Cuarto año Lesson 1 of 12 · about 15 min
How a VFD works
What you’ll learn
- The three sections inside every variable frequency drive and what each one does to the power.
- What DC bus voltage to expect on 208, 480 and 600 V drives, and why it is higher than the supply.
- How PWM turns DC into a motor current at any frequency.
- Why volts-per-hertz matters, and the danger stored in the bus after the power is off.
Before this: How an induction motor works · Motor starting methods
An induction motor’s speed is set by the frequency of its supply. A variable frequency drive (VFD) changes the speed by making its own supply: it turns the incoming AC into DC, then switches that DC back into AC at whatever frequency you ask for. Everything a drive does — soft starts, speed control, braking, protection — is built on those two conversions.
The rectifier and the DC bus
The front end on most drives is six diodes — a three-phase bridge. It lets current through only toward the bus, so it charges the bus capacitors from whichever pair of lines is highest at that instant. The capacitors charge toward the peak of the line-to-line voltage, which is 1.414 times the RMS figure you normally quote. Once the drive is carrying load, the capacitors sag a little between charging pulses and the bus settles around 1.35 times the line voltage.
| Supply (line to line) | Bus, running (× 1.35) | Bus, light load (× 1.414) |
|---|---|---|
| 208 V | ≈ 281 V DC | ≈ 294 V DC |
| 480 V | ≈ 648 V DC | ≈ 679 V DC |
| 600 V | ≈ 810 V DC | ≈ 849 V DC |
Those figures are the first thing to check when a drive misbehaves — a bus well below them points at a low or missing supply phase, and a bus that climbs high while the motor slows down is the motor pushing energy back in (the next lesson’s overvoltage trip).
The bus stays charged after the power is off. The capacitors can hold a lethal DC voltage for minutes. Open the disconnect, wait the time printed on the drive’s warning label, then verify zero across the DC bus terminals with a meter rated for the voltage — not just the “charge” light, which can fail.
The inverter and PWM
The inverter is six fast electronic switches (IGBTs), two per output phase. Each output terminal gets connected to the positive bus or the negative bus, thousands of times a second — the carrier frequency, often a few kilohertz. By making the “on” pulses wider in one part of the cycle and narrower in another, the drive makes an average voltage that rises and falls like a sine wave. That is pulse-width modulation. The motor’s windings are inductors, so the current through them cannot follow the sharp pulses — it smooths out into a near-sine wave at the frequency the drive is making.
That is why a normal meter gives strange readings on a drive’s output: the voltage is a string of full-bus-height pulses. Use a meter with a low-pass filter setting (often labelled VFD or LPF) and a clamp for current, which reads the smoother waveform.
Volts por hertz
A motor’s windings have less impedance at lower frequency. Give a 460 V motor full voltage at 20 Hz and it would draw far too much magnetising current and saturate its iron. So a drive keeps the ratio of voltage to frequency steady — for a 460 V, 60 Hz motor that is 460 ÷ 60 = 7.67 V per Hz. Constant V/Hz means constant flux, which means the motor can make full torque at any speed up to base. Above base frequency the drive has no more voltage to give, so V/Hz falls, the field weakens and the motor trades torque for speed.
Worked: a 460 V, 1750 rpm motor run at 30 Hz
- Nameplate: 460 V, 60 Hz, 1750 rpm — a 4-pole motor (synchronous 1800 rpm), slip 50 rpm.
- V/Hz = 460 ÷ 60 = 7.67 V/Hz.
- At 30 Hz the drive puts out about 7.67 × 30 = 230 V (plus a little low-speed boost on many drives).
- Synchronous speed at 30 Hz = 120 × 30 ÷ 4 = 900 rpm. With the same 50 rpm of slip at full torque, the shaft turns about 850 rpm.
- DC bus on its 480 V supply, running: 1.35 × 480 ≈ 648 V DC — the same at 30 Hz as at 60 Hz. The inverter varies the output, not the bus.
Why plants use them
Starting current stays low because the motor is started at a low frequency and never sees a locked-rotor start. Speed can follow the process — a pump holding pressure, a fan holding air flow — and on fans and pumps a small cut in speed saves a lot of power. Drives also ramp smoothly, can reverse without a reversing starter, and carry their own electronic motor overload.
En la obra
Tips
- Learn to measure the DC bus. It tells you at a glance whether the supply side is healthy.
- Use a meter’s low-pass (VFD) setting on the output; an ordinary AC range can read high or wander.
- Keep line and load wiring apart in the enclosure and use the cable the drive maker recommends — the output is electrically noisy.
- Check the heatsink fans and filters on every visit. Heat kills more drives than anything else.
Common mistakes
- Trusting the charge LED instead of metering the bus before touching anything.
- Meggering a motor with the drive still connected — the test voltage goes straight into the output transistors.
- Putting a contactor or power-factor capacitors between the drive and the motor without the drive maker saying it is allowed.
- Expecting the output to read like a sine wave on a basic meter.
- Forgetting that the motor’s own cooling fan slows down with the shaft at low speed.
Check yourself
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1.In what order does power pass through the three main sections of a VFD?
The rectifier turns incoming AC into DC, the DC bus (capacitors) smooths and stores it, and the inverter chops that DC back into variable-frequency AC for the motor.
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2.Roughly what DC bus voltage should you expect on a running drive fed from 480 V three-phase?
A six-pulse rectifier with a loaded bus gives about 1.35 × the line voltage: 1.35 × 480 ≈ 648 V. With little load the capacitors charge toward the peak, 1.414 × 480 ≈ 679 V.
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3.Why does a drive in V/Hz mode lower the output voltage when it lowers the frequency?
A winding’s impedance falls with frequency. Full voltage at low frequency would push too much magnetising current and saturate the iron. Keeping volts-per-hertz constant keeps the flux — and the torque it can make — steady.
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4.A 460 V, 60 Hz motor runs from a drive at 30 Hz in plain V/Hz mode. About what voltage is the drive putting out?
V/Hz = 460 ÷ 60 = 7.67. At 30 Hz: 7.67 × 30 = 230 V. Half the frequency, half the voltage (ignoring the small low-speed boost many drives add).
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5.What does the inverter section actually put on the motor leads?
That is PWM — pulse-width modulation. The voltage on the leads is a train of full-bus-voltage pulses; the motor’s inductance smooths the current into something close to a sine wave at the commanded frequency.
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6.You open the disconnect ahead of a drive. Why wait and meter before touching the DC bus or output terminals?
The bus capacitors store a lot of energy at several hundred volts DC. Wait the time on the drive’s warning label, then verify zero on the DC bus terminals with a meter rated for it before you touch anything.
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7.Why does starting a motor on a drive draw so much less current than starting it across the line?
Across the line, the rotor is at standstill while the field turns at full synchronous speed — 100% slip and locked-rotor current. A drive ramps the frequency up from near zero, so slip stays small and the current stays near its normal range.
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8.Raising a drive’s carrier (switching) frequency usually makes the motor quieter. What is the usual trade-off?
Every switching event costs a little energy in the output transistors. Switch more often and the drive runs hotter, which is why many drives derate at high carrier settings. Longer motor leads also see more stress from fast edges.
Preguntas frecuentes
Is a VFD the same as a soft starter?
No. A soft starter only ramps the voltage up at start (and sometimes down at stop) at the full supply frequency, then usually bypasses itself. A VFD changes the frequency, so it controls the speed the whole time the motor runs. A soft starter limits starting current; a drive also saves energy on fans and pumps by running them slower.
Can I put any motor on a drive?
Most three-phase induction motors will run on one, but the fast voltage pulses stress the winding insulation and can push current through the bearings. Motors built for drive duty ("inverter duty") have insulation and sometimes bearing protection for it. Long leads, high voltages and slow speeds with a shaft-mounted fan all need a look at the motor maker’s data.
Why is the DC bus voltage higher than the supply voltage?
The rectifier charges the capacitors toward the peak of the AC waveform, not its RMS value. The peak of a 480 V line-to-line supply is 480 × 1.414 = 679 V. Under load the capacitors sag between charging pulses, so a running drive reads nearer 1.35 × 480 ≈ 648 V.
Ver también
- VFD frequency & speed calculator — speed at any frequency, and V/Hz
- VFD parameters and troubleshooting — the next lesson
- How an AC induction motor works
- Motor starting methods
- Leer la placa de datos de un motor
- Rusty on VFDs?
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