Braking resistor on a PowerFlex 525: why the drive still trips F005, and why the resistor cooks at standstill

When a 525 with a braking resistor ends up on my phone, the call starts one of two ways. "We put the resistor on and it still trips F005 every time it stops." Or: "The resistor's too hot to touch and the motor isn't even turning." Neither one usually means the resistor is bad.

What the part does, quickly. When a load slows down faster than it wants to, or drives the motor itself (a hoist lowering, a conveyor running downhill), the motor turns into a generator and pushes energy back into the drive's DC bus. A braking resistor, or dynamic braking resistor if you like the long name, burns that energy off as heat so the bus never gets to the overvoltage trip. It doesn't stop anything on its own. The drive does the braking, the resistor just gives the energy somewhere to go.

So here's what a 525 does with one out of the box, how to size and wire it with real numbers, and why it cooks with nothing running.

Out of the box, a 525 ignores the braking resistor

Every PowerFlex 525 has the brake transistor built in, all ratings, all frame sizes. The resistor is always external and lands on DC+/BR+ and BR-. So people mount one, wire it up, hit stop, and get the same F005 they had yesterday.

A437 [DB Resistor Sel] ships at 0, "Disabled." Until somebody changes it the drive never fires the transistor, and the new resistor just sits there cold. It's the first thing I look at. Most of the time it's also the whole answer.

What the drive does instead depends on A550 [Bus Reg Enable], which ships enabled. With the regulator on, the 525 protects itself by quietly stretching the decel. Ask for 2 seconds and you might get 6, and nothing faults. Stuck regulating for a full minute, which an overhauling load can manage, it quits with F006, a stall fault. Switch the regulator off with no resistor and a hard stop runs the bus up to the trip, 810 V DC on a 480 V drive or 405 V on a 240 V one. There's your F005. The other bus faults have their own write-up in PowerFlex 525 fault codes.

Once A437 is set to anything but 0, the drive ignores A550 completely. Turning it off anyway does no harm.

One stop, three outcomes on a 480 V drive (simplified). The bus regulator stretches the decel, no regulator means F005, and an enabled resistor holds the bus under the trip while the motor follows the ramp.

Two readouts show what a stop really looks like. b005 [DC Bus Voltage] is the bus itself, filtered, around 680 V on a 480 V drive at idle. b013 [Contrl In Status] carries a digit for the brake transistor, the leftmost of the four. It goes to 1 every time the transistor fires and holds for at least half a second, so you can catch it. And for proof that the regulator is eating your decel time, set a relay or opto output to 5 "Ramp Reg." It switches whenever the drive reshapes the ramp to dodge a fault.

Parameter	Default	Set it to	Why
A437 [DB Resistor Sel]	0 "Disabled"	1 "Norml RA Res" for an Allen-Bradley AK-R2. 3-99 for a third-party resistor's duty rating in %. 2 "NoProtection" only for a resistor rated for continuous duty	At 0 the brake transistor never fires
A438 [DB Threshold]	100%	Leave it at 100%	Under 90% is rarely needed. Lower settings can keep the transistor switching with nothing to brake
A550 [Bus Reg Enable]	1 "Enabled"	Doesn't matter once A437 isn't 0	With A437 at 0 it stretches the decel, then faults F006 after a minute
P045 [Stop Mode]	0 "Ramp, CF"	0 or 4 (Ramp), or 8 or 9 (Ramp + EM brake) on a motor with a brake	A coast stop doesn't regenerate, so the resistor never comes into it
b005 [DC Bus Voltage]	Read only	Watch it through a stop	F005 trips at 810 V on a 480 V drive
b013 [Contrl In Status]	Read only	Watch the leftmost digit	1 means the brake transistor is firing

The PowerFlex 525 settings that decide whether a braking resistor does anything.

Motor data, start source and the rest of a normal startup are in our list of PowerFlex 525 parameters.

Do you need a braking resistor at all?

Not always. Plenty of 525s run their whole life without one. My quick test: coast-stop the machine from full speed and time it. If your decel ramp is longer than that, friction and losses do the braking and the bus hardly moves. If it's shorter, that energy has to go somewhere.

Load	What happens when it slows	Resistor?
Flat conveyors, most pumps	Friction stops it sooner than most ramps	Usually not
Big fans, flywheels, centrifuges, heavy rolls	Keeps spinning long after power is off, so a short decel turns the motor into a generator	Yes, or a longer decel
Hoist lowering, downhill conveyor, unwind stand	The load drives the motor the whole time it moves	Yes, sized for long duty
Conveyor transfers, anything where one drive pushes another	The slower drive regenerates while running, not only on stops	Yes on the slower drive, or fix the speed match
Category 1 stop (controlled E-stop)	Same energy as a normal stop, sometimes on a much shorter ramp	Size it for the E-stop ramp

Where the resistor earns its place, and where it's dead weight.

Sizing a braking resistor without guessing

Two hard limits, then heat.

The minimum resistance belongs to the drive. Go under it and the brake transistor carries more current than it's built for. The resistor survives, the transistor doesn't, and on a 525 the transistor lives in the power module. Minimums by rating are in the table further down.

How high you can go depends on the load. At a given bus voltage a resistor only takes V²/R watts, so if its resistance is too high it can't absorb the peak and the bus trips F005 anyway, resistor or not. For the math, use 790 V on a 480 V drive, 395 V on 240 V and 987 V on 600 V.

Heat is where people get surprised. The Allen-Bradley AK-R2 resistors that pair with the 525 are small parts, 86 W to 260 W continuous. The 120 Ω AK-R2-120P1K2 that goes on a 5 HP 480 V drive is a 260 W resistor. Put 790 V across 120 Ω and it's taking about 5.2 kW. 260 W is exactly 5% of that, which is what the 5% duty rating on these resistors means. Full braking for one second out of every twenty, then it has to cool.

A worked example: 5 HP at 480 V

Say a 5 HP, 480 V 525 stops a direct-coupled load from 1,750 rpm in 3 seconds, once a minute. Motor plus load comes to 0.12 kg·m² of inertia. The motor's share is on its data sheet; load inertia behind a gearbox gets divided by the ratio squared.

Step	Math	Result
Speed	1,750 rpm × 2π / 60	183 rad/s
Peak braking power	J × ω × ω / t = 0.12 × 183 × 183 / 3	about 1,340 W
Energy per stop	peak / 2 × t	about 2,000 J
Average power	peak / 2 × (3 s / 60 s)	about 34 W
Highest resistance that still takes the peak	790² / 1,340	about 465 Ω
Lowest resistance the drive allows	Table below	47 Ω

The math for the 5 HP example. The peak comes right at the start of the decel.

The AK-R2-120P1K2 is 120 Ω, well inside that window, and 260 W continuous against a 34 W average. Easy fit. Now run the same load with a stop every 6 seconds instead of every 60. The average jumps to about 335 W, the 260 W resistor runs hot, and the thermostat starts opening.

HP	240 V min Ω	240 V resistor	480 V min Ω	480 V resistor	600 V min Ω	600 V resistor
0.25	56	AK-R2-091P500	n/a	n/a	n/a	n/a
0.5	56	AK-R2-091P500	89	AK-R2-360P500	112	AK-R2-360P500
1	56	AK-R2-091P500	89	AK-R2-360P500	112	AK-R2-360P500
2	41	AK-R2-091P500	89	AK-R2-360P500	112	AK-R2-360P500
3	32	AK-R2-047P500	89	AK-R2-120P1K2	112	AK-R2-120P1K2
5	18	AK-R2-047P500	47	AK-R2-120P1K2	86	AK-R2-120P1K2
7.5	16	AK-R2-030P1K2	47	AK-R2-120P1K2	59	AK-R2-120P1K2
10	14	AK-R2-030P1K2	47	AK-R2-120P1K2	59	AK-R2-120P1K2
15	14	2 × AK-R2-030P1K2	43	2 × AK-R2-120P1K2	59	2 × AK-R2-120P1K2
20	10	2 × AK-R2-030P1K2	43	2 × AK-R2-120P1K2	59	2 × AK-R2-120P1K2
25	n/a	n/a	27	3 × AK-R2-120P1K2	53	2 × AK-R2-120P1K2
30	n/a	n/a	27	3 × AK-R2-120P1K2	34	3 × AK-R2-120P1K2

Minimum resistance and the matching Allen-Bradley braking resistor for PowerFlex 523 and 525 drives. "2 ×" and "3 ×" mean resistors wired in parallel.

Single-phase models match the 240 V three-phase drive of the same horsepower, except the 1.5 HP 120 V one: 41 Ω minimum, AK-R2-091P500. Every resistor in that table is rated for 5% duty.

Wiring it so a shorted transistor can't start a fire

The braking resistor goes between DC+/BR+ and BR-. On every frame size they sit side by side in that order: DC-, DC+/BR+, BR-. Land the second lead on DC- instead of BR-, and the resistor is across the entire bus the whole time the drive has power. Around 680 V across 120 Ω is close to 3.9 kW, going into a 260 W part. It won't last long enough to finish commissioning.

A DC fuse goes at BR-, and on a UL installation that isn't optional. Under IEC every one of those terminals you connect gets its own, so both resistor leads. Use the fuse specified for that drive rating, a PV-20A10F at BR- on the 5 HP 480 V drive for example, not whatever DC fuse happens to be in the crib.

Then the thermostat. AK-R2 resistors have one built in that opens at 227 °C (440 °F) and closes again at 182 °C (359 °F), with a contact rated 125 V AC, 15 A. The common move is to wire it to a drive input set to "Aux Fault." That catches an overloaded resistor. It does nothing about a brake transistor that failed shorted, because a faulted drive still has a charged bus and a shorted transistor keeps the resistor across it. The drive can't protect an external resistor from that. Only taking line power away does.

So I put the thermostat in series with the coil of the line contactor feeding the drive, inside an ordinary start/stop seal-in. When the thermostat opens, the contactor drops out and the bus bleeds down with nothing feeding it. And since it's a seal-in, the drive doesn't come back by itself when the resistor cools to 182 °C and the contact recloses.

Mounting: the surface can run past 200 °C. Give it at least 2 inches of clearance, keep plastic and wire duct away from it, and mount it where its heat leaves the cabinet instead of cooking everything else inside.

Resistor across DC+/BR+ and BR- with a DC fuse at BR-, and the thermostat in series with the line contactor coil.

When the braking resistor is hot and the motor isn't turning

It should be cold whenever nothing is regenerating. Hot with the motor stopped means something is putting bus voltage across it, and on a 525 b013 sorts that out in about a minute.

Power the drive, leave it stopped, and watch the leftmost digit of b013. If it shows 1, the drive is firing the transistor on purpose. Usually someone has pulled A438 below 100%, which can keep it switching with nothing to brake, and a line that runs high makes that worse. Put A438 back.

Zero there, with the resistor still heating? The drive isn't firing anything, so it's wiring or hardware. Kill power and check that the second lead isn't on DC-. If the landing is right, the brake transistor has failed shorted. Don't bother with a new resistor, it'll cook exactly like the old one. The repair is a power module, or a new drive.

What you see	Usual cause	Check first
F005 on every stop, resistor stays cold	A437 still at 0	A437, then b013 during a stop
F005 only on hard stops, resistor warm	Resistance too high for the peak, or a decel shorter than the load allows	The peak math. Lower ohms (never under the minimum) or a longer decel
Stops slower than the decel time, no fault	Bus regulator stretching the ramp because A437 is 0	An output set to 5 "Ramp Reg"
Fine for months, now F005 on every stop	Open element or a broken lead	Ohm it out, steps below
Line contactor keeps dropping on the thermostat	More duty than the resistor is rated for	Average power math. A bigger resistor or a longer cycle
Resistor hot, motor stopped	Lead on DC-, A438 below 100%, or a shorted transistor	Leftmost digit of b013

Trouble with a resistor on a PowerFlex 525, and where to look first.

Checking the resistor itself

1.    Power off, lock out, and wait at least three minutes.

2.    Prove L1, L2 and L3 are dead, then DC+ to DC- at zero (110 V single-phase drives have no DC bus terminals), then each power terminal to ground until it reads zero. A blank display doesn't mean the bus is empty.

3.    Lift one resistor lead. Measured in place, you're reading through the drive.

4.    Ohm it cold. AK-R2 resistors are ±5%, so a 120 Ω unit should read 114 to 126 Ω. Open, or way off, it's done.

5.    Ohm the thermostat leads cold. Closed is right; open at room temperature means a failed switch.

Hoists: the braking resistor is the easy part

Lowering a load is regeneration from top to bottom. Fifteen seconds of lowering is fifteen seconds of braking, way past what a 5% part is built for. Size the resistor for the real duty and set A437 to its rating.

The 525 can run a motor brake. P045 goes to 8 or 9 (the Ramp + EM Brk options), and one of the outputs to 15 "EM Brk Cntrl." On a start the drive sits at P043 [Minimum Freq] for t086 [EM Brk Off Delay], then energizes the output to release the brake and ramps up. Stopping runs it backwards: down to minimum frequency, output drops, the brake sets, and t087 [EM Brk On Delay] later the drive shuts off. Both delays default to 2.00 s and go up to 10. Make t087 longer than your brake takes to set.

The 525's EM brake sequence from start to drive output off. The shaded zones are the brake's own travel time.

On top of that, three things I always do:

·       An interposing relay between the drive and the brake coil. The 525's relays are rated 0.5 A inductive, the opto outputs 50 mA and non-inductive only. A brake coil wears contacts out, and a plug-in relay costs a lot less than a control module.

·       The brake coil on its own supply, not off the motor leads. At low speed the drive puts out a fraction of line voltage, and a coil hung on the motor terminals won't pull in reliably.

·       A PLC permissive in series with the drive's brake output, so the PLC can set the brake whatever the drive thinks it's doing.

Now the part that matters most. It's all timers. With P043 at its 0.00 Hz default, "minimum frequency" means zero hertz, and nothing in that sequence checks that the motor is carrying the load before the brake lets go. A 525 has no torque proving, and its Vector mode can't control torque. A PowerFlex 755 with TorqProve checks both ways, before it releases the brake and after it sets it. For a trolley, or a small lift with nobody under it, a 525 with a properly sized resistor and a good mechanical brake is a reasonable setup. If the load ever hangs over people, I'd use a drive that proves torque.

FAQ

What does a braking resistor do on a VFD?

It turns the energy a slowing or overhauling motor sends back into the drive into heat. Without one, that energy charges the DC bus until the drive trips on overvoltage or stretches the decel to save itself. Some people call it a brake resistor or a DB resistor; same part. It won't hold a load at standstill, that's the mechanical brake's job.

What causes a braking resistor to fail?

Heat, nearly always. More duty than it's rated for (a hoist, a fast-cycling machine, a lowered A438), or a brake transistor that shorted and parked the full bus across it. Below-minimum resistance is the odd one out: the resistor's fine and the transistor dies.

How do you check a braking resistor?

De-energized: wait three minutes, prove the bus is at zero, lift one lead, compare the ohms to the nameplate (±5% on AK-R2 parts), and confirm the thermostat is closed cold. With the drive running, watch b005 and the leftmost digit of b013 through a stop.

How do I connect a braking resistor to a PowerFlex 525?

One end to DC+/BR+, the other through its DC fuse to BR-, nothing on DC-. Thermostat into the line contactor coil circuit. Then set A437 to match the resistor (1 for an AK-R2) and leave A438 at 100%.

Replacing a braking resistor, or the drive behind it

If the math lands on an Allen-Bradley part, we carry the AK-R2-120P1K2, the 120 Ω resistor for 3 to 10 HP 480 V drives (in pairs or threes above that), and the AK-R2-360P500 for the 0.5 to 2 HP ones. If b013 says the transistor is off and the resistor still heats, no resistor will fix it. That's a power module or a drive, and we keep PowerFlex 525s on the shelf, including the 25B-D010N114, the 5 HP 480 V drive from the example above.