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  #1  
Old 05-09-2011, 08:15 PM
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iONic iONic is offline
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Default A Mobility Scooter Motor Controller issue(s)!!

O.K. Guys/Gals...

I have a mobility scooter that is acting up pretty darn good these days.
NOTE: No longer under warranty and Medicare will not pay for it.

The problem was this:

A) Intermittent(...no, don't say that!)
B) When it does not work:
When pressing the forward/reverse accelerator pot the motor
tries to go forward, but may only creep or not go at all.

I have since took a lot of variables out of the equation:
1) The "forward/reverse accelerator pot"
2) The "max speed pot"
3) The circuit board with the pot connectors
4) The cable wiring from the pots to the motor controller
5) The wires from the motor controller to the batteries
6) The wires from the motor controller to the Motor

I've pretty much narrowed it down to the motor controller

The catch: A new replacement motor controller for this
scooter runs in the neighborhood of $500 - $600 US.

That is a big ouch!

I even when into the motor controller and replaced the 3 24V
relays and ended up with the same symptoms.

Thus I am here in hopes for some suggestions and or ideas on
my next step.

My inclination is to build a new controller.

Meanwhile I thought of just building a very basic controller that will
enable forward movement only.

Haven't really done any testing in the motor itself other than running it from
a 24V bench power supply.

...more to come



Notes on this circuit:
1) D4 is not needed as the MOSFET has an internal diode
2) I need to verify the appropriateness of R3 (2K pot)
Changed from 2K to 500K(V Gate increased from 7.94V to 10.54V at full on pot)
3) I need to verify the pot is configured in the circuit correctly
Pot was incorrectly configured, fixed this.
4) I need to record the Gate voltage and make sure it is bringing
the MOSFET into saturation.
Not sure what nomenclature designates this in the datasheet.

MOSFET Datasheet

Problems with it thusfar...

With 24V bench supply or 24V Battery, the wheels spin and the speed varries while on a block but do not when on ground.
Not sure why that is.

...After making changes (In Dark Red, above) The wheels spin faster, but when on the ground, the scooter does not
move. What would I check from a motor perspective?
Still not sure the MOSFET is operating correctly. Should not the 555 pulses to the gate always be at full on Voltage, like 10V? It should not vary from 0V - 10.5V!?
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Last edited by iONic; 05-09-2011 at 09:26 PM. Reason: Added more details
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Old 05-09-2011, 09:27 PM
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Default

If the motor is a brushed type, have you inspected the brushes and the commutator?
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Old 05-09-2011, 10:59 PM
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Quote:
Originally Posted by beenthere View Post
If the motor is a brushed type, have you inspected the brushes and the commutator?
Been meaning to do that...

o I's say about 3/4 - 7/8 on both brushes, both smooth and not grooved.
o Commutator also looks clean, like polished brass, and smooth.
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Last edited by iONic; 05-09-2011 at 11:23 PM.
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Old 05-09-2011, 11:03 PM
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Quote:
Originally Posted by iONic View Post
Not sure how to check the commutator, but I can check the brushes...
look at it. if it's all black and/or gnarled and/or concave then that's a problem. you can clean it with fine grit sandpaper.

EDIT: the commutator is what the brush rides on in the center of the motor. there may be a 2nd plate so you can access it, or you may have to remove the brush holding assembly to access it.
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Old 05-09-2011, 11:09 PM
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I would omit D4 (the MOSFET has a built in one, anyway) and replace D5 with a high power fast Schottky diode, in the neighbourhood of 5-10 amps. Make sure to give the MOSFET good heatsinking. If you put both on the same heatsink, make sure to keep them isolated using a mica insulator or similar, or you will see sparks. A fuse in line with the battery for the motor would not be a bad idea.

With a 12V supply, do not worry too much about saturation: the MOSFET will usually be saturated above 10Vgs, anyway (the 555 will drop 1.5V on the output usually, so you're clear.) To be extra sure, you could use a 7815 instead of a 7812.

And, because discharging a lead acid battery too far can damage it, do you have some kind of under-voltage lock out?
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Old 05-09-2011, 11:20 PM
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@ Ionic - Quote "Notes on this circuit:
1) D4 is not needed as the MOSFET has an internal diode"

You will/do need the diode D4. Even though the mosfet shows that there is a diode in the mosfet, it is called 'an intrinsic diode'. It is just part of the internal structure of the mosfet, it does NOT take the place of the diode shown in the circuit.
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Old 05-09-2011, 11:38 PM
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@Ionic- your trying for a PWM circuit to control the motor,right? The 555 by it's self doesn't make a very good one to drive a mosfet/motor of much amperage. with a couple extra components you can make it much better

This circuit from Bill Marsden; http://forum.allaboutcircuits.com/blog.php?bt=678 Fig. 4.3 works and is a true PWM circuit. It gives an adjustable duty cycle square wave out put that the mosfet gate needs.
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Old 05-09-2011, 11:41 PM
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Quote:
Originally Posted by tom66 View Post
I would omit D4 (the MOSFET has a built in one, anyway) and replace D5 with a high power fast Schottky diode, in the neighbourhood of 5-10 amps. Make sure to give the MOSFET good heatsinking. If you put both on the same heatsink, make sure to keep them isolated using a mica insulator or similar, or you will see sparks. A fuse in line with the battery for the motor would not be a bad idea.
As per the next user post I will keep the Diode D4 in place. I do already have a fuse (not shown in schematic) in line with the battery (25A)

Quote:
Originally Posted by tom66 View Post
With a 12V supply, do not worry too much about saturation: the MOSFET will usually be saturated above 10Vgs, anyway (the 555 will drop 1.5V on the output usually, so you're clear.) To be extra sure, you could use a 7815 instead of a 7812.
Yes, I thought of the 7815.
The problem is that the voltage to the MOSFET seems to vary with the pot
position and do not know why this is.

Quote:
Originally Posted by tom66 View Post
And, because discharging a lead acid battery too far can damage it, do you have some kind of under-voltage lock out?
Since I live where I have to go uphill regardless of direction from home the scooter gets sluggish, letting me know that I had better get home.
I charge the batteries after every use and do not go more than 2 miles/use, generally. This will be considered in the new motor controller, with digital readout for individual battery voltage and current.


Quote:
Originally Posted by strantor View Post
look at it. if it's all black and/or gnarled and/or concave then that's a problem. you can clean it with fine grit sandpaper.

EDIT: the commutator is what the brush rides on in the center of the motor. there may be a 2nd plate so you can access it, or you may have to remove the brush holding assembly to access it.
They check out O.K.


Quote:
Originally Posted by shortbus View Post
@Ionic- your trying for a PWM circuit to control the motor,right? The 555 by it's self doesn't make a very good one to drive a mosfet/motor of much amperage. with a couple extra components you can make it much better

This circuit from Bill Marsden; http://forum.allaboutcircuits.com/blog.php?bt=678 Fig. 4.3 works and is a true PWM circuit. It gives an adjustable duty cycle square wave out put that the mosfet gate needs.
I will check this out ASAP.
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Old 05-10-2011, 01:24 AM
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Quote:
Originally Posted by shortbus View Post
@Ionic- your trying for a PWM circuit to control the motor,right? The 555 by it's self doesn't make a very good one to drive a mosfet/motor of much amperage. with a couple extra components you can make it much better

This circuit from Bill Marsden; http://forum.allaboutcircuits.com/blog.php?bt=678 Fig. 4.3 works and is a true PWM circuit. It gives an adjustable duty cycle square wave out put that the mosfet gate needs.
What would you suggest for the specs of the BJT's?



Is this circuit follow your suggestion??
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Old 05-10-2011, 02:49 AM
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24v on the gate of the FET will blow it up. I really don't like your gate drive circuit. I would use a regulated 12 for the push-pull pair, and still add a 13v zener diode from the FET gate to ground.

1N4005 for D3 is TOTALLY unsuitable, at medium speeds (50:50 duty) D3 will be conducting for as much time as the FET and will likely have a higher forward voltage drop and needs to be a large power diode, with better heatsinking than the FET.

At low walk type speeds D3 may be on for 2/3 of the duty cycle!

A 40 amp power schottky in TO220 or better still TOP3 pack would be a good choice. You can put D3 and FET on the same large heatsink (although isolated).
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