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Post by scooter76 on Jul 25, 2019 15:48:23 GMT
Hi gang! Recently came across this really nice early P5 with a nearly perfect Seeger cabinet and am getting it ready for use in my workshop (why someone left the condenser naked during a past (poor) repaint is a mystery - perhaps they thought it looked cool). I was pleased to note that the heater is still working and, after about an hour or so, heard SO2 merrily bubbling away in the crankcase. However, upon trying to start it, the starter contacts merely fluttered for about 10 seconds before the overload tripped. I checked the winding resistances of both the run and start windings - both good. Next I tried testing the big blue porcelain resistor in the start circuit and wouldn't you know it - it's open. What are folks using for a suitable (and safe for the start winding) replacement these days? There seems to be some conjecture (upon researching other posts) that these resistors were actually designed to be more like thermistors, wherein the inrush current is limited by a higher "cold" resistance and the resistor becomes more conductive if and when it heats. If that's the case, it would seem a linear replacement that does not limit inrush current like this could be detrimental to the life of the (already 91 year old) windings? Any suggestions are much appreciated. Thanks!
Ryan
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Post by ckfan on Jul 25, 2019 22:37:42 GMT
I’m glad your motor tested well. I think your best bet would be to look in the DR manual. It lists the different types of resistors used and what their values are. I always thought they were just a standard resistor and not like a thermistor.
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Post by scooter76 on Jul 25, 2019 23:04:07 GMT
I’m glad your motor tested well. I think your best bet would be to look in the DR manual. It lists the different types of resistors used and what their values are. I always thought they were just a standard resistor and not like a thermistor. Hi and thanks for the suggestion. I already looked there and it’s definitely either a #10 (3.4 ohms) or a #11 (6 ohms) as illustrated. Unfortunately, those two appear identical visually to me. I guess I’ll try 6 ohms first, as I’d rather pass less current than more, as long as that provides sufficient starting torque. Back to my original question, though - was hoping someone would have a suggestion on a proven modern replacement. I assume 50-100W would be sufficient for the little duty time it’s in the circuit? Has anyone seen these fail and, if so, what did you use? Thanks again! |
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Post by cablehack on Jul 26, 2019 1:11:16 GMT
I don't see why a capacitor could not be used as a replacement - the purpose of the resistor is to introduce a phase shift in the start winding, and a capacitor will do the same thing, but a lot more efficiently. As for the original resistor, if you can find where it has gone open circuit, you can measure each intact portion of the winding to find what the original resistance value was.
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Post by coldspaces on Jul 26, 2019 3:46:15 GMT
We have not really found a sure replacement other than another original resistor. If a capacitor would work as cable hack suggested that would sure make life owning a DR easier.
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Post by turbokinetic on Jul 26, 2019 11:09:24 GMT
We have not really found a sure replacement other than another original resistor. If a capacitor would work as cable hack suggested that would sure make life owning a DR easier. When we got Paul's DR going; it had an open resistor. We jumpered-in a start-capacitor from my parts pile, for a test. (Unfortunately I didn't look at the uF value). That worked perfectly. The resistor ended up needing the terminals on the bottom cleaned and it was good in the end - but the cap definitely worked. |
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Post by scooter76 on Jul 31, 2019 17:16:37 GMT
Thanks all. I'd like to try a capacitor as suggested. Can anyone suggest a recommended uF value?
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Post by turbokinetic on Jul 31, 2019 20:51:40 GMT
Thanks all. I'd like to try a capacitor as suggested. Can anyone suggest a recommended uF value? I believe we tried a 150 to 300 uF. |
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Post by cablehack on Aug 1, 2019 10:05:17 GMT
I wouldn't go below 80uF. The value isn't super critical for motor starting, but because it's feeding an inductive load, it's probably wise to see the voltage rating is up around 250V AC. Certain resonant effects can cause the capacitor voltage to be higher than the mains voltage. In this early thread, Coldspaces tried 108-130uF with success monitortop.freeforums.net/thread/105/starting-resister-dr-controls?page=5 |
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Post by scooter76 on Aug 2, 2019 3:13:08 GMT
Good info - I’ll obtain one and give it a try. Thanks!
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Post by scooter76 on Aug 22, 2019 19:00:47 GMT
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Post by turbokinetic on Aug 22, 2019 20:07:38 GMT
Hi. I can answer this, I believe! With a motor-start capacitor, it is given an alternating current which charges the capacitor in one direction, then reverses and charges it in the other direction. This charge-forward / charge-reverse cycle repeats 60 times per second as long as the motor start winding is in the circuit. The capacitor films in motor capacitors are designed so that they can be "roughly treated" in this manner, at least for a few seconds at a time. The capacitor is built heavily and has an ability to tolerate some heat buildup. If a polarized capacitor (sch as a filter cap for an audio power supply) is used in this manner, it will rapidly fail. These capacitors are insulated with a thin aluminum oxide layer, which is stripped away by reverse voltage application. Even if the reverse cycle wasn't a problem, these capacitors aren't intended to be charged and discharged over and over. They are meant to hold a relatively constant charge and filter ripple current. The heat buildup would get to it very quickly. When I was younger and not so cautious, I tried this actual thing. All it did was blow the capacitor in about half a second.... with copious amounts of horrible smelling smoke. You might see if they have two separate 50uF motor caps which would be easier to package inside the control. Just a thought. |
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Post by scooter76 on Aug 22, 2019 21:03:04 GMT
Thanks! That actually makes total sense. I hadn't thought about packaging two together, but that's a good thought.
Thanks again!
Ryan
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Post by cablehack on Aug 22, 2019 23:00:51 GMT
Great to know it works. Is it not possible to mount the capacitor inside the cabinet top? This is where it is located in the CK-3 machines.
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Post by Travis on Aug 23, 2019 1:20:42 GMT
Unfortunately a DR has totally different construction than a CK.
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