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Thread: Electromagnet Control Wiring

  1. #11
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    We used these

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    To release the load a small reverse current was used. I'll try to find the drawing.
    The reports of my death are greatly exaggerated.

  2. #12
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    Quote Originally Posted by Jraef View Post
    A simple bridge rectifier fed with 240VAC will give you 330V DC, but pulsating.
    It's actually, about 220Vdc average for a full-wave rectifier and fairly level current due to the inductance of the electromagnet.
    Si hoc legere scis nimium eruditionis habes.

  3. #13
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    Quote Originally Posted by gar View Post
    171001-0426 EDT

    drcampbell:

    In this application one puts the switch ahead of the bridge rectifier. Thus, the switch switches an AC current.

    However, with a reverse biased diode shunt across the inductor, if the switch was placed on the output side of the bridge, then after turn off the load voltage is only about -1.5 V. Thus, the maximum voltage across the switch is the instantaneous full wave rectified sine wave plus the approximately 1.5 V diode drop. The switch arc extinguishes quite quickly. But this is a bad idea because there is still some metal transfer between the contacts in one direction.

    When you have only an inductor and series resistor in a switched DC circuit, and the switch is opened, then the inductor will produce a voltage of whatever is necessary to maintain current flow (possibly many thousands of volts). This maintains the switch arc until the stored energy in the inductor is dissipated.

    If the switch is ahead of the bridge rectifier then you don't need a shunt (or flywheel) diode. The current will circulate through the bridge diodes until the 1/2Li2
    gets to zero.

    .
    Si hoc legere scis nimium eruditionis habes.

  4. #14
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    171001-1500 EDT

    Besoeker:

    You are correct that when the diode bridge output connects directly to the inductor that the bridge becomes a two diode reverse biased diode across the inductor when power is removed from the bridge input.

    .

  5. #15
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    I am going to take a measurement on my way home tomorrow. I really appreciate the input and insight. This forum is a great resource.

  6. #16
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    Quote Originally Posted by gar View Post
    171001-1500 EDT

    Besoeker:

    You are correct that when the diode bridge output connects directly to the inductor that the bridge becomes a two diode reverse biased diode across the inductor when power is removed from the bridge input.

    .
    Thank you sir.
    Si hoc legere scis nimium eruditionis habes.

  7. #17
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    Control

    The magnet in question is 18'' in diameter. This is a picture of the control box the previous owner was using. I see 4 diodes and understand that they are using a contactor to pull in the DC using the 240v as control. Sorry for the poor quality I had to reduce the image to get in to upload. The control box had, 1 switch to turn AC power on, one switch to turn DC power on and a momentary switch to control dropping of the magnet. I am grasping this I am just unclear as to which rectifier arrangement to choose. What are the advantages to full wave vs half wave rectifier in this application?
    Attached Images Attached Images  

  8. #18
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    Quote Originally Posted by crispysonofa View Post
    The magnet in question is 18'' in diameter. This is a picture of the control box the previous owner was using. I see 4 diodes and understand that they are using a contactor to pull in the DC using the 240v as control. Sorry for the poor quality I had to reduce the image to get in to upload. The control box had, 1 switch to turn AC power on, one switch to turn DC power on and a momentary switch to control dropping of the magnet. I am grasping this I am just unclear as to which rectifier arrangement to choose. What are the advantages to full wave vs half wave rectifier in this application?
    Oooo! Messy panel. Whoever did it might benefit from knitting classes........
    Full wave rectification needs just single phase and four diodes which is what you seem to have. And probably best suited for this application.
    Half wave would commonly use two diodes and a centre tapped supply transformer.

    I think half wave might have been more popular when semiconductors were expensive so there could have been cost savings in having just two diodes. But now they are as cheap as chips.........

    Another thing with half wave is that the current passes through only one diode at a time so it would have a lower voltage drop than full wave where the current path is through two diodes. Not really significant at 240Vdc.
    Si hoc legere scis nimium eruditionis habes.

  9. #19
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    171007-1123 EDT

    crispysonofa:

    Your photo implies a bridge rectifier.

    I don't believe you would want to put 5 kW of power (20*250) into a magnetic that size, temperature rise. However, average temperature rise is duty cycle dependent.

    Therefore resistance should be greater than 10 ohms.

    If the load was a pure resistance, then a full wave rectifier provides twice the average voltage of a half wave rectifier. Full wave is 0.636 of a sine wave peak, 0.318 for a half wave rectifier.

    I doubt the coil resistance is as high as 100 ohms, and probably not as low as 20 ohms.

    Why not use the existing rectifier and switching assembly?

    .

  10. #20
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    Electromagnet Control Wiring

    Some really good advice in the responses. Like you said, the picture isn’t very good and it is hard to tell for sure how the four Diodes are being used. Four would typically indicate a (Full Wave) Bridge Rectifier but the Diodes could be two doubled pairs forming a Full Wave arrangement. There has been responses explaining a Half Wave and Full Wave arrangements and what to expect for output voltages (DC).

    A Full Wave Bridge would produce right at 170 Volts DC from 120 VAC (RMS) and may be a little easier to set up. I use Surplus Heat Sinks like part number HS-433 from All Electronics [ https://www.allelectronics.com/ ] and machine them to Accept modern Bridge Rectifiers. They also have Bridge Rectifiers like part number FWB-358, 35 Amp, 800 Volt Peak Inverse Voltage (PIV) for $4.75.

    With an 18” Magnet it may not need to be ‘Bumped’ by flipping DC Polarity to assist the Drop Time as bad as larger models. If that happens there are solutions for that too.

    If you don’t havea way to machine the raw Heat Sink material I will make one for you and send itat no charge if you decide to go that way. I use 3/4” x 4” x 6” Electrical-Insulating FiberglassSheets and Strips from McMaster Carr to mount the Heat Sink / RectifierAssembly. It’s thick enough to Drill andTap to mount the assembly and wide enough to drill through and mount to anEnclosure. Be sure and use Heat Transfer Paste between the Rectifier and the Heat Sink.

    Here's a picture of what I am trying to explain. It is designed for connecting the Locomotive 74 VDC Control Voltage to a Machine that we build for the Rail Industry. The Full Wave Bridge assures that even if someone connects the DC Polarity backwards / reversed, it will be correctly polarized for the Solid State Meters.


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