If you’ve ever dabbled with Switched Reluctance Motors (SRMs) — or even just seen them work for EVs, industrial pumps, or renewable energy systems — you know they’re tough, efficient, and way less finicky than your average permanent-magnet motor. But here’s the thing: a good SRM is only as good as its position sensor. That’s the secret sauce our team bangs on about every day as an SRM supplier — get this part wrong, and you’re looking at messy torque ripple, wasted energy, or even stuck motors. Switched Reluctance Motor
Let’s break down what position sensors we actually use for SRMs, why each matters, and how we pick the right one for a job. No overly technical jargon, I promise — this is the stuff we talk about over our morning coffee, not a college lecture.
First off, let’s level set: SRMs don’t have permanent magnets on the rotor. That means the motor controller has to know exactly where each rotor tooth is at all times to energize the stator windings at the perfect millisecond. Mess up that timing, and you get nothing but noise and lost power. So the sensor isn’t just a “nice-to-have” — it’s the eyes of the whole system.
The OG: Hall Effect Sensors
If you’ve been around motor tech for even a hot minute, you know Hall effect sensors. They’re the first thing most people reach for when they need position sensing, and for good reason — they’re cheap, tiny, and super reliable. How do they work for SRMs? Simple: we mount three Hall sensors on the stator housing, spaced 120 electrical degrees apart (same as the stator poles), and as the rotor spins, each tooth passes a sensor, which sends a tiny voltage pulse to the controller. The controller uses those pulses to figure out where the rotor is, when to flip the windings on or off, and how much torque to apply.
We use Hall sensors for a ton of our lower-power SRMs — like small conveyor belts, light-duty pumps, and entry-level industrial automation. They’re not perfect, though. If you have a high-speed motor, the pulses can get a little jittery, and they only give you discrete position points (you can’t get super-precise, real-time position data between the sensor pulses). That’s fine for a conveyor that’s moving at a steady speed, but not so great for a 100kW SRM in an electric bus that needs smooth, fast torque changes. Also, they can be finicky if there’s a lot of electrical noise from the motor itself — but we’ve gotten pretty good at shielding the wiring to fix that.
Resolvers: The Workhorse for High-Power SRMs
For our bigger, badder SRMs — think electric vehicles, wind turbine pitch systems, and heavy-duty mining pumps — we go with resolvers. If you’re not familiar, a resolver is a rotary transformer that spits out two sine waves: one sine and one cosine, 90 degrees out of phase. The controller measures the amplitude of those waves and crunches the numbers to get the exact rotor position, way more precisely than Hall sensors.
What’s awesome about resolvers? They’re basically indestructible. They can handle extreme heat, vibration, and electrical noise — all things you deal with in heavy industrial or off-road EV environments. We put resolvers on pretty much all our medium-to-high-power SRMs now, because their precision is non-negotiable. For example, in an electric delivery truck, you need super smooth torque to avoid jerking when the driver hits the gas, and resolvers give you that. The only downside? They’re a bit pricier than Hall sensors, and you need extra circuitry to process the sine/cosine signals, but for the performance you get, it’s worth every penny.
Encoders: High-Precision for Specialized Jobs
If you’re working on something super precise — like a robot arm, CNC machine, or lab-grade test equipment — we use encoders. There are two main types here: incremental encoders and absolute encoders. Incremental encoders send a pulse every time the rotor moves a tiny step, so you can track speed and direction, but you lose your position if the motor turns off. Absolute encoders, on the other hand, give you a unique code for every single rotor position, even if the power cuts out. That’s a game-changer for robotics, because you don’t have to home the motor every time you turn it back on.
We don’t use encoders for every SRM — they’re overkill for a simple pump — but when precision matters more than cost, they’re our go-to. One of our clients makes surgical robots, and they needed an SRM that could move a tiny cutting tool with sub-degree precision. We paired a small SRM with a high-resolution absolute encoder, and it works like a charm. The only catch with encoders? They’re more sensitive to dust and grime than resolvers, so we have to seal them up tight in applications where there’s a lot of debris.
Sensorless Options: When You Can’t Fit a Sensor
Wait a second — what if you can’t mount a physical sensor on the motor? Maybe it’s too small, or there’s no space, or cost is super tight for a disposable application. That’s where sensorless control comes in. Instead of a physical sensor, the controller measures things like the current and voltage of the stator windings, and uses those readings to guess the rotor position. The basic idea is that the inductance of the stator windings changes as the rotor teeth pass by — when a rotor tooth lines up with a stator pole, inductance is high, and when it’s not, it’s low. The controller picks up on those inductance changes to figure out position.
We’ve been playing around with sensorless SRMs a lot lately, especially for low-cost, high-volume applications like small home appliances. The upside? No extra parts, simpler wiring, lower cost. The downside? Sensorless control is way trickier to pull off, especially at low speeds. If the motor’s spinning really slow, the inductance changes are tiny, and the controller can’t tell what’s going on. Most sensorless SRMs we work with only work above 10-20% of their maximum speed, so you still need a Hall sensor to get the motor started (that’s called a “start-up sensor”). We call this hybrid approach “sensor-assisted sensorless,” and it’s been a huge hit for one of our clients making smart washing machines — they don’t want a $5 sensor on every motor, but they need it to start smoothly.
How We Pick the Right Sensor for Your SRM
As an SRM supplier, this is the part we get asked about all the time: “Which sensor do I need?” The answer isn’t one-size-fits-all, of course. Here’s how we break it down for every project:
- If it’s a low-power, low-cost application (small pumps, conveyors, small appliances): Hall effect sensors are perfect. They’re cheap, easy to install, and reliable enough for the job.
- Medium-to-high power, harsh environments (EVs, mining, heavy industry): Resolvers are non-negotiable. They handle vibration, heat, and noise like champs, and give the precision we need for smooth, efficient operation.
- Ultra-precision applications (robotics, CNC, medical devices): Absolute encoders are the way to go. No homing, sub-degree accuracy, exactly what you need for delicate, precise moves.
- If cost is king and speed isn’t super low: Sensor-assisted sensorless control. We can eliminate the physical sensor without sacrificing start-up performance, which cuts costs a ton.
And let’s be real — we’re not just slapping a sensor on a motor and calling it a day. We test every sensor with our SRMs for weeks, making sure it works in the exact conditions you’ll be using it in. If you’re running a motor in a dusty sawmill, a resolver that’s sealed tight is better than an encoder that can’t take the grime. If you’re building a lightweight robot for warehouse picking, a small encoder is better than a bulky resolver that adds too much weight.
Wait, one more thing — common mistakes we see people make when picking SRM position sensors. A lot of guys just grab the cheapest sensor they can find, only to realize it can’t handle the electrical noise from the SRM itself. SRMs switch windings on and off really fast, which creates a lot of electromagnetic interference (EMI). Cheapo Hall sensors don’t filter that noise well, so you get position errors that make the motor vibrate or lose efficiency. Or they pick a resolver for a tiny, low-power motor, which adds $20 to the cost for no reason. We avoid that by chatting with you early on — what’s your motor’s power? What environment is it going in? How precise do you need it? That’s how we match the right sensor, not just grab whatever’s on the shelf.
At the end of the day, position sensors are the unsung heroes of SRM performance. They’re not the flashy part like the motor core or the power electronics, but without them, the whole thing falls apart. As an SRM supplier, we’ve spent years testing every type of sensor, knowing exactly what works and what doesn’t for different applications. Whether you’re building an EV, a robot, a pump, or anything else that needs a solid motor, we’ll help you pick the right position sensor — no jargon, no pressure, just straight talk about what will work best for your project.
Permanent Magnet DC Moto If you’re working on a project right now and need to talk through what position sensor is best for your SRM, or you want to learn more about our motors and how we integrate these sensors, reach out to our team to connect and discuss your requirements.
References
- Miller, T.J.E. Switched Reluctance Motors and Their Control. Oxford University Press, 1993.
- Bose, B.K. Power Electronics and Motor Drives: Advances and Trends. Elsevier, 2006.
- Griva, G., et al. “Position Sensorless Control of Switched Reluctance Motors: A Review.” IEEE Transactions on Industrial Electronics, vol. 62, no. 10, 2015, pp. 6103-6115.
- AC Propulsion. “Switched Reluctance Motor Design and Applications.” Technical Report, 2018.
- Vas, P. Sensorless Vector and Direct Torque Control. Oxford University Press, 1998.
Zibo Auric Mechanical and Electrical Technology Co., Ltd.
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