Brush Timing

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EV2Go
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Brush Timing

Post by EV2Go » Tue, 02 Nov 2010, 21:08

If we were talking engines, and we were talking igniton timing this would be a no brainer...the igniton timing is "usually" set x amount of degree before top dead centre (TDC).

Now with motors since there is no TDC, how exactly is the timing referenced? And what do you check it with, some kind of electric motor timing light?

If someone has pictures to explain it that would be great, and no I'm not looking to alter brush timing on my motor I am just curious about the process.

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Post by Electrocycle » Wed, 03 Nov 2010, 02:12

there is a TDC - it's relative to the field coils.

If the brushes are in line with the field coil poles it's neutrally timed.
Advancing it from there increases power and speed in one direction (very similar to how an ICE behaves!)
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Post by EV2Go » Wed, 03 Nov 2010, 17:37

I took a few pictures to try make this a bit clearer.

First assumption: All brushes line up with the own respective TDC, that there isn't any staggered offset like the old Mallory duel point dizzy to extend dwell. So if it has 2 brushes they are 180 degrees opposite, or 90 degree in the case of 4 brushes like mine?

Second Assumption: Unlike an ICE where the plug is fired x amount of degrees before TDC (generally), and advancing or retarding fires the plug earlier or later relative to the crank / piston position. Since the field coils and the brushes are both stationary and the only thing thing that moves is the stator, and it's movement has no begining and no end... I am guessing that advancing the brush timing has some kind of magnetic field effect that pushes or pulls the stator relative to a centre point on the stator to alter torque?

Third Assumption: That a degree is based of one rotation of the motor. So 1 degree is 1/360th of a rotation.

ImageImage
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Brush Timing

Post by 7circle » Wed, 03 Nov 2010, 18:42

The Field coil your pointing at may be an inter-pole.

This topic on DIYelectic: kostov-11-inch-dc-motor-construction has some good pics.
Image

If you copy and paste it into EDIT'er and invert the colours you can see things better.
I tried to look for the size difference between the inter-poles (AKA:commutating poles) and the Field Coil Windings.
Image


What's interesting is before EFI (oops that should be electronic dizzys for spark advance), ICE distributors advanced using inlet vacuum.
So I'm suprised people haven't built a dynamically rotating brush frame that adjusts to zero the field inline with the brushes using a hall effect sensor.

Of course the interpoles in the Kostov deal with this, bit like octane rating increase reduces pinging. (Not really a good comparison)

Image
The site Maritime.org web article is very good. explaining DC motor operation.

Image

Got a bit carried away with links but sometimes clear info is hard to come by.
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Brush Timing

Post by EV2Go » Wed, 03 Nov 2010, 19:16

With my limited understanding of motors you could be right, but I don't think so. Bit hard to tell since there isn't a great deal of light that shines into the motor.

I think this is the interpole that hangs below the field coils. The bolts appear to hold some big flat looking plate in line with the brushes, and from what little I can see I think the field coil goes around the outside of it.

Edit: Removed incorrectly label pictures to avoid confusion.
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Brush Timing

Post by EV2Go » Wed, 03 Nov 2010, 19:29

We're both wrong those are a couple of wires that go up to the terminal block Image

Think I found it now...

Edit: Removed incorrectly label pictures to avoid confusion.
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Post by Electrocycle » Thu, 04 Nov 2010, 23:10

Interpoles are usually smaller windings that are not as wide as the main pole windings.
What's interesting is before EFI (oops that should be electronic dizzys for spark advance), ICE distributors advanced using inlet vacuum.
So I'm suprised people haven't built a dynamically rotating brush frame that adjusts to zero the field inline with the brushes using a hall effect sensor.
It has been done, Killacycle used / uses a pneumatic ram to advance the timing as the revs increase. There's a photo kicking around somewhere on here.

First assumption: All brushes line up with the own respective TDC, that there isn't any staggered offset like the old Mallory duel point dizzy to extend dwell. So if it has 2 brushes they are 180 degrees opposite, or 90 degree in the case of 4 brushes like mine?
Yes, the brushes line up with the field coil poles, so the commutator bars are connected to the brush at the point where their armature windings are lined up with the field poles.

So, the armature field is generated on windings that are lined up with the field from the stator, so they push / pull against each other and create torque.

The field on the armature is being switched between different windings and opposite polarity as it rotates, and the field takes time to build and dissipate - just as fuel / air mix in an ICE takes time to burn - so by advancing the timing you can get the field to be at maximum strength when the alignment with the stator is optimal.
Obviously the higher the rpm, the more degrees the armature will have rotated before the field has reached full intensity, so the more advance you need to maintain the same torque.

The advanced timing also works against the back EMF from the motor, so you can keep pushing current in without having to go to massive voltages.

From what I remember, interpole windings effectively shift the location of the stator poles as the rpm increases (same effect as advanced brush timing, but from the other side, and automatic) so you should get a wider useful operating rpm range.
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Post by EV2Go » Fri, 05 Nov 2010, 01:56

And I still managed to get it wrong Image Going off 7C's updated post with pics, the bit i'm pointing at is the interpoles not the field coil.

Just comparing my motor with the photos the top lot of bolts (closest to the terminal box) seem to be in line with the middle of the field coil, and looking from the other end on my motor the brushes are in line with the bolt holes, so it would seem that the brushes are lined up with the middle of the field coil.

Part of a post from Plamen on DIY
Plamenator wrote:... Contrary to non interpoled motor, it is not advisable to advance the brushes of our motors as this will significantly worsen commutation...
The interpoles seem to make advancing or retarding redundant.

Sounds like the best comparision might be with a car ignition coil... takes x amount of time to get to coil saturation before the points break and create a jump to the secondary windings. The faster the RPM the more lead time it requires???

Just wondering if a motor was brushless and was using an optical pickup for (I was going to say crank position) for commutator position, couldn't you simply apply an electronic advance /retard to the optical signal?
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Post by Electrocycle » Fri, 05 Nov 2010, 02:36

yeah the coil saturation time is exactly what's happening in the armature windings.

The brushless motors using an optical pickup (usually hall effect) generally have the sensor timed neutrally, and the controller (nearly said ECU!) can advance the timing as necessary.

On my old bike the Sevcon controller had a timing advance setting, and I had to give it a fair bit of advance to get the best performance due to the low voltage I was running at.
That was just a static advance setting, so you can't go too far without killing the low rpm efficiency.
Ideally the controller should have an advance curve (ramp) to keep the motor happy over a wider rpm range.
No one seems to have done that though...

The Kelly controllers don't have advance control at all, which surprised me, but if you run the Mars motors on 72v they don't really seem to need it.
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Post by EV2Go » Fri, 05 Nov 2010, 02:41

Do eletric motor have an equivilant of pinging if you get to heavy handed with the advance? I assume some kind of arcing perhaps?

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Post by Electrocycle » Fri, 05 Nov 2010, 02:56

sort of,

if you go too far with the advance, the field rises before the armature is in the right position, and it pushes the wrong way - between two poles, and the motor is locked and will burn out.
If you advance 180° it'll run backwards :P

When you advance too far the motor will draw massive current at low rpm and won't have much starting torque.
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Post by EV2Go » Fri, 05 Nov 2010, 03:00

ok I'm with you the equivilant of kicking back on an ICE.

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Post by T1 Terry » Fri, 05 Nov 2010, 03:14

If the brush timing was retarded would the motor turn in reverse from stationary or become a generator if it was being driven on over run?

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Post by 7circle » Fri, 05 Nov 2010, 03:48

Electro Cycle are you taking about Brushed DC motors or Syncronous Motors like RC BLDC.

Brushless DC controllers need advance to cope with the incrorectly positioned hall effects and the delay in the hall pickup to turning on the next phased step of the 6 pulse bridge.

DC Brushed motors can't be advanced by the Controller.

Te Kostov and Warp type motors have a problem with arcing.
The brush advance is to move the brushes to time the when the inductive energy in the rotor coils that is carrying the current

Image

If motor doesn't have interpoles the nuetral plan rotates forward, advancing more with higher rotor current.
So to balance this a motor can dynamically advance the brushes, but this is too costly mechanically. So motor brushes are twisted forward to advance the brushes so at for a small motor load its not too advanced and when at max load the timing is not too advanced. A compromise is made.

Why does the brush arc? Its a bit like water hammer. The mass of the water has momentum. Just like there is momentum in the magnetic field.
Because there is a high voltage
Image

This next picture shows what's happening in a 4 pole like the Kostov.
Image

So all the above 2 pole info gets squeezed into 180 degrees and mirrored.

Got to go get Milk rfor coffee ....

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Post by Electrocycle » Fri, 05 Nov 2010, 03:52

if it's retarded enough it'll run backwards (ie close to 180°), but if you retard it a small amount the motor will just lose power and rpm.

Retarded timing would help in a generator situation I guess - but a normal series wound motor won't work as a generator anyway.
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Post by Electrocycle » Fri, 05 Nov 2010, 03:59

7circle wrote: Electro Cycle are you taking about Brushed DC motors or Syncronous Motors like RC BLDC.

Brushless DC controllers need advance to cope with the incrorectly positioned hall effects and the delay in the hall pickup to turning on the next phased step of the 6 pulse bridge.

DC Brushed motors can't be advanced by the Controller.



I was talking about brushless motors in the post about brushless motors, but the rest is about brushed motors.

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Post by 7circle » Fri, 05 Nov 2010, 05:03

T1 Terry wrote: If the brush timing was retarded would the motor turn in reverse from stationary or become a generator if it was being driven on over run?

T1 Terry


Retarding the brushes by 0 to 45 electrical degrees would not reverse the motor.

You would need to retard or advance past 90 degrees from the neutral plane.
At 90 degrees the current and the fluxes don't have a net interaction or have a resulting force on the rotor wires.

Understanding the armature reaction is what is important to get a grip on why interpoles are needed to reduce arcing.
The current flowing in the rotor bars/2-turn-coils creates its own magnetic field proportional to the current in the rotor.
Image

The current flowing in the B coil will not producing arcing if their is no net magnetic flux passing though it. As the armature reaction causes the neutral plane to shift, and move a net flux through the B coil. So when it disconnectes from the brush it has so much magnetic energy is will generate a large voltage, high enough to cause an ARC (30kV).

The interpoles solve the problem by opposing the armature reaction using the same current.

The span of rotation the field from the interpoles creates, only needs to be in the span where the brushes touch the commutating bars.

I hope this diagram helps. The red lines show how the field has been squeezed or pushed deeper into the rotor core neutralising the field in the span angle where the bushes (in line with the interpoles) commutate.
Image

I hope this helps, I think I've got it right.

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Post by EV2Go » Mon, 08 Nov 2010, 17:28

So the interpoles would work something like the condenser does in relation to the points, it acts as a buffer to absorb the shock (voltage spike) so the points don’t pit / brushes don’t arc?

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Post by woody » Mon, 08 Nov 2010, 17:34

No I don't think so - I think they are an electro-magnetic solution to the same problem which brush timing advance attempts to solve - the magnetic field takes a while to build up, so "Neutral" timing moves. In this regard I think they are more like vacuum advance or centrifugal advance.
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Post by EV2Go » Mon, 08 Nov 2010, 17:41

Ok I'm with you.

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Post by Electrocycle » Tue, 09 Nov 2010, 02:36

exactly like centrifugal advance :)
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Post by 7circle » Tue, 09 Nov 2010, 05:46

Hey, you found an analogy?
oh after pondering I'm not sure...
Because the increase in armature/brush current distorts field, that shifts the Neutral Plane, and so needs to be retarded or repelled, in the same proportion.

So the current that flows through the rotor also flows through the interpole/commutation windings.

I tend to think of this more like vacuum advance.
- more vacuum : due to faster air flow in carbie/inlet
- more armature reaction : due to larger armature current.

The arcing is a bit like exhaust flame out (not sure the right word ?? Back fire).
There is still fuel to be burned when the exhaust valves open.
This extra fuel is due to the spark firing to late and/or to much fuel/air.

So you try and make it all happen faster and earlier, you increase the octane, advance spark to burn more air-fuel mix, and lean up the air-fuel ratio.

The Interpoles effect is like making the burn time shorter, it's not really changing the brush timing. It's making sure the reaction is complete when the brushes need to change to the next commutator bar (Exhaust valve opening).

So instead of their being 180 degrees of field interaction it is reduced to say (there may 72 com'bars, so 180 - 1/72 x 360 =) 175 deg

This is a compromise.
I think mechanical brush advance would allow for stronger and more efficient field windings if there were no interpole windings in the way.
But more moving parts is more wear and servicing of moving parts.

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Post by EV2Go » Tue, 09 Nov 2010, 06:16

7circle wrote: I tend to think of this more like vacuum advance.
- more vacuum : due to faster air flow in carbie/inlet
- more armature reaction : due to larger armature current.
Except this is back to front Image
High vacuum is caused by pressure differential (caused by an almost closed butterfly) faster air flow in carbie (opening butterfly) increase air volume but reduces vacuum.
Also depends on which dizzy you use, as late model factory dizzy's used vacuum (or lack thereof) to retard the advance at higher revs. (vacuum port either side of diaphram)
But then again mechanical advance probably isn't the best of examples either, as it has to pass a given RPM point for centrifigul force to overcome the springs and move the weights.
Using dizzy advance as an example is probably best as just a "generic" reference.
7circle wrote: This is a compromise.
I think mechanical brush advance would allow for stronger and more efficient field windings if there were no interpole windings in the way.
But more moving parts is more wear and servicing of moving parts.
I don't think it would be all that complicated to make the brush advance mechanical. If you pull apart an old point type dizzy you would be amazed how simple the rotating mechanism is. Provided the brushes are mounted evenly, and the base plate is well lubricated, I think it could work fairly well.
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Post by EV2Go » Tue, 09 Nov 2010, 06:18

double posted
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Post by 7circle » Tue, 09 Nov 2010, 07:14

The Brushes in the Netgain or Kostove motors covers a large diameter and with the friction force of the brushes rubbing on the Comm'bars at 3000 rpm with E-M fields pulling them together the forces needed to advance the brushes is not small.

A little dizzy/distributor is a very different structure.

It needs to shift as quick as your right brick/foot wants more torque current.

This makes me wonder again about why the arcing is so large at high torque and rpm. Perhaps the Saturation and hysterisis is also causing the Neutral Plane to shift.

If the rotor core can't release the high magnetic field due to hysterisis or effects of storing the field, like iron becomes a permanant magnet, the field colapse is delayed and is proportional to RPM.

So the
Electrocycle wrote: exactly like centrifugal advance :)

Has more weight (tunza puns)

Bloody magic these motors, complicated blobs of metal.

Sorry EV2GO, the vacuum I think of is from fast air pulling in more fuel in the venturi, but injectors and such have made the vacuum level more related to the inverse of air flow like you mentioned.

I really haven't found any good explanations of how DC motors work at a high performance level. They really only go into it just beyond a basic (4rth year Electrical EngImage) level, not the Mad scientist theories I seem to be posting. Its no wonder the manufactures dare to go there.
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