Prop Pitch Motor: Motor Repair

For your Halloween treat we're going to disembowel a prop pitch motor. Don't worry, it won't be too gory! We begin with the emergence of trouble atop the high tower one dark and stormy night...

When the prop pitch motor was first installed on the new tower with the 15/20 meter stacks I mentioned that it turned slowly. After doing resistance checks on the cable and the motor there appeared to be a faulty internal electrical connection. The problem was dirty contacts between the motor contacts and the receptacles on top of the gearbox. This I was able to fix with contact cleaner, tiny swabs and correctly torquing the bolts on the motor retaining ring.

A guide to the motor wiring can be found on K7NV's web site. There should be lettering on the motor housing denotes the motor as right hand or left hand, and the motor voltage.

Trouble signs

Unfortunately the trouble didn't end there. After several minutes of bench testing with a 13.8 VDC supply the motor noise I had already been aware of became worse and the motor repeatedly bogged down. When I connected it to the 24 VDC controller the deterioration accelerated and within two minutes the motor pretty well ground to a halt. This was accompanied by loud squeaking coming from the housing. For a motor running at close to 10,000 rpm any mechanical resistance can be catastrophic.

The failure was accompanied by excess current draw from the controller which, I discovered, is inadequately fused. The audible sizzling from the overheating was so apparent that I shut it down before the power supply failed. The ham who built the controller back in the deeps of time must have been an optimist.

With no backup on hand I had to take on the unwanted task of pulling the motor apart. I had some trepidation because these motors are becoming increasingly rare and expensive and I was unsure whether I'd be able to diagnose and repair it. With no readily available alternative I went to work.

Disassembly and testing

The prop pitch motors used as rotators were designed decades ago, with the originals dating to WW II. They were used on USAF bombers and other aircraft. The motor typically runs on 24 VDC, which is the aviation standard. We call it a motor but it is a motor plus a gearbox. It is the gearbox that enables it to be used as a heavy duty rotator.

DC motors with commutators and brushes are no longer common and parts are not always easy to find. Some hams have replaced it with an AC motor of similar RPM and power. Commutator motors will operate at lower voltage, but not too low or the motor can bog down and draw excess current. Luckily my motor is in good condition with none of the unique components requiring replacement.

The first stage of disassembly is to separate the motor and gearbox. The motor can be powered using the pins on its base. I used a high current 13.8 VDC supply on the workbench for testing during the repair process.

Removing the 3 bolts that hold together the two halves of the motor housing is not enough to pull them apart. The bearings at each end of the motor shaft are press fit into the housing halves. Axial force is needed to separate them. Levering too forcefully from one side can damage the shaft. Without a specialized tool for the job I used two pries between the upper housing and the shaft end to achieve a net axial (vertical) force. I moved the pries around the shaft in small steps as I worked the housing apart. Once separated the drive end of the shaft can be pushed out of the housing with a few taps of a cushioned mallet. Don't lose the thin shims that come tumbling out.

That done you have the armature and bearings removed and you can access to the brushes and field coils. Inspect the commutator and brushes for excess wear. If the brushes are excessively worn replacements can be had from a good quality motor repair shop. If the commutator is worn you may have no alternative other than to replace the motor.

The armature and field coil surfaces were pitted and coated with debris but otherwise sound. I cleaned them up to make myself feel better although it doesn't really accomplish much. Have a look at K7NV's web site to see what a pristine restoration looks like. Very pretty.

The brushes were inspected and found to have lots of life. I removed the easily dislodged debris (mostly carbon dust from the brushes) with a soft brush to avoid damaging the coils. The sides of the armature were cleaned with 400x sandpaper and then carefully brushed away the fine particles. Keep the motor interior clean of metal dust.

The brushes are bypassed with ceramic capacitor on bottom of the housing (not shown) which largely eliminate RFI generated by the rapid circuit interruption as the commutator rotates over the brushes. I cleaned and checked them. All were good.

The input shaft of the gearbox should turn easily by hand. Use a wide blade screwdriver to make testing easier. With a suitable coupler that doesn't damage the splines you can perform a higher speed test with a drill. I was satisfied by a hand test alone.

Bearings

Now we come to the crux of the fault: bearings. In the picture above you can see the non-drive side bearing and in the picture below is the drive side bearing below the commutator. The upper bearing is a shielded (not sealed) 3201 (if I remember correctly) and the lower bearing is an open 6200.

Both bearings ought to be sealed. There is a lot of carbon dust and other particulate debris inside the motor. The shielded bearing tests good so I'll leave it there. The open bearing was dry and pitted and took force to turn. A little machine oil freed it but could not repair the pitting. 

This is not the original bearing and whoever put it in there made a poor choice. Sealed bearings cost only a few dollars more. It pays to check the specs since the RPM rating decreases going from open to shielded to sealed. The rating of 6200 shielded bearings from reputable manufacturers exceed 10,000 RPM, which is the minimum rating for this motor. Check the low temperature rating if, like me, you have cold winters.

Within days I had a new 6200 sealed and cold weather rated bearing for less than $10, including tax and shipping from an Ebay merchant located in VE2. The old bearing was given a shot of penetrating oil and pulled off without trouble. A little more oil and the new one was pressed on.

I should mention that both the 6200 and 3201 are shown with metric dimensions in all the catalogues I checked. This can be deceptive since they often round those with English dimensions to the nearest millimeter. A friend checked his US military parts list for the small prop pitch motor, dated 1944, to check the dimensions before I placed my order. I suspected an unsuitable substitution. It is very unlikely that the USAF specified a metric and Japanese bearing during WW II! Unfortunately the specified bearings were only listed with proprietary part numbers from long gone manufacturers. Further research into the bearings was not practical.

I reassembled the motor and powered it up. It spun nicely with 13.8 VDC and the current draw was 2 amps lower than with the bad bearing. With this positive sign I did another and longer test in the shack with the 24 VDC controller. Motor heating was nominal and the current draw was very good. Back on the gearbox the motor hummed along at a more typical 7.5 amps. The gearbox resistance (mechanical loss) increases motor current.

I have some concern about the grease in the gearbox despite it testing well in the cold and not having excess mechanical resistance. I may tear it apart next year if only to ease my mind. There is no time for this preventative maintenance before winter.

Current draw

Commutator DC motors are interesting devices. Their theory of operation is well outside of my areas of knowledge. For example, I hadn't realized that the starting current is quite high at around 20 A. It is a bad idea to operate the motor by switching high current DC, whether by manual switch or relay. The high starting current can and do weld together relay contacts. That can cause a lot of grief up the tower because the motor will not stop.

The controller I use switches the AC line, which is safer. The CW or CCW direction is selected by switch before activating the rotation lever which turns on the power supply. Commercial controllers designed for prop pitch motors use other means to deal with the starting current. I will have to keep this in mind when I design or purchase a better controller for my two prop pitch motor rotators.

As with any motor there is a kind of torque curve. The motor will operate quite well with less than 24 VDC, and even a little more if you're careful, but not at peak efficiency. The field coils and commutator are optimized for a smaller range of speeds. The current draw is not very different when driven with 13.8 or 24 VDC. Driving a load at the lower voltage will further decrease efficiency and increase motor heating. 

I suspect the greatest risk is to the bearing lubrication, brushes and commutator, though perhaps not in actual usage since on periods are typically under a minute. Risking motor failure is inadvisable unless you have a ready supply of spare parts or spare motors. They haven't been manufactured for decades.

One way to limit excess current draw is to not use the largest wire gauge you can afford. A modest amount of extra resistance puts a ceiling on the current draw even in the event of a dead short. I am using 125 meters of AWG 10 wire for the motor (3 conductors). The DC resistance of the 250 meter long return circuit is 0.8 Ω (confirmed with an ohmmeter). 

With a dead short at the motor the current draw at 24 VDC is limited to 30 A. This drops to 19 A for the 1.25 Ω of AWG 12 wire. These figures assume the power supply voltage doesn't drop under the high current draw. If it does the power supply can overheat and fail due to its internal resistance unless there is over current protection. Of course with greater wire resistance the voltage will drop at the motor terminals and mast rotation will be slower. 

There is no one correct answer to the system design. There are several parameters to consider.

Weather delay

It's cold, really cold. We are setting record lows. This is not weather in which I want to do tower work at great heights. Once the weather warms up again I will remount the prop pitch motor on the tower. That is also when I will deal with the upper 20 meter yagi of the new stack. Yes, it's up and mechanically secure though not tested because I had to remove an element during the lift. 

The weather is delaying a lot of work that needs to be done. I want the new stacks operational for CQ WW CW at the end of November. There is little chance they'll be ready much earlier. 

Being ready includes a working prop pitch motor for the rotator. At least it's working, and that's something. I would have preferred this lesson in prop pitch motor repair in warmer weather. However, when it comes to maintenance you rarely have a choice and most repairs are done in winter when the station is stressed by the activity spurred by the cold keeping hams indoors and by the schedule of major contests.