Synchronous Motors vs. Induction Motors - What's the Difference?
Friday - 07/01/2022 09:02
Electric motors come in hundreds of sizes, shapes, and varieties, and the sheer amount of choices can be paralyzing when looking for the best option.
The first step in finding any motor is determining its power source; is it powered by AC current, or DC? This will cut the options into two broad categories, AC motors, and DC motors, and will eliminate any motors what won’t work with your electrical supply. However, both of these categories still contain many kinds of machines, so this article will help further differentiate the AC motor class. AC motors can be split into synchronous motors and induction motors, and this article will give a brief explanation of both, and compare their working characteristics and applications.
Induction motors are considered one of, if not the most prolific AC motor used in industry today. They were one of the first electric motors invented, and so have had ample time to be optimized to work in many applications. They have a relatively simple build, consisting of an outer stator and an inner rotor, which interact via the electromagnetic induction effect to generate mechanical rotation. Specific types of induction motors achieve this rotation in different ways, and please feel free to read our articles on squirrel cage motors, wound rotor motors, and single-phase industrial motors to learn more. Generally speaking though, the goal of induction motors is to pass an AC current through coils in the stator which will produce a magnetic field, and the oscillating frequency of the AC supply will cause this magnetic field to rotate. This rotating magnetic field (RMF) will then induce opposing magnetic fields in the rotor – the free moving armature attached to the output shaft – and cause useful rotation.
These motors are also known as asynchronous motors, as the frequency of their AC current does not directly match the number of rotations of the output shaft. This phenomenon is known as “slip” and occurs because the rotor is always playing a magnetic game of “catch-up” with the RMF. The existence of slip means precise timing is difficult with induction motors. As said before, these motors can be found in household appliances, electric vehicles, and even large mechanized industrial equipment, as they come in hundreds of speeds, torques, voltages, sizes, and forms. For more information on these machines, learn more in our article all about induction motors.
Synchronous motors cover the bases that induction motors cannot, namely their “asynchronous” nature. Synchronous motors match the output rotational frequency to the input AC frequency, allowing designers to use these motors in precisely timed applications, such as clocks, rolling mills, record players, and more. They accomplish this feat by linking the magnetic poles (the north-south pairs in every magnetic field) of the stator and rotor, so that the stator RMF will turn the rotor at exact, synchronous speeds. There are many ways to lock these poles, and our articles on reluctance motors and brushless DC motors give specific examples of these mechanisms. Note that the brushless DC motor is not an AC motor; this is because synchronous designs do not inherently have to be powered by AC power, whereas induction motors are typically always fed by AC power.
Synchronous motors are not inherently self-starting – that is, these motors often require motor starters to excite their rotors to full speed. These starters are not often implemented with induction motors because those can start from rest without an initial “kick”. To learn more, feel free to read our article on the types of motor starters. Also, even though their speed is synchronous, the speed of synchronous motors is difficult to change, and requires an AC motor controller to allow designers adjustable motor speeds (more information can be found in our article on AC motor controllers). Synchronous motors, while generally more expensive than induction motors, sport higher efficiencies (>90%) and are great choices for crushers, mills, grinders, and other low speed, high-power applications.
Comparing Induction Motors & Synchronous Motors
Since these two types of AC motors are still quite broad, this article will give general comparisons between the operating characteristics of each type, so that designers can use this information to further define the best-suited machine for their specifications. Below, in Table 1, shows a qualitative comparison between certain characteristics shared between induction motors and synchronous motors and visualizes the advantages and disadvantages of each AC motor design.
Comparison of induction motors vs. synchronous motors.
No (always lagging)
Yes (can lead and lag)
The complexity (or lack thereof) of induction motors is the best advantage that they have over synchronous designs. They are very simple to manufacture, operate, and maintain, and is why induction motors are by and large less expensive than synchronous motors. Conversely, implementing a synchronous machine requires a more complex rotor, which is more difficult to manufacture/repair, and requires additional circuits that must be bought and installed so that these motors can operate effectively.
As previously stated, induction motors are generally self-starting while synchronous motors are not. This means that induction motors require less external peripherals to work effectively, bringing down both their cost and complexity.
Power-density is the amount of power (typically measured in units of horsepower HP or kilowatts kW) generated per unit volume of the motor. Synchronous motors generally have a higher power density than induction motors of comparable size, allowing them to provide more power at a smaller volume. This is great for size-constrained applications and is a reason to choose a synchronous motor over an induction motor.
Synchronous motors can achieve efficiencies of >90% in some cases and are generally more energy-efficient than induction motors. The efficiency depends on the specific motor type and size, but the lack of slip in synchronous motors means there is less energy lost in converting between electrical energy and mechanical energy. Power-factor is the ratio of working power to apparent power and is given as a percentage to show the efficiency of power distribution and its associated losses.
For example: A factory needs to run at 1000 kW (the working power), and the electrical meter connected to the power supply reads 1250 kVA (the apparent power, which has units of a kilo-Volt-Ampere, or kVA, and is used to express energy to inductive loads such as motors coils, wires, etc.).
The power factor for this factory is therefore 1000/1250 = 0.8 or 80%, meaning that only 80% of the current to the factory is doing useful work and 20% is lost to heat and other inefficiencies. Engineers can help recover these losses using synchronous motors to “lead” the power factor, or generate energy back into the system (remember, that motors can also work as electric generators if given an input rotation). Oftentimes, synchronous motors are paired in tandem with induction machines to correct the inductive power losses of the induction motor, which represents another huge benefit of synchronous motors.
Finally, a common theme between synchronous motors and induction motors is their price separation. For reasons previously explained, synchronous motors are more expensive to produce, implement, maintain, and repair than induction motors. However, a case can be made that their energy savings and power-factor correction abilities may make up for their higher initial costs. Whether ot not this holds true will ultimately depend on the specific applications at hand, but should be considered, as total life cycle cost should always be minimized in any project.