Relay problem (buzzing noise) on motor and solenoid control board for hydraulic bath

[Robert Hill]

Hi all,

I'm trying to resolve an issue with a circuit board.

It is the control board for several solenoid valves and a 24v dc motor all of which drives a gear pump to pump hydraulic oil around pistons in a reclining/lifting bath.

This issue I'm having with it is as follows. When I press any of the buttons on the handset I can hear a loud buzzing noise. It comes from either of the oblong clear plastic relays (depending on which button I press on the handset) and appears to be the contact bouncing back and forth between being connected and disconnected. The motor they are intended to connect power to does still turn, but I can stop it very easily with my fingers. When I connect the motor directly to a 24v supply it turns for more powerfully. My assumption therefore is that the relays are for some reason not receiving continuous power to the coil (or some other issue) and so are not staying switched on. The effect this has on the motor is I guess almost like a switched mode power supply where voltage is reduced because the switch it disconnected part of the time. This would explain its reduced power.

I have replaced both the oblong clear plastic relays (Schrack RP310024) which were making the noice but the same problem occurs.

Anyone able to suggest why the relays may be bouncing like this? I'm hoping it is something obvious that someone with more experience with relays than me will be able to quickly identify.

The other components are the two orange Schrack ZD 211 024 relay/switchs? and the two what I assume to be capacitors marked with 2 micro 2k 100v Philips 373 mkt hq 92 49. There are also various diodes and what looks to be a resistor.

Any help much appreciated!

 

[duke37]

If you can hear buzzing, it seems as if the power supply is not behaving properly. This will also explian the reduction in motor power. I doubt if it is a relay problem.

 

[Robert Hill]

Hi Duke37,

Thanks for your response. I did think about that. The whole system is mains powered (converted down to 24vdc) and it has a battery back up. I'm currently testing it on the 24v battery back up which should mean there aren't any issues with 'noisy' power from the mains. I have the same issue with the relays regardless of whether it is mains or battery. Could a low voltage on the battery cause a similar issue? What would you recommend to test the power supply?

Possibly incidentally I used a 9v battery to energise the coils on the buzzing relays and they just clicked once, no buzzing.

 

[Robert Hill]

If it helps here is the original schematic for the control system

 

[Minder]

Have you tried it with the motor disconnected, this may offer some direction to take.
M.

 

[Robert Hill]

Minder by name and Minder by nature!

With the motor disconnected the relays click over nicely and stay in place, no buzzing!

Guess that means that the motor is the issue then?

 

[Minder]

Not so much the motor necessarily but the load.
Have you tried the motor direct on to a DC supply to see if it operates OK? And voltage remains correct.
Also when the 'buzzing' occurs monitor the coil voltage and see what is happening there.
M.

 

[duke37]

I knew a Robert Hill from Derby - nice chap.

A relay may drop out if the load is too high for the power supply to hold the voltage up. A typical case is the starter solenoid on a car but it bangs in and out rather than buzzing.

Measure the power supply voltage.


Edit
It is upside down, perhaps the electrons are falling out.

 

[Robert Hill]

The motor certainly operated better (it was much harder to stop it turning with my fingers) when I connected it directly to the battery (though I didn't monitor what was happening to the voltage.)

I'm on my way home presently and won't be able to get back to this until Thursday now. However, thank you for your help so far and i'll update you when I've had a chance to do those checks you have suggested.

 

[Robert Hill]

Hi Duke,

The battery with no load was showing 23v which seems fairly healthy, but I shall check what happens to it under load. The lead acid battery back up batteries must be 20+ years old so perhaps they have reached the end of their life. I think i'll also try to run the system using an alternative 24v supply and see what effect that has.

 

[Minder]

Hi Duke,

The battery with no load was showing 23v which seems fairly healthy, but I shall check what happens to it under load. The lead acid battery back up batteries must be 20+ years old so perhaps they have reached the end of their life. I think i'll also try to run the system using an alternative 24v supply and see what effect that has.

It is starting to look like supply voltage problem.
M.

 

[Robert Hill]

Whilst i'm away from being able to actively do test etc could you point me in the direction of any resources I could read online to learn a bit more about the issues with loads and power supplies. Is it to do with whether a power supply might be considered stiff'?

 

[Minder]

One of the first things with a symptom such as this is to look at the power supply,
First find out the current capacity of the gear pump, this can be done with a supply that is well above what the pump requires in the way of current,
Then use an ammeter to take a reading to find out the required Va of the supply, the motor plate spec itself would not include the pump load, in this case.
Then take a voltage reading as to what is happening to the supply circuit you are using when the relay/motor is energized.
The above should give you a fairly accurate picture of what is happening and what corrections are needed.
And power supply requirements.
M.

 

[Robert Hill]

Hello again.

I've managed to take some voltage measurements at various points in the circuit which has lead to an interesting development.

So:
Battery voltage with nothing connected: 27.5v
Battery operating relays but not motor 23.3-22.8v
Voltage across coil in 1st relay without motor connected 24v
Voltage across coil in 2nd relay without motor connected 24v
Voltage supplied to motor connection points with not motor attached either 12v or 20v depending on which handset button I press.

This is where it got interesting. I connected the motor to the relay/control circuit and pressed buttons on the handset.

Initially the relays buzzed as normal but after 2-4 seconds the buzzing stopped and the motor began to spin at what sounded like full power.

I measured the voltage as this process happened:

The initial voltage across the coils of the relays (when they buzzed) was 7v. This quickly rose over the next few seconds and at about 8-9v the buzzing stopped and the motor span up to full power. The voltage continued to rise until it reached about 12-3v.

The voltage to the motor began at 4v and then rose in relation to the voltage at the relay.

I've not yet tried running it with a different set of batteries but I'm now expecting that that will resolve the issue.

What do you think?
Thanks

 

[Robert Hill]

Update: replaced the batteries, seems to have resolved the issue. Unfortunate thing is that I have now lost the coupling between the motor and the gear pump! grrrr

 

[Minder]

When motor is voltage is first applied there is no generated/BEMF developed at that point so the current is high, this initial high current is sucking the voltage down.
In motor-battery operated systems it is preferable to bring motor rpm up slowly by some method if possible and this prevents the high initial current.
M.