Lift Controllers

 The most frequent visitors to the workshop nowadays are printed circuit boards for a variety of applications. Mostly lift controllers, but also chillers, central heating, electric fences, and fuse panels for motorcars; all sorts of odd things arrive each week.
The faults are often not dissimilar to those found in old CRT TV sets, being cracked solder joints and their consequences. One of the types of fault however that is not found in domestic appliances is a damaged varistor. With a thing like a lift controller there's always the chance the installation will get struck by lightning and, to combat consequential problems, the designers generally fit a varistor across each vulnerable input circuit.
Varistors come in various shapes and sizes dependent on the amount of surge protection likely to be needed.
When a varistor has done its job it may be still in good order, however if a significant amount of energy has to be dealt with it can burn up and sometimes catch alight. When this happens the thing will turn into a very low value resistor and blow any fuse in the circuit.
Most times the device does its job perfectly, protecting hundreds of integrated circuits, but will require changing before the circuit will work. Because of the dratted harmonization of European mains supplies varistors will go pop because the designers didn't realise what "harmonization" really meant... but that's another story..

 

 Replacing a varistor without a circuit diagram or a components list for the board is a bit of a black art as usually little is left of the original markings and often there's no firm indication of the circuit parameters. If there's a row of the things, clearly protecting identical circuits, one can see the rating of the device on the side of one that's still nice and blue and shiny.
Unlike resistors and capacitors, varistors from different manufacturers, but nevertheless having the same rating, can be marked in different ways and some background knowledge is necessary when purchasing replacements.
Fixing things like lift controllers (pictured above) is not easy as sometimes the lift is a hundred miles away and it's not a good idea to have the circuit board winging its way backwards and forwards until the fault is fixed. Not just a pain for the engineer having to drive backwards and forwards, but certainly not nice for the lift users, who may be old ladies in a nursing home, having to walk up several flights of stairs while their lift is waiting to be fixed.
One lift I fixed was the one that went up to a main operating theatre in a hospital on the Isle of Wight. As the hospital is judged on the number of operations it can perform the chief executive was rather upset when his lift failed. Such was the panic that I was presented with, not one circuit board, but a huge box full of the things. I had to look at all of them even though it was most unlikely there were more than one or two faults in all. It's easy to diagnose a fault when there's a blackened and burnt diode but very tricky when faced with a faultless board full of microprocessors and logic devices. Finding that there's no fault present always takes a lot longer than changing a burnt diode.
Thankfully most faults are due to the failure of common components such as relays, diodes and varistors and very few due to failure of complex chips.
A lot of failures can be blamed on the original designer. These generally fall into the category of excess heating causing the solder to fail or the board material or the component to burn. Many faults are due to finger trouble such as connecting a wire wrongly or short-circuiting connections and some are due to lightning or surges on the mains supply. Rarely does a component fail when it is run within its ratings.
I recall that when I worked in Defence, many millions of pounds was spent calculating reliability figures so that spare parts could be made available to keep equipment running. Looking back I think that the figures that were turned out were absolute rubbish, as the most common reasons for failure were never considered.
I bet that there are MoD warehouses stacked full of components, costing many billions of pounds, that will never be used. No one will ever admit this, as goodness knows how many little empires are dependent on the calculations, supply and handling (and disposal) of the stuff.
 

 

Just horsing around!

The latest arrival (March 2016) is a large circuit board. It's come from a nursing home where someone thought it would be nice if they introduced a horse to an inmate unable to leave the first floor. The doors closed and the lift moved a few inches then just stuck and wouldn't go up or down. An hour later the people were rescued from the lift complete with horse, having burnt out the motor contactors which I duly replaced.

 Typical drive unit repairs

 Below: a picture of the chassis interior of a damaged lift motor drive unit. This one is known as a "Vacon" unit.

 

 The circuit boards and the IGBT module have been removed and you can see where a failure has occurred by the soot that's been ejected from the module.

In this instance the module was made by Semikron and contained all the major high power semiconductors necessary to control the 3-phase motor.

There's a 3-phase bridge rectifier, a set of IGBTs (insulated gate bipolar transistors) for driving the motor and operating the brake.

The circuit board carries the IGBT drive components, power supplies and interconnections for the module etc.

Unfortunately, when an IGBT module fails some 600 volts or so is placed on the drive circuits because the insulated gate often breaks down. This causes lots and lots of damage to the low voltage circuitry which is very difficult to repair because the parts are surface mounted and carry microscopic codes instead of part numbers.

Generally, one can reckon on a dozen replacement parts in addition to the expensive IGBT module.

This type of unit can provide typically from 7KWatts to 12 KWatts of power.

 

  Above: a view inside a typical blown-up IGBT module used in a Kone drive unit. It's about100mm x 50mm or in sensible units: 4 inches x 2 inches.

The plastic lid has been removed so you can see the inside.

The square white areas are power transistors and diodes.

The black smudges are made by soot which is centred on parts which have failed. The soot is underneath a jelly-like substance which is used to encapsulate the assembly.

Amazingly, a module like this can supply 60 or 100 amps at 600 volts or more to drive a lift motor.

A failure can occur for several reasons. For example if a motor bearing is seizing, excessive current flows and this results in the module getting too hot. The hotter the module gets the less is it able to provide the high currents taken by the motor and it fails catastrophically often taking out a large fuse rated at 100 amps.

A pair of IGBTs for each of the 3 phases supplying the motor is usually connected in series (a "totem pole" push-pull circuit). If the top transistor collector carrying 600 volt HT breaks down to its insulated gate, a considerable amount of damage will be caused to the control circuit board.

Early drive units used SCRs (Silicon controlled rectifiers) but more modern types use a set of very high power IGBT transistors (IGBT=Insulated Gate Bipolar Transistor).

An IGBT can control huge currents at the expense of almost zero power input, much like a high power thermionic valve used in broadcast transmitters.

 

  This is the top side of the board that connected to the module shown above.

The underside of the board is covered with tiny surface mounted parts and the IGBT module is soldered by over 30 tags which are located around the 4 blue capacitors left of centre.

The parts which are usually damaged when the module fails are located in 4 places. Below the 3 orange blocks (pulse transformers) top left; to the left of the pulse transformer at the centre, and underneath both these areas.

The damaged parts will be primarily the optical couplers (the white i/cs) the capacitors, diodes and zeners which feed the insulated gates in the module.

In addition the pair of contactors (bottom right) which are mounted on a second control board may have burnt or welded contacts.

These contactors govern the direction of rotation of the motor.

In this model of drive unit the contactors are not mechanically coupled together so, if both operate and connect the motor to simultaneously go forward and backwards (perhaps due to welded contacts) the IGBT module will fail catastrophically.

The six large black capacitors smooth the rectified 3-phase mains producing up to 600VDC or so; the HT supply to the IGBT module.

 Not all jobs are straightforward and clinical. Take this circuit board for example where the designer forgot to consider localised heating from feed resistors.

Two areas were affected similarly, and both sides of the board needed fixing.

Why not chuck it away? Well sometimes a replacement circuit board is no longer available and there's no option but to make repairs, no matter how messy, as the only viable alternative is to replace major parts of the lift system costing tens of thousands of pounds.

If the burning is too serious a new section of board material has to be grafted in place once the burned area has been cut out.

In this case the board carries a pair of numeric indicators showing the floor of the building. Sitting at ground "G" most of the time resulted in the pull up resistors for the relevant LEDs in the matrix getting very hot. Eventually the solder melted and the joints began to go intermittent. One or more resistors then got even hotter and one eventually failed. The only warning something was amiss was when part of the "G" disappeared.

 

 Here's an instance of what happens when a lift engineer connects a high voltage to a low voltage circuit. Because a connector was poorly numbered Pin 4 wasn't Pin 4 carrying a ground connection of zero volts it was Pin 21 carrying 24 Volts.

Unfortunately, in this case 24 Volts was connected to the local intercommunications bus network and damaged 3 boards.

On the left is a bus chip (an NXP PCA82C251T/N3) and on the right four zener diodes and a termination resistor. Clues to the mishap are the small holes in the top of the zener diodes and the burn mark on the 220 ohm resistor. Sometimes, one of the hardest jobs is to identify the damaged parts. Here the letter "J" lying on its side is a good clue as it's a date marker used in this particular orientation by one specific manufacturer. For example, the same marking "Y4" is used for completely different devices by 13 different makers on an SOT-32 or similar 3-pin package. Checking data sheets for each likely candidate reveals only one maker who marks the date of manufacture with a sideways "J". If this method fails to identify a likely candidate, or the markings are obliterated, the only option is to reverse engineer the circuit because circuit board schematics are not available. Some manufacturers even go to the length of filing off chip markings so board repairs are virtually impossible outside the manufacturers repair department.

 

 Now a Magnetek Drive Unit, an HPV900

 

 Before fault diagnosis these units generally need to be stripped down to constituent parts as above. Drives usually require 3-phase mains because they can consume lots of power. This model isn't particularly big, rated at only 5KWatt. As you can see the thing is not very old and all the parts are very clean, looking quite new.

 

 

 First to be removed is this input/output interface board which sits on top of the processor board.

Bothe relays were OK. In other makes similar relays can be under-rated causing contact burning and drive failure. I wonder how many units worth around £5,000 have been scrapped for want of a £2 relay?

 

 

 

 

 

 

Below, the processor board, connecting to the main board via a multi-way flat cable. The cooling fan for the low voltage power supply carried on the main board is screwed to the processor board mounting plate.

 

 

 Before the drive was dismantled, I checked the various test points and found that no low voltages were present. The 3-phase was connected and the HT of around 600 volts was present at that 25 Amp fuse below. The likely problem is failure of the low voltage power supply which is fitted on the main board shown below.

There are several possible reasons for the failure. Checking circuit components is problematic because the board carries conformal coating to protect it from damp, but I was able to confirm the fuses were intact and all the rectifier diodes and the chopper transistor were OK. This leaves two possibilities. The feed resistor between the high voltage line and the chopper chip (the resistor needs to drop not far from 550 to 600 volts and only one resistor is fitted instead of the usual chain of three which are necessary to divide the operating voltage to be within the rating of the resistor's ). The resistor was intact leaving only the decoupling capacitor which stabilizes the chopper supply voltage. Checking in-circuit gave doubtful results but removing it showed it was open-circuit.

 

Above, just below that orange relay, you can see where I've removed the dud capacitor. It has been carefully sited adjacent to a really hot component (that pink resistor) because the designer forgot that capacitor lifetime is inversely proportional to temperature.

Below, the underside of the same main board. That large component is the IGBT module plus 3-phase bridge rectifier. Low edge, centre you can see markings "+ and -" and resoldering where the new 100uF capacitor was fitted.

 

 Here's the old 100uF 25volt electrolytic capacitor.
 
 

 

 After reassembly the unit will be as good as new. The user manual had 180 pages and manufacturer's fault diagnosis is based on the information given on the front display. Alas, because this was dead, fault diagnosis needed a different approach. I did in fact find another version of the manual, containing an extra 40 pages. Two of these showed test points on the processor board. Because none of these carried any voltage led me to the chopper power supply.

Electrolytic capacitors have a finite lifetime... a bit reminiscent of valves in old radio sets. In this particular application (chopper chip stabilization) there are now a multitude of electronic items subject to its failure including TV sets, Sky boxes, light bulbs etc etc and it's about time manufacturers introduced something new and more reliable at a cost which doesn't force manufacturers to specify a cheap pathetically poor alternative. Many, if not all, designers specify ludicrous MTBF figures for reliability, totally ignoring this weak link in their products.

 Yaskawa 31KVA Drive Unit

 

 This drive arrived from Gatwick recently. Oddly I'd seen an identical drive from an adjacent lift from the same site back in 2015 so had a good idea what the fault was. The reported problem was the lift would only move a couple of inches instead of zooming up to the next floor. You can see below the sort of power levels involved. Note the 75Amp fuse and the sizeable thyristor modules... This drive is particularly well specified as it tolerates 380 to 460 volts and can draw up to 46Amps from the mains supply. Many continental designed drives have a much smaller voltage tolerance and frequently bite the dust if a 3-phase mains imbalance occurs. This particular model has a weakness in a smallish relay mounted on the chassis. To accommodate a high current the designers made the mistake of running relay contacts in parallel. In this case three paralleled sets. I sometimes see this mistake in small relays, perhaps when the designer cut costs by using a common relay for both double pole and single pole applications. Sorting out the bad relay put the lift back in operation. Not an easy thing on which to work as its covered in soot, runs 31KVA, and weighs nearly 65 pounds.

 

 

Unidrive

 I mentioned that some equipments, particularly those designed originally to work from "220 volt mains" might be vulnerable to damage if used in the UK (without a power conditioner). Of course there are other reasons a drive unit can fail, not just from a higher than average mains supply. I examined this "Unidrive" example recently. It was unrepairable as are many like this one. Below, the first picture shows the IGBT module detached from the circuit board and with its cover removed. The black stuff is soot and shows up like this because of the jellylike substance covering the circuit. Because this drive was beyond economic repair a new one will be fitted. Hopefully, the new one will have a better mains tolerance.

 

 Not just the IGBT module, but the circuit board is also knackered. The difficulty met by designers is the size of the protecting fuse. This needs to be 50 to 100 Amps depending on the drive's ratings. The black soot is a mixture of residue from copper tracks used to carry 3-phase mains to the circuitry and burning relays, capacitors and resistors. The fuse was intact.

The method of driving a lift motor with all this fancy circuitry is jolly clever but has a drawback of needing a method of reversing the motor otherwise people would have to descend by the stairs. The usual method for reversing a motor is switching the control voltage using two large contactors. A contactor is just another name for a relay and contactors can fail just like their smaller relatives. Some contactor pairs use a metal pin as an interlock so that they can switch correctly. Unfortunately some pairs do not have an interlock because the designer didn't think about failure modes. The result can be seen above and below. Some contactor pairs are external to the drive unit (like this example) and its to be hoped that when this drive was replaced the new example didn't go bang.

 

 

Just in case you like looking at these kind of things, here's a couple more...

 

 

 

Big trouble..tiny cause

 Schindler circuit boards are of very high quality, at least compared with those from lots of other manufacturers, but I suspect their designers go to town on the design as they seem to be over-complex to me. Also I've seen a few with pronounced weaknesses such as the SMIC range which end up rather charred after a few years use. Today's example (below) is a PEBO pcb sent to me some time ago and languished unrepaired until just now.

 

 Many times I see over-stressed components looking very sorry for themselves but, in this example, all looked pristine. To make matters tricky many pcbs such as this have conformal coating for protection from corrosion due to damp conditions and making simple resistance measurements needs some patience to ensure the meter probes penetrate the coating. Looking at the picture above suggests a fault-finding exercise based on comparisons on stressed parts is not too difficult because there are two indentical power circuits.

In fact two 24 volt batteries connect to two battery chargers and two DC-DC converters so comparing the pair of circuits should reveal any discrepancy and hence pinpoint the faulty component.

Nothing seemed amiss however and pressure of other jobs resulted in the PEBO pcb being set aside. Yesterday I picked it up and was determined to discover the problem perhaps by powering the thing up? Below is a drawing (click it to see a clearer image although this drawing is very much a simplification of actual circuitry) which might indicate how to get the pcb to work as intended. Alas this proved to be too difficult as it meant a virtual reconstruction of the entire lift wiring on my work bench. I initially tried by using a pair of 24-volt power supplies for batteries and another 24 volt PSU for powering the logic area but the need to add other wiring in too much complexity made me suspend operations, after at least proving some circuits were good (the pcb would detect the battery voltages as good or bad) and the microprocessor was alive and well.

I suppose I did also discover that there were no critical problems, in that supply currents were minimal, which led me to believe the fault was going to be less obvious than the run of the mill types. Of course I'd already tested all the electrolytic capacitors for poor ESR, diodes and transistors for shorts etc so time to don my magnifying goggles. Years ago when testing obscure and complex TV chips a trick was to compare a suspect chip with a known good chip by measuring across its pins with a diode test meter and an ohm-meter. Often one found a short between two pins and a new chip could be fitted with fingers crossed that it didn't suffer the same fate.
 

 I eventually spotted an anomaly and then identified the chip in the centre of the picture below (Converter 2) and, after removing the conformal coating I could see it was marked SG3524P. I compared the resistances of its 16 pins to ground against the same chip in the other converter. The clear anomaly is indicated in red below.

 

 Chip-1 Pin

 Ohms

 Chip-1 Pin

 Ohms

 

 Chip-2 Pin

 Ohms

 Chip-2 Pin

 Ohms

 1

 197K

 9

 4.3M

 

 1

 199K

 9

4.2M

 2

 3.6K

 10

 6.9K

 

 2

3.6K

10

6.9K

 3

 2.8K

 11

 4.7K

 

3

2.8K

11

4.6K

 4

 47

 12

 6.5K

 

4

47

12

6.5K

 5

 0.1

 13

 6.5K

 

5

0.1

13

6.5K

 6

 5.6K

 14

 4.7K

 

6

4.9M

14

4.7K

 7

 4.7M

 15

 1.8K

 

7

4.5M

15

1.8K

 8

 0.1

 16

 4.4K

 

8

0.1

16

4.4K

 

 It had been mentioned that Converter 2 output was being reported by the system as being outside its correct limits and it was Converter 2 that seemed to have the anomalous reading. Of course this might have been a red herring and Converter 1 had an incorrect reading but further analysis showed otherwise. It was virtually impossible to accurately trace the PEBO circuitry because it's a mult-layer board and the conformal coating made it awkward to check connections, but comparing the layout of components on the PEBO pcb indicated that the SG3524P manufacturer's recommended circuit below came pretty close. As you might notice above the designers have used extra components to allow control and monitoring and it's these which make testing quite complex.
 

 In fact the chip requires two timing components which set its operating frequency, being those at Pins 6 and 7. So there it is... Pin 6 needs a resistor to ground and in the "good" circuit this is 5.6K. In both cases each SG3524P has a chip resistor marked 5.61K but in one case this chip resistor read open circuit. I removed the bad chip and fitted a new resistor. To make things easier I used a slightly larger resistor* (shown in the pcb picture marked 5.6Kohm) which for sake of authenticity I selected from several marked 5.6K as reading exactly 5.61K. In fact the value is likely to be uncritical as the associated 10nF timing capacitor is unlikely to be closely toleranced.

* original 0603 @ 1.6mm x 0.8mm (see below) compared with new 0605 @ 2mm x 1.2mm

 

 Why did that resistor fail? I might look at it with my new USB microscope but I can't think of a reason other than a manufacturing defect. Once the pcb has been returned to the customer we'll see if it works....

 

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