Braking resistor evidence for a drive request

Decelerating a high inertia load pushes energy back into the drive. A VFD, the variable-frequency drive that controls motor speed, cannot return that energy to the supply unless a special input stage is fitted, so the voltage on its DC bus climbs as the motor slows the load. The DC bus is the drive's internal voltage link, and it has an upper limit the drive will defend.

Enlarged system diagram

×

The flow describes designed energy behavior, not the wiring in your cabinet. Which components are fitted is what the intake has to establish.

A braking chopper with a resistor gives that energy somewhere to go by burning it off as heat. Without the chopper and resistor, or without a long enough deceleration ramp, the drive protects itself with an overvoltage trip. That is general drive behavior, and it explains why two machines with identical drives can behave differently purely on braking. Nothing here is exotic; it is simply where the physics of stopping a load puts the energy.

The user manuals describe this path for each family, and the description is worth reading once. Every later observation makes more sense with the energy flow in mind, from the trip code to the resistor photo. What the manuals describe is the designed behavior of the hardware, not the condition of your specific unit, and the two have to be kept apart when you write the record.

The fault pattern that results is distinctive, and it is worth recording precisely. Overvoltage trips during deceleration, or when a load such as a crane or a lowering hoist drives the motor, read very differently from trips during steady running. Note the code with its full text, the ramp time in force, and the point in the cycle where the trip lands.

Enlarged system diagram

×

The pattern links trips to the braking path, it does not prove the resistor failed. Settings changes are recorded next to the pattern.

Note the machine speed at the trip as well, because the pattern tends to repeat at the same point in the cycle. A deceleration-linked pattern tells a reviewer the braking path is part of the story before anyone touches hardware. Steady-running trips point somewhere else entirely, which is exactly why the timing belongs in the record rather than in the summary.

The pattern also interacts with settings changes. A ramp that was shortened last month, or a process speed that was raised, can move a marginal braking setup over the line, and the trip history with dates makes that visible. The fault or motor article in this series covers the history capture in general; here the only addition is to keep the deceleration context attached to the code.

State the load type while you are at it, because it frames the whole question. A fan or a centrifugal pump usually slows on its own and rarely needs dynamic braking, while winders, cranes, centrifuges, and flywheel loads convert serious kinetic energy on every stop. Saying which kind of load your drive faces turns the braking question from a checkbox into a sizeable judgment, and the same drive reference can be correct for one and undersized for the other.

Braking hardware does not always sit where you expect. Some drive families build the chopper into certain frame sizes and treat the resistor as external, while others offer braking as a separate option module. A compact PowerFlex 4 installation may show nothing braking-related on the drive itself and still carry a resistor bolted elsewhere in the cabinet.

Enlarged system diagram

×

The trace is a visual follow of cables to terminals, not a continuity test. Live work and bus discharge stay with qualified personnel.

Record whether the resistor is internal, external, or part of an option module, and where its wiring lands on the drive. Follow the resistor cables back to their terminals so the connection record matches what you photograph. A resistor that is present but disconnected is a finding worth its own line, because it changes what the settings can achieve.

The hardware location also decides who can safely work on what. Resistors reach high surface temperatures during braking, and the DC bus inside the drive holds charge after power off, so anything beyond looking and photographing belongs to qualified personnel with the equipment isolated. The intake's job is to establish what exists and where, not to open it.

Photograph the mounting as well as the connections. A resistor bolted to a wall with no clearance, or lying on top of cable trays, is installed differently from the drawing, and the difference is a heat question that the next owner inherits. The wide shot plus the connection trace gives the reviewer the whole physical picture in two images, which is why both belong in the pack.

Photograph the resistor unit and its wiring before anything is disconnected. The frame should show the manufacturer's plate, the connection points at both ends, and how the resistor mounts in the cabinet. Include a wider shot with the clearance around the unit visible, because resistors radiate heat and their surroundings matter.

Enlarged system diagram

×

The photo set documents identity and installation. It does not measure resistance or condition, which needs the proper instruments and isolation.

Copy the printed values from the resistor exactly as shown. The resistance in ohms and the power rating in watts or kilowatts are the two figures any replacement or review needs, and both are usually printed or stamped on the unit. A resistor with illegible markings should be recorded as exactly that, because a guessed ohm value is worse than an admitted gap. Both figures usually sit on the same plate, so one clear photo often covers the whole set.

Take the photo with the equipment at rest and leave anything involving live parts to qualified personnel. Give the unit time to cool before anyone handles it closely, and record when the photo was taken relative to the last braking event, because a hot surface changes what can safely be approached. The photo set is the core of the braking evidence, and it is only available while the unit is still installed.

Link the braking evidence to the drive settings and history. Record the deceleration ramp parameters, any DC braking or chopper parameters shown in the drive, and the overvoltage fault entries with their timing. Together these show whether the installed system was tuned to the load or was tripping its way through every deceleration.

Enlarged system diagram

×

The story links documented behavior, it does not measure the braking circuit. Parameter values come from the backup or group photos, not from memory.

The drive's parameter backup covers most of this if one exists. If it does not, photos of the drive's parameter groups cover the same ground at a coarser level, and the parameter backup article in this series explains which groups matter most. The pairing matters because the hardware and the settings are two halves of one behavior: a correctly sized resistor with a wrongly short ramp still trips, and a generously long ramp can hide a missing resistor until the day someone speeds the cycle up.

The combined picture is what a reviewer actually needs. Settings alone suggest the system was designed with braking in mind or was not; hardware alone shows what exists today; the fault history shows how the pair behaved under real load. Recorded together, dated, and photographed, they form a braking story that survives being forwarded without you.

The record decides two things downstream. In replacement sizing, the candidate drive must either carry braking capability for this supply class or accept the same external chopper and resistor, and the resistor values must suit the new drive's ratings rather than being assumed across. In fault review, a documented deceleration pattern separates a braking problem from an unrelated drive fault.

Enlarged system diagram

×

The record informs both decisions without making them. Duty cycle gaps are stated rather than estimated, which is what keeps the decisions reviewable.

The resistor values deserve their own line in any replacement conversation. A candidate drive's chopper has limits on the resistance it can serve, and an old resistor that suited the old drive does not automatically suit the new one. The printed values from the photo set are what make that check possible, which is why an illegible plate gets recorded as a gap rather than rounded to a close-looking number.

The record also makes the sizing conversation concrete. Instead of asking whether a candidate supports braking in general, the question becomes whether it supports this resistor, with these values, on this supply class. That is a checkable question with a documented answer, and it is the difference between a shortlist that survives commissioning and one that does not.

What the record still cannot tell you is the actual braking duty cycle, the frequency and duration of hard stops, so note measured behavior where it exists and mark the rest as unknown. Duty cycle is what sizes a resistor thermally, and a record that admits it is unknown protects the sizing decision that follows. That honesty is the last item in the pack, and it is the one reviewers trust most.

• Record whether braking is internal, external, or an option module, and where it connects.
• Copy the resistor's printed ohm and power values, and mark it unrecorded if illegible.
• Log overvoltage faults with timing against the deceleration ramps in force.
• State the load type, because inertia decides whether braking is central to the request.

• the drives hub
• what to match when replacing a drive
• recording cabinet conditions