What a VFD does
A variable frequency drive (VFD) controls the speed of an AC motor by changing the frequency of the power it receives. Without a drive, a motor runs at one speed set by the 60 Hz supply. With one, it can run at whatever speed the process needs.
Inside, the drive converts incoming AC power to DC, stores it on a DC bus, then switches it back into AC at a new frequency using fast electronic switches. The motor sees an adjustable voltage and frequency that let it run anywhere from a crawl to full speed.
Where drives save energy
Drives save the most on fans and centrifugal pumps that do not need to run at full speed all the time. On these loads, power drops sharply with speed. In theory, a fan running at 80 percent speed needs only about half the power it needs at full speed.
Real savings depend on the system. A pump that must overcome a fixed lift, or a fan that already runs at full speed most of the day, saves less. A drive on a constant-speed conveyor saves little energy at all, though it may still help with soft starts and process control. An energy audit with logged motor data shows where a drive pays back.
Other benefits include:
- Soft starting, which reduces inrush current and voltage dips on start-up
- Less mechanical wear on belts, couplings and pipework
- Replacing throttling valves and dampers with speed control
- Precise process control tied to pressure, flow or temperature
Common problems, and how to prevent them
Drives are reliable when they are applied and installed properly. Most VFD headaches come from the installation rather than the drive itself.
| Problem | Why it happens | Prevention |
|---|---|---|
| Harmonics | The rectifier draws current in pulses | Line reactors, DC chokes or harmonic filters |
| Motor insulation damage | Voltage spikes on long motor leads | Inverter-duty motors, short runs, output filters |
| Bearing failures | Stray currents through motor bearings | Shaft grounding rings or insulated bearings |
| Interference | Fast switching noise on cables | Shielded VFD cable, good bonding, separate routing |
| Overheating | Drive losses in a closed space | Ventilation, correct enclosure rating |
| Nuisance trips | Voltage sags and short outages | Ride-through and auto-restart settings |
Our guide to power quality and harmonics explains why a building full of drives needs attention to harmonics.
Installation and code points
A drive installation must meet the Canadian Electrical Code as adopted in Alberta and needs an electrical permit. Conductor sizing, overcurrent protection, motor overload protection and the disconnecting means all have specific rules for drive-fed motors. A licensed electrician confirms these against the drive and motor nameplates.
Commissioning matters as much as wiring. The drive needs the motor nameplate data, sensible acceleration and deceleration times, minimum and maximum speeds, and any skip frequencies that avoid mechanical resonance. Save a copy of the final parameters; it makes replacing a failed drive much faster.
The DC bus capacitors in a drive can hold a dangerous charge after the power is switched off. Only qualified people should open a drive, and only after following the manufacturer's discharge time and verifying zero energy.
Questions to ask before adding a drive
- Does the load actually vary, and for how many hours a year does it run below full speed?
- Is the motor inverter-duty, and how far is it from where the drive will be mounted?
- Where will the drive live, and can that room stay within its temperature rating all year?
- How many drives are already in the building, and has anyone looked at harmonics?
- What happens to the process if the drive fails, and is a bypass or spare needed?
- Who will hold the parameter backup and the drive manuals?
Clear answers to these questions keep a drive project focused on the motors where it pays back, and avoid the installation problems that give drives a bad name.
Edmonton conditions
Drives in unheated buildings, rooftop enclosures or pump houses face cold starts, condensation and dust. Check the drive's rated temperature range, consider enclosure heaters or a heated room, and keep filters clean. In summer, electrical rooms packed with drives can overheat, so ventilation needs to cope with both extremes.
When a drive fails, keeping a spare for critical motors, or a bypass that lets the motor run across the line, keeps the process going. See motor controls and plant retrofits.
Common questions
Can any motor be run on a VFD?
Many can, but not all. Inverter-duty motors are designed for the voltage stress and cooling conditions a drive creates. Older motors may need checking, and long cable runs may call for filters.
Will a VFD always save energy?
No. Savings come from running below full speed on variable loads like fans and pumps. A motor that needs full speed all the time will save little.
What is a drive bypass?
A bypass is a set of contactors that lets the motor run directly from the supply at full speed if the drive fails or is being serviced. It suits critical fans and pumps.
Do VFDs need maintenance?
Yes. Keep them clean and cool, check cooling fans and filters, look for loose connections and watch fault logs. Capacitors and fans age and may need replacing over the drive's life.
Codes and permit rules change and every building is different. For work on your system, talk to a licensed electrician and your local permit authority.