VFD RFQ Checks: Motor Data, Control Mode and Braking

A variable frequency drive (VFD) inquiry should begin with the motor and the process, not with a kilowatt number alone. A drive can appear to match on power while still being wrong for the supply, overload profile, control method, feedback arrangement, braking duty, or installed cabinet. For a replacement or project RFQ, the goal is to preserve the required motor behavior and system interface with enough evidence for a technical review.

This checklist is intended for maintenance teams, system integrators, and purchasing teams preparing an industrial drive inquiry. It focuses on the information that helps a supplier assess a model, a replacement family, or a compatible configuration without assuming that visually similar equipment is interchangeable.

Capture the motor nameplate and incoming supply

Start with a clear motor nameplate photo and record the motor voltage, full-load current, frequency, rated speed, power, connection method, service factor, and any duty or insulation notes. Record the incoming supply voltage, phase, frequency, available short-circuit information when relevant, and whether the drive is fed from a line reactor, transformer, or shared DC bus.

Motor nameplate data affects drive sizing and commissioning. ABB explains that the nameplate provides the information needed for drive selection and vector-control autotune, including voltage, current, frequency, speed, and power. The value on the plate should therefore be captured exactly, including units and connection details, rather than retyped from memory. See the ABB motor nameplate guide for the fields commonly used during review.

Define load behavior and overload duty

Two motors with the same rated power can require different drive configurations when the driven equipment behaves differently. Describe whether the load is a pump, fan, conveyor, extruder, hoist, compressor, winder, machine tool, or another application. Note the normal speed range, required starting torque, acceleration and deceleration time, reversals, stopping frequency, inertia, shock loading, and any continuous operation near the upper or lower speed limit.

Also state whether the application is treated as variable-torque or constant-torque duty and identify the required overload percentage and duration if known. A drive selected for a light fan duty should not be assumed to satisfy a high-breakaway-torque or repeated acceleration application. When the duty cycle is complex, a sizing review is more useful than a simple nameplate comparison. ABB’s DriveSize tool describes drive and motor matching across industrial, HVAC, water, machinery, and motion applications.

Confirm the control method and feedback arrangement

State how the motor is controlled today and what the machine requires after replacement. The RFQ should identify whether the system uses scalar V/f control, sensorless vector control, closed-loop vector control, or another control mode. If feedback is present, record the encoder or resolver type, pulse count or interface, supply voltage, connector, cable length, and where the feedback card is installed.

Do not treat a keypad, display, or local operator panel as proof of drive compatibility. The keypad may be only one part of the operating interface, while the control board, option cards, firmware family, parameter set, and feedback hardware determine the actual system behavior. Include a photo of the existing drive terminals, option cards, and nameplate when available. If an autotune or motor-identification routine is required, note whether the machine can be stopped safely for commissioning.

Review braking, regeneration, and thermal limits

Braking information is often missing from an initial inquiry even though it can change the drive configuration. Identify whether the machine coasts, ramps to stop, uses a mechanical brake, returns energy during deceleration, or repeatedly handles an overhauling load. Record the existing brake chopper, resistor part number, resistor value, duty cycle, cooling arrangement, and thermal protection method if those details are available.

The drive, chopper, and resistor must be reviewed as a system. The ABB ACS800 hardware manual describes braking power and resistor selection, heat dissipation, and minimum resistance requirements. It specifically warns against using a resistor below the specified minimum resistance. For this reason, a substitute resistor should never be selected from physical size or a nominal power value alone. Include peak braking power, expected braking duration, repetition rate, and enclosure temperature in the RFQ whenever possible. See the ABB ACS800 braking resistor manual for an example of the technical checks involved.

Preserve the cabinet and installation context

Record the enclosure rating, ambient temperature, altitude, cooling method, cabinet dimensions, mounting position, cable entry, terminal arrangement, and available clearance. Long motor cables, frequent switching, sensitive instrumentation, and strict EMC requirements may require filters, reactors, shield termination, or an installation review. Note whether the drive is installed in a clean control room, a dusty process area, a washdown zone, or a high-temperature cabinet.

For a replacement inquiry, photographs should show the complete nameplate, front panel, side label, terminals, option slots, wiring labels, and cabinet location. A single front-panel photo may identify the product family but may not establish the installed rating, options, or connection requirements.

Check communication, safety, and parameter recovery

List the control system, PLC or DCS interface, fieldbus or Ethernet protocol, hardwired I/O, safe torque off or other safety circuit, and any external speed reference. Record the current communication module, I/O expansion, keypad, encoder card, braking unit, and auxiliary power arrangement. If the machine depends on a parameter backup, state the file format, firmware version, and whether the original parameter set can be exported before removal.

These details help separate a drive replacement from a broader migration task. A replacement that fits the cabinet but changes the command interface, safety wiring, or parameter structure may create additional commissioning work. The inquiry should therefore ask for an application review when the installed documentation is incomplete rather than assuming a direct interchange.

RFQ information to send for a faster review

Exact drive brand, full model number, revision, and nameplate photo.
Motor nameplate photo plus motor type, rated current, voltage, frequency, speed, and power.
Incoming supply, phase, application, normal speed range, overload duty, and acceleration requirements.
Feedback device, braking resistor or regenerative unit, fieldbus, I/O, safety circuit, and option-card details.
Quantity, destination country, target delivery window, replacement reason, and any cabinet or environmental constraints.

For an industrial drive review, you can also browse the KNMKS variable frequency drive category, review related automation system information, or use the KNMKS contact page to send the model and application details. The full product directory provides another route when the exact series is not yet known.

Need a VFD checked? Send the exact model, motor nameplate, application, quantity, destination, and any braking or feedback information available. A precise RFQ gives the technical review a better starting point and helps identify the evidence still needed before a replacement decision.