A variable frequency drive (VFD) controls motor speed by changing the frequency of the AC power it delivers to the motor. Motor speed follows frequency directly: RPM = 120 x frequency / poles, so a 4-pole motor runs 1800 RPM at 60 Hz and 900 RPM at 30 Hz — the drive changes frequency to get whichever speed the process calls for.
Press Start on the keypad in the trainer above and the drive ramps a rotary lobe pump up to speed. Turn the speed knob and the display tracks your command. Open the enclosure cover and you'll see the terminal bay wired the way the sections below describe: three-phase power in, three-phase power out, and the factory jumpers already landed. This is a keypad-only preview. The full VFD module, linked at the end of this article, moves control out to the terminals — a remote start toggle, an FWD/REV switch, and a remote potentiometer for speed — and you wire, program, and commission all of it. The Wire Block Diagram below the trainer names each terminal those applications land on.
How the Drive Controls Speed
Getting to a commanded frequency isn't instant. The drive first rectifies incoming AC into DC, and large capacitors smooth that rectified voltage into a stable DC bus that becomes the energy source for the drive's inverter section. Those capacitors store energy after input power is removed: opening the disconnect does not instantly make the drive safe to touch. The bus has to discharge through the drive's internal bleed circuit, and the required wait time comes from the drive's own manual, not from watching the keypad display go dark. The inverter section then switches that DC bus through six power devices, usually IGBTs, at high speed — a technique called pulse-width modulation, or PWM — to reconstruct AC at whatever frequency the drive has been told to produce.
Two parameters decide where the drive listens for its commands, and they're set independently of each other. P046 [Start Source] picks whether run/stop commands come from the drive's own keypad or from the terminal block. P047 [Speed Reference] picks whether the speed command comes from the keypad, an internal setting, or an external signal such as the 0-10V analog input this module wires up. A drive can take its run command from the keypad and its speed command from a wired potentiometer at the same time.
The Wiring Behind the Keypad
Open the cover in the trainer above and you're looking at the terminal bay this module wires from scratch. Three-phase power comes in at R, S, T, an older international convention for a drive's three line terminals; the PowerFlex 525 prints both R/L1, S/L2, T/L3 on the block. Phase order on the input side doesn't matter, because the rectifier inside converts all three legs to DC regardless of sequence. Output to the motor leaves through U, V, W, the IEC naming for the drive's three reconstructed output phases, which matches most motor lead labeling. That output is the drive's own PWM-reconstructed waveform rather than ordinary sine-wave AC, which is why VFD-rated meters and inverter-duty motor practices apply on the output side in ways they don't on a straight contactor circuit.
The drive ships with its factory jumpers already in place, and they aren't the same kind of jumper. One jumper ties I/O terminal 11 (the drive's +24VDC supply) to terminal 01, the stop input — the PowerFlex 525 manual documents it as shipped installed between I/O terminals 01 and 11. That jumper holds the stop circuit satisfied so the drive will accept a 2-wire run command from a simple toggle switch instead of a 3-wire start/stop station; in the field it gets replaced by whatever provides the stop function: an E-stop contact, a stop pushbutton, a safety relay contact.
The second factory jumper set is a hardware safety path, not a control input. The Safe Torque Off (STO) terminals — S1, S2, and S+ — arrive bonded together from the factory, keeping the STO circuit satisfied so the drive is allowed to produce output at all. Terminal 01 asks the drive to stop through its normal control logic; the STO terminals remove the drive's ability to produce torque at the hardware level, independent of what the firmware is doing. Open that circuit (jumper removed, guard switch tripped, E-stop wired in) and the drive can't run no matter what a parameter says, and it trips on an STO-related fault such as F059. The PowerFlex 525 manual counts three jumpers on the control board in total: the terminal 01-11 tie plus the two that bond S1, S2, and S+ together. It lists verifying all three in their factory positions as a step in its basic bench test.
Why the Drive Has Two Stop Paths
Terminal 01 and the STO terminals look redundant at first: two circuits that both end with the motor stopped. They do different jobs. Terminal 01 is a control input — the drive's processor reads it and carries out whatever stop it has been programmed to perform. An E-stop contact landed on terminal 01 therefore gives a commanded stop, not a guaranteed one; the request still runs through the drive's firmware. The STO path works below that level. Opening it blocks the gate-firing signals from reaching the output IGBTs, breaking the link with the drive's microcontroller, so torque production ends whether or not the software cooperates. Ordinary stopping belongs on terminal 01; removing torque no matter what the drive's electronics are doing belongs on STO. The manual rates the STO function to Category 3, PL d per EN ISO 13849-1 and SIL CL2, with a safety reaction time of 100 ms or less; it assigns no comparable safety rating to terminal 01.
The use cases follow from that split. Stop pushbuttons, ordinary E-stop contacts, and run permissives land on terminal 01. Guard-door interlock switches and E-stops that belong to a safety-rated circuit run through a monitoring safety relay into the STO terminals — the manual's application examples wire GuardMaster safety relays exactly this way. The two paths also behave differently at the end of travel. STO by itself coasts the motor; when an application needs a controlled deceleration, the manual's own example sequences a normal stop command first and opens the STO circuit after it. Restart after an STO trip is deliberate: the safety relay must be reset, then the drive needs a fresh, valid start command. None of this is mandatory. A machine with no safety circuit leaves the factory bonds in place, and the manual says exactly that: if not using safety, verify and tighten the jumper on S1, S2, and S+.
One boundary is easy to get wrong: STO removes torque, not voltage. The manual states that the function does not provide electrical safety, and hazardous voltage can still be present at the motor. Opening the STO circuit is not a substitute for disconnecting power and verifying zero before putting hands on the machine. Lockout/tagout still applies.
Reading is one thing — wiring it yourself is another. Open the interactive trainer and build this circuit from scratch.
Wire the terminals, program the parameters, and commission the drive in the full VFD module →Frequently asked questions
What is a VFD?
A variable frequency drive (VFD) is a device that controls motor speed by changing the frequency of the AC power delivered to the motor. Motor speed follows frequency directly (RPM = 120 x frequency / poles), so the drive changes frequency to produce whatever speed the process needs.
What does VFD stand for?
Variable Frequency Drive.
How does a VFD control motor speed?
It rectifies incoming AC to DC, stores that DC on a capacitor bus, then switches the bus through power devices (usually IGBTs) at high speed to reconstruct AC at whatever frequency it has been commanded to produce, a technique called pulse-width modulation (PWM). Motor speed tracks the output frequency: a 4-pole motor runs 1800 RPM at 60 Hz and 900 RPM at 30 Hz.
What is the difference between a VFD and a motor starter?
A contactor-based motor starter uses wiring as its logic: a physical connection breaks or holds to start, stop, or reverse the motor. A VFD moves that logic into programmed parameters. The wiring delivers signals (run, direction, speed reference), and the drive's parameters tell it what those signals mean. Soft starting is also inherent to a VFD, because it ramps frequency up from zero and inrush current stays near rated. That is why VFD installations skip the reduced-voltage starters, autotransformers, or wye-delta switches a contactor-based system would need for the same result.
Where are VFDs used?
Anywhere a process needs variable motor speed instead of a fixed on/off run: rotary lobe pumps, HVAC fans and blowers, conveyor belts, compressors, and water or wastewater treatment equipment are common applications.
Is a VFD's output safe to measure with a normal multimeter?
Use meters rated for VFD service on the output side. The drive's output to the motor is a PWM-reconstructed waveform, not clean sine-wave AC, so standard practices change. Do not megger the drive's output terminals; if insulation testing is required, disconnect the motor leads and test the motor and cable separately.