Synchronous vs Induction Motors: A Comparison Guide - LinkedIn

Synchronous motors are AC motors that rotate at a constant speed that is proportional to the frequency of the supply voltage. They have a rotor with permanent magnets or electromagnets that align with the rotating magnetic field of the stator. Synchronous motors have high efficiency, power factor, and torque. They can also be used as generators or for power factor correction. However, synchronous motors have some drawbacks, such as high cost, complexity, and maintenance. They also need a separate starting device, such as a variable frequency drive or a slip ring, to overcome the high starting torque and current.

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Let me share the -sometimes- disadvantage of using a synchronous motor. I say sometimes, coz not always the time, but just to highlight the possible additional maintenance occured. First, is carbon brushes. You need to ensure you maintain, or replace actually, the brushes on time. Alternatively, those with rotating diodes, the diodes sometimes fail thus planning to replace them at a certain interval may turn out to be the best bet instead of gambling on its life. Next is the exciter and its exciter panel. Believe it or not, the exciter panel's part also tend to fail here and there.

Induction motors are AC motors that rely on electromagnetic induction to create torque. They have a rotor with conductive bars or windings that are short-circuited by end rings. The rotating magnetic field of the stator induces a current in the rotor, which in turn generates a magnetic field that opposes the stator field. Induction motors have low cost, simplicity, and durability. They can start directly from the line and can operate at variable speeds and loads. However, induction motors have some disadvantages, such as low efficiency, power factor, and torque. They also produce more heat, noise, and slip than synchronous motors.

Induction motors have a high starting current of up to 6x the nominal current. This may lead to the power utility provider to limit the end user of its starting current (or more precisely, limit its voltage dip, especially if we are talking about large capacity motors, which usually sit on the medium voltage bus). Threfore, this shall end up with the end user to equip the motor with starters. The delima with starters are, what is the best type of starter to be used with your MV motor? Do u go with star-delta, if possible? Or do u use an auto-trans? How about a soft starter, or even a VFD? MV VFD is not so common, isn't it? Or how about RVS (reduce voltage starter)? Once finalized, you need to think of the starter's maintenance as well.

For more information, please visit How Synchronous Motor Works.

Choosing between synchronous and induction motors depends on several factors, such as the required speed, torque, power, load, and application. Generally, synchronous motors are more suitable for applications that need constant speed, high power, or precise control, such as pumps, compressors, or conveyors. Induction motors are more suitable for applications that need variable speed, low power, or simple operation, such as fans, blowers, or mixers. You should also consider the availability, cost, and maintenance of the motor and its components.

To compare synchronous and induction motors, one must understand some key characteristics that describe their performance and behavior. These include the rotational speed of the motor, measured in revolutions per minute (RPM). Synchronous motors have a fixed speed that depends on the supply frequency and the number of poles, while induction motors have a variable speed that depends on the slip. The rotational force of the motor is measured in newton-meters (Nm) or pound-feet (lb-ft), with synchronous motors having a constant torque that is independent of the load, and induction motors having a variable torque that depends on the load and the slip. The mechanical output of the motor is measured in watts (W) or horsepower (HP), with synchronous motors having a constant power that is proportional to the torque and the speed, and induction motors having a variable power that is proportional to the torque and the square of the speed. Lastly, power factor is the ratio of real power to apparent power, which indicates how efficiently the motor uses electrical energy. Synchronous motors have a high power factor that can be adjusted by changing excitation current, while induction motors have a low power factor that decreases with load and slip.

Another important characteristic to consider is the motor efficiency, which is the ratio of the mechanical output to the electrical input, expressed as a percentage. Motor efficiency depends on several factors, such as the design, size, speed, load, temperature, and losses of the motor. Synchronous motors have higher efficiency than induction motors, especially at high loads and speeds. This is because synchronous motors have lower losses, such as copper losses, iron losses, windage losses, and friction losses. Induction motors have higher losses, especially at low loads and speeds. This is because induction motors have additional losses, such as core losses, rotor losses, and slip losses.

The last characteristic to consider is the motor control, which is the method of regulating the speed, torque, direction, and other parameters of the motor. Motor control can be achieved by using various devices, such as switches, relays, contactors, circuit breakers, fuses, transformers, and controllers. Synchronous motors require more complex and expensive control devices than induction motors, such as variable frequency drives, slip rings, or brushless excitation systems. Induction motors require simpler and cheaper control devices than synchronous motors, such as rheostats, capacitors, or soft starters.

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