How to control holding force of electromagnet
Close
Login to Your Account
Results 1 to 14 of 14
  1. #1
    Join Date
    May 2011
    Location
    The Buckeye State USA
    Posts
    193
    Post Thanks / Like
    Likes (Given)
    15
    Likes (Received)
    20

    Default How to control holding force of electromagnet

    I have an electromagnetic chuck on my surface grinder. 6X12 Electro-Matic with an old tube powered dc rectifier. My question is can I add a device in line to control the power of the magnet? Would a variac control work? If so would it be used on the ac input side or dc output side. Appreciate any help or suggestions. I will add that this is for a hobby application so Im not looking to spend a bunch of $$on a new chuck and or rectifier.
    Thanks

  2. #2
    Join Date
    Nov 2013
    Location
    Eastern Massachusetts, USA
    Posts
    5,762
    Post Thanks / Like
    Likes (Given)
    5328
    Likes (Received)
    5447

    Default

    Quote Originally Posted by Sea Sick Steve View Post
    I have an electromagnetic chuck on my surface grinder. 6X12 Electro-Matic with an old tube powered dc rectifier. My question is can I add a device in line to control the power of the magnet? Would a variac control work? If so would it be used on the ac input side or dc output side. Appreciate any help or suggestions. I will add that this is for a hobby application so Im not looking to spend a bunch of $$on a new chuck and or rectifier.
    Thanks
    Variac should work - ac side only. A variac is a type of autotransformer and transformers only work with ac.

    An autotransformer is a transformer with a primary winding only (no secondaries). There are taps for different voltages (which may be higher voltage than the input). Instead of taps a variac has a low resistance wiper that runs across an exposed section of the windings.

  3. #3
    Join Date
    Mar 2011
    Location
    Geneva Illinois USA
    Posts
    6,452
    Post Thanks / Like
    Likes (Given)
    2826
    Likes (Received)
    2535

    Default

    The tube unit should have a means of controlling the force other than just on or off. Also, most units had a demagnetizing mode that resets the iron to neutral. Straight dc units will leave residual magnetism. The demag circuit momentarily applied a AC voltage to the magnetic decayed gradually.

    Tom

  4. #4
    Join Date
    Sep 2011
    Country
    GERMANY
    Posts
    3,154
    Post Thanks / Like
    Likes (Given)
    2038
    Likes (Received)
    1145

    Default

    I would think that feeding your old tube powered dc rectifier the wrong input voltage (as you would get from a variac in-line with the input power) is likely to damage it. That tube-powered rectifier has transformers in it which (for example) provide power to the filaments of the tubes. Feeding in the wrong voltage will then give the wrong filament voltages to the tubes, and they won't work correctly. As Tom suggests, look for another solution.

  5. Likes Paolo_MD liked this post
  6. #5
    Join Date
    Jul 2005
    Location
    central Illinois
    Posts
    248
    Post Thanks / Like
    Likes (Given)
    1188
    Likes (Received)
    99

    Default

    Steve,

    Can you just eliminate the old original power source to the chuck, I'm assuming it's an external unit(?).

    On my Thompson 6F grinder, I just use a variac rectified by one of those little cheap 4 terminal rectifiers. I also mounted a volt gage on the machine, but wasn't necessary as it seems to read the same as variac dial. Been working a treat for 30 years.

    Those chucks don't require many watts of power.

    Best of luck to you/ Gus

  7. #6
    Join Date
    Nov 2013
    Location
    Eastern Massachusetts, USA
    Posts
    5,762
    Post Thanks / Like
    Likes (Given)
    5328
    Likes (Received)
    5447

    Default

    Quote Originally Posted by ballen View Post
    I would think that feeding your old tube powered dc rectifier the wrong input voltage (as you would get from a variac in-line with the input power) is likely to damage it. That tube-powered rectifier has transformers in it which (for example) provide power to the filaments of the tubes. Feeding in the wrong voltage will then give the wrong filament voltages to the tubes, and they won't work correctly. As Tom suggests, look for another solution.
    Good point. I forgot he said it was a tube rectifier.

  8. #7
    Join Date
    May 2011
    Location
    The Buckeye State USA
    Posts
    193
    Post Thanks / Like
    Likes (Given)
    15
    Likes (Received)
    20

    Default

    Quote Originally Posted by TDegenhart View Post
    The tube unit should have a means of controlling the force other than just on or off. Also, most units had a demagnetizing mode that resets the iron to neutral. Straight dc units will leave residual magnetism. The demag circuit momentarily applied a AC voltage to the magnetic decayed gradually.

    Tom
    I have attached a pic of the rectifier I do not see a means by which the force can be adjusted The thing in the lower left is a broken bulb that indicates power on
    Is there a component that i can add?
    img_3411.jpg

  9. #8
    Join Date
    May 2011
    Location
    The Buckeye State USA
    Posts
    193
    Post Thanks / Like
    Likes (Given)
    15
    Likes (Received)
    20

    Default

    Quote Originally Posted by gusmadison View Post
    Steve,

    Can you just eliminate the old original power source to the chuck, I'm assuming it's an external unit(?).

    On my Thompson 6F grinder, I just use a variac rectified by one of those little cheap 4 terminal rectifiers. I also mounted a volt gage on the machine, but wasn't necessary as it seems to read the same as variac dial. Been working a treat for 30 years.

    Those chucks don't require many watts of power.

    Best of luck to you/ Gus
    Can you post a pic or specs on the rectifier? My rectifier is 50 watts

  10. #9
    Join Date
    Jul 2005
    Location
    central Illinois
    Posts
    248
    Post Thanks / Like
    Likes (Given)
    1188
    Likes (Received)
    99

    Default

    Here is a link to one like I used.

    https://www.mouser.com/ProductDetail...iABEgKHOPD_BwE

  11. #10
    Join Date
    May 2011
    Location
    The Buckeye State USA
    Posts
    193
    Post Thanks / Like
    Likes (Given)
    15
    Likes (Received)
    20

    Default

    Quote Originally Posted by gusmadison View Post
    Thank you Gus

  12. #11
    Join Date
    Mar 2011
    Location
    Geneva Illinois USA
    Posts
    6,452
    Post Thanks / Like
    Likes (Given)
    2826
    Likes (Received)
    2535

    Default

    Quote Originally Posted by Sea Sick Steve View Post
    I have attached a pic of the rectifier I do not see a means by which the force can be adjusted The thing in the lower left is a broken bulb that indicates power on
    Is there a component that i can add?
    img_3411.jpg
    It would take a like calculating but adding a potentiometer, one end connected to DC supply, the other to ground or through a resistor, and the wiper to the chuck. The reason for the "not sure" part is that the pot will add additional drain on the rectifier.
    The resistance value and wattage of the pot will have to be determined. Also, the range of the pot can be limited by connecting the bottom end of the pot to a fixed resistor and then to ground. Mechanically, this is not difficult, but would require some knowledge of basic electricity and making some calculations and measurements.

    If you want to go with a whole new design of a variac and a bridge rectifier, then an additional component is needed, a bidirectional switch that switches between hold and demagnetize. In the demagnetize mode, the output of the variac (AC) is coupled to the chuck instead of through the diode bridge. To demagnetize, the variac is gradually reduced to zero. This will reset the chuck to zero retentive flux.

    Tom

  13. #12
    Join Date
    Jul 2018
    Country
    UNITED STATES
    State/Province
    New Hampshire
    Posts
    75
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    14

    Default

    It seems that you could make a magnet control with 2 light dimmer switches,
    1 double pole double throw switch, and a full wave rectifier. The magnetize
    circuit would have one dimmer connected to the rectifier then to the DPDT
    switch then the magnet. The de-magnetize circuit would have a dimmer connected
    to the DPDT switch then to the magnet.

    One dimmer would apply DC current and the other dimmer would supply the AC.
    You could adjust both the AC and DC independently. The whole thing should
    cost $20 to $30.

  14. #13
    Join Date
    May 2011
    Location
    The Buckeye State USA
    Posts
    193
    Post Thanks / Like
    Likes (Given)
    15
    Likes (Received)
    20

    Default

    Quote Originally Posted by TDegenhart View Post
    It would take a like calculating but adding a potentiometer, one end connected to DC supply, the other to ground or through a resistor, and the wiper to the chuck. The reason for the "not sure" part is that the pot will add additional drain on the rectifier.
    The resistance value and wattage of the pot will have to be determined. Also, the range of the pot can be limited by connecting the bottom end of the pot to a fixed resistor and then to ground. Mechanically, this is not difficult, but would require some knowledge of basic electricity and making some calculations and measurements.

    If you want to go with a whole new design of a variac and a bridge rectifier, then an additional component is needed, a bidirectional switch that switches between hold and demagnetize. In the demagnetize mode, the output of the variac (AC) is coupled to the chuck instead of through the diode bridge. To demagnetize, the variac is gradually reduced to zero. This will reset the chuck to zero retentive flux.

    Tom
    Tom thanks for the reply. This will be beyond my electrical capabilities but I have a friend who is pretty good with this stuff and I will run this by him and see if he can help me out

  15. #14
    Join Date
    May 2011
    Location
    The Buckeye State USA
    Posts
    193
    Post Thanks / Like
    Likes (Given)
    15
    Likes (Received)
    20

    Default

    Quote Originally Posted by Mr_CNC_guy View Post
    It seems that you could make a magnet control with 2 light dimmer switches,
    1 double pole double throw switch, and a full wave rectifier. The magnetize
    circuit would have one dimmer connected to the rectifier then to the DPDT
    switch then the magnet. The de-magnetize circuit would have a dimmer connected
    to the DPDT switch then to the magnet.

    One dimmer would apply DC current and the other dimmer would supply the AC.
    You could adjust both the AC and DC independently. The whole thing should
    cost $20 to $30.
    Thanks for the reply as I posted above trying to engineer something from scratch will be a bit more than my skills will allow I will however show this to my friend and see if he can help me out with this


Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •