If you’ve ever stood next to a large industrial motor when it first kicks on, you’ve probably felt that sharp jolt of power, heard the sudden roar, and noticed the lights in the room dim for a split second. That jolt comes from direct-on-line (DOL) starting, one of the oldest and simplest methods to get AC motors up and running. As a motor supplier, I’ve spent thousands of hours talking to plant managers, maintenance crews, and engineers about this method—both the wins that keep them coming back, and the headaches that make them reach for alternatives. Today, I want to pull back the curtain on what DOL starting really is, its pros, its cons, and when it makes sense to use it (or skip it) for your next project. Motor
First, let’s get on the same page: direct-on-line starting means connecting a motor directly to its full power supply, with no intermediate equipment like variable frequency drives (VFDs) or star-delta starters to limit the initial inrush current. When you flip the switch, the motor gets full voltage immediately, and it accelerates to full speed in just a few seconds. It sounds straightforward, right? That simplicity is both its greatest strength and its biggest weakness—let’s start with the advantages.
For any motor user, especially small to medium-sized operations, the cost savings of DOL starting are impossible to ignore. There’s no expensive control equipment to buy, no complex wiring to install, no extra components to stock as spares. When a small woodshop needs a 5HP motor for a table saw, or a local water utility needs a 10HP motor for a booster pump, they don’t want to drop $1,000+ on a VFD just to start it. DOL starters are usually just a contactor, a few fuses, and an on/off switch—parts that cost pennies compared to alternative starting methods. I recently worked with a manufacturing client that needed to replace three 20HP motor starters for their conveyor system. A VFD-based setup would have run them $12,000 total; DOL starters were a $1,800 install, and they’ve had zero issues for two years now. That kind of upfront affordability is a huge win for tight-budget projects.
Next, DOL starting is incredibly reliable when you stick to its sweet spot. There’s less to go wrong with a DOL system than with any other starter—no microchips to fail, no capacitors to wear out, no wiring harnesses to fray from vibration. We see this all the time in applications where the motor is started infrequently, or where the load doesn’t need fine speed control. For example, a large industrial fan that runs 24/7 to vent a factory floor—you start it once, it runs for weeks, and you rarely need to touch the starter. DOL’s simple design means fewer points of failure, which translates to less downtime. When your motor only needs to turn on and off, not speed up or slow down gradually, DOL’s reliability is hard to beat. It’s also fast: a 50HP motor might reach full speed in 2 or 3 seconds, which is ideal for applications that need instant power, like emergency backup pump systems or lifting equipment that can’t afford to slow down.
Another often-overlooked advantage is torque performance. DOL starting delivers 100% of the motor’s full-load torque almost immediately, which is a big deal for high-inertia loads—think heavy mixers, large presses, or conveyors full of raw materials that need to start moving fast. With a VFD, you have to ramp up the voltage gradually, which means torque builds slowly, and if you don’t get the ramp time right, the motor might not be able to overcome the load at all. I had a client in the food processing industry a few years back who tried using a VFD on a 75HP mixer for cake batter. The VFD’s ramp time was set too short, so it tripped the circuit breaker every time they tried to start a full load. When they switched back to DOL, the mixer fired right up, no issues. For high-inertia loads where you need maximum torque at start, DOL’s direct power delivery is a clear winner.
Now, let’s talk about the disadvantages—and these are real, costly issues that I see clients deal with every week, especially when they use DOL for the wrong application. The biggest one is inrush current. When a motor starts directly on full voltage, it draws 5 to 8 times its full-load current for the first fraction of a second. For a 100HP motor, that means 600 to 800 amps of inrush current—way more than the 120-150 amps it uses at running speed. That surge doesn’t just affect the motor; it hits the entire power grid. If you have multiple motors starting on DOL at the same time, or if your facility has a weak power supply, that inrush current can cause voltage dips. I once worked with a small textile mill where three 50HP looms all started on DOL at shift change. The voltage dip was so bad that it knocked out the building’s PLC controls, stopping all production for two hours. Even in larger facilities, voltage dips can damage sensitive equipment: computers, sensors, and lighting systems don’t handle sudden drops in power well, and repairs can add up fast.
Along with voltage dips comes mechanical stress on both the motor and the connected equipment. That sudden jolt of torque we talked about earlier? It’s not gentle. It twists the motor’s shaft, strains the bearings, and puts stress on gears, belts, and couplings. Over time, that repeated shock shortens the motor’s lifespan. A motor that could last 20 years with a soft starter or VFD might only last 12 or 15 years with DOL starting. We track failure rates for all the motors we supply, and DOL motors see 30% more bearing wear and 20% more shaft-related failures than motors with controlled starting. That also means more maintenance—bearing replacements, shaft realignments, and premature part changes. For a plant running 20 large motors, that’s thousands of dollars a year in extra maintenance costs. And don’t forget the impact on connected loads: a conveyor belt that starts suddenly can stretch over time, leading to misalignment and more repairs, while a mixer that jerks at start can spill material or damage the mixing blades.
Another disadvantage is energy waste. Wait, you might be thinking—DOL is simple, how is it wasteful? The inrush current is part of it, but there’s more. When you use DOL starting, you’re also wasting energy because you can’t adjust the motor’s speed to match the load. If you have a pump that only needs to run at 70% speed most of the time, a DOL motor has to run at full speed, wasting 30% of the energy. VFDs are more efficient for variable speed loads, but DOL can only do on/off. But here’s the bigger energy hit from DOL starting itself: the inrush current creates extra heat in the motor windings, which means the motor has to work harder to dissipate that heat, leading to higher energy use over its lifespan. For a facility with dozens of large motors, that adds up to tens of thousands of dollars in extra electricity costs each year.
So when does it make sense to use DOL starting, and when should you walk away? From my experience as a motor supplier, DOL is the right choice when you have small motors (under 20HP), applications that start infrequently, high-inertia loads that need full torque at start, or operations with very tight upfront budgets. For example, a small garage’s air compressor, a greenhouse’s ventilation fan, or a small conveyor system in a warehouse—all perfect uses for DOL. But if you have motors over 50HP, variable speed loads, sensitive equipment nearby, or a weak power supply, you’ll want to look at alternatives like star-delta starters or VFDs.
I’ve seen too many clients cut corners by using DOL on large motors just to save a few bucks upfront, only to deal with costly downtime, repairs, and higher energy bills down the line. Last year, a client brought in a 150HP motor that had failed 18 months after installation; they’d used a cheap DOL starter to save $5,000, and the inrush current had destroyed the winding insulation. Replacing the motor cost them $12,000, plus 3 days of lost production—way more than they would have spent on a proper starter from the start.
At the end of the day, direct-on-line starting isn’t good or bad—it’s a tool, like any other, that works best for specific jobs. As a motor supplier, I don’t push DOL for every project, but I do make sure my clients have all the facts to make the right choice. If you’re working on a new system, replacing an old motor, or wondering if DOL is the right fit for your application, I’m here to walk you through the pros, cons, and best options for your needs. Whether you need a small 5HP DOL starter or a customized solution for a large industrial load, we can work together to find what’s best for your operations.
BLDC Square Gear Motor References
- Electrical Motor Controls for Industry, 3rd Edition. Schroff, D. C., Delmar Cengage Learning, 2018.
- Motor Starting Methods: A Practical Guide for Industrial Applications. International Electrical Testing Association (NETA), 2021.
- Efficiency and Reliability of AC Motor Starting Methods. IEEE Transactions on Industry Applications, Vol. 56, No. 4, 2020.
- Industrial Motor Maintenance and Failure Analysis. Bloch, H. P., Fairmont Press, 2017.
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