards for motor efficiency, there are several things that engineers can do. Here are a few steps they can take:

1. Determine the specific efficiency requirements: Before selecting a motor, it's important to determine the specific efficiency requirements for the application. This can be done by consulting the relevant standards and regulations, as well as considering the energy efficiency goals of the organization.
2. Choose a motor with high efficiency rating: Look for motors with high efficiency ratings, such as those labeled as "premium efficiency" or "IE3". These motors have been designed to meet specific efficiency requirements and can save energy and reduce operating costs over time.
3. Consider the load factor: When selecting a motor, it's important to consider the load factor, which is the percentage of rated motor power that is required for the application. Motors that are oversized or undersized for the application can lead to reduced efficiency and increased energy costs.
4. Proper installation and maintenance: Proper installation and maintenance of the motor can also play a role in ensuring high efficiency. This includes things like ensuring proper alignment and tension of belts, and regularly checking and replacing worn or damaged parts.

By following these steps, engineers can help ensure that the motors they select meet the required efficiency standards and provide maximum energy savings and cost-effectiveness over time.

Cost:

Finally, you'll want to consider the cost of your motor and how it fits into your budget. While it may be tempting to opt for the cheapest motor available, keep in mind that a higher-quality motor may offer better performance and a longer lifespan, ultimately saving you money in the long run. Additionally, consider any ongoing maintenance or repair costs associated with your motor.

Conclusion:

In conclusion, selecting the right electric motor for your application is critical to achieving optimal performance and efficiency. When choosing an electric motor, you need to consider various factors, including the power, torque, speed, and acceleration requirements of your application. Once you have determined these parameters, you can use the manufacturer's data sheets to select the right motor that meets those requirements. Additionally, selecting a proper drive that is compatible with the motor and has the necessary control capabilities and energy efficiency is crucial for optimal performance. By considering these key factors, you can ensure that you choose the right electric motor for your application, which can result in increased productivity, reduced energy consumption, and overall cost savings.

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