Protecting against dielectric fatigue
Like the engines in conventional vehicles, the electric motors in EVs are designed for longevity, but every device will wear out eventually. While motors have many fewer moving parts than ICEs, they are under considerable electrical and mechanical stress.
A properly designed electric motor should have a useful life that far exceeds the life of the vehicle, however, some fail faster than others. According to Stephen Tuckwell, VP and Business Line Manager at ELANTAS PDG, an electric motor’s lifetime can be highly variable.
“People have experienced motors failing within three to five years of use if they’re really badly designed,” he said.
ELANTAS is a 99-year-old company that produces insulating materials for the electrical and electronic industries. Motors are one of the prime markets for these materials.
One potential cause of electric motor failure is a specific weakening of the insulation system that Tuckwell describes as dielectric fatigue. This phenomenon occurs as the insulative coating around the motor’s magnet wires gradually degrades due to something called partial discharge, also known as corona.
“Partial discharge happens when you have a high voltage in the atmosphere,” Tuckwell explained. “Anything over 4,000 volts and you’ve got the potential of creating corona.”
Corona and dielectric fatigue
The variable speed of an electric motor can create high-frequency, high-voltage spikes. If the voltage of these spikes surpasses a critical threshold, corona will occur in the motor’s magnet wires. This degrades the insulation around the magnet wires and ultimately can lead to motor failure. Corona is particularly likely to occur when an EV’s motor accelerates from zero to a high speed.
“What happens is that continued occurrence of the voltage spikes and the generation of corona deteriorates the dielectric properties of the insulation system,” Tuckwell continued. “It forms kind of a carbon track, what they call treeing, so you get these little tracks that are developed in the insulation. Finally, when those tracks break through, you’ve then got a potential dielectric fault situation occurring.”
The insulation system Tuckwell describes is in the form of a thin coating around the thin magnet wires in the motor. The tracks caused by dielectric fatigue are holes in this coating that start at the magnet wire and branch outwards. If one of these holes breaches the exterior of the coating, it can be enough to cause the entire system to fail. Dielectric fatigue, therefore, isn’t a problem to take lightly. And it’s a problem that affects all variable-speed electric motors, not just those used in EVs.
“It was discovered some twenty or so years ago when the industry moved from constant speed motors, where they used gearbox and belts to change the speed, to power electronics to control the speed of motors,” Tuckwell recalled. “They started finding motors that were failing earlier than expected, and when they analyzed it they found that there was this faster deterioration of the insulation material.”
Combatting this deterioration is a step towards improving the longevity of a motor. Electric motors are used in many applications apart from EVs, and there are different expectations in different settings. But no matter the case, the longer the motor lasts, the better. Unfortunately, in the EV industry, motor lifetime is something of an open question.
“Within the industrial motor realm, we expect the motor to last for 20,000 hours of use,” Tuckwell said. “In the electric vehicle world we’re hearing anything from 8,000 hours to 10 years. I will say that’s an area in the industry which they’re still trying to decide.”
One can mitigate the damaging effects of coronas by making the insulation system of a motor more resistant to the problem of dielectric fatigue. This is the approach ELANTAS has taken with a system that it calls co-SHIELD, an insulation system specifically designed to resist the formation of trees.
co-SHIELD consists of two parts, a primary insulation layer called CORONA-Clear and a secondary insulation layer called CORONA-Protect. Both layers coat the magnet wires in the electric motor. CORONA-Clear is the magnet enamel, the layer directly on top of the magnet wire, and CORONA-Protect is an impregnating resin (also called varnish) that’s layered on top of CORONA-Clear. Both layers protect against dielectric fatigue using an interesting obstacle: nanoparticles.
“We have developed a nanoparticle-containing magnet enamel which is coated on the magnet wire,” Tuckwell said. “So you have a special magnet wire which is identified in the market as corona-resistant magnet wire. Then with that you apply an impregnation resin which has nanoparticles in it. So now you’ve got an insulation system which is composed of your magnet wire and impregnation resin, but it incorporates nanoparticles.”
So how do the nanoparticles combat dielectric fatigue? In a typical insulation system, a corona has the effect of energizing electrons to move through the insulative layer. Over time, the movement of many electrons carves out a track in the shape of a tree. In the co-SHIELD system, it’s not that coronas don’t occur, or that they don’t affect the electrons in the magnet wires. Rather, with nanoparticles embedded throughout the insulation, the electrons are not able to move as easily. Electrons will encounter nanoparticles and be deflected, which disperses their energy in different directions. Because of this, the electrons are unable to form tracks.
“Rather than having a consistent path being created by the electron, it keeps being deflected in different directions. So it doesn’t have the opportunity to develop a track,” Tuckwell said.
The insulative layers of CORONA-Clear and CORONA-Protect consist of more than just a single sheet apiece. The magnet wire will be coated in from five to twenty layers, depending on variables including wire size and the enamel machines used.
“We always say that if you take a cross-section it looks like an onion,” Tuckwell said. “You can see the multiple layers of the coating. And the reason why you do that is to ensure that you get a perfect insulation film, coating, for the final product on the wire.”
The benefits of co-SHIELD and co-SHIELD Plus
ELANTAS tests its insulative systems with a method called pulse endurance, in which electric pulses of ±1,000 V are sent through the system until it breaks down. The longer the system can withstand these pulses, the stronger it is. The co-SHIELD system performs very well in this test, according to Tuckwell.
“When we evaluate the lifetime of the system, we get an 18x improvement,” he said. The numbers on the datasheet are even more impressive: during the pulse endurance test at 150° C and 20 kHz, CORONA-Clear endures longer than 6,100 minutes. In contrast, a typical MW 35 coating with a THEIC Polyester base coat endures for less than 10 minutes.
While CORONA-Clear and CORONA-Protect comprise the co-SHIELD system, a third element in the mix constitutes what ELANTAS calls co-SHIELD Plus. That element is ELAN-Film HT-180, a flexible electrical insulation layer that adds an extra 10 kV per mil of dielectric protection and a phase-to-phase barrier.
“It also has a thermal endurance of 180° C for 20,000 hours,” Tuckwell said.
The co-SHIELD and co-SHIELD Plus systems are currently being used by some of ELANTAS’s customers with great success, according to Tuckwell. “We’ve been selling it for a little while, and we’ve got a few customers already using it,” he said. “Every application we’ve gone into, it’s worked as we said it would work.”
This article appeared in Charged Issue 41 – January/February 2019 – Subscribe now.