Multi-speed DC brushless motors?

[RichLeimbach]

I am currently using a brush-type DC motor to drive a relatively high inertia flywheel from 0 to 15K RPM in approx. .5 sec with a DC source (battery).
The problem I am running into is that with a single, always-on gear ratio, we spend too much time in the high current, low speed, low efficiency range (< 1/2 the max motor speed) and I'm having durability and heat related problems with the current motor. Electrically modulating the applied voltage does not help, as it just increases the time the motor spends in this inefficient range.
Besides finding a way to mechanically shift gear ratios, is there any way to increase the low-end efficiency of a DC motor? I am thinking of something along the lines of 2 speed AC induction motors, but in DC form. Any ideas?

 

[ve7brz]

Sorry Rich, I just came back to withdraw that notion. The approach I was following led me into the trap of looking only at the last instant of acceleration. I'll try not to let it happen again. (I'm so ashamed ~/).

Bruce

 

[ve7brz]

I got the smiley wrong (I can't do anything right today). I'll try again.

 

[RichLeimbach]

Bruce,
Happens to the best of us. By the way, the multi-speed auto-tranny from the RC car seems to be working. Right now it is shifting too soon (~2600 RPM) and not giving us the full benefit, but it looks like it will give us some efficiency improvements. I've had to dig back into the physics books to figure out how to change the shift point upwards, but we should be able to get this going. I'll let you know if we do actually see the 30% efficiency increase that was predicted next week, but now it is time to go out and have a beer and forget all about efficiency and motors and batteries. Have a good weekend. Thanks for your help.

 

[ve7brz]

Thats good news Rich.

As a suggestion, skip the C. shift mechanism and use a servo to accomplish the physical shift. Response time < .1 sec.. If you like I'll try to talk you into my constant mesh trany. It can easily use an externally triggered shift.

Bruce

 

[jbartos]

Suggestion to the original posting. If you are thinking of two speed AC motor in DC term, this DC motor would need either two different windings or two different amounts of poles. This is what I meant by customizing the motor characteristics. Apparently, it is still in the making.

 

[ve7brz]

Hi Rich,

Have't seen anything on this thread for a while. I'm curious as to how the two speed transmission turned out.

Thinking on the lines of the motor concept suggested on the other thread as well as multi-speed motors suggested on this thread, I'll toss out an old idea for a (miniature) wheel motor which I never pursued to the prototype stage.

The design was inspired by the ready availability of 6 to 20 VDC three phase AC controllers.

The concept is to wrap a permanent magnet (armature) linear motor into a circle, or wheel. Two forms are possible; with magnets on the rim of the wheel aligned radially or with the magnets aligned parallel to the wheel axis. Obviously, the radial alignment would require radially aligned stator coils, whereas the axially aligned magnets would provide much greater torque (and efficiency) using laminated iron 'U' shaped stator coils.

For this application you could distribute stator coils (in three's) around the periphery of the 'armature' at will (on a two to three spacing, obviously) within the constraints dictated by size.

This motor would have considerable torque at low RPM and might still offer reasonable efficiency at high RPM. Significant losses might be encountered at the controller at very high RPM. The control system could be designed to use only some of the stator coils at high RPM. If you wanted to become really creative you design your own controller which would only energize selected coils at selected times based on the back EMF from the other coils.

Just food for thought,

Bruce

 

[sancat]

Rich,
I did not looked at your problem from the electrical point of view. From what I remember from electrical machines, you should use a Series DC motor, with brushes in which the armature is in series with the rotor. That kind of motor has a very high torque at low speeds, which decreased with the increase of speed. This is the kind of motor that tramways used, and they were able to get electrical energy back from their kinetic energy.
I think that at this thread there should be more people who know best about this, and the way to get the electrical energy back. Their construction is similar to that used in small AC appliances, (blender, small drills, etc).
sancat

 

[jbartos]

Suggestion: The multi-speed motors can achieve the higher efficiency by the principle of their operation and associated characteristics rather than by changing electrical variables only.

 

[RichLeimbach]

Guys,
Sorry I haven't osted for a while. Experiments with the 2 speed tranny have been positive. We've seen efficiency increases of ~ 20% with the 2 stage gear system (based on a modified RC car trans). Because of the high speed that we need the shift to occur at (and the associated impact energies), we would need to redesign this significantly to increase durability of the production set-up. This is in the works.

Bruce... Interesting idea. not sure how I could fit all of that into the package size that I have, but I'll think about it some.

Sancat... Doesn't a DC series wound motor just replace the field that you get for free from the permanent magnets with one that you have to pay for (with electrical energy)? Maybe there is some hybrid permanent magnet / wound motor combo that could help us?

Jbartos... Not sure what you're getting at with your post. Can you expand on that?

 

[sancat]

Rich,
Well, I opened after 15 years my DC machines book, and this is what I found.
No it is not the same to use a magnet instead of a series winding. A permanent magnet will have the same behavior of a derivative winding.

The torque equation for a DC motor is
T = k . phi . I, (1)
where I is the current, phi is the armature field, and k is a constant.

The product phi.I balances with the torque, so a higher torque demands a higher intensity at a constant phi (a permanent magnet).

When the motor is in series, this current circulates through the armature windings too,
Phi = I . k3 (2)
reinforcing the field and therefore increasing the torque in a much higher factor.
However, on the other hand
V=E + I .r + other losses
where V is the applied voltage, and E is the counter electromotive voltage generated at the rotor, I is the current circulating, r is the total resistance of the motor, and other losses account for brush losses etc
And E= k2 . n . phi,
Where n is the motor speed and phi is the field again.
From there,
I = ( V - k2 . n . phi – other losses) / r
so each time the motor accelerates its current decreases,
And when the current decreases, it weakens the field (on 2) and thus decreases its torque with the second power (on 1).

I think you should not worry too much about a loss of power at the armature field since normally this would be a small percentage of the total power, because the main power is consumed producing work by means of this E counter electromotive voltage. The armature field is there just to help, but you need it to be very hard on the start, and weaker on the run. Indeed a series motor is very comfortable at 15k speeds, as any vacuum cleaner motor will tell you.
Well I am mechanic not electrical, so I hope I have not spoken too much sht
sancat

 

[jbartos]

Suggestion/comment. The motor set of parameters associated with a speed range given by winding connection has its efficiency range. Another motor set of parameters, of the same motor, at different speed range given by winding connection has its efficiency range that could be approximately close to the previous efficiency range. This would mean that the reasonable efficiency could be achieved by the winding connections that would discretely vary the motor speed.