Oh, I did try the motor on the weekend with a (poorly maintained) 7Ah SLA and it worked OK. Not fantastic but I reckon that it's pretty good for a first attempt on a limited budget.

I can imagine a few problems, will explain when I get a chance to think it through and do a longer post.

One thing that jumps out at me is the lack of a solenoid like R1 between the field and armature, I can understand you wonting to minimize the number of solenoids but I cant see it working without at least 2. As it is if you connect power into the junction of the field and armature via C1 you will have a very low resistance path through the armature. B1 closing with C1 will give you a circuit through the fields; Closing A1 on the return doesn’t seem to do a lot. I tend to think the armature will have lower resistance than the fields and even if you PWM a mosfet it is effectively a dead short. Adding any sort of resistor in series with the armature interferes with the forward stroke. I am curious about Andrews diode trick but one way of getting it working using 2 solenoids, 2 relays and a resistor and a diode is to.


Start by connecting the field to pos; then use a H bridge to run the armature with 2 solenoids for forward and 2 relays and the resistor in series for return, the diode connects across the field to take care of any transience but is probably unnecessary as there are so few turns.

This is rough as im tired and its done in paint but should give you an idea

http://robowars.org/forum/album_pic.php?pic_id=1348

I couldnt find any pics of my series wound motor mods . i think they are on one of my old hard drives there might be one in the photos section on this forum

I could move R1 to in between the field and armature windings and the split the supply to the field and armature (to become C1 & C2). I would also need to split the diodes (via A1) to their respective windings. The PWM supply (C1 & C2) would be contstant current regulated to say 40A. Since the supply is split, due to R1 being open, they can sit at different voltages. Another drawing is on its way.

A1, (soon to be A1 & A2) is there to prevent voltage spikes from the PWM since my fets are only rated at 60V.

@ Totaly_Recycled
I will think on it a bit. I did think about it in the first place but I don't remeber why I dismissed it.

Have you looked at disconnecting the field windings from the main motor power leads and run an extra two wires out. That way you just need a single relay to excite the field windings when you power the armature up... Control the motor with two circuits.
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quote:

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Originally posted by TDT:
Have you looked at disconnecting the field windings from the main motor power leads and run an extra two wires out. That way you just need a single relay to excite the field windings when you power the armature up... Control the motor with two circuits.

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Its not my field, I am an electro mechanical type rather than electronic but a piece of heavy copper bar in a stationary one turn starter motor armature is not going to have any resistance or back EMF to limit current if you try and drive it with a mosfet, if you only turn the mosfet partly on it is a resistor getting hot very quickly, not saying it cant be done.

The other thing that bothers me a bit is it takes time for relays and solenoids to drop out, when you are trying to get a motor up to speed and stopped again in half a turn getting everything tuned just right might be a challenge.

With both the Rotwang Hammer bots we set them up so the weapons could spin 360 degrees if necessary and used H bridges made of 2 NONC type relays.

Solenoids exist that work the same way but I have only come across them in electric winches on 4WD’s.

After a nice discussion over dinner last sunday, I have been convinced to drop the direct drive motor. Instead I will move to using a massive coil and it will look like a pneumatic axe.

Now if I use permanent magnets in the solenoid, it should be able to go forwards and backwards without additional mechanics. I'll take a grinder to my starter motor solenoid tonight, (after looking at my IBC) and see how much weight it can pull.

That'll be really cool if you could pull it off...
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YEAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAH

Starter motor solinoid can lift about 40-50kg on 24v... We have one for controlling the 5th wheel in Vertex
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Starter solenoids have 2 windings the heavy pull in coil and the lighter hold in coil that has more turns.

Run both in parallel for max pull.
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Satisfaction is proportional to effort and results.

Well I accidently killed the hold coil. Some of the top layer had some damage, so I took it all off. I might rewind it with the same size wire on the pull coil.

Now if Aaron is right, and his solenoid can hold 50kgs, and mine is similar then I'm in for some fun.

Test day is tomorrow.
I'll see how long it takes for 25g of magnets to hit the ground, if they're fired vetically. Best case (or worst, depending if I want to see my magnets ever again) is a launch speed of 60m/s (200kph) and a travel of 60 metres in the air. It will take 12 seconds for the magnets to hit the ground and about an hour to find them.

So I'll be in the park with my dad, wearing helmets, shooting magnets towards the sky, and running for our lives.

Remember to use 24v to get that force...
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