Synchronous motors use an exciter (or a permanent magnet) to create the field currents necessary to generate the magnetic field. The starter is then used to bring the motor up to its rated speed. Generally this is done by providing a voltage boost to the rotor winding.
This could be done with a set of resistors, or by using any of the various electrical starters available, including a reduced voltage starter, a soft starter, a direct-on-line (DOL) starter, an auto transformer starter, or a solid-state starter.
All of these methods have their own advantages and disadvantages depending on the application. Reduced voltage starters have the disadvantage of needing a large inrush current, and can be expensive to purchase.
Soft starters can provide a smooth ramp up of the motor, but must be able to handle the motor’s starting current inrush and the locked rotor current drawn by the motor. A DOL starter requires no special equipment, but has the disadvantage of providing a sudden full voltage boost which can over.
stress or damage the motor or its components. An auto transformer starter provides a step-less reduction in current, but again must be able to cope with the motor’s inrush current. Finally, a solid-state starter provides a smooth ramp up and additional monitoring and protection, but may require additional circuitry to be added.
How can a synchronous motor be self-starting?
A synchronous motor can be self-starting by using an auxiliary device to provide starting current. This would allow the rotor to follow the rotating magnetic field created by the stator. Common auxiliary devices used include an electromechanical starter, a static starter, or an adjustable-frequency drive (AFD).
An electromechanical starter is an AC motor that is used to provide starting torque. This motor connects to the motor windings by electrical contacts. As the starter motor begins to rotate, its magnetic field will interact with the rotor, causing it to spin as well.
A static starter is also known as a soft starter. This type of starter starts the motor in a softer, more gradual approach than an electromechanical starter. Static starters contain electronic switches and resistor networks that limit the amount of current that is allowed when the motor is starting.
An AFD is a type of electronic controller which allows the motor to remain synchronous with the power source. It increases the motor’s speed over time, and the speed can be programmed to match the full operational speed of the motor.
The AFD also helps to reduce mechanical stresses and motor wear.
All of these devices are useful for self-starting a synchronous motor. Depending on the application and the type of motor, one of these devices may be better suited than the others. It is important to match the starter with the speed of the motor and the voltage available from the power source.
How does a synchronous motor work?
Synchronous motors are AC motors that rely on the relationship between the stator and rotor to provide rotating magnetic fields in order to generate torque. This type of motor works on a basic principle: if a stator has a rotating magnetic field and a rotor is placed in the vicinity of that field, the rotor will tend to turn in the same direction as the field.
In a synchronous motor, the stator is equipped with a set of stationary electromagnets, which create an alternating magnetic field. This alternating magnetic field interacts with the rotor, which is also equipped with an electromagnet, to produce a rotational force.
When the stator’s field is aligned with the rotor’s, the magnetic force causes it to rotate synchronously with the field.
In most synchronous motors, the rotor magnets need to be powered separately. This is achieved by a process known as “magnetizing”, where electricity is passed through the coils of the rotor. These variables allow the user to control the speed of the rotor and the amount of torque it produces, which in turn can be used to control the speed of the entire synchronous motor.
At its core, synchronous motors rely on the electromagnetic principles to produce the rotational force, which in turn is transferred via the shaft to the load or transmission system. This makes them an efficient and reliable choice for a wide range of industrial applications.
Why the synchronous motor is not inherently to self-starting?
Synchronous motors are not inherently self-starting because they require a source of rotating magnetic field which is only provided when the motor is externally driven. In order to obtain the necessary rotating magnetic field, the stator winding must be excited by an (unbalanced) alternating current supply.
When the stator winding of a synchronous motor is energized, the rotor follows the rotating field at synchronous speed but will not reach the same position unless it is originally at the same position, when energized.
This is why the motor is not inherently self-starting; the external rotating field must provide the starting torque for the motor in order to reach synchronous speed.
Can a synchronous motor be started with a load on it?
Yes, a synchronous motor can be started with a load on it, although the starting process does involve some additional considerations. When a synchronous motor is started with a load on it, the motor must be accelerated to a speed close to its synchronous speed (>95% of its synchronous speed) as quickly as possible but with as little stress to the components as possible.
Since synchronous motor has engines that require high starting torque, the ability to accelerate from a dead stop to the synchronous speed quickly, with minimal stress, is important. Start-up must begin with the field winding connected to full voltage.
The voltage must then be steadily increased at a certain rate, allowing the motor to run with the load. This process, called “accelerating with load”, will take a bit longer than accelerating without load, and may require more voltage and current.
It is important to monitor the current and voltage through this process in order to prevent mechanical or electrical damage due to high and sudden temperatures as the motor reaches its synchronous speed.
What happens when synchronous motor is overloaded?
When a synchronous motor is overloaded, its speed is slowed down and it requires an increase in torque. This increase in torque can cause the rotor to be pushed back against the stator, resulting in increased friction and overheating, which could lead to motor damage.
To prevent an overload, the load current should be monitored, and the load should be reduced or the motor should be disconnected if the current limit is exceeded. Additionally, if the motor is running in a closed loop system, the speed of the motor should be maintained or the set point should be adjusted in order to reduce the load on the motor.
If the overload persists, the motor may need to be replaced.
What is the difference between induction motor and synchronous motor?
The main difference between an induction motor and a synchronous motor is their differing operational principles. An induction motor operates through the electromagnetic induction principle, where the currents that make up the motor are induced due to the interaction of two magnetic fields.
This interaction caused by the windings within the motor and the rotation of the rotor generates torque to drive the motor. A synchronous motor, on the other hand, operates in synchrony with the rotation of the rotor and its synchronal excitation from the stator.
This means that the AC currents that pass through the windings of the stator of the motor are in phase with the rotor poles, which are generated due to the excitation field. This generates electromagnetic torque which causes the motor to run.
The rotational accuracy of the synchronous motor is very precise and high in comparison with the induction motor’s accuracy. In addition, a synchronous motor has a much weaker torque than an induction motor when operating at no load or low load.
What is the effect of increasing the load of a synchronous motor running with normal excitation?
Increasing the load of a synchronous motor running with normal excitation will affect the synchronous motor in a number of ways. Firstly, as the load increases, the torque output of the motor will decrease, leading to a decrease in power output.
Secondly, due to the increased load, the motor output frequency will decrease because the motor will be running at a slower speed. As the load increases, the torque output will further decrease, leading to a further decrease in power output.
Additionally, the excitation current and field flux will decrease, leading to a reduction in the motor’s output power. The decreased power output will lead to a decrease in the motor’s efficiency which will result in an increase in energy costs.
Finally, as the load increases, the motor will require additional cooling to dissipate the increased heat energy being generated.
When load on a synchronous motor is increased provided it is?
When load on a synchronous motor is increased, the motor’s speed tends to drop due to the increased resistance. This creates a decrease in synchronizing torque or negative torque, which causes the motor to slip backward.
This slippage generates additional torque that resists the applied load, thus limiting the motor’s response to the increased load. Therefore, it is important to match the horsepower necessary for the application with the proper motor size and as the load requirements change, the motor can be re-sized as necessary.
As well, it is important to have a motor whose power supply will not be overburdened by sudden changes in load.
When synchronous motor is on no load the torque?
When a synchronous motor is running without a load, the torque is referred to as ‘zero load torque’. This is the amount of torque the motor generates when it is running without a load (when it is not driving any external loads).
It is normally less than the rated torque of the motor and is only enough to overcome mechanical and electrical losses. This torque is mainly dependant on the field winding current and the rotor position in the stator field.
It is normally expressed as a percentage of the full load torque and is typically 5-15%. However, depending on the design and the manufacturer, this can vary.
Why is the speed of the synchronous motor constant from no load to full load?
The speed of a synchronous motor is constant over its entire load range because of the synchronous speed of the motor which is determined by the frequency of the power supply and the number of poles in the motor.
The magnetic field in the motor is produced by the stator windings and the rotor has permanent magnets. The magnetic field rotates at the synchronous speed which maintains the rotor speed and prevents it from slipping or slowing down as the load changes.
When the load increases, the internal torque produced by the motor increases in order to maintain the constant speed, however the torque does not effect the speed of the motor itself. Synchronous motors provide the advantage of high efficiency, voltage regulation, and constant speed operation.
Which motors are self-starting?
Most AC induction motors are self-starting. These motors rely on the alternating current they receive to set up an alternating magnetic field inside the motor. This alternating magnetic field creates a rotating magnetic field inside the motor that causes the rotor to move and therefore the motor to start.
There are other types of motors that can be self-starting as well, such as DC brushless motors, switch-reluctance motors, and switched reluctance motors. Self-starting motors are important because they allow machines and other items of machinery to be started without the use of an external power source.
This helps to make machines and other machinery more efficient and user friendly.
What are the characteristics of synchronous motor?
The characteristics of a synchronous motor include its ability to be used in both clockwise and counter-clockwise directions, the ability to be synchronized with an existing power source, the use of magnets in their operation, and the overall good performance of these motors.
Operationally, synchronous motors maintain a steady, constant rotational speed when in operation, which is dependent on the line frequency of the connected power source. In addition, the speed of the motor can be varied by changing the frequency of the applied voltage.
A synchronous motor is powered by an electromagnet and uses a permanent magnet field for the rotor. This permanent magnet field causes the rotor to align and lock with the electromagnet, creating synchronous operation.
Synchronous motors are very reliable and efficient. The synchronous motor has a constant power factor, and the efficiency is usually quite high up to around 95% or better.
These motors are mostly used in applications where precise and consistent speeds are necessary, such as in pumps, conveyors, and compressors.
When starting a synchronous motor its field winding should be?
When starting a synchronous motor its field winding should be initially energized from an independent supply, such as a separate battery. This enables the motor to rotate at the same synchronous speed as the main supply, as the field winding is not subject to the main supply voltage fluctuations during starting.
Once the motor is running and up to speed, its field winding can then be reconnected to the main supply. This allows the motor to operate as a synchronous motor. The field winding provides the motor with the necessary torque to accelerate and continue accelerating up to its rated speed.
What precaution should be taken during the start up period of a synchronous motor?
During start up of a synchronous motor, certain precautions should be taken to ensure its safe operation. First, make sure that all mechanical elements, such as rotors, bearings, and lubricators are in good working order before powering the motor.
Perform a visual inspection to ensure there are no mechanical defects present. Additionally, check that the shaft is properly seated and its alignment with the electric motor is straight.
It is important to keep an eye on the temperature of the motor during start up. The motor should not be allowed to exceed its rating temperature. Additionally, make sure the voltage supply is correct and the phase sequence is correct.
An incorrect phase sequence can lead to an overload of the motor, which could lead to permanent damage.
Also, check the speed of the motor before turning it on as it should be running at the correct frequency. If the speed is higher than the rated frequency, the motor will draw more power to reach its rated frequency.
Too high of a voltage can lead to motor overheating and destruction.