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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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Dremel scroll saw DC motor
Posted here a while ago broken speed control: Well they sent me a new
one - no charge! but the damn thing still won't work. The motor just hums in all positions of the control. I measured 100VDC going to 2 of the 4 wires to the motor. The motor bearings and brushes look good. Any ideas what further diagnosis I could pursue? |
#2
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"Martik" wrote in message news:jeIHd.36644$06.34415@clgrps12... Posted here a while ago broken speed control: Well they sent me a new one - no charge! but the damn thing still won't work. The motor just hums in all positions of the control. I measured 100VDC going to 2 of the 4 wires to the motor. The motor bearings and brushes look good. Any ideas what further diagnosis I could pursue? There's 4 wires to the motor? I had assumed the brushes would be in series with the field coil, you need voltage to both pairs of wires for the motor to run. |
#3
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"James Sweet" wrote in message news:zy%Hd.13338$Os6.4292@trnddc08... "Martik" wrote in message news:jeIHd.36644$06.34415@clgrps12... Posted here a while ago broken speed control: Well they sent me a new one - no charge! but the damn thing still won't work. The motor just hums in all positions of the control. I measured 100VDC going to 2 of the 4 wires to the motor. The motor bearings and brushes look good. Any ideas what further diagnosis I could pursue? There's 4 wires to the motor? I had assumed the brushes would be in series with the field coil, you need voltage to both pairs of wires for the motor to run. The 2 wires to the brushes have the 100VDC, I do not know what the others are for. Here is a picture, the red and blue wires go to the brushes: http://www3.telus.net/martik/motor.jpg |
#4
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"Martik" wrote in message news:Hj3Id.133368$KO5.1002@clgrps13... "James Sweet" wrote in message news:zy%Hd.13338$Os6.4292@trnddc08... "Martik" wrote in message news:jeIHd.36644$06.34415@clgrps12... Posted here a while ago broken speed control: Well they sent me a new one - no charge! but the damn thing still won't work. The motor just hums in all positions of the control. I measured 100VDC going to 2 of the 4 wires to the motor. The motor bearings and brushes look good. Any ideas what further diagnosis I could pursue? There's 4 wires to the motor? I had assumed the brushes would be in series with the field coil, you need voltage to both pairs of wires for the motor to run. The 2 wires to the brushes have the 100VDC, I do not know what the others are for. Here is a picture, the red and blue wires go to the brushes: http://www3.telus.net/martik/motor.jpg Can't really see much there, that's definitly the commutator of a universal motor though. As I said before, you need power to the stator windings for the motor to run, otherwise you'll just burn up the armature. |
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On Fri, 21 Jan 2005 08:46:31 GMT "Martik" wrote:
"James Sweet" wrote in message news:zy%Hd.13338$Os6.4292@trnddc08... The 2 wires to the brushes have the 100VDC, I do not know what the others are for. Here is a picture, the red and blue wires go to the brushes: http://www3.telus.net/martik/motor.jpg While most small DC motors that I'm familiar with these days have permanent magnet stators, 2 extra wires imply that there must be a stator field coil. Your photo is very good, but it's too close up to see the whole motor and how it's built. I suspect, however, that it has field windings which are not getting any power. It needs voltage across BOTH the brushes AND the field windings. - ----------------------------------------------- Jim Adney Madison, WI 53711 USA ----------------------------------------------- |
#6
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I would bet that the extra pair of wires bring variable power to the
stator, providing a variable magnetic field and thus varying the speed. With no power on these leads (no power on the stator), there is no field, so the motor won't turn, of course. Since you just got a new speed control unit from Dremnel, you have to assume that the speed control unit is OK. This leads you to suspect that the problem is in the wiring inside the motor, or the cord from the speed control unit to the motor (assuming you didn't get a new one with the new speed control). So start at the speed control - does it put power on the yellow and white wires, as well as on the red and blue wires? If so, does the power make it through the cord to the pcb mentioned in another post? If so, does it make it to the motor? Find out where you are losing power in the yellow / white wire circuit. Bill Jeffrey =========================== There's 4 wires to the motor? I had assumed the brushes would be in series with the field coil, you need voltage to both pairs of wires for the motor to run. The 2 wires to the brushes have the 100VDC, I do not know what the others are for. Here is a picture, the red and blue wires go to the brushes: http://www3.telus.net/martik/motor.jpg It appears that the white and yellow wires are possibly connected to some other item (thermal sensor ??) on the other side of the fibreglass pcb which is clamped under the brush housings. Can you remove the pcb to have a look? |
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"Bill Jeffrey" wrote in message news:m3CId.43060$St.19486@fed1read03... I would bet that the extra pair of wires bring variable power to the stator, providing a variable magnetic field and thus varying the speed. With no power on these leads (no power on the stator), there is no field, so the motor won't turn, of course. Since you just got a new speed control unit from Dremnel, you have to assume that the speed control unit is OK. This leads you to suspect that the problem is in the wiring inside the motor, or the cord from the speed control unit to the motor (assuming you didn't get a new one with the new speed control). So start at the speed control - does it put power on the yellow and white wires, as well as on the red and blue wires? If so, does the power make it through the cord to the pcb mentioned in another post? If so, does it make it to the motor? Find out where you are losing power in the yellow / white wire circuit. http://www3.telus.net/martik/ Referring to the above pics: Motor1 is the PSB, the small white connector (upper right) with grn,brn,blu goes to the speed control. There is continuity to all 3 wires and voltage (2-3 v) and resistance (4-7k) vary when adjusting the control. The large white connector goes to the motor with 100v between the blu and red; 50v between blu and yel; 50 v between blu and wht, ov between wht and yel with NO change when adjusting the control for both 100 and 50v readings Motor2 shows the pcb in the motor. I get 100v between blu and red and 50v between blu and wht, 50v between blu and yel with no change when adjusting the speed control. I cannot see any connection under the pcb between the red/blu and wht/yel wires. So, I conclude that all the wiring for those 2 connectors is ok but don't understand why there is no change when adjusting the speed control. Motor3 show closeup of wiring from pcb to 'end cap' Motor4 shows other side of end cap with magnetic disc Motor5 shows commutator inserted Motor6 shows housing and armature. Those odd shaped thinks in the housing are just washers. Thanks again for your help! |
#8
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"Ross Herbert" wrote in message ... On Mon, 24 Jan 2005 03:43:50 GMT, Ross Herbert wrote: A great set of photos... The motor is definitely a permanent magnet motor, so no field winding... Since you know the controller is ok the problem can only be in either the armature or whatever is hooked up to the yellow and white wires on the brushes pcb in the end cap. Since the armature is new it is unlikely that there is an internal short in the winding, but this is still a possibility. My guess is that whatever is connected to the yellow and white wires provides some sort of feedback to the controller and if this device is not working then the controller doesn't know what to do. PS. Found this page http://www.scrollsaws.com/General/Chosing.html Extract. "Subject: Dremel scrollsaw Date: Fri, 03 Aug 2001 08:16:16 -0600 From: Rik White To: Have you had the opportunity to use this piece of junk? I've been through 3 since March. All suffer the same problem, they just plain quit running for no apparent reason. The saw has many flaws but this is the most frustrating. I thought you might want to add a review of this paperweight to your website. Rik. " It seems that someone else isn't too impressed with Dremel scroll saws either. Well, I'm getting pretty close to just bypassing the whole PS and replacing the motor with an AC induction motor. Don't need the variable speed anyway. |
#9
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"Ross Herbert" wrote in message ... On Mon, 24 Jan 2005 02:00:31 GMT, "Martik" wrote: "Bill Jeffrey" wrote in message news:m3CId.43060$St.19486@fed1read03... I would bet that the extra pair of wires bring variable power to the stator, providing a variable magnetic field and thus varying the speed. With no power on these leads (no power on the stator), there is no field, so the motor won't turn, of course. Since you just got a new speed control unit from Dremnel, you have to assume that the speed control unit is OK. This leads you to suspect that the problem is in the wiring inside the motor, or the cord from the speed control unit to the motor (assuming you didn't get a new one with the new speed control). So start at the speed control - does it put power on the yellow and white wires, as well as on the red and blue wires? If so, does the power make it through the cord to the pcb mentioned in another post? If so, does it make it to the motor? Find out where you are losing power in the yellow / white wire circuit. http://www3.telus.net/martik/ Referring to the above pics: Motor1 is the PSB, the small white connector (upper right) with grn,brn,blu goes to the speed control. There is continuity to all 3 wires and voltage (2-3 v) and resistance (4-7k) vary when adjusting the control. The large white connector goes to the motor with 100v between the blu and red; 50v between blu and yel; 50 v between blu and wht, ov between wht and yel with NO change when adjusting the control for both 100 and 50v readings Motor2 shows the pcb in the motor. I get 100v between blu and red and 50v between blu and wht, 50v between blu and yel with no change when adjusting the speed control. I cannot see any connection under the pcb between the red/blu and wht/yel wires. So, I conclude that all the wiring for those 2 connectors is ok but don't understand why there is no change when adjusting the speed control. Motor3 show closeup of wiring from pcb to 'end cap' Motor4 shows other side of end cap with magnetic disc Motor5 shows commutator inserted Motor6 shows housing and armature. Those odd shaped thinks in the housing are just washers. Thanks again for your help! A great set of phoros... Amazing what a macro setting and high F-stop(8), no flash and a self-timer can do The motor is definitely a permanent magnet motor, so no field winding... Since you know the controller is ok the problem can only be in either the armature or whatever is hooked up to the yellow and white wires on the brushes pcb in the end cap. Since the armature is new it is unlikely that there is an internal short in the winding, but this is still a possibility. My guess is that whatever is connected to the yellow and white wires provides some sort of feedback to the controller and if this device is not working then the controller doesn't know what to do. Could you suggest a way to just bypass the controller and run at full power? |
#10
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Martik wrote:
"Ross Herbert" wrote in message ... On Mon, 24 Jan 2005 02:00:31 GMT, "Martik" wrote: "Bill Jeffrey" wrote in message news:m3CId.43060$St.19486@fed1read03... I would bet that the extra pair of wires bring variable power to the stator, providing a variable magnetic field and thus varying the speed. With no power on these leads (no power on the stator), there is no field, so the motor won't turn, of course. Since you just got a new speed control unit from Dremnel, you have to assume that the speed control unit is OK. This leads you to suspect that the problem is in the wiring inside the motor, or the cord from the speed control unit to the motor (assuming you didn't get a new one with the new speed control). So start at the speed control - does it put power on the yellow and white wires, as well as on the red and blue wires? If so, does the power make it through the cord to the pcb mentioned in another post? If so, does it make it to the motor? Find out where you are losing power in the yellow / white wire circuit. http://www3.telus.net/martik/ Referring to the above pics: Motor1 is the PSB, the small white connector (upper right) with grn,brn,blu goes to the speed control. There is continuity to all 3 wires and voltage (2-3 v) and resistance (4-7k) vary when adjusting the control. The large white connector goes to the motor with 100v between the blu and red; 50v between blu and yel; 50 v between blu and wht, ov between wht and yel with NO change when adjusting the control for both 100 and 50v readings Motor2 shows the pcb in the motor. I get 100v between blu and red and 50v between blu and wht, 50v between blu and yel with no change when adjusting the speed control. I cannot see any connection under the pcb between the red/blu and wht/yel wires. So, I conclude that all the wiring for those 2 connectors is ok but don't understand why there is no change when adjusting the speed control. Motor3 show closeup of wiring from pcb to 'end cap' Motor4 shows other side of end cap with magnetic disc Motor5 shows commutator inserted Motor6 shows housing and armature. Those odd shaped thinks in the housing are just washers. Thanks again for your help! A great set of phoros... Amazing what a macro setting and high F-stop(8), no flash and a self-timer can do The motor is definitely a permanent magnet motor, so no field winding... Since you know the controller is ok the problem can only be in either the armature or whatever is hooked up to the yellow and white wires on the brushes pcb in the end cap. Since the armature is new it is unlikely that there is an internal short in the winding, but this is still a possibility. My guess is that whatever is connected to the yellow and white wires provides some sort of feedback to the controller and if this device is not working then the controller doesn't know what to do. Could you suggest a way to just bypass the controller and run at full power? get a rectifier of lets say 5 amps or morw and can handle 160 V AC or better. P.S. if you really don't know the actual max voltage to the brushes you could burn out the motor giving it direct source via a rectifier. you will know if it seems to really wind up faster then normal before the smoke comes out. oh btw, legion has it, if you let the smoke out then its a total lost! |
#11
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Could you suggest a way to just bypass the controller and run at full power? get a rectifier of lets say 5 amps or morw and can handle 160 V AC or better. P.S. if you really don't know the actual max voltage to the brushes you could burn out the motor giving it direct source via a rectifier. you will know if it seems to really wind up faster then normal before the smoke comes out. oh btw, legion has it, if you let the smoke out then its a total lost! He said it hums though and measures 100v at the motor? That would seem to indicate that the speed control is fine (heck he replaced it right?) perhaps it got overheated and demagnetized the field magnets? I can't think of anything else that could be the problem unless the thing is physically jammed or he's measuring wrong. |
#12
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"James Sweet" wrote in message news:JXiJd.12651$HT6.6579@trnddc04... Could you suggest a way to just bypass the controller and run at full power? get a rectifier of lets say 5 amps or morw and can handle 160 V AC or better. P.S. if you really don't know the actual max voltage to the brushes you could burn out the motor giving it direct source via a rectifier. you will know if it seems to really wind up faster then normal before the smoke comes out. oh btw, legion has it, if you let the smoke out then its a total lost! He said it hums though and measures 100v at the motor? That would seem to indicate that the speed control is fine (heck he replaced it right?) perhaps it got overheated and demagnetized the field magnets? I can't think of anything else that could be the problem unless the thing is physically jammed or he's measuring wrong. It turns freely and the field magnet is very strong. When I assemble the brushes and commutator in place I get zero (instead of 100) VDC between red and blue. Somehow that pcb on the motor end is blocking the voltage but I can see no traces on the backside of the pcb??? |
#13
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"James Sweet" wrote in message news:JXiJd.12651$HT6.6579@trnddc04... Could you suggest a way to just bypass the controller and run at full power? get a rectifier of lets say 5 amps or morw and can handle 160 V AC or better. P.S. if you really don't know the actual max voltage to the brushes you could burn out the motor giving it direct source via a rectifier. you will know if it seems to really wind up faster then normal before the smoke comes out. oh btw, legion has it, if you let the smoke out then its a total lost! He said it hums though and measures 100v at the motor? That would seem to indicate that the speed control is fine (heck he replaced it right?) perhaps it got overheated and demagnetized the field magnets? I can't think of anything else that could be the problem unless the thing is physically jammed or he's measuring wrong. I thinks it's the power supply: If i connect a 14v battery to the commutator the armature become strongly magnetized, however when I connect it directly to the red and blue leads from the main PCB it is NOT magnetized even though I read 300vdc from these two leads. Not enough amperage? I cannot check this as I have no amp meter. |
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