Re: CIM speed in reverse?
 

I might as well add in something about variable speeds through variable voltage...some folks in R/C car racing have dealt with a Manual Speed Controller (MSC). Essentially, it's a servo with an arm that feeds power to the motor and a set of resistors, doing exactly what Al described (giving X amount of voltage to the motor, then the rest is dissipated through resistors that create heat.

I've personally never used one (the ones I dealt with seem to have a tendency to release the magic smoke, according to my sources), but the fact that you're feeding a fair amount of your battery power through resistors simply to make that excess power into heat is grossly inefficient. Dangerous, in some cases. (That, and having taken one out of a truck I bought off eBay, they're obscenely bulky. Seriously--if weight is the enemy, these things have pictures in all the post offices.)

Hence roughly 99.999% of hobby-level R/C cars (and all of the racing versions) use Electronic Speed Controllers.

Just figured I'd give a practical example--the tip jar is at the door. :D

Re: CIM speed in reverse?
 

Al,
Thanks for the offer, but I know how they work. I got bored one day and looked them up.
Furthermore: You are correct in saying that they don't put out DC, persay, but I wouldn't say they put out AC either. AC as most think of it is a voltage travelling from a plus voltage to a minus voltage, and back, all at a frequency. If you hook that up to a DC motor, sparks will fly. Guaranteed. In conclusion, AC motors are hard to vary speed to (they use different windings for different speeds.) Victors do put out PWM, or, as I like to think of it, modulated DC, and I am sorry for any confusion I may have caused. Also, barring the fact that there isn't much in the way of alternatives, I think it's a really stupid way to vary motor speeds. Like I said before, it can shorten motor life considerably.

Sparks

Re: CIM speed in reverse?
 

Sparks,

When you try to control voltage in the analog world, you end up dissipating a lot of power.

As an example, let's assume that you are delivering 3V at 20A to a load from a 12V source. The power dissipated in the load is 60W but the power delivered by the source in 240W. 180W of power is dissipated in the analog speed controller in the form of heat.

A perfect switch is either open (full voltage, no current) or closed (no voltage, full current) which makes the power dissipated across it 0W. Yet with a 25% duty cycle, it can deliver the 60W to the load with only 60W delivered by the source. Of course, a power semiconductor is not quite a perfect switch, but it is close.

Without semiconductor power electronics, one of our 130 pound robots (with the same power delivered to the motors) would discharge the Exide battery in less than a minute and the 6 AWG wires would have to be 1/0 or 2/0 wire. Also, the robot would weigh over 300 pounds (the controls and wire would weigh more than the mechanical systems).

In short, we could not do what we do in FIRST…

In reference to AC motors, a variable speed AC induction motor requires both amplitude and frequency modulation. Without power electronics, making a variable speed AC motor required a motor/generator pair with a variable speed transmission coupling… Very inefficient.

Now, let’s talk about the effect of a PWM wave on a motor. Yes, it has an effect. However, that effect is mitigated by the fact that the PWM frequencies are high and getting higher with every new generation of power semiconductor devices. The impact on a motor is a function of the harmonic frequency of the switching due to the inductance of the motor. This falls off at 20 db per decade. Bottom line, there is some effect but it is minimal.

Let’s look at it from the perspective of music. When the CD player came along, many of us unwittingly destroyed the speakers of our stereos because the dynamic response of the digital music was beyond the ability of the speakers. Would you say that digital music was stupid and we should still be playing vinyl record albums? I think not… The industry began making speakers which could withstand the new requirements.

Without power electronics, we would not have portable personal computers, alternative power co-generation and flat panel TV sets (not to mention space stations, shuttles and communications satellites). It is a basic enabling technology second only to the transistor in its impact on society.

Stupid? Not in MHO… If you can invent a better way of doing it, patent it. There should be a Nobel Prize in it and you will be wealthier than Dean within a year!

Regards,

Re: CIM speed in reverse?
 

Sparks,
In our real world, anything that is termed DC is considered a steady state or non varying electric potential. Any voltage that varies is called AC, that does not imply it is a sine wave, just that the potential alternates.
Please read Mike's post above as he very nicely explains analog and digital controls systems. As to hooking up AC to a DC motor creating sparks, most wired drill motors are termed AC/DC motors. By their very nature, the current flowing through the motor windings can be either DC or AC. In our case, the drill motors and other motors are all DC motors. We are pulsing that DC on and off at a predefined rate and since that rate is fairly fast, the laws that hold for AC circuits holds for our circuit analysis as well as DC laws. For instance, a scope looking at the output of the controller will display the signal either in DC or AC coupling. An AC voltmeter will measure a voltage where it will not measure a voltage when connected to the battery.

Re: CIM speed in reverse?
 

I forgot to mention this...Using a speed controller does not shorten motor life. The sparks you see inside a brush type DC motor are caused by the collapsing magnetic field in the armature. As the brush moves off the commutator segment and the coil goes open, the magnetic field set up by the current flow through the coil immediately starts to collapse. This "AC" (since it is changing/collapsing) flow induces a very high voltage and that is what you see arc across the brush and commutator. Higher loads produce higher currents which produce higher voltages when the brush circuit opens. Just look inside any drill while you are drilling holes. You will see the arcing inside.
This is the same method spark voltage is generated with an ignition coil, a high voltage pulse is generated when the distributor points or electronic switch open the current supplied to the ignition coil. Oddly enough, this arc produces electromagnetic energy and was used in the early days of radio for transmission. Anyone who has heard the story of the Titanic knows that a radio was in use and that transmitter was a spark gap device.

Re: CIM speed in reverse?
 

Al,
I am not suggesting that the sparking has anything to do with the motor life. If one pulses the power, you shorten motor life, for the same reason that turning a light on and off repeatedly shortens life: Going from no power to full power in a very short amount of time is damaging. For example: On an RC helicopter, a standard motor life is about 50-70 hours, using typical speed control. On a new helicopter that uses a gyroscopic swashplate (it achieves this by pulsing the motor at specific intervals), the motor only lasts about 10 hours. I think this is an example of how motors are damaged by pulses. That is why I like analog voltages.

Sparks

Re: CIM speed in reverse?
 

Sparks,

Why don't you explain how a "typical speed control" works...

Re: CIM speed in reverse?
 

Sparks,
I need to respond to this so others will not be misled. Pulsing power to a motor does not shorten motor life. If you were to analyze DC motors, you would see that the brush and commutator already are pulsing current to the motor as normal operation.
A filament in a light bulb is a far different device. The metal in the filament has more than a thousand degrees thermal difference between on and off. As this rapid temperature change occurs the tungsten rapidly contracts and expands and it is this flexing that eventually breaks through the material due to metal fatigue. There is a domino effect that cuts in once a fracture has occurred. The point where the filament has narrowed due to the fracture, now has a much higher resistance and the heating at that point is much higher than the rest of the filament. Higher temperature, more flexing=failure.
If you were to inspect the motors that are failing on your RC helicopter, I am sure you will find that they are being used well outside their design specifications. It may be higher loads, more voltage, or restricted airflow. If you were to inspect an RC servo, you would find that it contains a DC motor, geared to the output shaft, and a pot for feedback all connected and driven by a PWM motor driver.
If you were to inspect and analyze the DC brushless motor that drives the fan on the CPU inside you computer, you would find a circuit that pulses the coils of the motor to make it spin.
To repeat, speed controllers and PWM drives do not damage motors. The motors supplied in our kits coupled to the speed controllers under the right design conditions will not prematurely fail. Failure of motors on FRC robots are due to entirely different circumstances not related to drive, input voltage or PWM.

Re: CIM speed in reverse?
 

I'd like to take this chance to thank Al and Mike for their contributions to this thread. Lots of great advice/knowledge being shared.


Thanks again guys, we really appreciate what you're doing. ;)

John

Re: CIM speed in reverse?
 

I've had three motors fail in the course of my RC hobbies. Once was in an RC airplane where the (non-replacable) brushes wore down really far and the motor did not spin nice anymore, lots of friction. I suspect this is what is happening in your helicopter because these motors do have a brush life on them. The second time was in an RC truck where for whatever reason the armature got bent and was slightly hitting on the magnets. The third time was in an RC buggy where something got lodged between the armature and the magents causing the motor to sieze up.

While I did have some failures, none of them were caused by pulsing.

Re: CIM speed in reverse?
 

if you look at recent high end rc escs, they have insane high frequencies. and compared to that, well analog would be the analog speed controllers. not good things. first, just plain eat up the batteries. because the resistors use the juice while escs just shut off the flow. and about those motors that last ten hours? thats alot already. 50-70? if that helicopters motor isnt just crap from the beginning, and its a as-long-as-it-spins-its-ok kind of motor, thats incredible. wish my motors would keep their perfomance up that long. my rc car motors. suppose one battery pack lasts say 5 min. id probably notice a loss in power after about at a maximum 50 packs. which is 250minutes, around 4hrs. and thats a BIG loss in efficinecy by then. the brushes would probably be almost gone by then. (im talking around 19t motors with 7.2v) then ud need to change brushes, cut comm, w/e. pros do it like every pack. are u sure the 50-70 hrs isnt with all the maintenance? cutting coms, changing brushes, reoiling the bearings/bushings? because if you ignore all that, you will finish that motor real quickly. it will stop just like sanddrags airplane's did. most good motors are openupable(sorry cant think up the right word just now) for a good reason.

Re: CIM speed in reverse?
 

Okay, I'm convinced.
Pulsing a motor, at least at the frequencies we're talking, does no harm to the motor.
Thank you for correcting me.

Sparks