Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Low voltage relay and AC motor questions. Heeeelp. Please!

  1. Feb 25, 2012 #1
    Hi, I have a central vacuum system that has blown it's circuit board up.
    Basically a hoover with a low voltage switching system to fire the motors up.
    Finding a new board will be difficult so I'm going to rebuild the control system. I can solder, can rewire a house, but the finer points of electricity elude me. Any pointers would be very helpful!

    Question 1 - I have a 30 amp relay (12VDC coil, contacts rated to 30A @ 300VAC), to be controlled from a 12 volt DC 300mA power supply. The relay will switch the AC vacuum motor power.
    The 12V goes to the vacuum wall points, through contacts in the hose to the operating switch at the wand you hold when hoovering.

    The 1st question - do I need a resistor or capacitor or anything in series with the 12V operating the coil, to protect the coil in any way? Or do I just stick 12V across the coil and off I go?

    Question 2 - I have two vacuum motors - both 1000W bare vacuum motor/fan assemblies with two wires, which were going to the original circuit board.
    I plan to hook those two wires to the 240VAC switched through the relay.
    Old board has a small induction coil (maybe mains interference suppression?) and what looks like a rectangular yellow box 20mm x 3mm x 8mm approx - I'm guessing this may be a capacitor, OR there's one inbuilt on the motor.

    The 2nd question - does each motor need a capacitor to aid starting, and if so, what size? Is this what is meant by soft-start?

    House is RCD protected, all will be protected by fuses (one biggish 10A one to the relay, plus a smaller 5A fuse to each motor, and a self resetting 7 Amp breaker to each motor as well ). Belts and braces sit well with me!

    Thanks in advance, regards, Andy
  2. jcsd
  3. Feb 25, 2012 #2

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Question 1:
    Measure DC resistance of that coil.
    Estimate its surface area.

    Power dissipated in coil will be volts^2 / resistance of coil
    144 / ohms you measure

    about a watt per square inch is i think all you want to ask of the coil
    DC relays sometimes switch in a resistor to reduce coil current after the relay has successfully closed.
    Trace coil wires and see if you find a power resistor in series with coil and a contact that shorts resistor out until relay has closed..

    Question 2:
    Vacuum cleaners are usually brush type "universal" motors that operate at very high speed, well above 3600RPM. Though with a large impeller you could certainly pump air at 3600 , which is fastest you can go on 60 cycles with an induction motor..

    So look at your motors - do thay have brushes? If so they'll run without start capacitors.

    But - sadly "Soft Start" means there is an electronic gizmo to apply voltage starting out low and increasing as motor speeds up. It both makes for a quiet, elegant start and reduces starting current surge.

    I dont know what will happen if you try a full voltage start on your motors.

    Do your motors both run at same time?
    Railroad locomotives start with their drive motors in series then switch them to parallel after they're rolling.
    So there's things you can do.

    I'd try first to find a board at salvage yard or on internet.

    keep us posted !
  4. Feb 26, 2012 #3


    User Avatar

    Staff: Mentor

    The 12v relay may have a diode across it to reduce sparking at the wand contacts (sparking when it opens).
  5. Feb 26, 2012 #4
    Hi, thanks for the replies, much appreciated.

    Q1 - DC resistance is 102 ohms.
    So power dissipated would be 12 x 12 / 102 = 144/102 = 1.4 Watts
    Coil is maybe only 0.5" x 0.5" x 0.5" in size.
    Therefore if I bung a 1.4W resistor in series with coil, I'm protecting the coil correctly, yes?

    Q2 - motors are brushed, and having had a really good google since posting here I'm kind of leaning toward capacitors not being required - if you reckon a brushed motor will run without caps, that'll do for me.

    Soft start does look the 'right' way to do things - elegant is a good description, but a suitable RS softstart module is equivalent to the cost of a pair of motors!

    Sparking at wand isn't really a problem - PS is only 300mA - but looking at the old board there is what could be a diode.
    I'll build and fiddle over the next couple of days and report back!

    Many thanks!
  6. Feb 26, 2012 #5


    User Avatar

    Staff: Mentor

    While you may not see it, the switch contacts will feel it and become pitted.
  7. Feb 26, 2012 #6

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    ""Coil is maybe only 0.5" x 0.5" x 0.5" in size."

    if it's a cube, that's 1.5 square inches so you're 1.4 watt/1.5 square inches = .93 watts / in^2 probably okay.

    If it's round with flat tops it's .78 sq inches around outside and .39 top & bottom...

    just about 1.2 w/in^2

    if you dont find evidence of a dropping resistor it's probably okay.
    But check you 12 volt supply - unregulated ones with a filter cap inside can be 15 volts lightly loaded.

    N.O's diode is a good idea - stripe toward + end of coil..
  8. Feb 27, 2012 #7
    To both of you - thankyou - just wish this place had been here 20+ yrs ago when I did my physics A level :)
    PS is pretty dead on 12V - I have a decent digital multimeter.

    Resistor - if I go to Maplin and ask for a 1.4Watt 100watt resistor suitable for 12VDC?
    http://www.maplin.co.uk/metal-film-2w-resistor-5729 - I'm going for metal film for improved audio performance ;)
    Sticking it in series with the switch, yes?

    Diode - Help! I'm guessing it'd be a signal diode? Which one?!
    http://www.maplin.co.uk/components/semiconductors/diodesAgain, diode in series with everything inc resistor?

    Had motors test running great just off a mains plug - so no capacitor requirement was a good call.
    30A relay is working ok, and will switch the motors no problem, but for longevity and for the sake of 'doing it right' the resistor and diode will be built-in before the whole lot gets installed later this week hopefully.

    Really appreciate this!
  9. Feb 27, 2012 #8


    User Avatar

    Staff: Mentor

    Signal diode could be a bit dicey. I'd go for the 1A Fast Recovery diode. It connects across the relay coil, but oriented so as to not short out the coil.

    Eh? What? Watt?

    You'd ask for a 10 watt resistor, so then it won't get very hot. It won't be metal film, it will be wire wound. But I think you haven't yet worked out how many Ohms you need. Jim better advise. In any case, this is applicable only if your relay has extra oppositely-operating contacts. Has it? You need a set of contacts which is closed until the relay is powered, as well as the ones to operate the motor, these will be open until the relay is powered. I think you'd better photograph the relay contacts.

    I think I'd try operating the relay just on the bench as a trial, when you have 12v available. Every 5 mins turn off the power and feel the coil. How warm does it get? You may not need a resistor.
    Last edited: Feb 27, 2012
  10. Mar 3, 2012 #9
    Well, it's working great.
    Maplin gave me a diode - the bloke there actually knew what he was talking about which was good - diode now strapped across relay in reverse.
    I haven't fitted a resistor - PS is only 300W - and the relay has been left on for hours with no sign of anything other than what I felt might be a very slight warming to the casing. It'll only be energised when vacuuming - so maybe no more than 30 mins continuous action. PS I've used is just a little plug-in jobby with switcheable voltage - currently only set to 9V. It'll be switched off when not in use too - previous board was live 24-7 for about 6 years, which probably didn't do it much good.
    Switch on wand operates relay, relay supplies motors, all working fine. Slight spark from mains voltage contacts on relay when switching off sometimes, but there is 2000W+ going through there.
    All good - many thanks all :smile:
  11. Mar 3, 2012 #10


    User Avatar

    Staff: Mentor

    http://imgcash1.imageshack.us/img853/3671/thumbsups.gif [Broken]
    You could try a 0.022uF 600v polyester capacitor across the contacts to suppress that. Sparks like this cause needless annoying blips on nearby radio and tv, etc.
    Last edited by a moderator: May 5, 2017
  12. Mar 3, 2012 #11

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    That's good news on the lukewarm relay coil.

    If the motor causes TV or radio interference while running, try a "R-C snubber" type suppressor right at the motor terminals.
    Stateside we use CDE "Quencharc", they're common as Coca-Cola and everybody stocks them. Your friend Maplin should know what's 'coin of the realm' where you are.....
    Just get one rated high enough voltage for your mains, and with capacitor on the generous side.

    Congratulations on the fix !
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook