Need Help!- I'm looking for ideas on controlling this DC motor

[NEATman]

Hello everyone-

I have this huge Brush DC motor on my latest Hardinge lathe project. It is not the original 3 phase spindle motor that came with the machine, as that had a constant speed AC motor and a variable speed belt drive.

I'm looking for suggestions on how to control this motor and possible sources for compatible drives, as the DC motor controller that was hacked onto this lathe is usually used to control motors in the 1/4 HP range. It was very underpowered and the spindle could be stopped by hand. It had worked for a few years on the very small diameter parts that had been made on this lathe, but won't work for parts I plan to make with it.

I have confirmed that it is indeed a brush motor, as the brushes are located behind the small football shaped cap adjacent to where the wires exit the motor. The brushes are at least 1/2" square.

There are 6 wires exiting the motor, two are twisted together with a wire nut, and the rest are labeled as follows: A1, A2, F1 and F2. I am assuming that they are Armature 1, Armature 2, Field Coil 1 and Field Coil 2.

There are no vendor markings anywhere on the motor - I even checked under the wiring cover - nothing but black paint. I think it has been rebuilt. I am guessing that this is at least a 3 HP motor, as it is a foot long and 8" in diameter. I would love to find a controller that would let me to have full power, variable speed and would allow me to control the spindle with Mach3.

Any help is appreciated-
Thanks
NEATman
(Keith)

[gar]

080205-1359 EST

You need a DC motor control that includes field control. One nice feature of a wound field motor is you can run above base speed by reducing field excitation, although at lower torque for a given armature current. You need loss of field detection because motor speed is inversely proportional to field excitation.

For controls look for GE and Reliance as a couple sources.

Figuring out the nominal voltage rating of the motor is probably going to require some measurement, experimentation, and analysis. Guess at the HP. Assume this might be a 3 HP like you did and that full field excitation is 10% of full power. Thus, we would need 0.3 HP or 746*0.3 W about 225 W. Assume field excitation is 200 V, then R = 40,000/225 = 177 ohms. I have no idea if my guesses are close.

See if you measure anything close to that. Hopefully you have a high accuracy Fluke (4 digits) because later I would want you to measure average temperature rise of the field by chage of resistance.

(edit) I do not think that is a 3 HP motor, maybe closer to 1 Hp. (end edit)

.

[holbieone]

if it's a 24 volt motor you can use a drive out of a golf cart and a 5k pot for speed controle

[Al_The_Man]

I would also guess it is probably about 1.5hp tops.
The wound field usually increases the physical size compared to PM types,
It probably is going to be 90v armature 100vdc field, or 180vdc 200vdc field.
You could start by applying a 120v supplied full wave bridge output to the field, (no cap).
Drive the armature externally up to a known RPM, and measure the output.
Extrapolate the findings for 2000~2500 rpm and if it is around 100v, then you possibly have a 90v motor.
You can get SCR drives from KB and Baldor, some that offer dual voltage output, eBay is a good source, if you buy new, get the chassis model, they do not include the expensive enclosure and reversing switch.
The also have the feature of allowing Tach input for very precise speed control for a spindle.
Other features are +-10vdc control and four quadrant SCR for reversing & regenerative braking if you desire.
One thing to keep in mind if experimenting, is if the field voltage is removed while the armature is being supplied with DC when the motor is running it can run away at dangerously high rpm.
Al.

[NEATman]

gar-
Thanks for the insight. I'll measure the resistance with my 4 digit fluke meter tonight. I believe that they had been controlling the motor with a permanent magnet brush DC controller - one that does not have field control.

Would it be safe to use a Brush DC drive and control the field seperately? Perhaps interlock the drive with a relay properly tied into the field to power down the drive if the field is lost?

For the field power (amperage at a voltage) I guess it really comes down to how much power the motor field can continiously dissipate. Perhaps I need a way to vary the power going in and monitor the temperature of the case - and stop when the temperature continues to increase steadily. Perhaps by strapping a temperature thermostat (like out of a dryer) to the top center of the case where it should be hottest? It would then act as a saftey and cut power to the field if things got out of hand. This test of course would be done without power going to the armature.

Holbieone-
It's at least a 100VDC motor. I like the idea though.

NEATman

[Mike Nash]

I would suggest you measure the field resistance and calculate the field current at 100V and 200V. E.g. If you measured 120 ohms, then at 100V the current would be (V/R=I) or 120/100=1.2 Amps 120/200=0.6 Amps. This will probably be off from the original rated current due to cold windings. I typically see around 220-240VDC on a 300V rated field when the motor is cold and the field regulator is set for rated amps. So you might multiply your values by 5/6 for a closer estimate. Now go check some motor manufacturer's web sites to see what kind of specs their motors have for one about the same size. But if you check more than one manufacturer, you'll likely find they vary quite a bit. Also be warned that 50/100 V fields are still available from at least Baldor.

The only problem I see with supplying 100V to the field, spinning the armature and measuring the armature voltage is that a 180V Arm / 200V Field motor with only half field current is going to give around half armature voltage at base speed, which I would initially guess to be around 1750RPM. But if you run a 180/200 motor at 90/100, you'll have a severe loss in torque.

Good luck. Are you sure there's no label under all that paint somewhere?

[gar]

080205-1936

NEATman:

From "Reference Data for Radio Engineers", fourth Edition, ITT.

The resistance of a copper conductor increases about 0.4% per deg C.
Suppose we are conservative and allow a 30 deg C rise as acceptable, then a 12% change of resistance from room temperature would be a reasonable estimate for this average rise. Note this is average rise and not maximum hot spot. Lets round down to 10%.

Power to a fixed resistor is proportional to the voltage squared. If the choices are 108 or 216 as expected values for field excitation, then since temperature rise will be approximately proportional to power dissipation, if we apply 108 for a test and get a resistance change of maybe 5% to 10% after maybe 1/2 hour, then we might guess this is a 100 V motor. If the rise is 2% or less, then it is probably a 200 V motor. You will only need to make one room temperature resistance measurement if you room temperature is relatively constant, within a few degrees. The thermal time constant is going to be long.

You can remove excitation periodically to check resistance because you have determined your base room temperature resistance.

Al:

I think your method will work if you try to estimate torque capability. But speed may not be a good indicator.

If it is a 200 V motor and we apply 1/2 the normal feild excitation we get 1/2 the flux. Now apply 100 V to the armature and we should run at nominal speed (2000) because at 100 V we get twice the speed from 1/2 the expect flux, but we are applying 1/2 the expect armature voltage. In other words speed will be approximately constant if both field and armature have the same applied voltage. Obviously torque capability changes and therefore firiction becomes a factor.

My professor for both DC and AC machinery was Dr. J. G Tarboux. He had a great skill to provide a simple intitutive approach to the subject. A basic equation for speed is Va/K*Phi, but if Phi is proportional to Va, then speed is constant vs Va. Va is armature voltage, K is some constant, and Phi is the flux density.

.

[vger]

I have a DC motor just about that size that I put on a wood lathe. It's a 1 HP motor and has a 180 V armature and 200 V field.

Al's recomendation on deriving the proper excitation voltages is great!

Best of luck.
Steve

[gar]

080207-1337 EST USA

NEATman:

Have you measured your field resistance yet? If so what is it?


Al:

Suppose this is a 200 V motor and I externally drive it at 2000 RPM as an example. Making it a 200 V motor makes it possible to do the test without overheating the field. Now apply 200 V to the field and I read 200 V from the armature. Next when I lower the field to 100 V ( 1/2 the flux density ) and the armature voltage drops to 100 V. So from that test I can not tell if it is a 100 V or 200 V motor. Did I understand what you suggested correctly?

.

[Al_The_Man]

Originally Posted by gar Suppose this is a 200 V motor and I externally drive it at 2000 RPM as an example. Making it a 200 V motor makes it possible to do the test without overheating the field. Now apply 200 V to the field and I read 200 V from the armature. Next when I lower the field to 100 V ( 1/2 the flux density ) and the armature voltage drops to 100 V. So from that test I can not tell if it is a 100 V or 200 V motor. .

Gar, It appears you are correct
Al.

[Motorcoach1]

Nice motor , Look at the positive side. You could throw a weedeater on it and have a cheep generator also Granger has fairly cheep speed controler made by Dayton that will operate your motor , I have use them in the past and are work horses for the money. Mike

[NEATman]

gar/AL-
I've finally gotten some time to get back to this. With working too many hours and happily married and having two young kids, etc...

Anyway, I measured the resistance of the field coil at 87.8 ohms at about 52 deg. F. (11 deg. C) (My garage isn't heated - yet another future project)
After 1 hour connected to a KB Multi-Drive with a 1/4 hp resistor installed (I have piles of stuff - I found it in my Misc. drive box) across the coils with an input voltage of 120 vac, (105.3 VDC measured) the resistance was 106.2 ohms and was dropping steadily by about .1 ohms every 2 sec as the field coil cooled. A change of 21%?
The temperature of the case of the motor was barely warm to the touch. I think it was even slightly below body temperature. This would suggest a temperature delta of about 26 deg. C.

My guess is that this is a 100 VDC motor, and most likely a 1 hp. This will be good enough for my needs, so I guess I'll need to order the proper power resistor element from KB to use this drive and motor at a 1 hp rating. The 1/4 hp resistor setting will easily drive the motor, but it seems low on torque. I've even managed to get the motor under variable speed control through Mach3 with a cnc4pc step/dir to analog converter.

Now I just need to find a 14" longer belt so it will work with my 7" riser block that I will be installing under the spindle.

Thanks to everyone for the assistance!

NEATman
Keith