@MisterBill2
There is the possibility the elevation and traverse gearboxes may be over greased, however, it is an oil/grease mix as per the manual. Also, the traverse ring gear and elevation gears can be rotated by hand, does take some force, but nothing too extreme. I am in the process of locating replacement bearings used in the gearbox to help reduce any wear induced load.

This is the load breakdown:

1. Full Load of Traverse and Elevation (All New belts) = 10.9V (160amps)
2. (Worth noteing I have run the mount with the old well used belts (All OLD belts) = 11.1v (130amps)
3. Elevation belts only = 11.3v (100 amps)
4. Gearbox Load Only no Belts = 11.75v (65amps)
5. Motor Only = 11.97v (24amps)

 

My money is still on a series field motor.

 

My money is still on a series field motor.

OF COURSE it is a eries field motor! That i because it MUST have great starting torque for that application. Not only i that a high starting friction load, it is a high inertia load so it had to be a series field motor.

 

Thanks everyone for your input so far it has been very helpful. There seems to be 3 factors that could be causing the voltage drop:

Would I be right in saying we think the motor is not at fault, the repairs were substantial (no rewind required) and are not causing a voltage drop?

It is in fact that the battery supply (130ah) is too small and as a result, the motor is drawing a high current?

Or the actual load on the motor is too high? It's a combination of new belts, newly greased parts, and generally old worn parts causing the added load.

It would be good to get a consensus on what people's thoughts are and any next steps to getting to the bottom of this. I'm looking at buying a larger supply, but conscious of not inflicting any unnessessary costs.

 

A series field motor possesses very high torque and can draw very high current due to the load , this is because the current produced also flows through the field, IOW the current and strength of the field produced is proportional to the load.
For this reason a very substantial sized battery is normally required.

 

It is in fact that the battery supply (130ah) is too small and as a result, the motor is drawing a high current?

Or the actual load on the motor is too high? It's a combination of new belts, newly greased parts, and

I don't see how too small of a battery could result in higher current. If the battery is undersized for this application, that would imply higher internal resistance in the battery, resulting in less voltage at the battery terminals under load and lower current, not higher current.

The battery may well be too small for this application, but if that's true, I don't think there's clear evidence to prove it yet.

What we know is that the motor claims to be rated for 100A max sustained load (presumably ignoring brief peak starting current and/or stall current) and that it's presently drawing much more than that when loaded. That would seem to imply either too high of a load (gear mesh issues, grease, pulley friction, etc) or a damaged/worn motor that's significantly out of spec.

The other thing we know is that it's drawing substantial power under no load. I could be wrong, but 24A seems like a lot of current for a motor with no load. With no gearbox connected, the only mechanical resistance should be from the bearings and the brushes. The only real load at this point is overcoming the friction of those components. I have very little experience with motors this large, but I'd be surprised if 24% of max current should be required just to overcome internal friction in the motor. Sounds to me like either there are mechanical issues adding unreasonable friction to the system, or there are leakage paths allowing wasted current.

As I said though, most of my experience is with smaller motors, and I'm sort of extrapolating from there in terms of what the no-load current should be. I could be way off base here. I totally defer to Max and the other experts here with regards to no-load current expectations.

Regardless, I don't see how an undersized battery could cause excessive current draws. It seems to me that you need to identify and fix the cause of your excessive current draws before determining whether or not you need a larger battery (or larger charge/recharge/boost supply.)

 

One thing would be to check the no-load current draw spec of similar size starter motors. That could let you know if the 24 amps no load is reasonable or not.
An undersized batery would certainly not be the source of a higer current, but rather the source of a voltage drop under load. Probably the original system specification could give a clue. Really, it looks rather heavy for an aircraft assembly., more like something for shipboard use. The 12 volt rating is starange for an aircraft system, usually they were 24 or 28 volts. But maybe this system was for use in the very back end of a plane. At any rate, an examination of the support systems willgive you a great deal of insight.

 

One thing would be to check the no-load current draw spec of similar size starter motors. That could let you know if the 24 amps no load is reasonable or not.

I would rate 24amps to be on the low side for a 12v starter motor off load.
They normally posses very heavy armatures requiring high torque to rotate.

 

So I've done a bit of digging and noticed the spec sheet in the manual. It's important to note the style number does not quite match up to the motor used on my mount(4470417). Nonetheless, it does say it was compound wound and the RPM is substantially less than what is quotes on my motor (5400RPM), possibly that is the reading underload?



@MisterBill2 from a quick google it seems the normal current draw of a 12v starter motor under no load seems to be 60-150amps which seems a lot. This would suggest the 24amps I am reading is low as @MaxHeadRoom mentions. Also worth mentioning this was not used in aircraft, rather it was used to shoot down enemy aircraft from the ground which may explain the 12v system.

I am going to the motor repair company today and run through any possibilities that could be causing this issue and possibly have them carry out further testing.

Thanks again for all your assistance.

 

They will certainly be able to tell if it is compound wound.The presence of a second winding is very obvious. But given the performance requirements of a gun turret it does not seem like it would be a compound wound motor.
OOPS! I had thought it was an aircraft turret. Thus both the weight and the power voltage were confusing. Now the 12 volts makes sense. And different motors will be quite different in performance.
The motor in your package might not be a correct replacement. Depending on the application that might be a problem for you.
Can you mention your application?

 

The compound DC motor combines the features of both the shunt and series field motor.
For fixed voltage supplied motor, it is still two conductors fed, so it is not always obvious from the outside.

 

I spoke to the motor repair company and they suggest trying a 15v 10amp power supply boost to the system when it is operational, hopefully counteracting the voltage drop we are experiencing. If that does not work, then they recommend having a rewind. I think this is a sensible approach.

@MisterBill2 The application is purely a restoration project to be used and enjoyed at military shows and eventually sold on.

I have to still locate replacement bearings and then reassemble so maybe a few weeks until I can update.

 

I spoke to the motor repair company and they suggest trying a 15v 10amp power supply boost to the system when it is operational, hopefully counteracting the voltage drop we are experiencing. If that does not work, then they recommend having a rewind. I think this is a sensible approach.

Can't quite see that working?

 

Max is certainly right!! If it draws 160 amps, or even just 90 amps,at 12 volts, The only thing putting a 15 volt 10 amp supply in parallel will do is overload the 15 volt supply. And unlike AC motors, DC motors generally to not increase current a the voltage drops, if the load does not change. Instead, they just run lower.

 

UPDATE:

The motor has been rewound professionally and is running at its optimum when it is not installed onto the gearbox and subsequent belts etc.. I will include some of the readings I have taken below. But it seems as though the load is simply too much for the motor by design. The manual states the generator must be running when operating and the whole system should be run 5mins ON, 5mins OFF. This makes me believe the high amps are not unlikely?

New Rewound Readings using 12v (12.4v) battery only:

1. Motor Only = 33amps - 11.8v
2. Motor & Gearbox = 70amps (40amp increase) - 11.7v
3. Motor, Gearbox & Traverse belts = 114amps (44amp increase) - 11.4v
4. Motor, Gearbox, Traverse & Elevation belts = 160amps (46amp increase) - 11.1v

Interestingly using the old slightly more loose/worn V belts reduces to 130-140 amps (11v).

Unless anyone suggests an obvious solution, I am going to use it as is with the older belts and during use, I will keep an eye on the amp draw. As a WW2 piece of history, it is not going to see anywhere as much usage as it did, so am confident no serious damage will be made. The more it is used the more the new parts will bed in and hopefully become more efficient and I can easily upgrade the batteries later on if required too.

Thanks for your help!

 

I still recommend checking all of the rotating elments for excess friction or binding. Specificly, sleeve bearings not perfectly aligned, gears engaging excessively tightly, and any section not moving smoothly. Or maybe you have already done all of that. Using the broken in belts is certainly the better choice. And it would be moore important to pay attention to the motor temperature, or at least as important as watching the current.