It is an age-old, simple and cheap technology, yet shelved for nearly two centuries because it was never easy to use. It is a technology centred around reluctance.

The first ‘switched reluctance motor’ was made in the early 19th century. Reluctance is the property of a material to resist the flow of magnetic flux — similar to ‘resistance’ to ‘current’. The physics behind it is fairly simple. Magnetic flux — the invisible magnetic lines passing through a surface — likes to travel down the path of least resistance (‘reluctance’). A reluctance motor uses this principle. It has a rotor with alternating regions of high and low reluctance and a stator featuring electromagnets. When electricity is supplied through them, the rotor will rotate.

And now, switched reluctance motors are being brought back, mainly to serve the electric mobility revolution.

Why now

The comeback is for two broad reasons: Cost and China.

Motors need magnets; today the best magnets are made of rare earths — neodymium, dysprosium, samarium, strontium, cobalt and so on. China controls the supply and trade of rare earths. Wary of China’s grip, the world has been trying to find other ways of making efficient motors (see ‘The Big Attraction’, in Quantum dated August 2, 2021).

So, if you can make magnets without rare earths, you are a winner. But you are a bigger winner if you can make motors without permanent magnets.

This is where switched reluctance motors (SRMs) come in. All you need to make them are copper and steel. That means lower costs. Steel costs around ₹100 a kg, copper costs ₹800 a kg, while magnets cost ₹6,000 a kg. SRMs, overall, are 30-40 per cent cheaper than conventional motors.

In a motor, when you supply current, you get a torque — the twisting force that creates rotation. To put it simply, the more the current the greater the torque. However, the relationship between current and torque is also dependent on the rotor position and the current through the windings. This relationship, in an SRM, is non-linear, which means if you double the current you don’t double the torque. You have to send just the right amount of current in the windings to make it work, which is difficult.

“Owing to the non-linear behaviour of SRMs, the motor parameters need to be constantly monitored,” says Bhaktha Keshavachar, Founder and CEO of Chara Technologies, a Bengaluru-based start-up, which designs SRMs. “But now, with better computing power and machine learning-based algorithms, it is possible to better control the SRMs,” he told Quantum .

The power of algo

This is how it works: By creating a ‘digital twin’ of the motor in software and mapping to it the observed parameters such as current, voltage and position, the current applied at any instance can be controlled optimally to enable smooth operation and extract the desired performance. “With the increased computing power, it is now possible to perform on-the-edge, real-time optimisation of currents based on the electromagnetic behaviour of the motor,” says Keshavachar.

EVs and beyond

Electric vehicles need high-powered motors, and those that fit the bill contain rare earth magnets — which means dependence on China. Further, the motors in electric vehicles heat up more, which is a problem because degmagnetisation occurs above 150 degrees C.

SRMs can withstand extreme temperatures. A Bengaluru-based start-up called Bounceshare, which rents out scooters at a per km charge, has said it would manufacture 30,000 electric scooters featuringSRMs.

“SRMs are coming back with a bang,” says Amith Bysani, Head of Products, Chara Technologies. Two companies, Turntide Technologies and Advanced Electric Machines, both based in the UK, are bringing out SRMs. Advanced Electric Machines’ technology even allows the use of the cheaper aluminium instead of copper for the windings.

Though SRMs are often hyphenated with EVs, they can gainfully replace any motor. SRMs will “create a new revolution in India,” says Ravi Singh, Partner, Kalaari Capital, which has invested in Chara.

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