Asynchronous and synchronous motor drives in comparison
Medium voltage variable frequency drive (MV VFD) systems mostly utilize two types of electric motors: asynchronous and synchronous machine. This article compares some of the characteristics of both technologies. Aspects such as power and speed range, efficiency, system dimensioning, control and instrumentation and fault behavior are discussed.
Introduction
Two motor types dominate the medium voltage drive systems: asynchronous and synchronous motor. Both serve the industry for decades and have proven track records. Historically, asynchronous motors took the stage from DC drives. However, in high-power range the market availability of asynchronous machines gets limited and the synchronous motor is an attractive alternative. Yet, there is also a trend of some manufacturers to push asynchronous machines to higher power. The debate might get heated sometimes. Thus, in this article we look at both classical motor drive options and compare them in areas such as rated power and highest speed, efficiency, dimensioning considerations, control strategy, required instrumentation, fault reaction etc.
Abbreviations
AFE … Active Front End
DFE … Diode Front End
DFIM … Doubly Fed Induction Machine (Motor)
DOL … Direct On-Line
DTC … Direct Torque Control
EESM … Electrically Excited Synchronous Machine (Motor)
LCI … Load-commutated Inverter
PDS … Power Drive System
PMSM … Permanent Magnet Synchronous Machine (Motor)
VFD … Variable Frequency Drive
VSI … Voltage Source Inverter
WRIM … Wound Rotor Induction Motor
Motor Types
This paragraph provides a very brief characteristic of subject motor types.
A. Asynchronous motor
Asynchronous motors are also called induction motors. Both names refer to the principle of rotation of this machine. Rotor spins asynchronously with respect to the magnetic field on the stator. Thus, a voltage is induced into the rotor. This voltage drives current flowing in the rotor winding. The interaction of stator and rotor flux produces torque. The relative difference between the frequency of stator field and the actual rotation of the rotor is called slip. It shall be noted that slip of a larger industrial asynchronous motor is very small as
illustrated in [1].
From rotor construction point of view an asynchronous motor can be realized with a squirrel cage or with a wound rotor (WRIM). The later design requires slip rings and brushes to provide electric contact. Main purpose is to connect external resistors to shape the torque-speed characteristic and eventually to control the motor speed (resistive control with high losses that is considered as obsolete today). Squirrel cage asynchronous motors are ideally suitable for supply from a VFD (inverter duty). If a wound rotor type of motor shall be used with a VFD, it is recommended to remove the sliprings and install a short-circuit device.
Doubly fed asynchronous machine is a very specific type. Such machine typically has stator directly connected to the grid and rotor supplied through a VFD. It is beneficial in case of a narrow speed range since it results in a relatively small VFD. Such topology is utilized in very specific applications and is not addressed in this article.
B. Synchronous motor
Synchronous motor works on a different principle. In this case the rotor spins in synchronism with the stator field. The produced torque is a function of so-called load angle, i.e. relative angle between stator and rotor flux vector.
Also synchronous machines differ based on the rotor construction. The rotor can be electrically excited (EESM), i.e. having electric winding in the rotor. Instead of winding the rotor can be equipped with magnets. We talk about permanent magnet synchronous machines (PMSM). Finally, the rotor may have neither winding nor magnets and uses variable inductance. Those motors are called synchronous reluctance motors (SynRM).
For our purpose we will just consider the electrically excited motor. Permanent magnet motors are popular in traction applications but in the high power range they are still very rare. Synchronous reluctance motors are even more seldom at medium voltage and higher power. The author is not aware of any running reference in the industry (not talking about research test stands).
EESM requires an excitation unit (a unit supplying the rotor with electric power). Based on operation principle two types of excitation exist: direct (DC) excitation with brushes and sliprings and brushless excitation. The choice depends on factors such as motor speed, application etc. Details on excitation exceed the scope of this article.
Comparison of asynchronous and synchronous motor drives
Asynchronous and synchronous motor drives will be compared using multiple aspects from general capability over performance up to fault behaviour. Note that the comparison is made on a drive system level, i.e. not just comparing the two motor types.
Rated power
Asynchronous machines cover rated shaft power up to some 25…30 MW. Higher power may be possible but the number of available manufacturers reduces drastically. There is one company pushing asynchronous machines up to approximately 80 MW. Special case present doubly fed induction machines in specific applications such as pumped hydro power plants. These machines can reach power of 200 MW or even higher. However, it is a very special machine design and a niche application.
Synchronous machines can be built to almost any power rating, being derived from large synchronous generators in thermal power plants (generators reaching 1’500…2’000 MVA). Above some 20…25 MW, synchronous machines are usually the choice.
High speed design
Both motor types can reach wide range of rated, resp. maximum speed.
Synchronous 2-pole turbomotor can reach up to some …
Low speed & high torque design
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Input power factor
The input power factor of asynchronous machine results from the design. Power factor is always inductive and is not controllable. Typical power factor ranges 0.86…0.94 at rated power. Higher pole number motors usually have somewhat lower power factor.
Synchronous machines have adjustable power factor. Direct on-line (DOL) motors sometimes operate with capacitive power factor (overexcited) to compensate for other loads that are mostly inductive. In combination with VFD, the most meaningful power factor is 1.0 as it leads to the lowest stator current for a given power. The only exception is a load-commutated inverter (LCI) that requires overexcited motor to supply the reactive power for the inverter.
VFD control strategy
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Efficiency
– Motor efficiency
To compare efficiency of asynchronous and synchronous motors we shall use the same base: same or similar rated power and speed. Provided that rated values are similar, synchronous machine typically achieves 0.5…1% higher efficiency at full load. That sounds like a small number but over the lifetime it integrates as a considerable amount of energy. Let’s illustrate it using following example:
– VFD efficiency
The motor technology has also some impact on the VFD efficiency and losses. Asynchronous motor due to its inductive power factor (~ 0.9) requires higher inverter current for the same motor output power than synchronous machine. Thus, the inverter losses when running an asynchronous machine are likely higher. Thus, on a drive system level the synchronous motor drive benefits twice with regards to efficiency: higher motor efficiency (0.5…1%) and slightly higher VFD efficiency (0.05…0.1%).
Torque quality
Torque of inverter fed motors depends on the modulation type (e.g. PWM versus hysteresis control), switching frequency and motor parameters.
Specific challenge is the controllability of torque at very low speed, eventually at standstill. This is traditionally manageable for synchronous machines but is very challenging in case of asynchronous motor.
Additional hardware
Synchronous motor with electrically excited rotor (not a permanent magnet motor) requires a static excitation unit. In case of direct excitation it is basically a thyristor rectifier. In case of brushless excitation it is a triac (two anti-parallel connected thyristors per phase). Otherwise, the key system components of asynchronous and synchronous motor drives are the same.
System cost
Both motors and VFDs are standardized in certain frame sizes with discrete steps. The hardware cost is often project-specific to some extent depending on specific requirements. Thus, we cannot provide a generic statement or formula. However, the considerations are as follows:
– Asynchronous motors require higher stator current for the same shaft power and stator voltage. Considering the typical power factor of 0.91 it results in about 10% higher current. Therefore, a larger inverter is required. Depending on the discrete steps of inverter sizes, the user may use the exact same VFD hardware as for synchronous motor or one frame size larger VFD.
– Synchronous motors are generally somewhat more expensive compared to their asynchronous counterparts. Moreover, they need an additional excitation unit to supply the rotor. On the other hand, the excitation system is relatively inexpensive when referring to the total cost of the drive system.
Fault behavior
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Summary
Medium voltage industrial motor drives in the megawatt range are dominated by squirrel cage asynchronous motors and electrically excited synchronous motors. The choice of motor type affects several operational characteristics, ranging from efficiency, inverter dimensioning, motor control strategy up to fault behavior. Some of the key aspects were listed in this article. The aim was to provide a technology neutral comparison as far as
possible.
References
[1] Challenges of high-speed machine design, MB Drive Services, April 2022, available online, https://mb-drive-services.com/challenges-of-high-speed-machine-design/
[2] Power, speed and torque, MB Drive Services, October 2020, available online, https://mb-drive-services.com/power-speed-and-torque/
[3] What type of asynchronous motor suits best for inverter duty?, MB Drive Services, September 2023, available online, https://mb-drive-services.com/what-type-of-asynchronous-motor-suits-he-best-for-vfd-duty/
[4] Energy efficiency – Part 5: What efficiency can you expect from your drive system?, MB Drive Services, August 2019, available online, https://mb-drive-services.com/energy-efficiency-part-5/
[5] Energy efficiency – Part 6: Power factor and reactive power consumption, MB Drive Services, August 2019, available online, https://mb-drive-services.com/energy-efficiency-part-6-power-factor-and-reactive-power-consumption/
