Motor - stated RPM, windings question?

[Bikes4two]

My understanding of BLDC theory is poor, so some clarification on a couple of motor parameters will be gratefully received.
1. My TSDZ2 for instance, has on its rating label '4000 RPM'
- what is this telling me? Is it that the motor gives its max torque at this RPM or something else?
- and if the motor is rotating at less than 4000 RPM, is this less efficient e.g. outputting something less than the max torque?
- conversely, what if the motor runs over 4000 RPM?
(4000 RPM on the TSDZ2 motor equates to a pedal cadence around 95 RPM)

2. Another parameter I haven't got a grasp on is when people talk about a motor's number of windings - can someone explain what that is all about please?

 

[Sturmey]

Grin has an article on (hub) motors that might help. The principles are the same. Your mid drive motor is much smaller because it runs much faster and uses the 2 stage internal gears to multiply the torque by approx 40.

Hub Motors - Learn

This page is a bit outdated from 2007-2009, but it has a lot of good information about Direct Drive and Geared hub motors. Other more recent information about our current line of motors can be found on our <a href="getting-started/hub-motor-options.html">Hub motor Options page</a> and why we...

ebikes.ca

Below is max power/torque curve of the 48v TSDZ2 measured after the gear reduction. Notice how power tapers down as motor reaches full speed. Also notice that max power (blue line) is between 51-78 rpm. But notice the red line is torque but this is also proportional to motor current. Motor current drops as rpm increases (due to motor back voltage/emf) so the optimal (most efficient) cadence/rpm of this motor is 78 rpm and above when motor is at full power. Not so important at lower power as controller reduces current)
(Link above discusses this (80%)

 

[Bikes4two]

Thanks for the graph and link @Sturmey - I found the Grin piece on BLDC motors rather beyound my understanding - I'll look for something simpler me thinks .

The graph is more easily understood - what is the source for this please?

As an 'unassisted' rider of many years, I use my derailleurs extensively (on the TSDZ2 bike) and I would gues my normal cadence is around 60 +/- 10 RPM for most of the time and I ride in ECO also for most of the time. Looking at that graph explains why I get such good mileage out of my 10Ah battery (around 60+ miles - that's an estimate based on my typical 40 mile rides having monitored amp hours used through a power meter).

 

[soundwave]

hi speed motors want more rpm like those 3kw+ direct drive ones tho that's fine if all you want is top speed but will be pretty useless climbing steep hills.

 

[Sturmey]

The TSDZ2 seems to have a small but very dedicated following who are prepared to go to all lengths in terms of both hardware and software modifications to perfect this motor for there needs.
On the other hand, some people seem to hate this motor. Personally, I think its all about choice and what suits your requirements, level of fitness etc. Anyhow, a guy on the Polish forum with a dynamometer came up with this graph.(use Google translate).

https://pojazdyelektryczne.org/viewtopic.php?t=2599&start=500

 

[Nealh]

The 4000rpm is how fast the motor armature can run (the motor in the tsdz2 is very small), internally the reduction gearing is approx. 42.1 giving the final rider cadence /rpm of about 95max. The high 42.1 gear reduction means the motor is quite easy to pedal wth no power on the tsdz2 which can't be said of the BBS models with serious blood busting efforts.

 

[soundwave]

 

[Bikes4two]

I'm always suspicious of folks who wear their caps backwards and I'd no intention of watching 22 mins of his prattle.

So I'm wondering what might have been in that video that went to answering the questions I raised in this post - anyone?

Thanks to those who replied to my original questions and I'm ploughing through the links (and what they lead to).

I'm coming around to an idea that Tong Sheng limit the RPM to 4000 (pedal cadence of 92 ish) as going beyond that the motor cuts out because the motor reaps no particular benefit, in terms of assistance after this point, (now into the realms of flux density, back emf, field weakening etc - but I need to be an a dark room to get to grips with all of that!).

There's an interesting vid on YT by jbalatutube here where he demonstrates the max cadence on the OEM software (92 ish) versus achieving a higher cadence of around 115 (with assistance) using OSF. Now whether the higher cadence (and hence motor RPM) brings any great advantages in terms of power/ torque is not clear. (And I don't want to pedal anywhere near that cadence anyway).

There's lots of chatter on Endless Shere about the whole subject but I'll need that dark room again.

 

[matthewslack]

I'm always suspicious of folks who wear their caps backwards and I'd no intention of watching 22 mins of his prattle.

So I'm wondering what might have been in that video that went to answering the questions I raised in this post - anyone?

Thanks to those who replied to my original questions and I'm ploughing through the links (and what they lead to).

I'm coming around to an idea that Tong Sheng limit the RPM to 4000 (pedal cadence of 92 ish) as going beyond that the motor cuts out because the motor reaps no particular benefit, in terms of assistance after this point, (now into the realms of flux density, back emf, field weakening etc - but I need to be an a dark room to get to grips with all of that!).

There's an interesting vid on YT by jbalatutube here where he demonstrates the max cadence on the OEM software (92 ish) versus achieving a higher cadence of around 115 (with assistance) using OSF. Now whether the higher cadence (and hence motor RPM) brings any great advantages in terms of power/ torque is not clear. (And I don't want to pedal anywhere near that cadence anyway).

There's lots of chatter on Endless Shere about the whole subject but I'll need that dark room again.

There are some vast simplifications that might help, but for full understanding you'll need that darkened room.

No load rpm for example. For a given voltage and with a standard controller i.e. without those fancy tweaks you mentioned, just basic commutation, at no load rpm the losses equal the output power, efficiency drops to zero, and so the motor can't go any faster.

Apply higher or lower voltage and more or less linearly the no load rpm goes higher or lower too. If you plot no load rpm against voltage the slope of the line gives the parameter 'rpm per volt'. In radio control and drone circles, this is given the symbol kV.

For a given magnet and stator configuration, the number of turns of wire in the coil around each stator pole affects the no load rpm. This is what is different between the 'fast, 'standard' and 'slow' versions of the same motor. More turns for a slower motor. Why? That's a darkened room question!

When load is applied, speed drops below no load rpm, and there is more output power than losses, so motor becomes useful. There is a limited range of rpm in which efficiency is high, and in general efficiency is higher at lower loads.

If Tongsheng actively limit rpm rather than the motor approaching no load rpm at 4000, it will be because there is no advantage. There is no point operating on the steep down slope on the right hand side of the efficiency curve.

Much as I would like a deeper understanding, I'm content just knowing that there is a useful rpm range where efficiency is high, and working out how to stay there as much as possible.

 

[Sturmey]

It all comes down to Faraday's formula for induction.

https://en.wikipedia.org/wiki/Faraday%27s_law_of_induction

But I can make it simple.
The electric motor acts like a generator when its in motion. It generates a voltage often called a back emf (electro motive force) that opposes the battery voltage and this limits the speed. As per Faradays law, this voltage is proportional to (1) The motor speed, (2) The number of turns and (3) The magnetic flux (caused by the permanent magnets in our case.
So we have a very basic formula (take equal symbol to mean proportional)

Voltage = Speed X Turns X Flux.......if we rearrange the formula we get

Speed = Voltage X 1/Turns X 1/Flux.

Therefore to increase the speed of a given motor, we have 3 choices according to Faraday's Law
1. Increase the Voltage.
2. Decrease the number of Turns.
3. Decrease the Flux. e.g Flux weakening

 

[Bikes4two]

Some great info there @Sturmey - thanks. I've now re-read the Grin tchnology piece and I sort of understand it and your last couple of posts have helped too.



And I can't remember where I read it now but as a rule of thumb the article said that 80% of nominal RPM was generally the most efficient area of operation (4000RPM and 80% divided by 42.1 gives a pedal cadence of 76RPM) which fits in well with the graph you posted up-thread.

Now where's that dark room...........

 

[sjpt]

The simulator at https://ebikes.ca/tools/simulator.html may be of interest to show the practical results of the theory on a variety of motors under different conditions.

It appears to be related to GRIN, I'm not quite sure what the relationship is.