DC Motor control SCR and Pulse Transformer Failure

Discussion in 'The Projects Forum' started by Drocke, Nov 14, 2015.

  1. Drocke

    Thread Starter New Member

    Nov 4, 2015
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    Hi,
    I am restoring a Bridgeport milling machine and trying to repair the table power feed system. The table is driven by a DC brushed motor with 100V DC going to the field and a variable up to 100V DC to the armature. Although I have not fully tested the motor its field DC resistance and armature resistance at approx 500 ohms and 10 ohms seem reasonable. My problem is in the control board that provides the field DC supply and the armature variable supply.
    It is 40 years since I did any Electronics but I believe it is a PWM (Pulse width modulation) unit. The circuit diagram is attached along with a separate list of component.
    It is being fed by a 110V AC supply and I get 100V DC across the field output at Tp 8 and 9 but 0 volts across the armature at 7, 8.
    My initial attempts at diagnosing the fault centred around the various electrolytic capacitors but all tested ok using an ESR tester. I also checked R1 and R3. I then found that the secondary windings of the pulse transformer T2b and T2c were open circuit and prematurely jumped to the conclusion that this was the problem without thinking why they should have failed in the first place. I was not able to identify the pulse transformer part number as the number shown in the circuit diagram and the number stamped on it appears to refer to a Mullard E ferrite core only and does not give any clue to the windings. So I made a bit of a "guess" and went for a 1:1+1 unit from Vigortronix VTX-110-006 on the basis that it had roughly the same DC primary winding resistance, the only thing left form me to measure, a 200mA max pulse current and that it would physically fit the board.

    I fitted it applied power and bang the pulse transformer blew immediately. So lesson learnt, why did I not find out why the original transformer had blown in the first place! To do that I turned my attention to the Thyristors (SCRs) Th1 and Th2 and tried to test them with a multimeter.

    There is an open circuit between anode and cathode as one would expect but briefly connecting the gate to the anode does not switch the thyristor on. The multimeter is putting out about 2.2V so it should be enough to turn the SCR on. I then put the meter across the anode and gate and get a virtual short circuit either way the leads are connected. Surely this cant be right and think it is why the pulse transformer blows as it is getting best part of 110V AC across its secondary windings.

    Sorry this is rather long but my questions are:

    1. Is my diagnosis of faulty SCRs correct and if so what might have caused them to both fail? Sorry but I have no history of the unit at time of failure.

    2. Is it possible to specify the pulse transformer rather than me just guessing.

    Any help / suggestions on how to proceed further most welcome.

    David
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    These are SCR phase angle control, not PWM in the customary sense.
    These have been largely replaced by the PWM mosfet control.
    One of these I worked on, I ended up replacing with a similar KB unit, I replaced the smaller capacity bridge with an external components on a heat sink.
    The other thing to do was make it reversable.
    Max.
     
  3. Drocke

    Thread Starter New Member

    Nov 4, 2015
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    Max,

    Thanks for enlightening me on it being a phase angle control not pwm. I agree I could replace the unit with a modern one and that has indeed successfully been done by others. I do however hate throwing things away for the sake of replacing a few relatively inexpensive components to fix a faulty one and I get a certain kick out of repairing things. However there has to be a limit! As regards reverse this is currently achieved by a mechanical lever linked to a pair of micro switches to change the polarity of the field winding.

    Not withstanding it is a phase angle control and not a pwm, my first question is have I got my diagnosis right?

    David
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Have you checked the diodes of the bridge, if the SCR's are blown, that would be my first suspects, if the SCR's are blown open, then they would likely blow the pulse transformers.
    On these drives it is generally the bridge components that go.
    Also D5 the BEMF diode.
    Max.
     
  5. Drocke

    Thread Starter New Member

    Nov 4, 2015
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    I think I have checked all the diodes with the diode setting on the multimeter but I will have another look tomorrow. I assume the bridge is made up of D1,2,3,4. I think they are ok, I am getting 100V DC between Tp8 and Tp9 for the field winding.

    David
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The armature bridge consists of D3 D4 D5 and TH1 TH2.
    I would check the armature output with a 120v lamp first.
    Max.
     
  7. Drocke

    Thread Starter New Member

    Nov 4, 2015
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    I did have a 100Watt 240v lamp across the armature output and the field windings of the motor across the field output when I connected the 110v supply that was stepped down using a transformer from 240V AC. I would have at least expect the lamp to have come on.
    Can you please confirm or otherwise that a working SCR should not show a short circuit between gate and anode when they are the only terminals connected to a multimeter, irrespective of polarity, with the SCR removed from the circuit altogeter. If this does not confirm that the SCR is duff then I will put together a proper test circuit.

    David
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There should be no short circuit, you should have a different reading in one direction.
    Max.
     
  9. Drocke

    Thread Starter New Member

    Nov 4, 2015
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    Thanks Max that has been a great help and I have learnt a lot.
    I can set about sourcing a pair of replacement SCRs and also D5 but I am still unsure about the specification of the transformer T2.
    Do you, or anyone else out there, think that the 1:1+1 unit VTX-110-006 from Vigortronix will do the job or do I need something different?

    David
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Usually the dual secondary type are 1:1+1 and the windings should indicate a polarizing dot.
    I have used one from Hammond Transformers for a similar application.
    Not sure if they are avail in UK.
    BTW I recalled later the resistance between the K & G on the Scr's was around 50Ω's.
    Max.
     
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