Variable Frequency Drives
Explain how a VFD changes motor speed, set its key parameters, use the affinity laws, wire it correctly, and respond to common fault codes.
Table of contents
A variable frequency drive (VFD) controls motor speed by changing the frequency and voltage it sends to the motor. Instead of running a pump flat out and throttling a valve, you slow the pump down. VFDs start motors gently, save a lot of energy on fans and pumps, and give you fault data a plain starter cannot. They also bring their own hazards, because a drive holds a dangerous charge after the power is off, and it can damage a motor or itself when it is wired or set up the wrong way.
What You Will Be Able to Do
- Name the three sections of a VFD and say what each one does.
- Predict motor speed from drive frequency.
- Use the affinity laws to estimate energy savings.
- Set the key parameters for a basic start-up.
- Find the likely cause of common drive faults.
Warning: The DC bus of a drive holds a lethal charge after the power is removed. Follow the wait time on the drive's label, commonly 5 minutes or more, and a qualified person must measure the DC bus with a suitably rated meter to prove it has discharged. A dark keypad does not prove the drive is safe.
How a VFD Works
Every drive has three main sections:
| Section | What it does |
|---|---|
| Rectifier (converter) | Turns incoming AC into DC, using a diode bridge |
| DC bus | Stores and smooths the DC in large capacitors |
| Inverter | Turns DC back into AC at a variable voltage and frequency |
On a 480-volt supply, the DC bus sits at around 650 to 680 volts. The peak of a 480-volt AC wave is 480 x 1.414 = 679 volts, and the bus reads a little lower under load.
The inverter uses fast electronic switches called insulated gate bipolar transistors (IGBTs). They switch the DC bus on and off thousands of times a second, typically at 2 to 16 kHz. The width of the pulses is varied, so the motor sees something close to a sine wave. This is called pulse-width modulation (PWM).
Speed and Frequency
An induction motor's speed follows frequency:
New speed = rated speed x (output frequency / rated frequency)
A motor rated 1,750 RPM at 60 Hz, running at 45 Hz, turns at about 1,750 x 45 / 60 = 1,312 RPM.
The drive also lowers the voltage along with the frequency. On a 460-volt, 60 Hz motor, that ratio is about 7.7 volts per hertz. Keeping this ratio steady keeps the magnetic flux steady, so the motor makes its rated torque without overheating. At 30 Hz, the drive applies about 230 volts.
The Affinity Laws
Centrifugal fans and pumps follow three rules, known as the affinity laws:
| Quantity | Changes with | At 70 percent speed |
|---|---|---|
| Flow | Speed | 70 percent |
| Pressure | Speed squared | 49 percent |
| Power | Speed cubed | 34 percent |
Because power follows the cube of speed, a small reduction saves a lot.
Worked Example: Fan Savings
A 25 HP fan runs at 90 percent speed instead of full speed.
- Cube the speed: 0.9 x 0.9 x 0.9 = 0.729.
- Power needed: about 73 percent of full-speed power.
- Savings: roughly 27 percent, for only a 10 percent drop in airflow.
At 80 percent speed the same math gives about 51 percent of full power, which is about half. These rules do not apply to constant-torque loads such as conveyors and positive-displacement pumps, where power falls only in proportion to speed.
Setting Up a Drive
Parameter numbers and names differ by brand, so keep the manual close. Most start-ups need these settings:
| Parameter | What to set |
|---|---|
| Motor nameplate data | HP, volts, FLA, RPM and frequency, exactly as marked |
| Acceleration time | Time from zero to full speed, often 10 to 30 seconds for fans and pumps |
| Deceleration time | Time from full speed to zero, long enough to avoid overvoltage trips |
| Minimum frequency | Lowest allowed speed, often 20 to 30 Hz for a self-cooled motor |
| Maximum frequency | Usually 60 Hz, unless the motor and load are rated for more |
| Control mode | V/Hz, sensorless vector or closed-loop vector |
| Control source | Keypad, terminals, analog signal or network |
Control modes:
- V/Hz: the simplest. It works for most fans and pumps, and it is the mode to use when one drive runs several motors.
- Sensorless vector: better torque at low speed. It needs accurate motor data.
- Closed-loop vector: uses an encoder on the motor shaft for the most exact speed and torque.
Minimum speed matters. A TEFC motor is cooled by a fan on its own shaft. At low speed that fan moves very little air. A motor run slowly at high torque can overheat, even if the current looks fine. Inverter-duty motors are built to handle this better.
Auto-tune. Many drives can measure the motor's electrical values during start-up. Run this with the motor connected, and follow the manual. Some methods turn the shaft, so the machine must be safe to rotate.
Wiring a Drive
Drive wiring is different from a plain starter, and most early drive failures come from wiring and installation.
- Input power: size conductors and protection from the drive's input current, using the manual. NEC 430.122 sets conductors at 125 percent of the drive's rated input current. Use the fuses or breaker type the manufacturer names.
- Output power: use VFD-rated shielded cable, or conduit dedicated to the motor leads. Keep output leads away from control wiring.
- Grounding: run a dedicated grounding conductor with the motor leads, and connect the drive and motor as the manual shows.
- Control wiring: use shielded twisted pair for analog signals, and ground the shield at one end unless the manual says otherwise. Cross power wiring at right angles.
- Enclosure cooling: drives shed heat. Keep the filters clean and leave the clearance the manual requires.
Things Never to Do
- Never connect power to the output terminals (usually T1, T2, T3 or U, V, W). Line voltage on the output destroys the inverter.
- Do not open a contactor between the drive and motor while the motor is running, unless the system is designed for it. It can cause a trip or damage the output section.
- Do not put power factor capacitors on the output. The fast switching can destroy them and the drive.
- Do not cycle input power to start and stop the drive. Use the run command. Repeated power cycling wears the charging circuit.
- Never test insulation with a megger while the drive is connected. The test voltage can destroy the electronics. Disconnect the motor leads at the drive first.
Problems Drives Cause
Long motor leads. Fast pulses can reflect at the motor and raise the peak voltage, which can break down winding insulation. Keep lead lengths within the drive maker's limit, or add an output reactor or filter.
Bearing damage. Stray currents can pass through motor bearings and pit them. You can hear this as a rising whine, and the marks look like a washboard. Shaft grounding rings or insulated bearings are the usual fix.
Harmonics. The rectifier draws current in pulses, which distort the supply. Line reactors, DC chokes and filters reduce this. Facilities with many drives may need a harmonic study.
Electrical noise. A drive can upset sensors, radios and PLC analog inputs. Good shielding, grounding and separation of cables prevent most issues.
Common Faults
Drives record what they were doing when they tripped, so read the fault history before resetting.
| Fault | Likely cause | What to check |
|---|---|---|
| Overcurrent | Binding load, short in the motor or cable, acceleration too fast | Mechanical load, insulation, acceleration time |
| Overvoltage | Deceleration too fast, overhauling load | Longer decel time, or a braking resistor |
| Undervoltage | Supply sag or loss | Input voltage, fuses, connections |
| Ground fault | Insulation failure in the motor or cable | Disconnect at the drive, then insulation test |
| Overtemperature | Blocked airflow, hot room, failed fan | Filters, cooling fan, ambient temperature |
| Motor overload | Load too high, or wrong motor data | Current, parameters, mechanical condition |
| Output phase loss | Open lead or a bad connection | Motor connections, cable continuity |
Why deceleration trips on overvoltage. When you slow a heavy load quickly, the motor acts as a generator and pushes energy back into the DC bus. The bus voltage climbs until the drive trips to protect itself. Lengthen the deceleration time. If the load must stop quickly, use a dynamic braking resistor, which burns off the extra energy as heat.
Keeping a Drive Healthy
- Keep the enclosure filters and heat sinks clean.
- Check the cooling fans, which are a common wear part.
- Keep connections tight.
- Save the parameter list after start-up, so a replacement can be set up quickly.
- If a drive has been in storage for a long time, follow the maker's process for reforming the capacitors before you power it up.
Common Mistakes
| Mistake | What it costs |
|---|---|
| Working inside before the bus has discharged | A lethal shock |
| Connecting line power to output terminals | A destroyed drive |
| Entering the wrong nameplate data | Nuisance trips and poor motor protection |
| Running a TEFC motor slowly under heavy load | Overheated windings |
| Using the same conduit for motor leads and signal wires | Noise and erratic behavior |
| Resetting faults without reading them | A drive that fails again, or a burned-out motor |
Check Your Understanding
1. A 3,500 RPM motor is rated for 60 Hz. At 40 Hz, what speed do you expect?
3,500 x 40 / 60 = about 2,333 RPM.
2. A supply fan is slowed to 70 percent speed. Roughly how much power does it need compared with full speed?
About 34 percent, because 0.7 x 0.7 x 0.7 = 0.343.
3. A centrifuge trips on overvoltage each time it slows down. The decel time is already long, and it cannot be longer for this process. What is the next option?
Add a braking resistor or another way to absorb the regenerated energy.
4. A technician wants to megger a motor fed by a drive. What comes first?
Lock out the drive, wait for the DC bus to discharge and verify it, then disconnect the motor leads at the drive. Test only the motor and cable.
Key Takeaways
- A VFD rectifies AC to DC, stores it on a bus, and inverts it into variable AC.
- Motor speed follows frequency, and voltage is lowered along with it.
- Fan and pump power follows the cube of speed, so small slowdowns save a lot.
- The DC bus holds a lethal charge after power is off, so wait and verify.
- Never put line power on the output terminals, and never megger through a drive.
- Deceleration overvoltage faults are fixed by a longer ramp or braking.