Heating valve microswitch fault

[Guineafowl]

TL;DR Summary

A microswitch in a heating valve keeps sticking shut. Is this due to low quality, thermal stress, inappropriate rating, or something else?

Here is the valve in question, an Enolgas R2835P00:

The default state is shut, as shown by the red indicator in the picture, oriented crossways. When activated, either by the frost ‘stat, or the user, the valve opens 90deg, and at the end of travel, switches on a pump:



Rating plate:

The recurring fault, (this is the third valve in 6 years), is that the microswitch (SPDT) sticks shut, such that, on activation, the valve doesn’t open, but the pump is switched on, resulting in its pumping against a shut valve.

Here are the internals:

The offending switch is top left (normally situated underneath the switch with blue wires, which has been pulled to the side). You can see how it’s activated by the cam, once the shaft has rotated 90deg anticlockwise.

The switch is made by Crouzet, MPN: 83170012. SPDT. Rated 10A.

I have, in the past, squirted switch cleaner/lubricant (Servisol) into the switch, and worked it. This solves the problem for a variable time, but it still recurs.

What could be the problem? I thought perhaps the grease inside the switch was drying out (valve is generally lukewarm for most of its time), but the latest application of Servisol only lasted a few days. I think the switch contacts are welding together. They shouldn’t, assuming Crouzet is a decent brand, and that the switch is correctly rated, which it seems to be.

Although the pump power rating is well inside the 10A capability of the switch, it is an inductive load, so this may be a factor.

I have considered trying a different brand of switch, but which one, and how do I ensure the switch fits and operates correctly? The dimensions must be exactly the same to fit the existing setup.

 

[Baluncore]

Although the pump power rating is well inside the 10A capability of the switch, it is an inductive load, so this may be a factor.

It is a snap action switch with an 0.35 mm gap.
https://au.mouser.com/datasheet/2/92/SW_MS_SM_V4_83170_EN-2886939.pdf
I think the inductive load details are important here.

Examining the curves in the data sheet predicts a life of 30,000 cycles at 10 amps for an inductive load. Those 30k cycles would be consumed in 2 years, if there were an average of 42 cycles per day.
How often do you expect it should switch ?
How long does it take the motor to cycle off/on/off once?
Could the system be cyclically threshing ... on/off/on/off .... ?

 

[Guineafowl]

How often do you expect it should switch ?
How long does it take the motor to cycle off/on/off once?
Could the system be cyclically threshing ... on/off/on/off .... ?

- Hard to say exactly, but during the frosty months, every few hours to keep the heat exchanger from freezing. At other times of year, never. Nowhere near an average of 42 times/day.

- About 20 minutes.

- It’s an unattended system, but when I’ve seen it operate, the ‘stat appears to have a good amount of hysteresis.

That said, in the hope that the switch is adequately specced but just worn out, I’ve ordered some new switches, and once fitted I’ll dismantle the old one and see what’s inside.

 

[Dullard]

I always use an interposing device (PLC, relay, or SSR) in this situation. I don't always "need" to do that, but I never need to worry about the ratings of the cheapest micro-switches that a valve actuator manufacturer can find.

 

[Guineafowl]

I always use an interposing device (PLC, relay, or SSR) in this situation. I don't always "need" to do that, but I never need to worry about the ratings of the cheapest micro-switches that a valve actuator manufacturer can find.

That’s a thought.

Something like this would do:
https://uk.rs-online.com/web/p/power-relays/4500403

However, I’d need one with spade terminals. Is that what this has? They look right, but are described as ‘PCB mount’.

 

[Guineafowl]

Further to this, I see the maximum switching power of the relay above is listed as 1VA. What does this mean? It implies, at face value, that the relay is essentially useless.

 

[jrmichler]

Look for a relay designed for use in HVAC systems. HVAC relays are designed for the loads and duty cycles of your application. This relay has been controlling the HVAC blower in my shop for ten years now with zero problems:

There are many companies that make suitable relays. I suggest asking your nearest HVAC technician what they use, and get one of those relays.

 

[Guineafowl]

We don’t really have HVAC technicians over here, nor the forced air type of heating that I assume that relay is controlling. The plumbing firm who installed this system (a six-figure woodchip boiler setup (not mine, I just live here!) have been told of the fault several times.

First time, they gave me a new actuator, which failed. Second time, they promised to get a new one on warranty, and didn’t, now they’re not showing any interest at all. I’m sure we’ve all seen that sort of thing.

So I’m on my own a bit. Am I looking for a relay with ac switching power of at least the 425W peak listed on the pump? I wasn’t aware that switching current and switching power could be different things.

 

[jrmichler]

The normally open contacts of the relay in Post #7, as can be seen on the label specifications, are rated for up to a 1/2 hp (0.37 kW) motor at either 120 or 240 volts AC. A motor is a motor whether it drives blower or a pump. It is always best if the relay rating is larger than the load. The relay in Post #7 is driving a 1/15 hp blower.

If your pump is rated at 0.425 kW, that would be 0.6 hp. This relay would be too small for such a load. If that 0.425 kW is just the inrush current at pump start, then the actual motor power would be less. A photo of the pump data plate would help us.

 

[Guineafowl]

The normally open contacts of the relay in Post #7, as can be seen on the label specifications, are rated for up to a 1/2 hp (0.37 kW) motor at either 120 or 240 volts AC. A motor is a motor whether it drives blower or a pump. It is always best if the relay rating is larger than the load. The relay in Post #7 is driving a 1/15 hp blower.

If your pump is rated at 0.425 kW, that would be 0.6 hp. This relay would be too small for such a load. If that 0.425 kW is just the inrush current at pump start, then the actual motor power would be less. A photo of the pump data plate would help us.

It’s looking like that little microswitch is nowhere near enough for what it’s switching. The actuator is intended for switching a pump, but perhaps not one as large as this.

Pump data plate is third photo in post #1.

 

[Tom.G]

You are fighting two problems here trying to control a motor with a low power switch.

1) The Inrush Current to the motor when power is first applied and the motor has not yet starting turning. Rule-of-Thumb is the Inrush, or Starting current, is 10 times the full load rating. (Larger motors will have rating on their nameplate called Locked Rotor Currrent (or LR Current) that will be more accurate than the factor of 10 above.)

2) The Inductive Kick when power is removed from the motor. The collapsing magnetic field in the motor can create a voltage several times the line voltage. I don't have a Rule-of-Thumb for this.

For 1), the high current starts: At the instant of turn-on, the switch contacts are just slightly touching and not making good contact with each other. This relatively high resistance and the high current rapidly heats the contacts for a fraction of a second at each turn-on. The high temperature causes oxidation of the contacts and even higher resistance... and more heat, the next time.

For 2), the Inductive Kick: The high voltage from power removal causes arcing across the contacts. The arcing erodes the metal contact surface causing pits and often high spots or spikes on the mating surfaces, greatly increasing the contact reistance and heating at the next switch closing.

The two factors above working together will eventually cause switch failure, either by welding the contacts closed upon starting, or burning them enough that they will not make electrical contact at all.

The common solutions are:
a) Use a switch or relay with a higher contact rating (preferred method)

b) Suppress the high current and voltage spikes

1) Put a (Negative Temperature Co-efficient) NTC resistor in series with the motor (not generally recommended)
2) Put a surge suppressor across the switch contacts or across the motor to limit the Inductive Kick voltage at turn-off (this can be mostly effective, but is usually done as an after-thought or when a properly rated switch is not available.)​

Relays designed especially for motor switching are available, they are often called "contactors" and are designed to handle the higher currents and voltages from switching a motor.

There is slight complicating factor here, driving any relay you use will also present high current & voltage conditions to the switch controlling it. But the conditions will be much smaller because the relay needs much less power to operate that the motor does.

There are also Solid State Relays (SSR) for motor control that address the overvoltage and overcurrent situations. They could be a better bet but I have not used them. Hopefully others here with SSR motor drive experience will throw in some ideas on how practical they are.Well, there is motor control 101. Hope it helps!

Cheers,
Tom

 

[Guineafowl]

@Tom.G Thanks for the advice. I‘ve actually set up quite a few machines with new motors, DOL starters with contactors/overload, VFDs, etc.

The contactors are, being designed with motors in mind, quite easy to specify: FLA on the motor plate, or kW rating, plus some headroom.

What has foxed me here, going to a smaller relay, is that the current rating is more nuanced and specific to circumstances. I was hoping to pick up a standard 240V, 10A+ relay which should do the job, but have found that they are significantly derated for inductive loads.

I’ve found a 20A relay, that will switch *only* a 550W inductive load, so will try that.

Here is the pump documentation:
https://product-selection.grundfos....pumpsystemid=2221187445&tab=variant-quotation

The text below, which I‘ve just found, suggests there’s a variable speed drive inside, implying soft start and speed control. It also might mean an initial current surge to charge the capacitor bank:

“The pump incorporates a 4-pole synchronous, permanent-magnet motor (PM motor). This motor type is characterised by higher efficiency than a conventional asynchronous squirrel-cage motor. The pump speed is controlled by an integrated frequency converter.”

 

[Guineafowl]

Right, I’ve installed the relay and replaced the microswitch. You can clearly see the deterioration of the old contact vs the new: