If you’ve ever walked through a factory floor, you’ve probably heard the low hum of asynchronous motors—those workhorses that power conveyor belts, pumps, and compressors across industries. As a supplier of low-voltage asynchronous electric motors, I spend a lot of time answering questions from plant engineers who need to customize motor setups for unique operational needs. One question that comes up again and again is about series connection: “What happens when I wire two low-voltage异步 asynchronous motors in series, and what are their key characteristics?” Низковольтные асинхронные электродвигатели
First, let’s ground this in basics. Low-voltage asynchronous motors (LVAMs) run on 1,000V or less, use a rotating magnetic field to turn their rotor, and are designed for direct line or variable frequency drive (VFD) operation. When you connect two of these motors in series, you’re linking the stator windings of one motor to the stator windings of the other, so the same current flows through both, and total voltage is split evenly (ideally) across each unit. It’s not a wiring choice to make casually—misalignment can lead to overheating, uneven torque, or unexpected shutdowns. But when done right, series-connected LVAMs have distinct characteristics that solve specific problems standard parallel setups can’t touch.
The first big characteristic is voltage division under load. Let’s say you have two identical LVAMs rated for 480V each. If you wire them in series across a 960V supply, each motor should see exactly 480V at steady state—no single unit has to handle the full line voltage, which is a huge win for motors that might otherwise be pushed beyond their insulation or voltage rating. This is especially useful for retrofitting older facilities that need more power than a single LVAM can provide, but don’t want to upgrade their entire electrical infrastructure to support a higher voltage. For example, a food processing plant I worked with last year needed to replace two 10HP motors driving a dual conveyor system, but their existing panel only supported 960V (they couldn’t afford to rewire for 1,920V). Series connection let them use two identical 10HP LVAMs to get the total power they needed, without overloading their electrical system. But here’s the catch: voltage division only works if the motors are well-matched. If one motor is slightly worn (its stator windings have higher resistance), it’ll draw more voltage, leading to uneven load. That’s why I always advise clients to use motors from the same production batch, or at least the same model and age, when wiring in series.
Next, series-connected LVAMs share current, but their torque response is different from parallel setups. In parallel, each motor draws its own current from the line, and total torque is the sum of each motor’s individual torque. In series, current is the same through both motors, so total torque is limited by the weakest motor in the chain. Wait, that’s a downside, but it’s also a feature. If you’re powering a system where both motors need to stay synchronized (like a conveyor carrying delicate food products, where uneven torque could spill goods), series connection enforces sync because current is identical. If one motor slows down, current spikes until both adjust. Parallel setups rely on mechanical couplings to sync, which can wear out over time, but series connection creates an electrical sync that’s more reliable for constant-load applications. I once had a mining client use series-connected LVAMs to drive a dual hoist—they had issues with mechanical coupling slippage in parallel, but series connection eliminated that problem almost entirely, reducing downtime by 30% in the first six months.
Another key characteristic is reduced starting current. Direct-on-line (DOL) starting of LVAMs can draw 5–7 times their full-load current, which strains the supply grid and can trigger breakers. When you wire two motors in series, each motor gets half the starting voltage (for a 960V line, 480V per motor) during the initial startup phase, so starting current is cut roughly in half. That’s a massive benefit for facilities with weak power grids, where high starting currents cause voltage dips that disrupt other equipment. For example, a small manufacturing plant I supply to has a 480V grid that can’t handle a 20HP DOL start. They used two 10HP LVAMs in series, so starting current dropped from 120A to 55A, which was well within their grid’s capacity. Now, they can start their entire assembly line without tripping breakers mid-shift. That said, you can’t just wire any two motors in series for soft starting—you have to make sure their locked rotor current (LRC) ratings are the same. If one motor has a higher LRC, it’ll draw more current during startup, which can cause overheating.
Heat management is a big part of LVAM performance, and series connection changes how heat is distributed. Normally, parallel motors each generate their own heat, with cooling tied to their individual shaft speed. In series, both motors run at the same speed (since current is identical), so their cooling fans operate at the same rate. But because voltage is split, each motor’s power output is half the total, so each generates roughly half the heat of a single motor for the same total power. Wait, but if the motors are mismatched, the higher-resistance motor will generate more heat, leading to uneven temperature. That’s why I always recommend adding temperature sensors to both motors when running in series—if one sensor detects a 10°C or higher difference from the other, that’s a sign they’re mismatched and need to be serviced. I had a client skip this step a few years ago; their motors ran for three months before the overheated one burned out, leading to a $20,000 repair bill and a two-week shutdown. Since adding sensors, they’ve had zero issues with series setups.
Now, let’s talk about limitations, because no characteristic is all good. Series-connected LVAMs are less flexible than parallel setups if you need to adjust power on the fly. In parallel, you can add or remove motors without disrupting the others, but in series, the entire circuit is one loop—if one motor fails, the whole system stops. That’s a problem for applications where uptime is non-negotiable, so series works best for auxiliary systems, dual conveyor drives, or hoists where redundancy is built into the design (not just the motors). Also, series setups are not ideal for variable speed operation unless you use a VFD designed for series connection. Standard VFDs are set for parallel loads, so running two series motors on a regular VFD can cause voltage imbalances. That’s why I partner with VFD manufacturers to provide custom drives for series LVAM setups, ensuring smooth speed adjustment without damaging the motors.
Another point: efficiency. When properly matched, series-connected LVAMs have slightly higher efficiency than parallel setups for low-power, dual-motor applications. Because there’s no extra wiring for parallel power distribution (each motor shares the same current path), there’s less resistive loss. But if the motors are mismatched, that efficiency gain vanishes—even turns into loss from uneven load. I’ve tested this with a sample batch of motors: two identical 10HP LVAMs in series had an efficiency of 92.1%, while the same motors in parallel had 91.3%. But when I tested two mismatched motors, efficiency dropped to 89.7% for series, compared to 90.2% for parallel. So again, matching is everything.
As a supplier of low-voltage asynchronous electric motors, I’ve seen firsthand how the right series setup can transform a facility’s performance. But I also can’t stress enough that series connection isn’t a one-size-fits-all solution. It’s best for: retrofits with limited grid voltage capacity, applications requiring electrical synchronization, soft starting on weak grids, and dual-load systems where mechanical sync is unreliable. It’s not best for: high-flexibility power systems, applications needing built-in motor redundancy, or setups with motor models from different production batches.
If you’re considering series-connected LVAMs for your operation, don’t hesitate to reach out to our team to discuss your specific needs. We can help you match the right motors, design the wiring and VFD setup, and even provide on-site testing to make sure everything runs smoothly. Whether you’re looking to reduce start-up current, upgrade an existing system, or solve sync issues, we have the expertise to help you implement a series setup that fits your facility’s goals.
Rail Transit Direct Current Motor References
- Boldea, I. (2017). Electric Drives, 4th Edition. CRC Press.
- Fitzgerald, A. E., Kingsley, C., & Umans, S. D. (2003). Electric Machinery, 6th Edition. McGraw-Hill Education.
- International Electrotechnical Commission (IEC) 60034-1: Rotating electrical machines – Part 1: Rating and performance.
- National Electrical Manufacturers Association (NEMA) MG 1-2021: Motors and Generators.
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