I have no idea about the jumper. But you surely can use a resistor to drop excitation voltage. If you have a 100 ohms winding, you need a little more than 200 W dissipation. That produces a lot of heat that you need to think about.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

I'm wondering if you have the motor nameplate data available? I've never seen a stock DC motor large enough (in HP) to be hooked up to a drive offering a 300 VDC field supply that didn't have dual voltage (150/300V) field capability.  

Such a motor brings out two pairs of shunt field wires, often labeled F1 and F2 for the one coil, and F11 and F22 for the other. To wire it for 150V operation the shunt field coils are parallelled (F1, and F11 to the + supply, and F2, and F22 to the - supply), or placed in series (F1 to the + supply, F2, and F11 connected together, and F22 connected to the - supply).

Sometimes a motor shop may repair a dual field voltage motor, and bring out the field on two wires (although, in this case, the customer should be the one choosing the voltage!).

If the (cold) field resistance is really 100 ohms when connected for 150V then I'm guessing the motor is in the 5 to 10 HP range (150V/100 ohms - 1.5 amps; fairly typical for ~ 7-1/2 HP motor).

You could put a resistor equal to the resistance of your field in series with it as you describe, but one thing to be aware of is field current falls as the coils heat (and resistance increases). I don't know whether the temperature coefficients of the field winding and power resistor (which are almost certainly not the same) might buy some trouble.

Also, in addition to generating a fair amount of heat (heat that has to go somewhere, I'd recommend put them in a separate, ventilated enclosure bolted on the outside of the main enclosure), it throws money down the drain. This would be roughly equivalent to turning on a couple of 100 watt light bulbs any time the drive is powered - for an order of magnitude feel for it, $0.07/kWh and 200 watts at 7000 hours a year is about $100.

If you don't use field weakening, and the drive doesn't do field current fault testing (or otherwise need a shunt field connected in order to operate) you might consider building or buying a fixed 150V supply of sufficient current capability.

The motor is a 75hp GE dc motor, 150/300 (field) Vdc, 500 (arm) Vdc, 1750 / 2100 rpm. It is currently running with the 300Vdc field however I would like to run at the higher rpm.

giselle: If you can't find four same lead size and length in the terminal box, then probably you have no choice to utilize its voltage duality for 150V. However, you may but laborious  such as dismatling the machine and reconnect or bringing out the shunt field wire to the terminal box to be able to come out as rawelk stated above for terminal marking of field winding like; F1,F2, F11 and F12(take not field polarity). What speed you're machine operating at the moment?

"Whenever you are asked if you can do a job, tell them, certainly I can! Then get busy and find out how to do it." Theodore Roosevelt.

Running on the 150 volt connection will not increase your speed.  The 150 volt fields are simply a parallel connection giving you the same speed as properly applied 300 volts on the series connection.  Simply reduce the voltage from 300 or 150 if you have them connected series parallel and the speed increases.

Careful now!

Running with half the rated excitation may be bad to the motor. You really have to get someone knowing about DC drives to help you - or you may destroy the machine.

Do not do anything to the field if you are not 100 % sure that you understand what you are doing!

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

Respectfully,
I agree that one should not take chances and should follow manufacturers recommendations.  This is a GE machine with a 300/150 volt field circuit.  It also has a field weakened speed range indicated by the nameplate.  The motor should run at base speed with 500 volts applied to the armature and rated field current on the fields plus or minus 5%.  The 300 volt field is usually a series arrangement and the 150 volt field is a parallel/series parallel arrangement with double the current for the same speed.  Our small motors like this are usually 4 field machines with all 4 in series for the 300 volt connection and 2X2 at double the current for the lower voltage.  Based on the nameplate data provided, one should be able to weaken the field to achieve the upper nameplate speed without becoming unstable or creating other problems.  The model number would be invaluable in ascertaining the original design parameters. This sounds like one of our standard type CD designs, probably a 500 series frame.  They are common and plentiful in industrial applications in the US.

Fine. You have the information needed to move on now, giselle. Make sure your machine is what oftenlost says.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

Thanks oftenlost. Just to confrim, the model number is SCD185VA001B001, type CD329AT, Fld amps: 2.4 / 1.6.

giselle
No problem with getting the speed you want provided the load is proper.  Reduce the field voltage until the unit speeds up to speed within the nameplate range.  You should be able to maintain stable operation down to 1.6 amps on the fields. These machines provide base nameplate torque up to base speed and nameplate horsepower above that speed.  Monitor armature current to make sure that you do not overload the machine.  This is particularly important if you have a load that increases as to the square of the speed.  These units can withstand overload and have been tested at 150% for short periods at the factory during final testing but it is best to keep it within nameplate amperage and ensure that the fan, if external, is providing the required CFM.

The GE motor part number should start with '5'CD..., and not 'S'CD.

I don't know anything about the WER drive, but, if it doesn't offer field weakening control you can add a seperate controller. For instance, although the Control Techniques FXM5 is normally used with their line of Mentor drives it can also be hooked up and configured for stand-alone operation.  

Do a Google search on ' "Control Techniques" FXM5  manual ', and you'll find a manual available in PDF format you may audit.

One thing to keep in mind is you'll need use tachometer speed feedback (as opposed to armature voltage feedback) because, once you hit the armature spillover voltage, and enter into field weakening, armature voltage will no longer track with shaft speed.

Add as much protection as possible, and shut down the drive if certain conditions are violated. The aforementioned FXM5 controller has a 'field loss' relay that will drop out if field current drops below a setpoint limit, and must be added to the drive stop circuit as a minimum level of protection.

It is also a very good idea to monitor and shut down for overspeed and tach loss conditions, because a motor spinning at full field weakening is only a stone's throw away from spinning so fast it'll fail in a catastrophic and rather colorful way - ranging from developing extreme low and high commutator bars and blowing up the brush set, through throwing the comm bars completely from the motor, and on up to ejecting the rotor like a rocket.

Quote:

motor spinning at full field weakening is only a stone's throw away from spinning so fast it'll fail in a catastrophic and rather colorful way - ranging from developing extreme low and high commutator bars and blowing up the brush set, through throwing the comm bars completely from the motor, and on up to ejecting the rotor like a rocket

This event would require a total loss of load to occur concurently with the field failure, not particularly likely in normal circumstances. I've only seen one motor in this state and it was a real mess around the commutator box. I wasn't involved in whatever caused the damage - we had a lot of strange stuff lying around in that company - but I was told by a colleague that it had oversped following loss of field.
 

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  Sometimes I only open my mouth to swap feet...

Scotty,
I would have thought so, too, but I saw the aftermath of a 400HP/1750 base speed motor eating a set of brushes after developing a severe high/low comm bar condition (up to 1/8th inch bar-to-bar!). This was on a plastics extruder screw application, and occurred during a production run, so at a minimum the screw/barrel frictional load was still there when it happened. The drive was an old engineered Reliance MaxPak (pre-MaxPakPlus).
 
It took me a while to arrive on the scene (my car decided to break down on the way to the plant), and it happened in the late 80's, so my memory is a bit fuzzy, but I recall the field controller had failed, measured field resistance was congruent with the nameplate field current & voltage, and field insulation resistance was OK (several hundred of megohms).

After the motor was replaced I checked the tach loss shutdown, and it was set to a hair under 2000 RPM.

My only speculation as to how this level of motor damage occurred sounds a bit nuts, but here goes - the field controller didn't fail 'cleanly', but rather went to zero output for some span of time (and the motor sped up, but not up to the tash loss/overspeed level), then re-established field current for a time before failing complete (essentially, trying to drop motor speed back to normal levels instantaneously), and it was this abrupt change in velocity at near full speed that threw the bars. Yep, still sound nuts ... I've never been able to come up with a satisfactory answer.

That said, in extruder screw applications drive setup on older analog drives is often done with the motor uncoupled to prevent excessive wear on the screw and barrel (no polymer; no screw lubrication), and must be done very gingerly, and with full knowledge of the bad things that can happen.