12 Volt Reversal Switch with Dynamic Braking

As you may or may not know my father has a few mobility problems, and a while back he built a winch controlled platform to help him get in and out of the bath.  Up until now there haven't been any problems with it, but recently the momentary switch has started to show intermittent problems.  It still raises and lowers the winch, but it has an extra function that isn't working well.  When the switch is released it returns to the centre position and places a short across the motor.  This is important because it adds something called "Dynamic Braking" to the winch.  When you short out a motor like this it's equivalent to putting a brake on it, if you leave it open circuit the winch won't hold loads and will slowly lower what it's holding.  In this case that is my Dad.  Getting in and out of a bath on a platform that slowly sinks is almost impossible.

We've had this problem before, but were able to clean out the switch and get things working again.  I would have liked to replace the switch but the dynamic braking feature can't be replicated with a normal DPDT switch, and I can't find a replacement anywhere.  Even though I'm holding one in my hand everyone I've spoken to says they don't exist.

Original Winch Switch

I could have bought something like the product below, but with shipping it would have cost me around $100, and if it were to brake somehow, it's not easy to get hold of another one.

http://www.cncelectrical.com/servlet/the-13234/Cole-Hersee-Solenoid-Reversing/Detail

Last time this happened I came up with a contingency plan using standard off the shelf automotive relays, so I dusted that off, made some improvements and started building.

I had a few requirements when designing the system.  It had to made of easily obtainable parts, and it had to be reasonably easy to service.  In case I'm not around Dad can fix it himself.  It also had to be as waterproof as practicably possible.  This was to prevent the same problems with water ingress that caused the last one to fail.  This is what I came up with.

Relay Operated 12 Volt Reversal Switch with Dynamic Braking

This was a difficult project, technologically, it's far from ground breaking, the biggest problem was trying to source quality components from places like ebay and the local electronics store at a reasonable price.  Nothing was exactly what I wanted, I just had to make do with what I could get.

The first thing I had to do was determine the current draw of the motor.  Since I didn't have a meter that would measure such a high current I had to build an adapter.  It turns out that the maximum current draw is about 25 Amp.  Although I would have liked a bit of headroom, I used this as my design current.  All the components I could easily source were rated for 25 Amp or higher, and since it's not in continuous operation, I'm happy with that.  It might only operate for a maximum of 30 seconds at a time.

The operation of the circuit is pretty simple, it consists of two relays and a momentary SPDT switch with a centre off position.  The diagram below shows the operation of the circuit.  It should be familiar to anyone with an electronics background.  It's a H bridge motor controller, but with one important benefit.  By using relays instead of semiconductors, it's impossible for shoot through current to occur.

When the switch is in the centre position none of the relays are energised, connecting both of the motor terminals to the negative supply line via their normally closed contacts.  This shorts out the motor implementing the dynamic braking feature.

Reversal Switch in the Braked Position

When relay 1 is energised, the motor moves forward.  It's negative terminal is still connected to the negative supply line via the normally closed contact of relay 2, while the positive motor terminal is connected to the positive supply line via the normally open contact of relay 1.

Reversal Switch in the Forward Position

When relay 2 is energised, the current though the motor is reversed.  It's negative terminal is connected to the negative supply line via the normally closed contact of relay 1, while the positive motor terminal is connected to the positive supply line via the normally open contact of relay 2.

Reversal Switch in the Reverse Position

To control all of this I used a cheap pendant switch from ebay.  It's rated to 250 Volt, 5 Amp, personally I wouldn't trust it at that voltage, but for a 12 Volt, 100 mA signal it's fine.  It claims to have a mechanical interlock that prevents both the up and down connection being simultaneously pressed, but I've been able to press them both at the same time.  In this design that doesn't matter, all it does is active both relays and connects both terminals of the motor to the positive supply effectively braking it.

Pendant Switch

The inside is straight forward.

Pendant Switch Internals

Annoyingly I couldn't get a rubber boot to fit a small cable.  Apparently they exist but I couldn't source one.  I intended to use an old extension cable I had, but it was too small.  The easiest solution was to just get a half inch 4 core cable and use only three of the cores.  A bit wasteful, but it got the job done.

Pendant Switch Rubber Boot

Finding a cheap enclosure with a rubber seal was hard enough, so it was rather annoying when it came time to install the cables and the threads on the glands were too small to fit through the case.  I had to hack together a bit of a milling machine to create a recess in the case to make everything fit properly.

The Milled Recess to accommodate the Cable Glands

Installation of everything required a base plate and some mounting brackets.  How I went about that is described in a couple of past posts.  DIY Relay Brackets and Waterproof Enclosure Mounting Plate.  I did have to slightly change how the relays were mounted to create more room.

There isn't anything special about the relay.  It's something that can be sourced from pretty much any automotive supply store.

30 Amp Changeover or SPDT Relay

Even though this is personal project, I wanted to try to do everything as professionally as possible.  Within reason of course.  One thing that really bugs me are uninsulated crimp terminals, so all of mine are insulated.  If something works itself loose, it can't short out on other terminals.

Insulated Terminations

Even the large terminals on the terminal block are heat shrunk.  I initially tried to make my own terminals, but came to my senses pretty quickly. DIY Eye Terminal Proof Of Concept and Bending an Eyelet Terminal Without Tearing the Metal describe how I went about doing things.

The terminal block may seem a bit redundant, and your right, it is.  In most cases the cabling could be easily connected straight to the relays, but I feel this will make it easier for my father to service it.  The main board can be easily removed and serviced on a bench.

Terminal Block

After wiring up all the connections I realised I had made a mistake.  It wasn't bad, but when the circuit was in the braked position, both terminals were connected to the positive terminal of the battery to give dynamic braking.  There is nothing wrong with that electrically, but from a corrosion protection stand point it isn't great.  The constant 12 volts on the motor in a humid environment is likely to promote corrosion.  This was easy to solve though by swapping the normally closed contacts of the relays to the negative supply rail.  This also reversed the direction of the motor and was fixed by swapping the positive and negative motor connections on the terminal block.  The images above reflect the way the circuit was originally connected before the changes, but the circuit diagrams above are correct.

A bill of materials and a wiring schedule for this project can be found here.  Hopefully this will make future maintenance easier.  The diagrams below are used as a reference in the wiring schedule.

Annotated Terminal Block diagram

Annotated Pendant Controller Diagram

I'm quite happy with how this turned out.  The total cost was 140 dollars.  Although that seems high, by the time I purchased an off the shelf solution and put it in an enclosure it would have cost more and replacement parts would have been harder to get.  Most importantly Dad can get out of the bath easily again.

Onga JM100 Pump Teardown

The pressure pump connected to our rainwater tank failed the other day.  It didn't stop working, it just sounded really wrong and got too hot for the short amount of time it was running.  I'd previously replaced the motor run capacitor on this particular pump and was reasonably familiar with its operation.  As a first step I checked the things I know best, the electrical components.  The 10 uF motor run capacitor was fine, it was reading a perfectly acceptable 9.8 uF.  The winding resistances were however way out of whack.  When I last repaired the pump I took note of the readings, and they had changed significantly since then.  They were now about one sixth of what they used to be.  If some of the windings are shorted it pretty much means it's beyond repair, but I decided to pull it apart for fun, and to see if there was something obvious that could be causing the problem.  So even if the pump can't be repaired there's at least a guide that others can use if they need to pull their pump apart.

Warning - This article describes equipment and circuits that operate at high voltages.  Don't attempt to repair any high voltage circuits if you're not trained to safely work with electricity.  You may be seriously injured or even killed.  For further information read the blog's Terms Of Use.

I hadn't done anything like this before so I tried to find some sort of service manual.  Although not an exact match but the parts diagram in this manual was a fairly accurate representation of how the pump was constructed.

I started by removing the pump head and taking off the fan cover.

Fan cover removed

Pump head removed

At this point I took a moment to inspect the impeller on the motor and the diffuser in the pump head for any signs of wear.  Everything looked fine.

Brown diffuser in the pump head

The next step is to remove the pump stand from the bottom and disconnect all the electrical wiring.  If your not sure how to reconnect everything take lots of photos for reference.  Removing the fan at this stage is a good idea.  It may take some doing, but it can be pried off with a flat bladed screw driver.

With the fan removed, a screwdriver can be used to lock the shaft in place while the nut holding the impeller on to the shaft is removed.

Pump stand removed

Removing the impeller seemed to disturb a colony of ants inside the motor.

Ants inside the motor

After removing the impeller you'll be faced with the layout in the image below.  To remove the black plastic plate the seal on the shaft needs to be removed.  Once again it may take a while to figure it out, but it can be pried off with a flat bladed screwdriver.

Seal on the shaft

Ignoring the ants for the moment, all that's left to do now is to remove the end plate of the motor.  There's another rubber seal on the shaft that can be removed easily.  Once this is done the bolts holding the plate on can be undone.  To remove the plate, gently tap the other end of the shaft where the fan used to be attached, this will push the plate off.

Motor end plate

You now have access to the rotor, bearings, and stator coils.  Apart from the ants, I couldn't see anything out of the ordinary here.  The bearings seemed to turn freely and there was no obvious damage to the motor windings.

Disassembled motor

I used a compressor to clear all the ants out and then reassembled the motor.  It didn't sound any worse, so I'll take that as a win.  I'm still not entirely sure what caused it to fail though.  It could have just been a random breakdown of the winding insulation, that caused part of the motor to heat up causing the insulation on other windings to break down.  Maybe the ants damaged the winding.  Who knows.  One thing is for certain, this motor has reached the end of its life.

Replacing a Motor Run Capacitor

Lately I've had a bit of trouble with a pump connected to our rain water tanks.  Not turning on, stalling, just generally not working.  Catchup on my previous attempts to fix the pump in my last post to bring yourself up to speed.  The pump in question is an Onga JM100 pump.  The specs are shown below.

Pump Specifications

Warning - This article describes equipment and circuits that operate at high voltages.  Don't attempt to repair any high voltage circuits if you're not trained to safely work with electricity.  You may be seriously injured or even killed.  For further information read the blogs Terms Of Use.

Well it turns out that I was wrong about the cooling fan cowling.  It wasn't too tight and causing the motor to stall.  How do I know?  I took it off completely and let the pump run, but after cycling a couple of times it stopped working again.  All I had left to check was the pressure switch, the motor capacitor, and the motor itself.  If it was the motor I didn't have a hope. So hoping for something easy, I decided to take a look at the pressure switch and check if the contacts were dirty. I thought the humming noise I was hearing may be the contacts chattering or the motor humming because it couldn't get enough current because of high contact resistance.

Pressure switch

The pressure switch is a mechanical switch is by a company called Square D that is now owned by Schneider Electric.  The pressure of the water controls two sets of contacts that break the neutral and active lines.

Pressure switch mechanism

In the above photo, just above the screw terminals where the wires are terminated, you can see the set of contacts that break the neutral line.  Sometimes after a long period of operation the contacts can become dirty and  increase the resistance of the switch, but after listening closely to the switch and testing it with a multi meter it was clear that the switch was working fine.

The only thing left to check that I could actually fix was the motor run capacitor.  It's housed in a box that's mounted on the top of the motor.  In a single phase induction motor a second winding that is 90 degrees (or as close as possible) out of phase with the main winding is needed to create a rotating magnetic field.  The capacitor in series with the start winding is used to produce this phase difference

Motor run capacitor housing

The capacitor is connected via standard spade connectors, so it's easy to remove, but before doing so I labelled all the wiring to make sure I knew how to reconnect the windings.

Removed capacitor

As you can see from the markings on the capacitor above it's meant to be 10 uF.  So I pulled out my multimeter to test if it really was.

Removed capacitor measurement

I know I don't have the best multimeter in the world, but come on, 2.1 uF?  To make sure, I double checked the multimeter against a know capacitor and the measurement agreed to within 5 percent.  Just to be extra sure, I set up an RC circuit, fed it with a low frequency square wave, and measured the time constant with a scope, and once again I got 2 uF.  So it looked like this capacitor was the culprit.  That was great, after all, this was the first time I could point at something that I knew was definitely wrong.  Two days later I had new capacitor in my hands from RS components.  Not exactly the same part, but its specs were equal or better.  Like the old one it's a four terminal device with two terminals connected to either side of the capacitor which allows you to jumper connections off it.

New capacitor

New capacitor measurement

Before installing the new capacitor I measured its value to make sure it agreed with the marked value of 10 uF.

New capacitor installed

Although I marked the leads on the capacitor, it doesn't hurt to double check the connections.  By measuring the resistance between the leads of the motor you can tell how the motor is wound.

Wiring diagram

On this motor if you measure the resistance between the black and blue wires you measure the resistance of the main and start windings in series.  If you measure between the brown and black wires you get the resistance of the start winding, and if you measure between the brown and blue terminals you get the resistance of the main winding.  The measurement with the lowest resistance is typically going to be the main winding.

Confident that I had reconnected all the wiring correctly I replaced the covers, turned the pump back on, primed it and got the outlet line up to pressure.  I then turned the tap on and cycled the pump about 10 times.  Each time the pressure switch cut in and the pump started.  What was immediately noticeable was the increased flow rate of the pump and a reduction in the amount of noise it produced.

The reduction in capacitance causes a drop of current in the start winding, which reduces the starting torque at certain rotor positions.  This also has a side effect of reducing the power output of the motor, and creates a fluctuating rotating magnetic field within the motor which causes increased operating noise.

So far the pump has been operating correctly for the past two days, which I think is a good indication that the problem is fixed.  I learned quite a lot during the repair process, and as usual the last thing I checked was the actual problem, but as with any problem worth solving it came down to the electronics.

Fixing a Pressure Pump

Today I did something a bit different and did some mechanical work on a pump that we use to supply our washing machine and toilet with rainwater from tanks.  Recently it has been making some strange sounds, and sometimes it doesn't cut in at all.

Pump without the water outlet

The pump is part of a standard set-up, water comes from the rainwater tank and is pressurised and sent to where it needs to go.  A pressure switch on the outlet port of the pump turns the pump on when it senses a drop in line pressure created by a tap being opened, it then cuts out when a certain pressure is reached.  To stabilise the system, and stop the pump constantly cycling, a pressure tank is connected to the output.  This consists of a pressurised bladder of air in a tank, as the water is pressurised it compresses the bladder.  If you want to think of it in electrical terms, the pressure tank acts like a capacitor and the pressure switch adds hysteresis, creating an oscillator.  The larger the capacitance, or pressure tank in this case, the lower the oscillation frequency.

After a few tests I could tell that the pressure tank was fine and only needed to be topped up with some air.  The strange sounds that I heard sounded like something was rubbing when the motor turned.  This could have been a dry bearing, something wrapped around the impeller shaft, or a object obstructing the cooling fan.  The only way to find out was to remove the pump and pull it apart.

To start with I removed the outlet of the pump to make sure there was nothing wrapped around the impeller causing the sound I was hearing.  There was a bit of slime, but in general it was clean as you can see in the image below.

Pump Impeller

The next most likely culprit was the cooling fan that blows air over the motor casing.  After removing the cowling I was able access to the fan and found a build up of dirt.  What's basically happening is air with dust in it gets draw into the fan, the dirt being heavier hits the blades and gets flung into the cowling where it builds up until it rubs against the blades and causes a rubbing sound.  Pieces then break off and rattle about the fan housing.  If your really unlucky the pieces jam the fan blade and stall the motor.

Dirt on the fan and cowling

Dirt on the fan blades

After a clean up with an old toothbrush it was as good as new.  The cowling was replaced and the pump was reconnected.  It worked perfectly.  The pump was pumping and the strange sound had disappeared.  Then it stopped working again.  I tapped the cowling off a little bit and all was good again for a while.  From what I can see, I think the cowling is a tight fit and if it is slightly misaligned it will touch the fan blades.  The next step is to remove it completely, let it go for a week and see if it works.  If it does, which I think it will, I'll have to make my own fan cover or file a tiny bit off each of the blades.

Update - 20 May 2012
With further investigation and testing I have discovered the fault with the motor.  The problem and how to fix it can be found here.