Based on the most conservative estimates, electric motors account for more than 50% of all industrial electricity used globally, and 45% of all global electricity. Making industrial motor drive systems even a fraction more efficient would significantly impact global energy consumption and reduce environmental impact. Increasingly stringent efficiency standards are emerging to address energy consumption on a global scale, posing new challenges to power electronic designers.
Wolfspeed silicon carbide provides an excellent solution to improving efficiency in industrial motor drives by enabling efficiency gains of 2.4% and higher by simply replacing traditional IGBTs with silicon carbide. Further redesign with silicon carbide can enable the integration of drives and motors to create smaller, lighter embedded industrial drives.
In this article we will explore how Wolfspeed’s WolfPACK™ power modules deliver up to 50% reduction in losses while enabling smaller, lighter, more thermally stable embedded 25 kW three-phase industrial low voltage motor drives.
Achieve higher efficiency with smaller heat sinks with SiC
A typical motor drive system consists of an AC-DC (Active Front End) stage followed by a DC-AC (inverter) stage. In a 25kW motor drive system with a six-switch active front end (AFE) switching at 45 kHz, designers can realize a 1.3% efficiency improvement in the front-end stage when benchmarking against a silicon switching at 20kHz. A similar improvement can be achieved in the inverter when Wolfspeed’s 30 A rated power module is conservatively benchmarked against a 100 A rated Si-IGBT module, both switching at 8 kHz. Together, these two changes yield an impressive 2.6% in efficiency improvement, 50% reduction in losses system-wide, and help an integrated motor achieve IE4 efficiency standard given the original system was IE3.
One of the most noteworthy improvements that can be made in the inverter with silicon carbide is a significant reduction in system-generated heat, enabling designers to use smaller heat sinks and design overall smaller, lighter industrial motor drive systems.
Fig. 1: 25 kW inverter, Fsw = 8 kHz, 77% reduced SiC MOSFET heat sink: 0.31L (1.6°C/W) vs. 1.37 L (0.73°C/W)
The graphs above demonstrate an improvement in efficiency when using Wolfspeed’s silicon carbide six-pack WolfPACK™ modules vs traditional silicon IGBT modules in a 25 kW inverter with an 0.8 L heat sink. As the power level increases, the junction temperature of 50 A and 100 A rated silicon IGBTs increases, causing them to fail, while Wolfspeed’s 32 A silicon carbide MOSFETs remain stable and well below the failure threshold.
It is important to note that the efficiency improvement above is not only at peak loads but also at partial loads. At some partial loads the efficiency improvement is higher which is ideally suited to the typical load profiles of these machines. Additionally, the silicon carbide device being tested is a lower current-rated part with a junction temperature at max load of 105°C, creating a significant buffer to maximize the permissible system limit, whereas the 50 A IGBT modules is significantly over the limit and the 100 A IGBT is slightly over the limit at maximum load. “Limit” here is defined as 150°C and is based on usual system requirements for max allowed junction temperatures in such systems for power modules.
Fig. 2: 25 kW inverter, Fsw = 8 kHz, larger Si IGBT heat sink: 1.37 L (0.7°C/W), smaller SiC heat sink 0.8 L (0.99°C/W)
To ensure a viable, functioning, and optimized system we increased the IGBT heat sink size from 0.8 L to 1.37 L using a different heat sink and reduced the silicon carbide heat sink by 61% to ensure its junction temperate is increased to reduce the buffer. This resulted in a 77% smaller heat sink for the silicon carbide solution as compared to IGBT. Despite these modifications, the 50 A IGBT is still significantly above the 150°C temperature limit, but our 32 A part and the 100 A IGBT end up at the same junction temperature of around 129°C. Also noteworthy is that efficiency in the silicon carbide inverter increases by 1.1%. In summary, using a reduced and more optimized-heat sink with silicon carbide in a 3-phase supplied, 25 kW system results in an overall 2.4% efficiency improvement with a 600 W reduction in losses while still achieving IE4 efficiency standards for an integrated motor that was IE3 initially.
Achieve up to 50% less losses system-wide at no additional cost
Silicon carbide presents a tremendous value at the system level in industrial low voltage motor drives. While the upfront cost of a silicon carbide device might exceed that of traditional silicon IGBTs, the higher switching frequency and lower losses means less investment on passives and heat-sinks.
This optimized system can result in a savings of up to 605 W, which when considering a varied load profile operating annually for 8200 hours, would result in an annual saving of 1,297.8 RMB based on electricity costs in China as of November 2023. for a 25 kW system and accumulate up to ~19,000 RMB over the next 15 years. Replacing IGBTs with silicon carbide devices might be more expensive upfront, but when we consider the overall system cost, the higher cost of silicon carbide is offset by a reduction in passives all while achieving a new level of efficiency for the industrial motor drive end systems at the same time.
Fig. 3: 25 kW inverter, FSW = 16 kHz, 41% reduced SiC MOSFET heat sink: 0.80 L (0.99°C/W) vs 1.37 L (0.73°C/W)
In Fig. 3 we further support how silicon carbide is enabling superior performance at even higher switching frequencies. Here we increase the switching frequency from 8 kHz to 16 kHz and use a 41% smaller heat sink than the comparable IGBT heat sink. With Wolfspeed’s silicon carbide FM3 six-pack power module we are still above or close to 99% efficiency and close to the 150° C temperature limit at peak load. With a 50 A and 100 A IGBT we begin to thermally fail at round 10 kW and 15 kW respectively, due to increased switching losses. To make these higher current rated IGBTs perform as effectively as Wolfspeed’s FM3 silicon carbide modules designers would need to include a far larger heat sink or higher current rated parts. Interestingly, inverter efficiency with Silicon Carbide at 16kHz is still higher than inverter efficiency with IGBT at 8kHz.
In Conclusion
In conclusion, replacing traditional silicon IGBTs with silicon carbide can achieve up to 2.6% overall efficiency improvements in a 25 kW industrial low voltage motor drive system. High efficiency improvement at higher power levels is possible throughout the load profile, resulting in massive energy savings. Silicon carbide also offers improved power density due to smaller passive components and heat sinks and leads to overall system cost and size optimization. Additionally, high junction temperature possibilities and improved thermal dissipation of SiC devices together with lower losses allow designers to build more compact systems, enabling an easy integration of drives and motors.
Learn more about how Wolfspeed is powering the evolution of industrial low voltage motor drives at Wolfspeed website.