DC resistor overheating passive protection - Page 1
Forum section: electronics / projects

author: r0d3z1
author_rank: Regular Contributor
timestamp: April 03, 2020, 07:40:07 am
content: Hi,I need to protect a brake resistor from overheating. It must be a passive solution that avoids overheating (and consequently fire) in case of MOSFET short circuit.The resistor is connected to a 40V supply and during normal working conditions it is choppered with a 1khz PWM, the current can reach 20A.My ideas are:slow blow fuse with a rating near the 20A in a way that in case of MOSFET short circuit the long term (tens of seconds) 20A current will burn the fuse beforeresettable PTCNC thermal switch in series with the resistor (hare the normal working high current could be a problem)NO parallel thermal switch + fast fuse, here the NO thermal switch trigger a fast short circuit which causes the fuse openingwhat do you think about these solutions?the 1st is the most simple to implement, but will it be effective?the 4th is probably the most secure but it is not easy to put the thermal switch near the resistor.Thanks

author: Siwastaja
author_rank: Super Contributor
timestamp: April 03, 2020, 08:06:45 am
content: Fusible resistors may be available. They blow up on overheating in a way which causes little external heat damage, like fuses; in other words, the surface temperature before it finally fails open will be much lower than with "standard" resistors. Often, a thermally insulating glassfiber/silicone tubing is added around the resistor to protect nearby parts.If this is something that dissipates a lot under normal operation and is heatsinked, then I'd just add a single-use (non-resettable) thermal fuse, coupled to the heatsink, i.e., your idea 3 or 4.Your idea #1 should be implemented in any case as an extra measure, because it is fairly cheap and small yet effective.Have you thought about what happens if the resistor goes open? You have some kind of overvoltage cutoff which stops energy from being generated when you can't dissipate it in the brake resistor? What are the consequences of not being able to dissipate this energy?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 03, 2020, 09:57:10 am
content: Ain't gonna get PTCs big enough for that directly, but you could put one in front of the transistor, or use an NTC to shunt its gate drive.Preferably with a comparator, so the transistor doesn't spend extra time in the linear range, burning a sizable fraction of that 800W load. And with the comparator output feeding back to the controller to command it to stop braking, or warn the user, or some kind of useful feedback or control.Also, might it be a... rather horrific idea, to consider disabling a braking element? I have no idea what this is braking, but if it's braking motive equipment as the name suggests, it's probably a good idea it comes to a safe, zero-energy condition under such a failure mode. In which case, the resistor should be designed to handle this easily in the first place, but also could be designed with a redundant backup perhaps, or using especially robust components (e.g., vitreous enamel or bare wirewound types versus metal-case types) with the understanding that, they will probably sustain one or a few excessive operations, but should be replaced promptly after such stress.A good mission-critical example being commercial jet brakes. They have to handle the enormous energy of stopping, well, however many hundreds of ton(ne)s is moving at ~ludicrous ground speed. They can do it, magnificently so, but they get unbelievably hot in the process, just barely holding together as the craft comes to a halt. Brake fires are an immediate grounding order and maintenance step (AFAIK; or at least, I would hope so..?!). Normal landings of course (with long runways, and often, thrust reversers to help out) have enough time to keep them cool during use so only need regularly scheduled maintenance.And if it's, like, an electric vehicle application, why not use a regenerative motor controller? Batteries make fantastic brake dumps, dissipating a fraction of the heat! The cheapest watt to dissipate is the watt you didn't have to dissipate in the first place.Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: April 03, 2020, 10:39:23 am
content: Quote from: T3sl4co1l on April 03, 2020, 09:57:10 amThere are very awkward corner cases though, like the guy who lives on the top of the hill, charging the pack full there, or the another poor guy who's living in a cold country (charging disabled or limited below 0 degC). Maybe the braking resistor is a backup for these corner cases.Another option is to make sure the mechanical brakes can work every time, using the same physical controls than the e-brakes, so that there is insignificant difference in how the controls react when the e-brakes (regenerative or dissipative) are out, maybe the driver only needs to push the brake pedal a bit further down than normally, for example. In this case, getting rid of the brake resistors completely might be the most sensible choice, you have fewer corner cases now.

author: duak
author_rank: Super Contributor
timestamp: April 03, 2020, 08:37:22 pm
content: I repaired a CNC lathe recently where the braking transistor failed shorted so the braking resistor was always under power. The resistor is a hollow cylindrical type with the long axis vertical and mounted about 10 - 15 cm from the servo drive. The operator noticed a burning smell but there were no alarms. He opened the electrical panel door, some smoke came out and saw the resistor glowing red so he shut the machine off. The wiring and the nylon connectors on the side of the servo drive were melted and charred and some components on the PCB appeared to be overheated. The high temperature wiring, probably PTFE and not glass fiber, was charred close to the resistor. The resistor itself actually looked fine and was still the correct resistance. The resistor is rated for 200 W but I calculate it was dissipating 1500 W for probably 30 minutes! The shop had another unit so I just replaced the servo, resistor and wiring.This tells me that this type of resistor has quite a large overload capacity. A number of years ago I designed a similar servo drive. The braking resistors were the aluminum case style and were mounted on a plate for cooling. During testing, the brake circuit failed and applied 5X to 10X power for about 10 seconds. All the resistors were damaged with their end terminals pushed outwards. When the unit went to production, I used the cylindrical ceramic type.Some things I would think about are:1.) can a shorted or open braking transistor or resistor be detected?2.) what problems can be caused if the fault is not detected? ie., will resistor overheat? Will the motor drive, drive mechanism or payload be damaged if braking circuit does not work?3.) should a failure in the braking circuit prevent the drive from working? ie., an interlock.I think the best design would monitor the voltage on the switched end of the braking resistor and determine if the braking circui is good.The second best is use a thermal switch on the resistor. Some braking resistors already have thermal switches that can be used to disable the drive. These switches are normally closed that open when there is an over temperature. If the switch is resetable and in series with the resistor, it would have to be inspected to see if it had been tripped. I don't know how important the braking circuit is.I don't know if a fuse will have the characteristics needed and still be reliable over the long term. However, the current & time curves for fuses are available from the manufacturer and it may be possible to find a solution. Note that an open fuse may not be obvious.Here's an idea that might be useful: If one end of the braking resisitor is connected to the DC-link voltage, perhaps a thermal switch or even a thermal circuit breaker or fuse could be mounted close by the resistor and supply DC power to the drive. If the resistor gets too hot, enough heat could be coupled into the device to cause it to open and disable the drive.Best Wishes,

author: Gyro
author_rank: Super Contributor
timestamp: April 03, 2020, 09:42:27 pm
content: When heat dissipation transitions from convection to radiation, its efficiency increases greatly. This gives an open construction wirewound resistor a significant advantage under fault conditions - However, as you observed, it's pretty catastrophic for things that happen to be in the vicinity!Metal shielding and suitably rated wiring might be all that are needed to cope with fault conditions.

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 04, 2020, 03:10:32 am
content: With industrial drives, the accepted method is a normally closed thermostat close to the resistor controlling a contactor/relay that removes the source voltage upon chopper failure.In some higher power systems, the power supply diodes are replaced with thyristors and the devices serve two purposes. One, capacitor pre-charge is controlled with gate control and two, shutdown by way of the same nc thermostat if the brake chopper fails and the resistor overheats.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 04, 2020, 06:21:50 am
content: Protecting the resistor is the easiest part. The difficult part is, what do you do when the resistor goes out? It has some important task, right, otherwise you wouldn't have it in the first place? And often it is something safety-related.

author: r0d3z1
author_rank: Regular Contributor
timestamp: April 07, 2020, 06:53:33 am
content: thanks for the answers.The thermal switch seems to be the best solution.Here is the answer to the questions:1) the fault is not critical, the regenerative load can be stopped safely if a failure happens.2) there is already software protection to prevent overheat.3) it is caps bank with an AC/DC, a regenerative load and a brake resistor, all of them connected to the so-called "dc-link"3) if the MOSFET shorts I can't stop current flowing through the resistor because the ac/dc is still working and I have no way to stop it.