Old Forklift DC motor help

I have an old stand-on forklift that I use inside my shop because my 8000 pound Forklift is too big. It has a 12VDC Battery, large battery in metal case that would be at least 12 large car batteries in size. Battery claimed to be new with a new battery charger when I purchased it and they look it. Hydraulic motor for lift runs good and I have no problem lifting max weight and slightly more. (haven't tried exceeding max but lift works great when ever lifting anything) My problem is drive motor, it seem to be week. Been a while since I played with it but if my memory is correct 3 relays are switched to provide 3 forward/reverse speeds. A metal type flat bar is used as a resistor that taps are connected to the 3 relays, Hi speed is direct, no resistance connected. A while ago I cleaned the contacts on all the relays but it didn't help, Brushes and commutator look OK but I haven't removed the motor.
With fully charged battery drive motor is very weak. Has a hard time getting started but once it's moving will continue but having limited space I never get it up to speed, getting started is very weak, if I have the wheels cut hard it might not move.
Any suggestions where I should look? Remember it's old there is no solid state controls!

Does the motor have brushes that you can inspect? I had an old DC motor that lost power and it turned out one of the brush springs broke. Other motors I have had to free up brushes that got sticky in their holders.

Check battery voltage, how much voltage drop when you engage motor? Check cable connections, check for voltage drop at motor, old cables can become resistors, they turn green under the insulation and may feel stiff. Check brushes and springs as mentioned above, plus commutator.

What kind of drive motor? Permanent magnet or wound rotor? Shunt/series wound or separately excited? Armature or field resistors?

Shorted field resistors on a field-controlled motor will make it run at minimum output. Shorted armature resistors on a permanent magnet motor will make it run like a bat out of hell.

Thanks guys, I'll do some checking. I doubt there is any issues with the battery or charger, they are quite new. Been a while since I looked inside so I will open it up and take a good look at what I have. I doubt that it is a PM motor but not sure if field or amateur is controlled. From what I remember the resister that controls the speed is shorted by the speed control relays so that as speed is increased the relays remain activated until full speed by-passes the resister completely. I'll do some voltage measurements. I do have an Amprobe that will measure DC Amps so I'll get amperage readings also.

I did some checking. I noticed a lug connection screw loose. Removed all the wires and rebuilt the quite large relay. But I think it's supposed to be the reversing relay, has 2 coils but one is energized all the time when stepping on the brake pedal that removed the brake and activates one coil on the above large relay. Making a wiring diagram and from what I see the motor is wired so that the armature and field are in series. One small relay looks as if it energized all the time, NC contact is advisably open looks as if the swing arm is up against the coil but I removed all power and swing arm don't move. NO contacts might be welded. Like most forklifts there is little room and that small relay is the one most difficult to get to.
Would series DC motor with resistor in series with the + be used in such an application. There are 4 terminals on the motor (large studs about 3/8" at least in diameter) and + thru resistor is connected to only one terminal. Other 3 motor terminals are connected to that large relay mentioned above. Only power connection to the contacts is the Negative side of the battery.

Froneck said:

... I doubt there is any issues with the battery or charger, they are quite new.....

Click to expand...

No offense intended, but prob best to not assume. And anything new is unproven and therefore suspect, in my book .

Measure the voltage at the battery terminals directly. If they can't sustain voltage, then the batteries are either shot or the motor has shorted windings.

If the voltage is fine, check the voltage directly at the motor terminals while driving. If it's good, then the motor has an issue. If it is low, then the control circuitry has an issue.

I did check voltage at the battery using my Amprobe as a volt meter (switches positioned to measure voltage with digital Amprobe but reading is only in full volts no decimal places, 2 AAA batteries were dead in my Digital Fluke multi meter)Voltage at battery 12VDC unloaded 11 volts under load, other places measured the same reading. Unfortunately I don't have alligator clips for my Amprobe meter probes so some locations like on the motor terminals were not taken.
There seems to be issues that need to be addressed. First I want to know if a motor wired in series like I mentioned is normal or will work!
That large relay is constructed so that the NO contacts for each coil are connected together. It consists of a flat copper bar about 3/4" wide 1/8" thick and about 4.5" long to go from one side of the assembly to the other, It's attached to insulated support with 2 8-32 screws. Between the screws is another 5/16" X18 hex head screw that attaches the Neg. terminal to the copper bar, at each end there is a round silver colored contact. The NC side of the relay is constructed the same way and one motor terminal connected to the center. To each swing arm is connected a similar copper bar and it becomes the common for each side of the relay. I'll make a photo and post it later tonight. To each common of each rely relay is connected the other 2 motor terminals. Being that the relay operates one side as soon as the break pedal is depressed the motor seem to lurch forward. Turning the speed control handle activated 2 solenoids similar to those found on older car starting systems that short sections of the resistor therefore lowering the resistance in series with the Positive connection to the motor eventually to Zero.(as near zero depending on contact and wire resistance) I will check to see if contact is made but they do click! However when rotating the speed control to reverse the other side of the above mentioned relay activates, activated side does not deactivate therefor applying Neg to 2 of the motor terminals and the 3rd is totally disconnected. The truck will move forward with very little power, attempt to move reward but with less power and will not move probably because of a flat spot on the solid rubber tire made from sitting in one place for a long time.
Please do not tell me about voltage reading, I am well familiar with and understated electrical power circuitry. I'm looking for function help. Yes I tested the battery and it functions well under load!
I have purchased the batteries needed for my multi meter plus another fluke digital Multi meter, I have a few other analog volt meters and will connect a few of them with alligator clips I have someplace or will buy more! I will determine how the system is operating and if it's functioning. The large rely I mentioned seems to be the reversing relay in the it will reverse the connections of the other 3 wires if one side drops out when the other side is activated. I don't know if its reversing the field or the armature. Motor end cover is not that easy to get to. I suspect the hard to get to small relay that seems to be welded in the closed in the NO connection is the problem. and possibly the solenoids not making contact another.

That's a lot of text to unpack there. A picture of the contactors would help.

But yes, series wound DC motors are commonly used in vehicles because they have very high low speed torque and aren't speed limited by the supplied voltage. Oversimplification but they exhibit more of a constant power output in theory, as opposed to constant torque, so they'll bog down but won't stall (usually).

They also can draw a lot of current when perfectly fine, so high current draw may be totally normal under load.

Here is the photo of the relay/contactor. I unstuck the small relay and now the large unit in the photo operated more like it should but still has issues. Some times both coils are energized and other times they are both off. Photo is sideways left side is up, wire on the left side on the top of the relay is the Negative from the battery. I didn't change any wiring but I think that small wire conned to the terminal in the center of the photo should be connected to the top Negative from the Battery.

[ATTACHCONFIG]335889._xfImport[/ATTACH]

Do you have a DC clamp ammeter? That will tell you a lot about the relative current draw of the three speeds, plus the lift motor.

This is all pretty high current, so if you have a bad connection somewhere, stuff is going to either get pretty hot fast, or just not conduct. I would be looking carefully at whether the starting resistors are being appropriately shorted out, assuming that's the speed control used.

I suggest labelling the relays and making a truth table of which activate at what settings

Yes I do have DC Amprobe, will measure both DC and AC depending on what the selector switch is set to.
My concern is the small wire connected to the center terminal on the right side. I assume that to be control wire but it's connected to one of the 3 wires going to the motor. Voltage at that point will change depending on current flowing thru the motor and the resistor. What purpose that will serve I don't know but the center terminal to the left is connected directly to the Neg. side of the battery. (Both Neg. wire from battery and wire leading to the left side of the relay is connected together on the stud for the lift hydraulic motor)I will remove that wire to see if it will extend to the Neg. terminal. Could be the solenoids have 6VDC or less applied.
Resistor resembles a spring, wire is at least 5/32" thick maybe 3/16". Very much like a spring in that wire is very difficult to bend. 2 springs in series, 2 assembles paralleled. I'll make photo. Connections very good, some time ago I disassembled it,
cleaned the surfaces and used a pure copper coating when assembled.
Lift motor runs very good, always has! In fact it's too fast, I don't use it to move packages, it's prime use is rigging machinery or putting something heavy in the machines.

Oh, measure the voltage across the resistors when the contactors across them are closed.

Strostkovy said:

Oh, measure the voltage across the resistors when the contactors across them are closed.

Click to expand...

I will do that to determine if the speed solenoids are working. But my concern is why a connection of a small wire is made to the terminal right side center. I see no use for a varying voltage connecting to that terminal will create.

Froneck said:

I will do that to determine if the speed solenoids are working. But my concern is why a connection of a small wire is made to the terminal right side center. I see no use for a varying voltage connecting to that terminal will create.

Click to expand...

Honestly I'm only skimming this thread because I'm at work. I can look more in depth when I'm off.

Is the commutator OK?......clean between the bars?....solder connections (if any).....armature connections are sometimes welded in high current motors,but can break.....Weak motr pulling high amps may be a grounded/shorted armature winding.

I made voltage test at the resister today. Photo is side ways, I rotated it and it's correct on my computer but the file sent is still sideways. Wire on the left is connected to + side of battery, wire on the right is connected to the motor. Solenoids are between the wires. at low speed bottom wire measured +3.8VDC to Neg. side of battery, When High speed solenoid activated voltage went up to 12VDC.
I did not make any current measurements. I was looking at the large relay I posted photos of yesterday. I was confused at what I was seeing as I turned the speed control handle and like yesterday the truck attempted to move but didn't. Thinking I should look at the speed switch I removed the cover, marked the micro switches as to location and removed them.
The speed switch is a solid metal bar with about a 1" hole thru the center. A cam to push on 4 micro switches is in the 1" hole. I tried to disassemble the cam but could not figure out how to remove it. The first micro switch connected to the forward coil on the large relay is NO, as the speed handle is rotated clockwise the push rod rises but does nothing when handle is rotated Counter Clockwise. The nylon push rod was sticking. Not being able to remove the cam I used Kroil to clean the hole, push rod no longer sticks. Next micro switch is connected to reverse coil and works just like the previous except opposite direction. Shims are placed under each switch for adjustment. The 3rd switch has me scratching my head! 2 sets of contacts having both NO and NC connection. Push rod is at the top when spring holds the speed handle in the center of rotation and drops when handle is rotated! Connecting the battery, turning the speed handle, nothing happens but if I push on #3 micro switch NO contact closes and forklift will move! But if I were to install #3 micro switch cam action will not allow power to be switched to operate the motor! Problem seems to be in the switching! Tomorrow I'll remove the wires from the micro switch to determine exact operation of the switch. Seems to me someone wired the speed handle wrong! At least the forklift will now move!