DC Motor Question

[driberif85]

I have a very nice older drill press. Some were in its life time it was converted to DC. It has reverse so I was using it to tap some holes. It just stopped working. The controller is home built and unfortunately I have yet to wrap my brain around electronics. I understand enough identify the 4 diodes set up as a rectifier bridge to convert AC to DC. Other than that it is all gibberish to me. I did test the motor and there is voltage at the brushes when it is turned on. The brushes look OK to me. If any one could help I would really appreciate it. I do have a multimeter and I have a simple knowledge of using it. Thank you

[battwell]

what voltage is the motor? and what voltage are you getting at the brushes?

[Doug Baker]

I think the first thing you have to do is establish if the speed controller is working.

I would connect a lamp accross the output of the speed controller after disconnecting from the motor and then wind the voltage up and down to see if the lamp reacts.

If it doesn't then you have to either fix the speed controller or replace it.

In the event that it is fine then the electric motor is at fault and it would be my guess a brush is not touching the commy.

Hope this helps,

Regards,

Doug

[mlogical]

Does the motor have 2 or 4 wires going to it? Is the voltage present at the brushes DC or AC? On a digital meter, you can test the DC or AC component of a voltage using those scales. Is the motor warm at all or is there any hum or vibration felt in the shaft? How much voltage is observed at the brushes?
Respectfully,
Ron Moore

[lclore]

An option would be to buy a KB Electronics #KBMG212D. They are about $125. It's a regen drive with anti-plug reversing and regeneration. Good Speed-Torque Range. $125 may be worth it to not learn how to be a drive designer for a one time application...

[Al_The_Man]

I would also investigate the Baldor or KB unit, you can get them in a chassis version to save money if you already have the enclosure and speed pot.
To test the motor first, either run it off a straight bridge across the mains, or an automotive battery will also give you some idea of the performance, test in both directions.
It would also be wise to check the brushes while you have it off.
The KB and Baldor can be had on ebay usually, they come in either 2 quadrant (single direction, no braking) or 4 quadrant, which gives you the dynamic braking and reverse.
Al.

[KEN BANGERTER]

IF ALL ELSE FAILS AND IF THE DC MOTOR IS A PERMANENT MAGNET FIELD MOTOR YOU REVERSE IT BY REVERSING THE POSITIVE AND NEGATIVE WIRES TO THE MOTOR. FOR A POWER SUPPLY, UP TO 2 HP, YOU CAN USE A VARIABLE SPEED CONTROLLER BUILT FOR ROUTERS, TYPICALLY LESS THAN $50. ADD A 120 VOLT DC RATED DOUBLE POLE, DOUBLE THROW CENTER OFF SWITCH FOR INSTANT REVERSING WITH OFF. THAT WAY, THE SPEED CONTROL CAN BE LEFT ON AND THE SPEED SETTING UNCHANGED.

IF THE MOTOR HAS A WOUND FIELD, THE REVERSING SWITCH MUST BE WIRED TO REVERSE POLARITY TO EITHER THE FIELD OR THE ARMATURE, AND MUST NOT HAVE A CENTER OFF POSITION, SO AS NOT TO HAVE THE FIELD OR ARMATURE ENERGIZED WHILE THE OTHER IS DIS ENERGIZED. ON/OFF IS DONE AT THE SPEED CONTROLLER POWER SWITCH.

IF THE MOTOR HAS ONLY 2 WIRES, AND WAS REVERSED AT THE CONTROLLER, YOU CAN BE SURE IT IS A PERMANENT MAGNET FIELD MOTOR.

IF IT HAS 4 WIRE, IT IS A WOUND FIELD MOTOR AND IT MAY BE SERIES OR PARALLEL WOUND. A SERIES WOUND MOTOR, AT NO LOAD HAS A VERY HIGH SPEED, LIKE A ROUTER MOTOR. A PARALLEL WOUND MOTOR HAS A NO LOAD SPEED NOT MUCH HIGHER THAN ITS LOADED SPEED.

IT IS MOST LIKELY YOUR MOTOR IS EITHER A PERMANENT MAGNETIC OR PARALLEL WOUND FIELD TYPE.

[Al_The_Man]

Originally Posted by KEN BANGERTER FOR A POWER SUPPLY, UP TO 2 HP, YOU CAN USE A VARIABLE SPEED CONTROLLER BUILT FOR ROUTERS, TYPICALLY LESS THAN $50.

The problem I see with that is Router motors are typically Universal motors, (AC/DC Series connected).
Almost all of these controllers work in the AC mode by means of a Triac control, which means they will not work with a regular DC motor.
Al.
BTW turn you caps lock off.

[KEN BANGERTER]

SORRY, I FORGOT TO MENTION THE NEED FOR AT LEAST A 1/2 WAVE RECTIFIER IN THE OUTPUT. MANY DC MOTORS ARE RUN ON 1/2 WAVE AT TYPICALLY 90 VOLTS. CAPS LOCK MAKES IT EASIER FOR ARTHRITIC HANDS TO TYPE.

[Al_The_Man]

I think you will find the DC controllers like KB and Baldor run full wave, Diode/SCR bridge, with no smoothing the output is just over 100VDC and with the current detection resistor they have in series, the voltage out is around 90VDC.
I have never tried a Triac controller working into a bridge, but it should work.
Al.

[Al_The_Man]

Originally Posted by driberif85 I did test the motor and there is voltage at the brushes when it is turned on. The brushes look OK to me. If any one could help I would really appreciate it. I do have a multimeter and I have a simple knowledge of using it. Thank you

I just reread and noticed you mention a voltage at the brushes, so there could be a problem with the motor if it is not turning, another test is to back feed it with another motor source and see if it generates a DC output, the output will be relative to speed, if it is a 90vdc motor at 2000rpm, turning it at 2000 rpm should generate 90vdc.
This applies to a permanent magnet field motor.
Al.

[lclore]

As you make your decision, please note that polarity switching isn't an optimal dc motor control method, especially if its immediate (via DPDT Relay or whatever, with a minimal time delayin contact changeover). It works, but doesn't control the field build up or decay of either a permanent magnent or shunt wound motor. The end result can be a more severe arc when switching which shortens motor life.

Polarity switching of a fixed source voltage also negates accel or decel control and fixes speed (voltage is speed to a dc motor). A regen drive (4 Quadrant as Al mentions) provides accel, decel, variable speed, and better torque control at low speeds. Possible mfg's of these include KB, Baldor (many of the Baldor units are KB), Dart, Minarik, Fincor (now part of Emerson), etc. While I have put a pile of the KB #KBMG212D out there successfully in many automation projects, I am sure any of those manufactures would have a suitable offering.

If you decide to spend the money on a real drive, skip the non-regenerative (2 Quadrant) types. A KB #KBIC120 can be had new for $80... but the performance isn't as nice. Good Luck!!