Most motors in a building, from fans and pumps to compressors and conveyors, are three-phase induction motors. They can be started directly across the line, eased up to speed with a soft starter, or run at variable speed by a variable frequency drive (VFD). The choice affects energy use, the stress on the electrical system when the motor starts, mechanical wear, and how well the motor can match what the building actually needs.
What every motor circuit needs
Whatever the starting method, a motor circuit has the same basic parts:
- A disconnecting means so the motor can be isolated for maintenance, within sight of the motor or lockable, as the Canadian Electrical Code requires.
- Short-circuit and ground fault protection, usually a breaker or fuses, to protect the circuit against faults.
- Overload protection, which protects the motor from overheating if it is overloaded or stalls. This is separate from the short-circuit protection because motors draw high current briefly when starting, which the overload must tolerate.
- A controller that starts and stops the motor: a contactor, a soft starter or a VFD.
In larger buildings, many of these are grouped in a motor control centre (MCC). See panelboards, switchboards and MCCs.
The three main ways to start a motor
Across-the-line starters
A contactor connects the motor directly to full voltage. The motor draws a large inrush current, commonly several times its running current, and reaches full speed quickly. Across-the-line starting is simple, reliable and inexpensive. It suits small motors and larger ones where the supply is stiff enough and the driven equipment can take the sudden torque. Its drawbacks are the voltage dip it causes, which can flicker lights or upset other equipment, mechanical shock to belts, couplings and pipes, and the fact that it can only run the motor at one speed.
Soft starters
A soft starter uses power electronics to ramp the voltage up over a few seconds, reducing inrush current and mechanical shock. Once the motor is up to speed, many soft starters close a bypass contactor and the motor runs at full speed as if started across the line. Soft starters are useful where starting current or water hammer is a problem but speed control is not needed, and are common where a motor must start on a generator. See how generators are sized.
Variable frequency drives
A VFD converts incoming AC to DC, then uses an inverter to produce AC at a variable frequency and voltage. Since an induction motor's speed follows the supply frequency, the VFD controls speed continuously. It starts motors smoothly with low inrush, and it can run them at whatever speed the system needs.
Why VFDs save energy on fans and pumps
For centrifugal fans and pumps, the power needed falls very steeply as speed is reduced: running at a modestly reduced speed takes much less power than running at full speed. Older systems often run motors at full speed and throttle the flow with dampers or valves, wasting energy. A VFD lets the motor slow down instead, controlled by a pressure or flow sensor or the building automation system. In buildings where fans and pumps run long hours at variable load, the savings are often significant, and energy codes increasingly require variable speed control on larger HVAC fans and pumps.
What to watch for with VFDs
- Harmonics. The rectifier draws current in pulses, which distorts the supply. In buildings with many drives, this can overheat transformers and neutrals and upset sensitive equipment. Line reactors, filters or drives with lower-harmonic front ends address it. See power quality and harmonics.
- Motor compatibility. The fast switching of the inverter stresses motor insulation. Motors rated for inverter duty are preferred, especially on long cable runs.
- Bearing currents. Drives can induce currents that pit motor bearings. Shaft grounding and proper cable and grounding practices reduce this.
- Cable length and type. Long runs between drive and motor may need filters, and shielded VFD cable is often specified.
- Heat. Drives give off heat and need ventilation or cooling in their room or enclosure.
- Bypass. For critical fans and pumps, a bypass lets the motor run across the line if the drive fails.
- Settings. Minimum and maximum speed, acceleration and the control signal must be set at commissioning and documented.
Choosing the right option
| Across the line | Soft starter | VFD | |
|---|---|---|---|
| Starting current | High | Reduced | Low |
| Speed control | None | None | Full |
| Energy savings at part load | None | None | Often significant on fans and pumps |
| Cost and complexity | Lowest | Moderate | Highest |
| Harmonics | None | Brief, during ramp | Continuous, needs attention |
Maintenance and troubleshooting
Motors and their controllers fail in predictable ways, and most failures give warning. A sensible maintenance program for motor circuits includes:
- Checking that overload settings match the motor nameplate and were not raised to stop nuisance trips
- Recording motor current and comparing it with nameplate and previous readings
- Looking for heat at terminations and contactors with infrared thermography under load
- Cleaning drive cooling fans and filters, and checking the drive's fault log
- Listening and feeling for bearing noise and vibration, or using vibration analysis on critical motors
- Testing insulation resistance on motors that have been idle or exposed to moisture
- Keeping drive parameter backups so a replacement drive can be set up quickly
Repeated overload trips are a symptom, not a nuisance. They usually point to a mechanical problem, a supply problem such as a lost phase or unbalanced voltage, or a motor working harder than it was sized for.
A typical case
A mid-rise office has two large supply fans that run at full speed during occupied hours, with inlet dampers adjusting airflow. Lights dim briefly every morning when the fans start. A review recommends VFDs on both fans, controlled from duct static pressure by the building automation system, with the dampers locked open. The drives remove the morning voltage dip, reduce fan energy whenever airflow demand is below the maximum, and reduce belt wear. Because the building already has several other drives, the design includes harmonic mitigation and checks the transformer feeding the mechanical room. The existing motors are checked for suitability on inverter duty, and one is replaced.
Drives store energy. The DC bus inside a VFD stays charged for some time after the supply is switched off. Only qualified workers should open drives, following the manufacturer's waiting time and verifying absence of voltage, within the site's lockout procedure.
How ehZee can help
Our power distribution design work covers motor circuits, MCCs and drives: choosing starting methods, sizing protection, addressing harmonics and coordinating with the mechanical design and controls.
Common questions
Should every motor have a VFD?
No. Drives make sense where load varies and running at reduced speed saves energy, or where smooth starting and speed control matter. For small motors running at constant load, a simple starter is usually the better choice.
Can a VFD be added to an existing motor?
Often yes, but check the motor's insulation, bearings, cooling at low speed and the cable length. Older motors not rated for inverter duty may need replacement or protection.