Choosing faster motor or using field weakening ?

[Doctorbass]

When building an fast ebike there is many possibility offered.

A few years before field weakening was getting popular and available, when you wanted to get higher speed ebike this also was meaning loosing torque.

Now with that new feature you can choose a motor that is rated higher torque ( high turn count winding) but you can also get high speed with field weakening wich make a kind of timing advance with the hall sensor wich is like lowering the BEMF of the motor and increasing the Kv for a moment, this open the possibility to get faster ebike without the need of high voltage battery pack.

Creating a high speed ebike will also mean high power required. Your ebike wheel can turn 200kmh off the ground this does not mean the ebike is fast. In fact at high speed your ebike will need to fight against the wind wich require power ( sustained torque at high speed.)

The possibility for high speed ebike are:

-Choosing a higher voltage battery wich is more complicated and also the requirement higher voltage controller that cost more but you will be able to keep all the low speed torque of your motor without blowing controller..

-Choosing a higher speed low turn motor and running lower voltage, but this will require a monster controller capable of high phase current otherwise you will see your controller mosfet making firework.

Different fun:
High speed is fun as well and the powerband is larger and offer you to still feel acceleration even at high speed
High torque also offer a different pleasure with very fast acceleration at low speed allowing you to beat some cars on acceleration from a stop and feel all that force that pull you from your static state! but as soon as you reach 2/3 of your max speed you begin to feel that the fun was short and the lost of acceleration at higher speed cut your fun pretty quick.

As well it is possible to get a fast ebike that also have enough torque by choosing a motor for torque ( high turn winding) and using a controller with the field weakening feature to continue giving torque at higher speed, but that is not without sacrifice! Nothing is 100% perfect and that great feature will work at a cost of loss in efficiency. in fact you will burn crazy Wh/mile to get this high speed also possible.

The other way to get both torque and high speed is getting a fast motor and a high current controller. But to get your torque your controller will have to provide very high phase current witch usually make them to heat pretty fast. But your fast motor will remain ice cold.

Now with the question of High speed motor or field weakening, I would say it depend if your controller does have or not the field weakening, if your battery is high voltage enough ( maxed to your controller spec) and if you accept scarifying efficiency at high speed. Efficiency loss is less when choosing high speed motor and high current controller but the cost is higher.

As well.. do not forget that the trade off does exist :wink:

Doc

 

[gogo]

As well it is possible to get a fast ebike that also have enough torque by choosing a motor for torque ( high turn winding) and using a controller with the field weakening feature to continue giving torque at higher speed, but that is not without sacrifice! Nothing is 100% perfect and that great feature will work at a cost of loss in efficiency. in fact you will burn crazy Wh/mile to get this high speed also possible.

What percentage energy use increase is typical for field weakening with permanent magnet hubmotors? The context I'm interested in is a 750W limit output to the ground (after all losses) and using the field weakening during tailwind conditions. We get 30 MPH steady winds often enough in Iowa.

 

[John in CR]

when you wanted to get higher speed ebike this also was meaning loosing torque.

This myth has been proven incorrect.

 

[hydro-one]

neither, use more volts.

re feild weakening, i expect it to use alot of amps

 

[Doctorbass]

when you wanted to get higher speed ebike this also was meaning loosing torque.

This myth has been proven incorrect.

John, Please explain?

For the same battery current limit a 5305 will have more torque than a 5302.. check on the ebike simulator.

Doc

 

[teslanv]

I'll make the popcorn.

 

[NeilP]

I've got one of those field weakening /timing advance boards sitting here if any one wants one?

I never used it more than a few experiments with it.

[youtube]fPkwkEbuXno[/youtube]

[youtube]Fpr_o0zCAec[/youtube]

 

[John in CR]

when you wanted to get higher speed ebike this also was meaning loosing torque.

This myth has been proven incorrect.

John, Please explain?

For the same battery current limit a 5305 will have more torque than a 5302.. check on the ebike simulator.

Doc

You didn't say anything about using the same current, which is a ridiculous restriction anyway, since the faster wind motor can handle proportionately higher current. It's phase current you should really talk about when talking torque anyway. If you wanted to fix the current, then why not use a smaller lighter motor while you are at it, since as long as you aren't into saturation of either motor, then motors with the same Kv make the same torque with the same current.

 

[NeilP]

You didn't say anything about using the same current, which is a ridiculous restriction anyway,

popping in late here, but in a real world build it could be a restriction caused by physical space, not allowing a bigger controller, hence limiting current. Depends on the initial reason for the same current, maybe if that is the max current that is available on that particular controller. Just my quick thought, not having read the full thread.

 

[John in CR]

You didn't say anything about using the same current, which is a ridiculous restriction anyway,

popping in late here, but in a real world build it could be a restriction caused by physical space, not allowing a bigger controller, hence limiting current. Depends on the initial reason for the same current, maybe if that is the max current that is available on that particular controller. Just my quick thought, not having read the full thread.

I can appreciate that, especially since space is typically the biggest premium on our ebikes, but even using the same controller at the same voltage and settings, the phase current pushed through identical motors wound to different Kvs will be different. ie the faster wound motor has a lower resistance and inductance, so the controller will deliver more current (I don't know if this is true for the FOC sine wave controllers). This is why virtually everyone who has gone to the significant trouble of Delta/WYE or Parallel/Series switching for their motors ends up hugely disappointed, because they expect a big difference in torque but it ends up a small difference.

 

[Doctorbass]

I'll make the popcorn.

lol :wink:

 

[friendly1uk]

Doesn't field weakening require power? Making the question a play off between extra battery weight or extra motor weight.

Maybe I wasn't listening. I just glanced at the subject 25 years ago. It wasn't running with less power, it's using power to collapse fields quicker

 

[gogo]

when you wanted to get higher speed ebike this also was meaning loosing torque.

This myth has been proven incorrect.

John, Please explain?

For the same battery current limit a 5305 will have more torque than a 5302.. check on the ebike simulator.

Doc

The difference seen in the graph above was explained by Justin in this post:
Re: Science, Physics, Math, & Myth

…
Here for instance is a comparison with the slow wind H3525 and the fast wind H3540, using a controller with 50 mOhm combined mosfet + lead resistance. Clearly the actual torque output at low speeds is about 10% higher on the slow motor:

Now replace the controller on the Faster 3540 winding with one that has fatter gauge wire so that it's only 10mOhm of resistance, and you can see that the torque difference between the two systems almost completely vanishes

Hopefully that gives some clarity on why you do both in practice and on the simulator see more torque with the slower motors. It's not that the fast motors produce less torque, but as a system with the same controller and phase wiring gauge then no doubt the motor that draws fewer amps will have less losses outside the motor, and that means more total power flows into the hub.

To me, an apples to apples comparison means that when you select a low turn motor, you are running at a lower voltage and higher current in general, so your phase wire gauge, controller mosfet resistance etc. should scale down accordingly so that the external losses are the same. In that apples to apples sense, motor winding really makes no difference as the graph above shows. The blame for lower torque with the fast motor does not lay in the motor, but in the controller and external wiring.

 

[NeilP]

Doesn't field weakening require power? Making the question a play off between extra battery weight or extra motor weight.

Maybe I wasn't listening. I just glanced at the subject 25 years ago. It wasn't running with less power, it's using power to collapse fields quicker

I don't understand why it is called field weakening at all...maybe 'Field Weakening' and Timing advance' are two different things..maybe Field weakening is just the effect that advance the timing has.

I am not sure how it all works, but this is how I have been thinking about it to make any sense of it.

With Burties Timing adjuster board, what is happening is that the hall signals from the motor are intercepted, and rather than being passed straight through to the controller, they are 'advanced'' .
The field in the motor windings is created by the current through the phase wires, and the phase pulses from the controller are sent out exactly the same, just at a different time. so the field 'sent' from the controller is the same. but its timing in relation to the position of the stator and rotor is changed

Since the Timing advance board can not actually see in to the future to know when a signal is coming from the motor halls, then I guess there is some sort of 'shift' going on, so the hall signals are being delayed by exactly one pulse when they are being passed directly' straight thru' and when the timing is 'advanced' that delay is lessened, to give the appeareance that the timing is advanced.

Think of a car ignition system, as the engine speeds up, to the spark is fired earlier and earlier,it needs to be earlier since the motor is turngin faster. and the fuel mis taeks time to burn and expand. if the spark fired at the same time as the engine got faster, then it would get to the point that the fuel would be burnign too late to give real power. SO it is fired earlier to maximum expansion happens at the top dead centre or jsut after of the piston.

I have thought of this timing advance in the same way for these motors. As the motor gets faster, the pulse of current from the mosfets to each set of phase winding has to be happen earlier to allow the buidl up in the magnetic field to give maximum 'push ' against the fixed magnets in the rotor.

That may not be exactly how it does work, but it makes sense to me.

 

[d8veh]

Some controllers change the timing, but they don't advance it. They keep the FETs open longer. There used to be an explanation on the LuLii Shui site in the FAQ, but it's gone now. It only seems to work with hall sensor motors. The KU series controllers from BMSBattery could do it.

 

[Hugechainring]

What percentage energy use increase is typical for field weakening with permanent magnet hubmotors? The context I'm interested in is a 750W limit output to the ground (after all losses) and using the field weakening during tailwind conditions. We get 30 MPH steady winds often enough in Iowa.

The tailwind you are talking about will allow your motor to spin past it's top speed (as shown on the Grin simulator under normal conditions) on it's own. You can model the same effect by inputting negative slope (downhill) on the simulator. A more predictable way to do it is to lie to the simulator about the aerodynamics of the bike. Use the "Full Recumbent" setting. Your motor will spin faster on it's own to a point. Adding field weakening substantially cuts efficiency, creates heat. Usually not recommended for continuous use.

 

[friendly1uk]

The moment the power is switched on will become advanced, but the moment it switches off the same.
When moving from field to field, you can wait for the first to collapse or help it by switching in drains, or cancel it using power. The first thing we will see is normal controllers using different software for cancelling. In the future they will put in more fets and resistors to run the more efficient draining circuits. It won't be long, we just have to look interested.

25 years ago a lecturer was showing off his knowledge to these who finished there work first and needed entertaining. I probably overheard lol. I'm just not qualified to be talking about this. I can see the holes in it myself.

Whenever a motor coil is powered it's first job is cancelling out the remnants of previous fields. It is best to build a motor that gives time for this to happen naturally. Though you can force the situation.


I hope nobody marks my work lol

 

[Doctorbass]

Doesn't field weakening require power? Making the question a play off between extra battery weight or extra motor weight.

Maybe I wasn't listening. I just glanced at the subject 25 years ago. It wasn't running with less power, it's using power to collapse fields quicker

I don't understand why it is called field weakening at all...maybe 'Field Weakening' and Timing advance' are two different things..maybe Field weakening is just the effect that advance the timing has.

I am not sure how it all works, but this is how I have been thinking about it to make any sense of it.

With Burties Timing adjuster board, what is happening is that the hall signals from the motor are intercepted, and rather than being passed straight through to the controller, they are 'advanced'' .
The field in the motor windings is created by the current through the phase wires, and the phase pulses from the controller are sent out exactly the same, just at a different time. so the field 'sent' from the controller is the same. but its timing in relation to the position of the stator and rotor is changed

Since the Timing advance board can not actually see in to the future to know when a signal is coming from the motor halls, then I guess there is some sort of 'shift' going on, so the hall signals are being delayed by exactly one pulse when they are being passed directly' straight thru' and when the timing is 'advanced' that delay is lessened, to give the appeareance that the timing is advanced.

Think of a car ignition system, as the engine speeds up, to the spark is fired earlier and earlier,it needs to be earlier since the motor is turngin faster. and the fuel mis taeks time to burn and expand. if the spark fired at the same time as the engine got faster, then it would get to the point that the fuel would be burnign too late to give real power. SO it is fired earlier to maximum expansion happens at the top dead centre or jsut after of the piston.

I have thought of this timing advance in the same way for these motors. As the motor gets faster, the pulse of current from the mosfets to each set of phase winding has to be happen earlier to allow the buidl up in the magnetic field to give maximum 'push ' against the fixed magnets in the rotor.

That may not be exactly how it does work, but it makes sense to me.

That's exactly what i'm thinging too!

Why it is called weakening... ? what is becoming "weak" exacty and how? if not only a question of playing with timing between the magnetic field to the motor and hall detection

Maybe the true motor guru, Biff on electricmotorcycleforum ( and Ryan from zeromotorcycle) could answer! Luke is fairly advanced in this technology but he also needed help to program his Sevcon controller on the deatbike and get the right tuning.
maybe both could give us some advanced field weakening class

 

[cycborg]

Why it is called weakening... ? what is becoming "weak" exacty and how?

My understanding is that the RPM limit occurs because of the back EMF (which is proportional to RPM). So when Vbatt = IR + Vemf you can’t go any faster.

So with field weakening, the BEMF is reduced so that you can go beyond this limit. I don’t understand how this is done, but this is what the terminology means - the “field” that is “weakened” is the BEMF.

 

[Doctorbass]

I don’t understand how this is done, but this is what the terminology means - the “field” that is “weakened” is the BEMF.

That's the question i'm wondering.. we already know that the field weakening is to compensate for the BEMF that reach Vbatt so torque is reduced...

Doc

 

[major]

I don’t understand how this is done, but this is what the terminology means - the “field” that is “weakened” is the BEMF.

That's the question i'm wondering.. we already know that the field weakening is to compensate for the BEMF that reach Vbatt so torque is reduced...

Doc

The BEMF is proportional to the field, or actually to the flux produced by the field. In your motors, this field or flux comes from the magnets on the rotor (why it is called the field). The coils in the stator are the armature.

I come from a wound field motor background. Commutator DC motors have the armature as the rotor and the stator is the field holding steel poles wrapped with field coils. In these motors, it should be easy to see if you reduce the current in the field coils, you reduce the field and reduce the flux flowing thru the armature coils. This weakened field then reduces the BEMF in the armature coils.


For permanent magnet motors, you can't simply turn down the field of the magnets to get less flux thru the armature coils. For all intents and purposes, the flux produced by the magnets is constant. But you can by devious means alter the flux passing thru the armature coils. One method would be to physically displace the magnets from the armature perpendicular to the flux path (called mechanically field weakened). Another way is to alter the commutation of the armature coils in a way as to reduce the net flux thru the coil. If you were to rotate the commutation angle 180ºe, the torque would reverse because the flux thru the armature coils would reverse. If you rotate that angle 90ºe, torque is zero because equal flux in both directions pass thru the armature coils (so net flux is 0). At angles between 0 and 90ºe, the net flux thru the armature coils is less than at 0º.

You haven't actually weakened the field, but you have altered the effective position of the armature coils in that field such that the net flux thru those coils is less. And that reduces the BEMF.

 

[NeilP]

That's the question i'm wondering..
Doc

Phew ! if the Doc is not sure exactly how this works, I suddenly don't feel so bad :lol:

Another way is to alter the commutation of the armature coils

You haven't actually weakened the field, but you have altered the effective position of the armature coils in that field such that the net flux thru those coils is less. And that reduces the BEMF.

So is this what happens when we alter the 'firing time' of the MOSFETs by playing with the timing of the hall signals?

 

[madin88]

I guess its calld field weakening because torque will be lower at same current or more current is needed for same torque (thats because efficiency drops).
I see it like an electrical "pre ignition". The controller switches few degrees earlier to the next phase than usual. As example on adaptto the value is degree, on sabvoton additional amps can be set.

Many trapezoidal RC controllers also have such timing setting (nomally in the range of 0-30 degree). They have this setting to make the controller fit to the motor. Also infinion 120% setting must do some kind of this "pre ignition" but i'm not sure if this is something completely different here.. I believe it does roughly about the same what a FOC controller does, though it works better with a sine waveform than trapezoidal waveform.

 

[Doctorbass]

thanks major, this is perfectly clear now :wink:

When you talk about mechanical field weakening I see yo are talking about this old classic popular video right?:

[youtube]6_41btVawMc[/youtube]

Doc

 

[Doctorbass]

That's the question i'm wondering..
Doc

Phew ! if the Doc is not sure exactly how this works, I suddenly don't feel so bad :lol:
?

Ah! :lol: I never stop learning ! :wink: There is some things I feel really familiar with but some else that I'm not sure and I preffer asking! 8)