WW2 Anti-Aircraft Gun Motor Voltage Drop

Thread Starter

GreenEagleKing

Joined May 21, 2021
10
Hi,

I am restoring a WW2 M45 Quadmount and have run into a few electrical issues regarding the 12v electric motor which runs the traverse and elevation gears.

Currently, I am testing the motor and receiving a voltage drop of around 1.7v (from 12.6v) under full load. As a result, it's pulling around 160amps with the motor cables getting warm as a result. I have included a basic wiring diagram including voltage drop readings with 1v at the motor and around 0.7v through the wiring. Note it is currently powered by 2x 6v 130ah batteries connected in series.

I have recently had the motor refurbished and when the motor is run without load it reads 11.8v.

I am worried the high amp readings and the voltage drop is going to reduce the life of the motor, is this something to be worried about? Is the drop because the batteries are not up to the job?

I've also included a few photos for context.

Thanks for your help,
Ben
Motor Circuit.png
IMG_20210222_163620.jpg
IMG_20210503_190320.jpg
 

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crutschow

Joined Mar 14, 2008
27,243
Do you know what the normal motor current is supposed to be?
Otherwise that voltage drop sound normal for a 160A load from those batteries.
 

scorbin1

Joined Dec 24, 2019
70
Unless something is stalling the motor mechanically, then it sounds like your supply isn't up to the task. Can you turn the drive gears pretty easily by hand with the motor physically removed? If not, it could be that the load is too much for the motor/supply, perhaps a bushing or bearing going bad or starting to seize. What's the condition of the motor? Since this is a restoration job, the motor could require some attention or replacement. Also check to make sure all connections are tight, though if the drop accross the battery is falling to 1.75V then that's probably not the issue.
 

Thread Starter

GreenEagleKing

Joined May 21, 2021
10
No not sure of the normal motor current. But I do know the data plate on the motor states 100amps and when the motor is run without load (11.8v) the amp reading measures around 70amps which makes me think 160amp is too high?

Yes, gears turn freely when the belts are not attached. The motor has been refurbished and runs with no trouble out of the mount (annoyingly never took readings in this state). If supply is the issue what would be recommended in this setup? The current batteries 2x 6v 130ah is what is originally stated in the manual.
 
Last edited:

scorbin1

Joined Dec 24, 2019
70
70 amps at no load?! Either that's a pretty heft motor or that sounds really high to me. I would think they would gear it down so as not to require so much current. Not sure what the original power source was, perhaps it was a nuclear powered war boat or something, but I would think your gonna need quite a battery to supply that much current or your run time will be very low. Is the 100 amp rating the locked rotor or the full load rating? If that's locked rotor amperage then I think your motor needs a rewind.
 

Thread Starter

GreenEagleKing

Joined May 21, 2021
10
So when I say no load, the motor is connected to a gearbox that runs 4x pulleys so the motor would have some load on it. If it makes any difference I have taken a reading of 50amps in that state. Originally there was a 12v generator that provided up to 12vs 20amps charge when it was running. For the sake of testing, I am using a 240v to 12v 15amp supply to 'boost' the circuit which when running provides around 0.2v boost.

I am not sure if it is a locked rotor or full load rating I can only say it is what is stated on the motor casing? IMG_20201031_140031.jpg
 

scorbin1

Joined Dec 24, 2019
70
If the original source was a 12V Generator rated at 20A I am guessing it would never normally see full load rating. A battery in there would add a buffer so to speak for inrush currents and peaks but it wouldnt be able to operate too long at that rating. Of course if the military grossly underrates their generators like they do their trucks, I wouldn't be surprised if it could actually handle 80A. Still it's pulling WAY to much current. I would remove the motor, try again, and take some more readings. If it works just fine then I think you have some mechanical resistance in the gearing. Otherwise the motor might be in need of replacement or rejuvenation. Have you megged out the motor windings to see if the insulation may be experiencing some break down? That would definitely pull extra current.
 

Thread Starter

GreenEagleKing

Joined May 21, 2021
10
Yes so the generator is to charge the batteries so most of the supply comes from the batteries. Ok so what would you expect the 100amp motor to be pulling under normal conditions?

I have had the motor professionally rejuvenated (not a rewind) and they tested it (just motor no gearbox or pulleys) and found no obvious issues with it. Finding a replacement motor would be very difficult. It seems to me that the full load of the belts and gears on the motor is the driving factor behind the high current and voltage drop. As testing without the load of the belts is 11.9v at around 60amps. Can more supply e.g. larger batteries overcome the load? or is it simply the case of the motor has deteriorated over the last 75years?

Also, I thought I'd mention I have been taking the amp reading using a clamp meter. Is this an accurate way of taking current readings?

What do you mean by megged out the motor windings?

Appreciate your help,
Thanks
Ben
 

MaxHeadRoom

Joined Jul 18, 2013
23,150
It would be a wound field for that vintage, it could possibly be a series field motor, i.e. very high torque.
Also hence the very high RPM's.
 

SamR

Joined Mar 19, 2019
3,424
Megging puts a High Voltage on the windings to check for insulation breakdown and shorts. Should have been done when the motor was "rejuvenated". Did that entail more than just brushes and cleaning the commutator surface? Bearings?
 

scorbin1

Joined Dec 24, 2019
70
I honestly don't know what to expect as a normal current draw, however drawing 60Amps without a mechanical load is pulling 3 times the amount of power the generator is capable of and presumably isn't even doing any work yet. For short intermittent use this may be perfectly normal, and possibly what the original design counted on. However for continual use, as I think would probably be expected during times of battle, you would quickly drain those batteries and the generator would not be able to replenish it leaving you with a useless tool. Typically military designs are grossly underrated for exactly this purpose, to ensure there is way more than enough head room to account for extreme conditions. I could be completely wrong here, but I feel like 60A is way to much.

Is this 60A draw the inrush current you're seeing or the sustained current load after a couple seconds or so? Initial inrush current is typically around 20 times the full load current for the first half cycle and you can expect 4-8 times the full load rating for the first several seconds after wards. When you say "without the load of the belts" does this mean that absolutely nothing else is connected to the output shaft of the motor? I know you said you had the motor "rejuvenated" so that should rule out most issues with the motor, but unless I'm wrong it seems to be pointing to the motor. With a 75 year old motor I would be really suspicious of insulation break-down, but I can't imagine that they wouldn't have tested for that. Did they provide you with a report of what tests they did and what the results of each test were?

Also, is the shaft actually rotating when you measure 60A or is the motor stalled?

Clamp-on ammeter is just fine for this application.
 

MaxHeadRoom

Joined Jul 18, 2013
23,150
For accurate testing you would really need to measure the motor current off load completely, as it is unknown at this stage what degree of torque is require when the motor is installed.
Whether series or shunt field, the current should be reasonably low once it has reached running RPM with no load.
The only load at that point would be due to friction and armature inertia.
If there is only two leads to the motor, it may most likely indicate a series field motor.
 

Thread Starter

GreenEagleKing

Joined May 21, 2021
10
So I have removed the motor completely from any exterior load which read 11.97v (battery terminals 12.6v) with a draw of 24amps. I'm waiting to hear back from the motor repair company with the receipt of what was done exactly, I do know however that they replaced bearings and brushes and cleaned it up. No rewind was carried out either.

To reply to Scorbin the 60amp pull was the sustained current and when I say "no belt load" there is still the load of the gearbox which simply translates movement of the two output shafts of the motor onto two pulley shafts at each end, so there are 4 pulleys in total (hope that makes sense, photo at the start of the thread will help). The shaft is in full rotation at 60A, the motor has never stalled, it can be a bit slow to warm up, but otherwise, it's fine.

I will post when I get the results of the motor testing.
 

BobaMosfet

Joined Jul 1, 2009
1,780
@GreenEagleKing

Voltage and Current work in inverse proportion to one another- Ohm's Law. Voltage is not real- it is a manmade value, that describes the ratiometric relationship between two aspects of physics: The movement of electrons, and the impedance to that movement.

If voltage drops, you are drawing more current than your power-supply can provide.

130Ah only means they can provide that much current cumulatively, not at once. 130Ah = 361mA every second output. That's as fast as the battery chemistry can provide current. Cold-Cranking-Amps - like with a 12V car-battery provides a value for instantaneous max output immediately.

1622551721935.png

500 CCA, or 625 when warm.
 

Thread Starter

GreenEagleKing

Joined May 21, 2021
10
Thanks for the response @BobaMosfet. Would there be a minimum CCA for an application like this? Also, would you consider Deep Cycle batteries?

In other news, I have a response of what repairs were carried out on the motor (sadly no readings):

"It was tested then stripped down in order to enable us to assess the condition of its individual parts. After inspection, we found that the armature was not burnt out but was in need of degreasing, re-soldering, varnishing, baking and matching to enhance its performance.
The field coils were shorting to earth which explained the motor sluggish performance, they were removed from the casing and the old insulation was stripped off and replaced with new insulation the casing was sandblasted and painted before the coils were replaced.
The carbon brushes were replaced with new made to measure ones, the bearings were replaced with new ones. The motor was then re- assembled and tested with much increased performance. "

Any red flags?
 

SamR

Joined Mar 19, 2019
3,424
Didn't specificly say it was megged but it was. That's good. Sounds like a thorough rebuild without having to completely rewind.
 

MisterBill2

Joined Jan 23, 2018
8,808
Since the motor current is proportional to the torque load, once it is spinning, a logical step is to see if some part of the load is taking more torque than it should be. It might be that the gear gease is too thick, or that the gear mesh is not right.
I see two Vee belts and those always.provide a fair amount of friction load. How much current does the motor draw with no belts in place?
Really, it gets down to checking each stage in the drive until you find the part that has the drag.
 
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