Every three-phase motor needs a way to start, stop and (sometimes) change speed. The two options you will meet most often are a direct online (DOL) starter and a variable speed drive (VSD). A DOL starter is simple and cheap. A VSD costs more, but it controls speed and torque and cuts the energy a motor wastes. This guide explains how each one works, how they compare with star-delta starters and soft starters, and how to decide which one belongs on your motor.
VSD vs DOL at a glance
| DOL starter | Star-delta starter | Soft starter | Variable speed drive (VSD) | |
|---|---|---|---|---|
| Starting current | Typically 6 to 8 times full-load current | About one third of DOL | Adjustable, usually 2 to 4 times full-load current | Close to full-load current |
| Starting torque | Full (and sudden) | About one third of DOL | Reduced, adjustable | Full torque available from low speed |
| Speed control while running | No | No | No | Yes |
| Energy savings on variable loads | None | None | None once running | Often significant on pumps and fans |
| Controlled stopping | No | No | Soft stop | Controlled ramp-down and braking |
| Upfront cost | Lowest | Low | Medium | Highest |
| Typical use | Small motors at constant speed | Larger motors that start unloaded | Reducing start-up stress without speed control | Pumps, fans, conveyors, process lines |
The figures above are typical ranges. The real values depend on the motor design, the load and the supply, so always check the motor and starter data.
What does VSD mean in electrical terms?
VSD stands for variable speed drive. In electrical and industrial settings it refers to an electronic motor controller that changes the speed of an AC motor by changing the frequency and voltage of the power fed to it. You will also see the terms VFD (variable frequency drive), AC drive, inverter and adjustable speed drive. For standard three-phase induction motors, these all describe the same kind of device.
How a variable speed drive works
A VSD has three main stages:
- Rectifier. Converts the incoming AC supply into DC.
- DC bus. Capacitors smooth and store that DC energy.
- Inverter. Fast-switching transistors (IGBTs) turn the DC back into a variable-frequency AC output, using pulse-width modulation.
The speed of an induction motor follows the supply frequency. Running a four-pole motor at 50 Hz gives roughly 1,500 rpm; drop the frequency to 25 Hz and it runs at about half that speed. The drive keeps the voltage-to-frequency ratio in step so the motor still produces the torque the load needs.
What is a direct online (DOL) starter?
A DOL starter connects the motor straight across the full supply voltage, usually through a contactor with an overload relay for protection. Press start and the motor accelerates to full speed as fast as the load allows.
That simplicity is its strength. There is very little to go wrong, it is inexpensive and it is easy to fault-find. The trade-off is the start itself. The motor draws a large inrush current and delivers a sudden jolt of torque. That shock goes into couplings, gearboxes, belts and pipework, and the current spike can cause voltage dips for other equipment on the same supply.
Star-delta and soft starters: the middle ground
Star-delta starters start the motor in a star connection and switch to delta once it is up to speed. This cuts the starting current to around a third of DOL, but starting torque drops by the same amount. They suit loads that start lightly, and the changeover can cause a current and torque transient of its own.
Soft starters use thyristors to ramp the voltage up gradually, which limits inrush current and mechanical shock. Many also give a soft stop, which helps with water hammer on pumps. A soft starter does not change the running speed, though. Once the motor is up to speed it runs exactly as it would on DOL.
If you only need a gentler start, a soft starter may be enough. If you need to control speed, or you want energy savings while the motor is running, you need a VSD.
Key benefits of a VSD over DOL
1. Energy savings on pumps and fans
This is where VSDs usually pay for themselves. Centrifugal pumps and fans follow the affinity laws: flow is proportional to speed, but the power drawn is roughly proportional to the cube of speed. Running a fan or pump at 80% speed needs only about half the power it draws at full speed. A DOL-started pump running flat out against a throttling valve wastes that energy as heat and pressure loss. See our guide to VSDs for pump control for a worked example.
Constant-torque loads such as conveyors and positive displacement pumps do not follow the cube law, so savings there come mainly from matching speed to demand and avoiding idle running.
2. Lower starting current
A VSD starts the motor from low frequency, so it draws little more than its normal running current. That matters in South Africa, where many plants run on standby generators or have limited supply capacity. A motor that trips a generator or causes a voltage dip on DOL start will often start without trouble on a VSD.
3. Less mechanical stress and longer equipment life
Adjustable acceleration and deceleration ramps remove the torque shock of a DOL start. Belts, couplings, gearboxes, bearings and conveyor splices last longer, and pumps suffer less water hammer on start and stop.
4. Better process control
A VSD can hold a pressure, flow, level, tension or line speed by adjusting motor speed in real time, often using its built-in PID controller or a signal from a PLC. With DOL the motor is either on or off.
5. Built-in protection and diagnostics
Modern drives monitor motor current, voltage, temperature and load, and log fault codes. That information makes breakdowns quicker to diagnose and can warn you about a problem before it stops production.
When DOL still makes sense
A VSD is not the right answer for every motor. DOL is often the better choice when:
- The motor is small and the supply can handle its starting current comfortably.
- The load always runs at full speed, with no need to vary flow or throughput.
- The motor starts infrequently and the mechanical shock is not a problem.
- The environment is very hot, dusty or wet and there is nowhere suitable to house electronics.
- Budget is tight and the running hours are too low for energy savings to recover the cost of a drive.
Things to plan for when fitting a VSD
- Heat. Drives lose a few percent of their power as heat, so panels need adequate ventilation or cooling.
- Cabling. Use screened motor cable, keep runs as short as practical and follow the manufacturer’s earthing guidance. Long cable runs may need output filters.
- Harmonics. Drives draw non-sinusoidal current. On larger installations, check whether harmonic filtering or a line reactor is required.
- Motor suitability. Older motors, or motors run at low speed for long periods, may need extra cooling or insulation rated for inverter duty.
- Setup. A drive only saves energy and protects equipment if its parameters, ramps and control loop are set up properly.
Frequently asked questions
Is a VSD the same as a VFD?
For AC induction motors, yes. VFD (variable frequency drive) describes how the drive works, by varying frequency. VSD (variable speed drive) describes what it does. Both terms are used for the same product.
Can I replace a DOL starter with a VSD?
Usually, yes. You will need a drive rated for the motor’s current and duty, suitable cabling and enclosure, and a check that the motor can run at the speeds you plan to use. It is a common upgrade on pumps, fans and conveyors.
Does a VSD save energy on every motor?
No. The biggest savings come from pumps and fans that run below full speed for much of the time. A motor that runs at full speed all day will see little saving, and the drive’s own losses can make it slightly less efficient than DOL.
What is the difference between a VSD and a soft starter?
A soft starter only controls the start and stop. A VSD controls the start and stop and the running speed. A soft starter is cheaper when all you need is a gentler start.
Why does a VSD trip or fail?
Common causes include overheating from blocked ventilation, supply surges, failed DC bus capacitors, damaged IGBTs and incorrect parameters. Our VSD maintenance tips cover how to prevent most of them.
Need help choosing, commissioning or repairing a drive?
Motion Tronic has supported industrial drives and automation since 2003. We supply Inovance variable speed drives, commission drive upgrades on existing machines, and carry out component-level VSD repairs on most major brands. Repaired drives are load-tested on our in-house dyno unit before they go back to site.
Contact Motion Tronic to talk through your motor application, or to log a drive repair.
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