Motor Starter vs VFD: When Direct-On-Line Is Enough and When It Is Not
Motor Starter vs VFD: When Direct-On-Line Is Enough and When It Is Not
TL;DR: A motor starter (DOL) connects a motor directly to the mains supply — the motor goes from zero to full speed in one step, drawing 6–8× full-load current at start. A VFD (Variable Frequency Drive) controls the motor by varying the voltage and frequency supplied to it, enabling controlled acceleration, variable speed during run, and energy savings on loads where speed matters. Use DOL when speed is fixed and starting inrush is acceptable. Use a VFD when you need variable speed, controlled ramp, or significant energy savings on fans and pumps.
Every AC induction motor application is a choice between these two approaches. Most motors in a facility run at one: on or off, fixed speed. A smaller number need variable speed, soft ramp, or energy optimisation. Knowing which application demands which solution prevents over-engineering and under-specifying.
What a Direct-On-Line Motor Starter Does
A DOL (Direct-On-Line) motor starter — a contactor plus an overload relay — applies full mains voltage directly to the motor terminals. The motor accelerates from rest to full synchronous speed (minus slip) in a fraction of a second for small motors, and a few seconds for larger ones. Simple, robust, cheap.
The downside of DOL starting is inrush current: a three-phase induction motor draws 6–8 times its full-load current during the first acceleration phase. On a 7.5 kW motor (FLC ~15A), that is 90–120A for 1–3 seconds. In facilities with weak supplies, transformer-limited sites, or many motors starting simultaneously, this inrush causes voltage sag that affects other equipment.
DOL starters have no ability to control speed during run. The motor runs at its rated synchronous speed minus slip, full stop. For a 4-pole motor on 50 Hz supply, that is approximately 1480 RPM.
For full details on the motor starter circuit — contactor, overload relay, control circuit, and PLC interlocking — see the motor starter vs contactor post.
What a VFD Does
A VFD (Variable Frequency Drive, also called an inverter or adjustable-speed drive) converts the fixed mains frequency and voltage to a variable frequency and voltage. The motor sees whatever frequency the drive outputs — 5 Hz, 30 Hz, 50 Hz, 60 Hz, 80 Hz — and runs at proportional speed. The basic relationship is:
Motor speed (RPM) = (120 × output frequency) / number of poles
A 4-pole motor at 50 Hz input runs at ~1480 RPM. At 25 Hz, it runs at ~740 RPM. At 60 Hz (on a 50 Hz supply), it runs at ~1780 RPM (with appropriate motor/VFD rating for field weakening above base speed).
The VFD also controls the V/Hz ratio (or uses vector control) to maintain rated flux at all speeds, protecting the motor.
Key things a VFD does that a DOL starter cannot:
- Variable speed: the fundamental capability. Fans, pumps, compressors, conveyors with variable throughput.
- Controlled acceleration: the VFD ramps frequency up over a set acceleration time (1 second to several minutes). The motor current during acceleration is limited to the drive's current limit setpoint — typically 150% FLC, far below DOL inrush.
- Controlled deceleration: ramped stop, not coast. Important for conveyors, centrifuges, and applications where abrupt stops cause mechanical or process problems.
- Built-in overload protection: the drive monitors motor current and trips on overload. The separate overload relay in a DOL starter is not needed.
- Energy savings: on centrifugal fans and pumps, reducing speed by 20% reduces power consumption by ~50% (cube law). A pump or fan that previously ran at full speed and was throttled by a valve or damper can instead be slowed by the VFD with dramatic energy savings.
Side-by-Side Comparison
| | DOL Motor Starter | VFD | |---|---|---| | Starting current | 6–8× FLC (inrush) | Limited (typically 150% FLC) | | Speed control | No — fixed speed only | Yes — variable 0 to rated+ | | Acceleration ramp | Immediate (mechanical shock) | Configurable (0.1 s to minutes) | | Deceleration | Coast or mechanical brake | Ramp or DC inject braking | | Energy savings (fans/pumps) | None — motor runs at full power | Significant — cube law on centrifugal loads | | Overload protection | Separate overload relay required | Built-in, often with logging | | Harmonics (power quality) | None | Yes — current harmonic injection into supply | | Motor compatibility | Any standard AC motor | Standard AC motor; VFD-rated for long cables | | Cost | Low | Higher — 3–10× a DOL starter for same rating | | Maintenance | Almost none | Drive cooling fans, capacitor life | | PLC interface | Digital outputs (coil) | Analog + digital + fieldbus | | Protection class | Contact Siemens/ABB for IP options | IP20 panel-mount; IP55–66 for enclosure mount |
The Soft Starter: The Middle Option
Between DOL and VFD sits the soft starter — a device that ramps up the voltage applied to the motor during starting, reducing inrush current without providing variable speed during run. Once the motor reaches full speed, the soft starter typically bypasses itself (internal bypass contactor) and the motor runs across the line.
A soft starter is appropriate when:
- Inrush current reduction is the only requirement (no variable speed needed during run).
- The application is a conveyor or compressor with a high-inertia load that needs gentle starting.
- Budget is between DOL and VFD.
Soft starters are simpler than VFDs (no DC bus, no PWM harmonics), but they do not save energy during run and do not provide variable speed. If there is any chance you need speed control later, buy a VFD.
Energy Savings: Where the VFD Pays for Itself
The energy saving case for VFDs on centrifugal fans and pumps is compelling and often pays back the investment in 1–3 years.
The affinity laws for centrifugal machines state:
- Flow is proportional to speed.
- Head (pressure) is proportional to speed squared.
- Power is proportional to speed cubed.
Reducing a fan or pump from 100% speed to 80% speed reduces flow by 20% and reduces power by nearly 50%. Reducing to 60% speed reduces power to 22% of full-speed power. A DOL fan running at full speed and throttled by a damper wastes all that power in the damper. A VFD fan at 60% speed uses 78% less power.
For a 30 kW pump running continuously, the energy savings from dropping from 100% to 80% speed exceed £10,000 per year at typical UK industrial electricity prices. VFDs in pump and fan applications typically pay for themselves within 18 months.
PLC Integration with a VFD
A VFD offers more PLC interface options than a DOL starter:
Analog speed reference: the PLC outputs a 4–20 mA or 0–10 V signal to the VFD speed reference input. The VFD maps 4 mA (0%) to zero frequency and 20 mA (100%) to maximum frequency. This is the simplest variable-speed interface.
Digital run/stop: a PLC digital output to the VFD Run/Stop input. The PLC controls whether the drive runs; the analog signal sets the speed.
Fieldbus: PROFINET, EtherNet/IP, or Modbus TCP gives the PLC read/write access to every drive parameter — speed setpoint, direction, acceleration time, current feedback, fault codes, run state — in one communication transaction. This is the standard approach in modern systems.
Feedback to PLC: the VFD outputs actual motor speed (via analog or fieldbus), drive status (running, faulting, at-speed), and fault codes. The PLC reads these to confirm the motor is running at the commanded speed and to detect and respond to drive faults.
Practice reading VFD status and writing speed setpoints in the VFD conveyor speed control scenario and the VFD fault detection scenario — both run in the browser and auto-grade your ladder logic.
Common Confusions Cleared Up
"A VFD replaces the overload relay." Yes, in the VFD circuit — the drive has built-in overload protection. The separate overload relay in a DOL starter is not needed when a VFD is in the circuit. However, if the motor circuit includes a bypass contactor (for DOL operation during VFD maintenance), the bypass circuit needs its own overload protection.
"I can use any motor with a VFD." Standard NEMA B or IE2/IE3 squirrel-cage induction motors work with VFDs. However, very long cable runs (over 50 metres) between VFD and motor can cause standing wave voltage peaks that stress motor winding insulation. Motors labelled "inverter duty" or "VFD-rated" have reinforced insulation. For long cable runs, use a dV/dt filter or output reactor at the VFD output.
"A VFD saves energy on any motor." Energy savings are significant only on centrifugal loads (fans, pumps, centrifugal compressors) where the cube law applies. Positive displacement pumps, conveyor motors, and process loads with constant torque do not follow the cube law — the energy savings are minimal. VFDs on constant-torque loads still provide controlled starts and variable speed, just not dramatic energy savings.
Frequently Asked Questions
Q: Do I need a contactor on the input to a VFD?
A: Not strictly required by the VFD operation, but recommended for isolation — a line-side isolation contactor allows the drive to be electrically isolated without switching off the supply to the rest of the panel. Some panel designs include a contactor on the output of the VFD (between VFD and motor) for bypass switching, but this must be switched only when the drive output is zero to avoid damaging the drive's output transistors.
Q: Can a VFD run a motor at above the nameplate speed?
A: Yes — most VFDs can output frequencies above the motor's base frequency (typically above 50 Hz on a 50 Hz motor). Above base frequency, the VFD operates in field weakening mode — voltage stays constant while frequency increases, so motor torque falls. Running above nameplate speed is only safe within the drive and motor manufacturer's stated limits, and the driven load must be mechanically rated for the higher speed.
Q: What is the difference between a VFD and a soft starter?
A: A soft starter ramps motor voltage on starting to reduce inrush current, then bypasses itself when the motor is at full speed. It provides no variable speed during run. A VFD controls motor speed from zero to rated (and beyond), limits starting current, and provides energy savings on variable-torque loads. Soft starters are cheaper; VFDs are more capable. If you need variable speed, buy a VFD.
Q: How does a VFD connect to a Siemens PLC?
A: The standard modern approach is PROFINET. The VFD (SINAMICS G120, G130, S120) has a PROFINET interface card. The PLC engineer configures the drive in TIA Portal using the drive's GSDML file. The PLC then writes a speed setpoint and run command to the drive via PROFINET cyclic I/O, and reads motor current, speed feedback, and fault codes back in the same cycle.
Practice VFD control logic and fault handling with the free browser scenarios.