Induction vs Synchronous Motors: Advantages & Disadvantages comparison

The purpose of this article is to simplify the comparison, differences, similarities, advantages (benefits) and disadvantages of AC Synchronous and AC Induction Motors.

70% of all the energy produced in the world are utilized by electric motors. 70 % of all industrial machines in the world account for 3-phase AC Induction Motors. The vast majority of motors used globally are induction motors. These motors are used everywhere in our daily lives in homes and offices.

Thus, it is essential to know the basic information about these motors in order to make an informed and intelligent decision when buying.



AC SYNCHRONOUS MOTOR Advantages: 

Efficiency:
- higher efficiency than induction motors (Permanent Magnet Synchronous Motor efficiencies are between 93% and 98%)
- more efficient than induction machines because there are no conductor losses in the rotor  
- synchronous reluctance motors have no rotor losses

Power Factor:
- flexible power factor (lagging, unity or leading)
- can be used to improve the power factor of the system
- used to make the overall system power factor higher (e.g. utility companies charge a power factor penalty if your power factor falls below 0.90 )
- can operate from lagging power factor to leading power factor by simply changing its excitation
- possible to alter the power factor by varying the excitation of the rotating DC field supplied to the motor
- synchronous motors can supply reactive power to counteract lagging power factor caused by inductive loads
- when DC field excitation is increased, the power factor (measured at the motor terminals) becomes more leading
- when DC field excitation is decreased, the power factor of the motor becomes more lagging

Power:
- synchronous motors are best suited for very high power at lower speeds
- power varies linearly with the applied voltage
- provide higher power density (ratio of output power to physical size or volume of motor) because of higher magnetic flux compared to induction motor
- no external power supply for mechanical braking needed

Torque:
- switched-reluctance (SR) synchronous motors produces 100% torque at “stall” indefinitely

Speed:
- speed is constant at any load
- speed is not dependent on the load
- high speed operation helps to eliminate power transmission elements such as gearboxes
- more accurate, more precise speed control (even though voltage, temperature, and load fluctuate constantly)
- SR synchronous motors can operate at higher speeds

Current:
- inrush currents are low

Life:
- Permanent Magnet Synchronous motors operate cooler, which means longer bearing life and longer insulation life
- wider air gaps (less vibration, more stable)


*** SynRM: The Motor of the Future
Synchronous Reluctance Motor (SynRM) are predicted to be the future of motor technology and will be the global dominant motor especially in the
industrial facilities as well as in electric vehicles (electric cars, e-bikes, traction electric vehicles, etc.). The following are its main advantages:
- more economical than an induction motor of equivalent frame size (lesser motor size but provides equivalent power output)
- highly efficient
- 6% energy cost savings compared to traditional induction motor
- 75% reduction in audible noice (quiet operation)
- 58% reduction in frame temperatures (cooler operation)
- no permanent magnets (rotor is made of ferro-magnetic material; no need for rare-earth magnets)
- no windings in the rotor
- no rotor losses
- cooler bearing temperature, longer bearing life (bearing failure accounts for 70 percent of all motor failures)

Syn RMs with variable speed drive (VSD) are cheaper, more efficient, smaller, lower maintenance, lower operational cost and with longer lifetime than any type of motor.



AC SYNCHRONOUS MOTOR Disadvantages:

Efficiency:
- permanent magnet synchronous motors are subject to demagnetization (loss of magnetic properties) when operated at high current or high temperatures

Power:
- synchronous motor is doubly excited (stator is supplied with AC power & rotor is supplied with a DC source)
- needs DC excitation from external sources

Torque:
- starting torque is zero

Speed:
- to achieve variable speeds, it needs Variable Frequency Drives to adjust supply frequency

Starting:
- not self starting (needs starting devices)
- cannot be started with a load applied

Control:
- commercially available permanent magnet motors need a variable frequency drive (VFD) to function

Cost:
- more expensive than induction motor with same power rating
- more complicated and more expensive to build

Loading, Stability:
- hunting (when sudden or variable loads are applied, the motor hunts (swings/seeks equilibrium) 
- when overloaded, the motor stops

Safety:
- synchronous motors with collector rings and brushes produce sparks/arcs, ozone gas (O3), wear due to friction and require maintenance



AC INDUCTION MOTOR Advantages:

Efficiency:
- used when huge power at the highest efficiency is required

Power:
- an induction motor is singly excited (AC power is supplied to the stator only)

Speed:
- induction motors are best suited for high speed

Torque:
- 3-phase induction motors (IMs) have high starting torques (maximum slip & maximum torque at zero speed)

Starting:
- three-phase induction motors are self-starting

Maintenance:
- minimal or no maintenance
- lesser parts to maintain
- no brushes, commutators, slip rings

Construction:
- rugged, sturdy and strong construction
- simple, easy to manufacture
- flexible, robust, operates in any environmental condition

Safety:
- no noises (electromagnetic interference, EMI)
- safer in explosive applications (no arcing, no sparking, no fire hazards)

Operation:
- rotating magnetic field is very smooth
- can operate in most severe environments (harsh environmental conditions, extreme temperatures, etc.)

Availability:
- available in a wide range of sizes from a fraction of a horsepower to thousands of horsepower

Cost:
- cheaper cost

Life:
- longer operational life (average 30 years) compared to synchronous motor that has brushes
.


AC INDUCTION MOTOR Disadvantages:

Efficiency:
- high I^2R losses
- lower efficiency compared to synchronous motors
- single-phase induction motors are less efficient than 3-phase induction motors
- IMs with a short circuit rotor cage (squirrel-cage) have rotor losses of 20–35 percent of the total motor losses

Power:
- 3-phase induction motors require 3 power supply lines (not usually available in residential homes)
- single-phase motors are limited to power ratings up to 15 HP
- larger power are available at three-phase (if you only have one-phase power supply, you need a phase converter device)

Power Factor:
- power factor is always lagging (low power factors at light loads)
- power factor of 0.85 to 0.90 at full load
- power factor of 0.20 to 0.40 at no-load
- power factor correction (e.g. static capacitors) are needed

Speed:
- motor speed is always lesser than the synchronous speed
- do not rotate at the same frequency as the alternating drive current
- turns slightly slower than the AC current frequency because of slip
- speed is dependent on the load
- speed decreases when the load increases
- because their speed is hard to control, variable frequency drives are necessary for effective speed control

Torque:
- cannot produce torque without slip (the difference in the stator’s magnetic speed and the rotor speed; rotor "catches up” with the magnetic field)

Current:
- huge starting current (inrush current) 8-10 times the rated full load current (this causes line voltage drop)

Starting:
- single phase induction motors are not self-starting (requires starting device, e.g. capacitor)





Glossary:
 Rotor = rotating part of an alternator, generator, motor, dynamo

 Stator = stationary part of an alternator, generator, motor, dynamo

 Armature = power-producing component of an alternator, generator, motor, dynamo (can be located on the rotor or stator)

 Field = magnetic field component of an alternator, generator, motor, dynamo (can be located on the rotor or stator; can be an electromagnet or a permanent magnet)

 Reluctance = is magnetic reluctance, or magnetic resistance property in which magnetic energy is stored (rather than dissipated as heat, etc); a magnetic field causes magnetic flux to follow the path of least magnetic reluctance.