@ IIT Madras, Chennai
Atul from PluginIndia visited the IIT Madras campus and met Prof. Ashok Jhunjhunwaala, who spoke about Electric Vehicles, startups, education and manufacturing in India.
This is a blog and findings from his trip and the interview.
Naturally, we started by talking about batteries. When IITM started work on battery packs 5 years ago, they were quite underwhelmed. Throw in a few cells, add a BMS and voila, you are done. But the real story unfolds when you start using it. The IITM experience with durability was disappointing. 500-600 cycles was what was seen as a realistic life, much lower than the claims made by the cell manufacturers. To provide a context, even the much older lead acid chemistry lasts for 300 to 500 cycles.) So what went wrong? One factor that affects life is capacity. Given cell costs, most Indian manufacturers undersize their batteries. So you are discharging it at higher C rates. (If you draw 1 kW of power from a 2 kWh battery pack, the C rate is 0.5) High C rates hurt batteries. If the C rate were to be 0.2 to 0.3, the battery would deteriorate at a much lower rate.
Another culprit is temperature. Western / Chinese battery materials are optimised for performance in a temperature range of 15 to 30 C. What was needed was customisation specific to Indian temperature ranges. Better thermal designing was required. The other Chhupa Rustom was in the battery mechanicals, which turned out to be as important as the electricals. One parameter that affects battery performance is clamping pressure. For the chemistry to perform, you require an optimal pressure. Vehicle vibrations can change this clamping pressure, leading to a deterioration in performance.
In cylindrical cells, pressure does not matter, but here’s the catch: a high number of contacts leads to a challenge in electrical resistance management. You also need to keep in mind the relatively shorter life of cylindrical cells compared to other form factors. Tesla has done a lot of engineering work in the cylindrical cells area, but it is unlikely that others are going to follow suit. One relative advantage of cylindrical cells though is that they are the easiest to manufacture. So with any new chemistry, the cylindrical cells are the ones that come out first, with the pouches following suit 6 months to a year later.
Now moving to electro-mechanicals – the battery terminal joints. A 2.5 kW motor operating at 48 V will draw around 50 A of current. Say, thanks to improper joining, there is a 1 milli ohm increase in the resistance at a joint. This changes voltage. The potential drop is the product of current and resistance = 0.001 * 50 = 0.05 V. What needs to be kept in mind is that the range of operation of a typical Lithium cell is merely 0.6 V, from 3.5 to 4.1 V. So for a car cell delivering 100 A the voltage drop goes up to 0.1 V. This extra resistance can also be sitting in the current path. As with any system, garbage in is garbage out. This voltage reading is supplied to the Battery Management System. (BMS) Faulty information coming in, implies the BMS taking wrong decisions. Most BMS’ do a good job of protection, but not too great a job of cell balancing, because of the voltage problems mentioned above. The same inaccuracies also lead to inaccuracy in State of Health (SOH) prediction.
The discussion veered towards battery voltages. Ashok swears by 48 V as the highest safe battery-voltage. In fact not only does he advocate 48 V DC in vehicles, he even wants 48 V DC in your homes! As more and more of us put solar panels on our roofs, we can move away from our grid driven AC to battery driven DC lights and appliances. One of the big reasons for this magic number of 48 is safety. You can put your finger into a 48 V line – and not get shocked to death. The upper limit for safety from a human perspective is 60 V. A 48V battery may go up to 57V when fully-charged; a battery with nominal volateg higher than 48V may cross the safe limit of 60V when fully charged and must be avoided, unless there is insulation. However as vehicle weights increase, one needs higher power and therefore may chose higher voltages. The first set of Indian electric cars (Mahindra and Tata) decided to use a 72 V architecture in their EVs. When you do that, you need to take good care of insulation. Ashok reckons that these extra precautions added about ₹50 K to such vehicles.
Next gupshup was battery chemistry. Ashok feels that NMC (or NCA) will come out tops in the immediate battery chemistry race. The reason – energy density. 40% of a battery’s cost is materials. So the lesser you use of materials, the better off you are. Cobalt is an expensive material, but per unit wh, the amount of material required is less. (Today the cell cost with NMC is about $ 100 per kWh. ) NMC batteries can store upto 300 Wh/kg, and 350 Wh/kg seems doable in the near future. LFP, in contrast, currently stores about 140 Wh/kg – with a theoretical storage upper ceiling of 160 Wh/kg. The real killer is going to be solid state batteries which can have energy storage densities of up to 500 Wh/kg. Incidentally, the Suzuki Dentsu Panasonic JV in Gujarat is working on Lithium Titanate Oxide (LTO) cells. LTO cells are more expensive, but have a longer life. LTO is faster to charge, but has the disadvantage of a lower energy density. So LTO is good for petrol-battery hybrid cars, which end up having relatively small batteries.
The Chinese government has an energy density based subsidy on batteries. Anything which is less than 200 Wh/kg is not entitled to government subsidy. One of the advantages touted for LFP is its higher thermal run-away temperatures, 150 C vs 120C for NMC. But Prof AJ feels that when battery thermal runaway starts, in a few milliseconds the temperature will climb from 120 to 150 C. A good BMS should be able to shut down the battery, irrespective of the chemistry, as soon as temperatures cross 70 C. And yes, it is important to ensure that multiple temperature sensors are present in the battery pack, so that battery safety is not compromised.
Ashok feels that Chinese manufacturers are keen to promote LFP in the Indian market, so that their manufacturers can dump their LFP spare production here. He believes that there is no capacity addition happening in LFP production in China. Niti Aayog till recently was aggressively promoting LFP, but off late they are trying to be chemistry agnostic – and trying to follow the Chinese subsidy strategy. So LFP seems to be on its way out in India too, according to Prof AJ.
Prof AJ introduced me to one of the senior team members at the Research Park, Prof L Kannan. Apart from being associate faculty at IITM, Kannan is also COO at Motorz Design and Manufacturing Pvt Ltd. (www.motorz.in) Was lucky to get a crasher on how EV motors work – and what work IITM is up to in the motors space. The permanent magnets are usually placed in the part that rotates, the rotor. It is easier for all electrical connections to be made in the stationary part, the stator. There are two types of permanent magnet motor designs based on the relative position of the stator and rotor. One: stator outside, rotor inside. Two: rotor outside, stator inside. A hub motor, and also a ceiling fan motor, fall into the second category.
Then there are also the axial flux motors, which are becoming popular with heavier vehicles like trucks. As the name indicates, In the axial flux motor, the magnetic lines run parallel to the axis of the motor shaft. This requires lesser use of copper. In order to connect the windings in the radial flux motor, you need some of the copper conductors to be placed outside the magnetic field. This adds to the resistance and cost – while not contributing anything to torque. Axial flux motors do not require end winding conductors, so there is no overshooting outside the magnetic area. Per kg weight, axial flow motors have lesser losses and higher speeds. Higher speeds mean more efficiency as you drive fewer amps in the copper.
Having said all this, the reality is that most low-power electric bikes sold in India use hub motors. Kannan went on to give me a tutorial on the evolution of the BLDC motor. BLDC stands for Brush less DC Motors. To start with, we need to understand the basics of AC motors that use rings for commutation. If you want to know more about fundamentals of motor operations, you can watch this video. The next step in motor evolution was the PMDC or Permanent Magnet DC motors. In a permanent magnet motor, you get a magnetic field for free, as you don’t have to use electricity to generate the field. In these motors, brushes and commutators are used to switch the direction of the current in the rotor.
These permanent magnet motors needed brushes because the rotor had the copper winding and you had to transfer the current to the rotor through brushes. However if you put the windings on the stator, life becomes easier. One problem does crop up though. A rotating commutator makes for an automatic switch, but with a stationary winding you need to have electronic switching. The advantage of electronic switching is the elimination of the brush. And this is what gives the BLDC its name, brushless. However, the DC is a misnomer. The only relevance in the DC is that the input is from a DC battery. But what the electronic switching does is to convert the battery DC to AC, so in theory the motor should have been called the very Afro sounding BLAC. But the BLDC baptism was done by the Americans, and being marketing gurus, they have ensured that the branding stuck.
In motors today what is important is the torque density, which is the ratio of torque that the motor can deliver to the volume of the motor. Rare Earth magnets have smaller form factors. So using a rare Earth magnet makes the motor smaller and more efficient.
IITM is doing work on motors, but that is on the next generation PMS motors. PMS stands for Permanent Magnet Synchronous. The difference between BLDC and PMS is simply in the way the switches convert DC to AC. The easiest way to convert DC to AC is to just change current directions once after every sixth of a rotation. This is what will give you in electrical jargon, a square wave. If you know your AC, then you realise that it is a rough approximation of the sine wave that characterises AC. But this approximation can be made better. How do you do that? Using what in electrical jargon is called Pulse Width Modulation.
Let’s use geometry to illustrate. You have to approximate a semi circle, using rectangles. The simplest way is to have half of a square, and that is what is a square wave. Now imagine doing it with 3 rectangles of equal widths, one at the center – and two smaller ones on the sides. The height of the rectangle is such that it is cut by the circle at the center of its width. Reduce the width further, and you get 5 small rectangles, and continue the process till you have a hundred small rectangles, and now you will need to look really hard to even realise that it is actually not a smooth curve, but just a collection of rectangles.
The height of each rectangle is the voltage. Let’s say we want an AC current which is 48 V. A square wave will have + 48 V across the motor conductors for half the time, and -48 V for the other half. (This will happen in bursts of, say, 0.01 s). Now if we want the voltage to be reduced to 24 V, what do we do? We just change the time that we fire the bursts. In the 0.01 second interval, we only do our firing for 0.005 seconds and we do nothing for the remaining 0.005 seconds. This means that our average voltage is now (48 + 0)/2 = 24 V. Manipulating the averages this way, we can get whatever voltage we want – and in effect, this is what is PWM.
In the initial days, there was a limitation on the hardware and the software, so the switching was clunky, leading to the BLDC square waves. Pulse Width Modulation nowadays changes the voltage to a motor in short pulses that are injected in at the rate of 10,000 times per second. In a synchronous motor the current flow is nearly sinusoidal. This results in better efficiency and smoother operation. The flip side is that it requires a lot more complicated electronics to do all that switching. The same pulse width modulation principle is also used to create variable frequency AC drives.
Another innovation that the IITM lab had done is related to magnet placement. The operating principle of motors (or generators) is based on the interaction between force, magnetism and electricity. A magnet creates flux around it,and when a magnetisable material passes through this flux, it faces something akin to an electrical resistance, which in magnetism is called reluctance. A compass has a tendency to minimise its reluctance in the earth’s magnetic field. This causes the needle to align in the north south direction.
When you have raised magnets on the rotor surface, the reluctance of the magnetic material and the reluctance of the air, which is present in the gaps between the magnets, is the same. But the reluctance of a magnet is more than the reluctance of steel. So when the magnets are recessed into the surface, there is a change in reluctance as the rotor rotates (reluctance asymmetry). This causes the buildup of a reluctance torque, which is a bonus over the conventional torque, adding to the motor efficiency. Another advantage of this process is that since the magnets fit into a slot, there is no tendency for the magnets to fly away. By the time Kannan came to this part, lunch time was approaching, and it was time for me to fly away to the canteen, but not before another of Prof’s colleagues, Anson, took me around the research park.
Excerpted from Wikipedia: When Dr. Jhunjhunwala joined IIT Madras in 1981, as an academician he observed that there was not enough interaction between industry and academic institutions. He recognized that if products and services were to be used by the bulk of Indian Society they had to be made affordable. To further this objective Dr. Jhunjhunwala initiated the setting up of the IITM Research Park, adjacent to the IITM campus. The 1.2 million sqft space today has about 90 R&D centres of industries.
The Research park was set up with an initial capital investment of Rs. 500 cr, out of which Rs. 400 cr was borrowing from banks. There are five buildings in the research park. One each is devoted to software, mechanical and healthcare sectors. The other two buildings house incubation cells and whole variety of areas and also has the common facilities areas like auditorium, amphitheater, food-court and a hotel. There are 7 centres of excellence. Today the Park generates an annual cash profit of Rs. 30-40 cr. The rentals at the research park are lower than what neighbouring office complexes charge. This is in part to incentivise collaboration with academia. In order to quantify a collaboration metric, corporate research labs have to earn 150 credit points for every 1000 sq ft of space that they occupy. And the top management of these research labs have their variable salary linked to these credit points.
I started with a visit to the battery excellence centre. One of the IITM projects at this center is a swap station. Anderson connectors are used because there is a CAN bus included in the port, which is important for transferring data from the battery to the cloud. We also had a look at an interesting public charging station, which does not require any internet connectivity. It’s based on an OTP system. And this OTP is a derived value from a certain public parameter. At the lab, battery testing was being done at temperatures ranging from 5 to 60 degree Celsius to see the effect of temperature on battery life. I was shown a battery pack which has been developed at IIT Madras itself. The interesting part of this pack is the use of a phase changing material to absorb excess heat from the cells. You can think of this phase changing material as something like butter or wax, which melts and absorbs heat. I was a little unsure about how the absorbed heat is released to the environment. But I guess this is only an emergency device.
One insight that I got into charging was that the battery life is enhanced when you charge at a slow rate and at a colder temperature. So in Indian conditions, charging overnight is best for the health of the battery. One lateral thinking idea: If heat pumps become popular for water heating, then we can look at delivering the cold ‘waste’ air of the heat pump to the EV charging station area. This will ensure a cooler environment for battery charging. I was told that Sun mobility does all its battery charging in air conditioned areas.
We then moved on to the machine tool centre for excellence. A team was working on deriving a lookup table for a temperature related spindle drift compensation in CNC lathes. This lookup table talks to the sensors and motor controllers and modifies the feed/speed. As a result, surface finish has been improved from 35 microns to 5 microns. An orbital cutting machine was being developed for the Chennai-based Metco. Orbital cutting is required when you are looking at taking samples for metallurgical analysis in steel Mills. The most interesting part was to see a robotic arm being worked on for MTAB. It can lift jobs ranging from 6 kg to 14 kg weight. The price ranges from 9 lacs to 23 lacs. I am sure my friend Sudeep will be interested. The machine centre was also working on a hydrostatic bed for Delhi based Micromatic grinding. They have filed three patents in this area. One of the patents is about an algorithm which changes wheel speed based on other grinding parameters.
And when the potential entrepreneurs walk in, the center does the reverse. It dehypes the glamour of the entrepreneur. It talks of the financial sacrifices. It reminds them that being engineers they are clueless about how marketing and people management work. The students are grilled about how they have handled failures in the past. After the team is convinced about the mettle and the grit of the student, they are let in. The incubatee company is then assigned a mentor, who they meet once in 15 days. A board is also created which meets every month to take stock of work and help decide future milestones.
Incubatees stay at the incubation centre for time periods ranging between 18 to 36 months. There are 75 companies, including Ather, that have graduated from the centre. 93 are still in the incubation phase. 30 have failed to make it and have shut down. All in all, an amazing startup survival rate! I met with one of the incubatees, a company working on an air taxi. The team talked of using a thrust to power ratio of 4 to 6 for their craft, using ducted fans. Compared to a propeller a ducted fan has better efficiency and lower noise. They are in the first prototype stage now, where they are looking at carrying 1 kg payload. The next prototype will be a 6 kg payload one, made of carbon fibre. Another interesting incubatee is the Pimo electric budget bike is priced at Rs, 30 K.
We had a chat about my favourite incubatee, Ather. Professor informed me that they have an eight month order book which is full. They expect to break even next year. Ather tried working on cell manufacturing. They hired 9 people from IIT Bombay and were working on NMC cell chemistry. But they closed down cell manufacturing research after a few months. Ather’s motor is being imported from Mahle’s Stuttgart plant.
We ended with a lovely sambar-rasam-rice meal at the Research park canteen. As I came to the end of this visit, I reflected on the anecdote Ashok had narrated about the days he was starting off the work on the research park. His mentor at IIT Madras, a senior professor, questioned the wisdom of inviting Laxmi into the house of Saraswati. But for our frugal professor, the personal Laxmi has always been irrelevant. The bigger task has always been one of increasing the nation’s wealth. And as we look back on his long journey, I am happy to conclude that the sisters, Laxmi and Saraswati, have led a happy life together at IITM.