The devil made me do it

Spent about 6 hours finding and wiring a reversing relay for my 8 Amp, 58/105 DC volt, shunt wound GE motor. Worked like an absolute charm but I decided to see if it could stand being reversed without stopping. The smoke came out!

The field's still getting the full 58 volts but the armature voltage (sadly) is now only working between about 19 and 36 volts. (no longer up to 105 V)

Probably asking the impossible but can anyone offer any suggestions as to the probable cause of failure. And might it be possible to strip out the speed control circuit (if it is screwed) and run the motor at its full rated speed using current directly from the full wave rectifier?

Had planned to to run this motor on a small lathe.

Thanks T

(Spam address)

Reply to
Ted Samuels
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You _cannot_ plug reverse a DC motor. When the motor is running it acts as a generator in opposition to the supply voltage. The current through the armature is due to the difference between the supply voltage and the generated voltage, all divided by the armature resistance.

When you reversed the supply, that reverse voltage due to the rotation of the motor was added to the supply instead of being subtracted from it

So you probably went from about one volt, or less, driving the current through the armature to 115 volts... Exit magic smoke stage left.

If you need to reverse the motor rapidly, you need to think in terms of a forward-stop-reverse switch. the stop position can have a biggish resistor shorting the armature. The resistance can be sized to allow about twice the rated current at the rated voltage. The motor should be stopped before reversing. Alternatively, It is possible to buy PTC based soft starters that might be used to limit the armature current with this sort of treatment, but those might slow down the stopping and starting process. YMMV

If the speed control circuit is designed smart enough, it can cope with reversing the motor by absorbing the braking power itself and controlling the armature current so that you change speed at the fastest rate that the motor can handle (e.g. running at maximum torque decelerating and accelerating). But it sounds like your circuit isn't doing that :-(

HTH

Mark Rand RTFM

Reply to
Mark Rand

Thanks Mark

RTFM! Sure, good advise, if I had one. The motor and control were something that was in a lot that I bid on a dozen years ago and to give you an idea of it's age, there was a penned in note mentioning a repair done in 71.

I was hoping to get a steer in the direction of what's most likely damaged . In any case I gave myself a good lesson on reversing.

Thanks again T

Reply to
Ted Samuels

The spindle VFD's on the DC motor driven lathes will decelerate stop and reverse in a minimum amount of time if they are set up right. You can go directly from forward to reverse with no problem. If the deceleration times are not set up right you start blowing fuses.

John

Reply to
John

if there is electronics, it's most assuredly damaged - power the motor separately from the electronics and see if it works, if yes, then you at least know where to look - a modern DC motor control is not too expensive though and there are a lot on ebay, otherwise look at Minarik.com they make nice ones

Reply to
William Noble

Thanks William (Bill)

Yes, I expect my adventure will cost me the price of a new controller. Kind of a shame. It was a nice setup. I suppose I could still use it to provide the field current and a variac with a rectifier for the current to the armature. I enjoy tinkering.

The lathe is already powered so there is no urgency. I just thought the reverse and variable speed of the DC motor would have been a nice touch.

Thanks for the good advise. T

Reply to
Ted Samuels

FWIW I've had no problems "plug reversing" (actually it was a classic DPDT switch wired as reversing switch) PM DC motors, Both fractional gear motors I've used for powerfeeds and a good sized 90VDC I've used for spindle motors, but my speed control was not electronic, but a big assed Variac and diodes from hell. I did it all the time on the power feeds running at about half speed and ocassionaly on the spindle motor usually running pretty slow when threading, (made threading metric to a shoulder a snap. -) A big caviet here is that doing this to PM DC motors WILL demag them over time... shrug ...

I suspect that the issue here is that reversed voltage blew the SCR's, TRIACS or DIODES in the speed control.

Yes you should be able to drive the armature directly from some from of rectifier, but remember if you use a full wave bridge, yer gonna be hitting that thing with around 150VDC.. (the 120 is an RMS value, 120 x 1.4 is about

170 peek)

You might consider just a half wave... I know those, 50/100's are "growlers" any way so....

Also if you are still going to "hot reverse" this motor you better get 400V Diodes since you'll be adding your generated voltage to the 170 when you hit the reverse switch.

--.- Dave

Reply to
Dave August

All I know from my own experience is plug stop reversed motors fail far far sooner than ones that come to a full stop before reversing, At least this is the case on saw chain and mower blade grinders.

Reply to
beecrofter

Reply to
William Noble

Yes! Thanks Dave!

I'm going to cut my losses on this and go for a set up like yours. I have a half dozen

90 volt pm dc drives and three 10 amp variacs and possibly one more if I could ever find it. Got to find some "diodes from hell" and I'll be all set.

I don't understand the math involved in calculating ("if you use a full wave bridge, yer gonna be hitting > that thing with around 150VDC.. (the 120 is an RMS value, 120 x

1.4 is about 170 peek) )" but I think you are saying that if I pit 120 ACV across the Variac and out through a full wave rectifier, at full rotation of the variac the output DC voltage will be 150 DCV and peek at 170 V

This being the case,, I could limit the rotation of to variac and utilize the

20 volt tap on these variacs.

Better run, have to respond to a couple of other guys that were good enough to lend a hand.

Thanks T

Reply to
Ted Samuels

I never should have done it. I had the damn thing working perfectly. I slowed the motor to a crawl and hit my newly minted reversing relay switch. I half way knew something would go wrong but like I said " the devil mad me do it."

Well now I know for sure what would happen. Better find out before I went to the bother of replacing the existing motor.

Got to find some good in what I did. Perhaps it will save some one else from the same fate.

Besides, I am retired and I like messing around.

Thanks T

Reply to
Ted Samuels

As I mentioned earlier, I think I'll cut my losses and scrap the controller. BTW the controller will still drive the motor in forward and reverse but the armature is only getting about 19 volts at minimum speed and 36 volts at a slightly higher RPM. Perhaps 150 RPM. In addition to that the "ON" breakers are now chattering and I have to hold the "ON" switch for a second or so to get it to latch.

Better scoot now. Thanks Bill

William Noble wrote:

Reply to
Ted Samuels

ted - why don't you try and fix the controller? it sounds like it has a bad diode or two in a full wave bridge, or a smoked SCR -

  1. do you have a schematic? if yes, several of us could suggest probable parts to check/change
  2. if no schematic, can you tell an SCR from a resistor? if yes, just change the SCRs and diodes, you have about a 60% chance of getting it working that way, if no, maybe a photo onto binaries? Can you solder? if no, better not mess with it, but if yes, then you can probably fix it

Reply to
William Noble

I'll see if I can read the part numbers from the devices in the heat sink and look them up on google if I can.

Left to see what was in the controller.

Yes there are a pair of 2N5165, pressed into the sink, they are about 1/2" dia with two terminals each and two 1/2" dia A448 single terminal (diodes??) pressed into a second sink.

it sounds like it has a bad

I can only wish! (I looked up the only number printed on the case and it turned out to be the part no of the fuse

Yes I am a bit familiar with diodes resistors and caps but I would be hard pressed to identify an SCR. I assume it would pass for a diode with an extra connection for a gate (if thats the right name, IE to control the flow through the SCR)

Question is do they have to be removed to be tested or do you have to pull them to test?

Better run got fish to fry

T
Reply to
Ted Samuels

A448 should read A44B Eyes are getting old!

Reply to
Ted Samuels
2N5165 is an SCR (google is your friend) - I will bet you that one of them is destroyed. the summary description is:

Silicon controlled rectifier. Reverse blocking triode thyristor. Peak repetitive forward or reverse blocking voltage 200 V. in 2-pin 310-02 package. Operational temperature range from -40°C to 100°C.

an SCR in a more modern package will be cheaper, but usually these parts are quite inexpensive. See if you can find a source and get a price - easiest to just change both if they are just a couple of $$. The other two things are diodes - you can use any stud mounted diode with a reasonable PIV (say

200 to 400v) and a current of at least the max current you will draw through the controller.

What is c>> William Noble wrote:

Reply to
William Noble

Is there any way to test the operation of the SRC without removal ? I read up on how to test and I think I could manage to do the test

Thanks for the offer to source but I have found them , to be special ordered through a local guy at $9.95 ea and $5.78 ea for the diodes.

The dealer's a good guy,, (says he will special order any time.(without having to go across town and make a deposit)

Better run T

Reply to
Ted Samuels

I de-soldered the four components and tested them all. I followed the test guide for the SCRs and wasn't totally sure they were shot until I tested an SCR from an old power supply. It behaved exactly as my instructions said it should. The two SCRs in the controller acted nothing like the good one. So I am confident that I am on the right track.

Had a little fun pressing the components out of the heat sinks, but I have a set of

12 LC Vise Grips which have the reach to get in between the heat sinks and with the aid of a small pin taped to one jaw and a socket on the top end of the component the all pressed out slick as you could wish for. The internet instruction suggested I pry them out with a flat tip screw driver.

When pigs fly!!!

Will be 3 to 4 weeks to get the SCRs.

Gotta run

T

Change the . to a 1 to contact me direct.

Reply to
Ted Samuels

now that you know the SCRs are bad (were the diodes also bad?) I'd suggest you replace them with parts having a different form factor that is more common - stud mounted diodes are common (I have some used ones for sale soemwhere on my web page, but they are cheap), and most low power SCRs nowadays mount via a single screw through a flange - your electronics supply place can help you select modern parts that are electrically equivalent to your old dead ones.

Check whatever was connected to the gate of the SCRs - it may have been hurt also - typically SCRs fail short circuit, then blow an internal wire and go open circuit (if they don't just explode their case)

T
Reply to
William Noble

The diodes were dead short in on direction and both about 200 K ohms in the opposite. That seemed OK to me but all the other diodes I had were in the 900 K ohm range so I went ahead and ordered replacements

Well I'll just have to see if replacing th four components does the trick or not.

Got quite a surprise today. Tried running one of my 60 volt DC motors (I thought they were 90 volt motors) with a variac and my setup tripped the breaker in my power bar. Checked and rechecked the circuits from the variac through the bridge and motor and everything seemed logical. tried it again and again it tripped (not very smart really). Then I happened to notice in small print on the very front of the variac "360 cycles per minute" 6 HZ

Live and learn!

Thanks again T

Reply to
Ted Samuels

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