What a Variable Frequency Drive Does
A variable frequency drive (VFD), also called an AC drive, controls the speed of an AC motor by varying the frequency and voltage supplied to it. That makes it the standard choice for fans, pumps, conveyors, mixers, and machine tools where speed must change, starting must be gentle, or energy use should follow demand rather than running flat out. A VFD also brings soft starting, which reduces mechanical shock and inrush compared with direct starting.
This guide explains VFD selection in generic terms: the concepts, the questions to ask, and the mistakes to avoid. It does not give specifications for any single model. If you have a model in mind, send it to us for a quotation — see the inquiry options at the end of this guide.
Start With the Load, Not the Drive
The load decides almost everything about the selection:
- Variable-torque loads such as fans and centrifugal pumps need less torque as speed falls. They are the classic energy-saving VFD application.
- Constant-torque loads such as conveyors, mixers, and positive-displacement pumps need full torque across the speed range, which places higher demands on the drive and on motor cooling at low speed.
- High starting torque or shock loads such as crushers and heavy conveyors need careful attention to overload capacity and starting behaviour.
- Hoisting and vertical loads need a deliberate braking and holding strategy; a standard drive selection without it is incomplete.
Describing the load honestly — including the worst case, not just the normal case — is the single most useful step in the whole selection.
Key Parameters, Explained in Plain Terms
Motor matching. The drive must match the motor it serves: supply voltage, motor power class, and full-load current. Current, rather than nameplate power alone, is the figure to check, especially for older or rewound motors whose current may differ from a new equivalent.
Overload capacity. Drives are rated for a continuous current plus a short-term overload. Confirm that the application’s starting peaks and occasional load peaks sit within the overload rating, not just within the continuous rating.
Control method. Simple voltage/frequency control suits many fan and pump applications. More demanding applications — tight speed holding, high starting torque, or low-speed running — call for vector-type control. Match the control method to the application’s accuracy and torque needs, not to the most advanced option on the market.
Speed range and low-speed running. A motor cooled by its own shaft fan loses cooling at low speed. If the application runs slowly for long periods under load, check motor cooling and consider whether forced cooling or a different motor arrangement is needed.
Braking. Loads that decelerate quickly, or that can be driven by the process, generate energy that must go somewhere. Confirm whether the application needs a braking resistor or another braking arrangement, and size that part of the system deliberately.
Supply and harmonics. A drive draws non-sinusoidal current from the supply. On weak supplies, or where many drives share a supply, check harmonic and power-factor implications and whether line reactors or filters are recommended for the installation.
Environment and installation. Ambient temperature, dust, moisture, altitude, and ventilation around the drive all affect its rating and life. Confirm the enclosure and cooling arrangements suit the real installation, including inside a cabinet on a warm day, and allow space for airflow and cable access.
Communication and integration. If the drive must report to a PLC or a plant network, confirm the communication interface and protocol before ordering, rather than discovering a missing option at commissioning.
A Practical Selection Sequence
- Describe the load: type, worst-case torque, speed range, duty cycle, and starting behaviour.
- Identify the motor: supply, full-load current, and cooling arrangement.
- Shortlist drives that match the motor current and the load’s overload demand.
- Choose the control method the application actually needs.
- Decide braking, filtering, and communication options.
- Confirm the environment, enclosure, and ventilation, then plan commissioning and parameter setting.
Common Mistakes to Avoid
- Sizing by motor power alone. Full-load current is the safer check, especially for older motors.
- Ignoring the overload rating. A drive that covers continuous current but not starting peaks will trip at the worst moment.
- Running slowly under load without checking cooling. Shaft-fan-cooled motors overheat at sustained low speed.
- Forgetting braking. Fast deceleration and overhauling loads need a braking plan, not just a drive.
- Skipping the supply check. Harmonics and weak supplies cause problems that look like drive faults but are installation issues.
- Leaving commissioning to chance. Motor data entry, ramp times, and protection settings decide whether the installation runs well or trips repeatedly.
AC Drive Models Listed on This Site
Our AC Drives range covers multiple brands. By way of example, it includes models such as the Siemens 6SL3210-1KE11-8AP2, and brand-specific ranges such as Schneider Electric AC Drives and Eaton AC Drives. Model names are given only as examples of what is listed; always confirm the drive against your motor current, load type, and installation conditions before ordering.
Can’t Find Your Model?
If your model is not listed, or you are unsure which drive suits your load and motor, send us the model and a short description of the application. Every product on this site is covered by a 12-Month Warranty, and we aim to reply with a competitive quotation within 12 hours. Reach us on WhatsApp at +86 17749794237 or by email at [email protected].