High Voltage - Low Heat... always?

[dirty_d]

the best thing you could do to get best efficiency and at the same time have the best acceleration would be to use a computer controlled CVT, from a stop it would start from the lowest gear ratio and gradually shift higher as you speed up to keep the motor rpm at the peak power output to maximize acceleration, once you are up to cruising speed it would shift lower to keep the motor at the peak efficiency RPM, that would be a balance of acceleration and efficiency, if you just care about efficiency it could just keep the motor running at peak efficiency RPM all of the time. of course thats probably too complex and expensive for an electric bike but it would be cool.

 

[safe]

the best thing you could do to get best efficiency and at the same time have the best acceleration would be to use a computer controlled CVT, from a stop it would start from the lowest gear ratio and gradually shift higher as you speed up to keep the motor rpm at the peak power output to maximize acceleration, once you are up to cruising speed it would shift lower to keep the motor at the peak efficiency RPM, that would be a balance of acceleration and efficiency, if you just care about efficiency it could just keep the motor running at peak efficiency RPM all of the time. of course thats probably too complex and expensive for an electric bike but it would be cool.

That's the ideal. I don't think it would be that hard to do... you could program the computer to know what rpm to be in given different circumstances (like you said) and then simply have a tachometer and adjust the CVT to do what it should do.

Gears do a good job too... I know it's not "bleeding edge" technology, but it is well developed and there is something to be said for a technology that is fully refined. And people know how to use gears already, so there is an existing educational basis for people knowing what to do. We really don't know how well the CVT concept is going to work out. The NuVinci concept introduces a whole new technology (that strange liquid that turns into a sort of solid under the right circumstances) and so we need to see how that works out.

One observation I've made is that people involved in the electrical world are prone to wanting to "reinvent the wheel" all the time. One example is the hub motor which (though it's history actually goes way back) doesn't reflect the geared world that we live in today. When the hub motor was first invented (for automobiles) they were still fiddling around with the Model A or even earlier than that. Was it Ford or Porsche that was building hub motor prototypes before he went with the gasoline motor? I forget... one of those guys... I suspect it was Porsche...

Anyway... there's a lot of history to experiment with and this whole process is one of trying things and seeing what works... for now the regular old geared approach gets me "close enough" to perfection.

The rest will have to wait... :wink:

Update - Yes it was Porsche after all:

Porsche's first engineering training was electrical not internal combustion based. As a result he developed his first cars as electric cars with electric wheel hub motors that ran on batteries . The Lohner Porsche, fitted with one wheel motor in each of the front wheels, appeared at the World Exhibition in Paris in 1900 and created a sensation in the young automobile world. In the following years, 300 Lohne Porsches were made and sold to wealthy buyers.

http://en.wikipedia.org/wiki/User:A-Hrafn/Wheel_motor

 

[dirty_d]

perhaps an easier way to get the same effect would be to use a separately excited dc motor. instead of shifting gears you alter the field current to change the K constant of the motor. lower current gives you more rpms and less torque while more current gives you more torque and less rpms.

 

[cadstarsucks]

perhaps an easier way to get the same effect would be to use a separately excited dc motor. instead of shifting gears you alter the field current to change the K constant of the motor. lower current gives you more rpms and less torque while more current gives you more torque and less rpms.

What? Torque is proportional to current, period. Speed is proportional to voltage plus IR drop in the windings.

Looking at motor curves you can calculate the rotor resistance from the speed change verses the current change at a given voltage.

Dan

 

[dirty_d]

thats for a permanent magnet DC motor, in a separately excited motor torque is proportional to field current and rotor current and speed is proportional to voltage and field current.

 

[safe]

And you can do weird stuff like change the order that the windings are used. Some of the "psuedo geared" electric motors work like that where they shift from one sequence to another and that in effect changes the internal configuration of the motor.

For the simple DC brushed motor things are very simple. The only trick you can play is to use mechanical gears and then do the MCL (motor current limiting) concept to allow a higher current in the peak power area while suppressing the current in the low rpms. Such a setup "forces" the rider to react and downshift when the load is too high. The standard BCL (battery current limited) controller allows the user to load up the motor at low rpms and this is what actually burns them up.

The MCL solution is very narrow... it only applies to brushed DC motors (well brushless is possible too) and it's best used with gears and some degree of overvolting, overamping and forced air cooling in order to really exploit it's benefits.

MCL isn't for everyone... :wink:

 

[cadstarsucks]

thats for a permanent magnet DC motor, in a separately excited motor torque is proportional to field current and rotor current and speed is proportional to voltage and field current.

So you are essentially using car alternators as motors?

One of our customers uses them for brakes on self powered bikes, but this is the first time I heard of them used as motors. It is certainly feasible but not very efficient. Why not just use induction motors?

Dan

 

[fechter]

I think he's talking about separately excited brushed motors. These are used on many golf cars with a special controller that handles the field current. It has many advantages, but the peak efficiency is not as good as a permanent magnet or induction motor. The power that goes into the field winding is essentially all wasted.

They are much less expensive than a typical induction motor setup.

A car alternator is similar in some ways, but does not have a commutator and is a 3-phase AC winding.

Induction motors have a similar characteristic of having a wider useful rpm range than a permanent magnet motor.

Permanent magnet motors can extend their rpm range by advancing the timing. This would require a mechanism to change the timing dynamically while the motor is running. This apparently only extends the rpm range slightly based on Maxwell's experiments.

 

[cadstarsucks]

I think he's talking about separately excited brushed motors. These are used on many golf cars with a special controller that handles the field current. It has many advantages, but the peak efficiency is not as good as a permanent magnet or induction motor. The power that goes into the field winding is essentially all wasted.

They are much less expensive than a typical induction motor setup.

A car alternator is similar in some ways, but does not have a commutator and is a 3-phase AC winding.

Induction motors have a similar characteristic of having a wider useful rpm range than a permanent magnet motor.

Permanent magnet motors can extend their rpm range by advancing the timing. This would require a mechanism to change the timing dynamically while the motor is running. This apparently only extends the rpm range slightly based on Maxwell's experiments.

That might well be... I personally would have thought that 3 phase was beyond most hobbyists comfort zone.

Dan

 

[fechter]

That might well be... I personally would have thought that 3 phase was beyond most hobbyists comfort zone.

Dan

It's way easier to use one as a brake. Using a car alternator as a motor gets complicated.