ERRATA

ERRATA: |Oops! Sorry guys. I got carried away with the usual in electronics to treat "k" as kilo. In brushless motors, the K as in Kv is not for kilo but for "constant". So when you have a 1100Kv motor, it simply means rpm per volt. So, if you are using 10 volts, an 1100Kv motor runs at 11,000 rpm. 10 x 1100. Forget the K.

Thanks,
BK

THE ESC

The ESC for brushed motors is purely that, a speed controller. But for a brushless motor ESC, it is first a "switcher" of battery polarities to the motor, then a speed controller. Speed controllers actually pulses the current with MOSFETS, the faster the pulsing, the more current is allowed to reach the motor. In this manner, no power is wasted as heat as what happened if you merely dropped the voltage/current with a resistor.

Modern ESCs for brushless motors can be programmed. Many newbies do not know this. I found a box thrown away which obviously was for a remote control airplane and guess what, the programming instruction was thrown with the box. Some of the things that can be programmed in the ESC are setting the throttle stick range, identifying what battery you are using, setting quick start or soft start, voltage cut off, current cut off, among others. Cut off problems are sometimes the result parameters set and there is nothing wrong with the ESC or motor or Batt. Related thio this topic is a motor which labeled 2S or 3S. It might as well be labeled up to 3S as there is nothing in the motor which automatically sets it for 2S or 3S. It might run ok with 3S and cuts off with 2S. It is premature to say the label is wrong. If you use a smaller prop, it just might run as well too but of course at a lower power output. Overprop also draws more current which triggers the cut-off when the threshold is reached.


to be continued.....

Ok friends, I am back. So let us review. It is E=IR, then power in watts is the product of V and I or V x A or simple VA. So, if you want more power, increase the V or the A or both. In the RC Car world, when you wanted to run the car faster, increase the cells. That would have been increasing the voltage. You have done the same thing by increasing the amps, especially if you were forbidden to add cells. To do this, you will remove some turns to lower the R. You will realize that we only have control over the V and the R really. The AMP draw will be the result.


The now popular KV measure is is really the RPM for every volt and K is just refering to kilo, which is 1000. They say V if to increase speed, A is to increase torque. Well put it this way. Take two similar motors with pinions turning a common spur. Or, join the shafts at two ends. Will you increase the speed. No, you will double the torque. Of course if they are the same motors and you were running them parallel(same cells or different cells), the current draw was doubled. See? double the current and you double the torque and not the speed. If you saw a speed increase, it was because load was halved on the first motor running singly. The RPM of motors is also the result of design like the number of poles and the spacing of the magnets.

When do you increase both V and A? Well, if you merely put more cells, the V went up and since the wires are the same, you have the same R. Current is Voltage/R. So, Amp draw will go up too.

LESSON: When you want more power, get a bigger motor because you can only stretch a small motor so far. You really can't make the Beetle a Benz by putting a Benz grill in front. When you want more power, you can increase the voltage but you have to increase the number of turns to keep the amp draw the same and keep the motor cool. . If you do not want to add turns, the Amp draw will go up and you have to fatten the wire. So, you see, it is putting more wires in there by either fattening it or lengthening it. Either way, you have to put it more bulk into the motor. Precisely, your motor will just have to be bigger to house the extra wires. A bigger diameter motor also add to torque. But since we can convert speed to torque and torque to speed, we can use a small diameter motor but longer and faster. Gear up a high speed motor and you can get torque out of speed. This is how it is done in helicopters. The motors used run up to 3800/V. Look at the gearing. It uses a large spur(wheel) turned by a pinion in the motor. That is why heli motors may not be suited for airplanes. The Blue Wonder brushles comes in 1100,1300, and 1500KV. Many will think the 1500 to be better. Not really, the 1100 turns the same prop size better.

Is stranded wires better than a single wire. Yes, they claim. That is why many motors use stranded wires instead of a single wire. Yet, in a forum, someone swore that the difference is minimal and rewinds motors using single wires. It has something to do with wires in strands has lower resistance than a single wire. The stranded wires tend to break more easily though. TIP:When you wind a motor or even for new ones, use "rugged" thread for maong pants to tie the 3 wires to the stationary case of the motor so you will pull on the thread and not the wires. By the way, before you rewind, dope the cores with resin. 5 mins epoxy will do. This will help to preven abrasion to the enamel coated copper wire and thus shorts to the casing.


P.S. I welcome any corrections from readers if you will just send it to my private email, rcdevices@yahoo.com.ph. For most, pls send your queries. You may request topics too.
Next topics will be ESCs, wiring configuration for multi motor airplanes(parallel or series). Thanks. Oops, errata!K IS NOT FOR KILO NOR FOR 1000. IT STANDS FOR CONSTANT. PLS DISREGARD ABOVE.

continuation MOTORS REWINDING

We talked of rewinding the RC car motor. Now, lets see what we usually did to customize our motor. We read of 27 turns stock wind, then we hear of 24 turns.
Everytime the number of turns is lowered, the RESISTANCE of the coil is lowered. Thus , if we were using the same battery pack or 8.4 volts, the OHMS law says this:

8.4V/lower resistance=Higher current draw

No wonder the run time got shorter. And there is a limit to how many turns you want to remove from your motor. The higher current will make it hotter and hotter. What should have been done. Well, lower the turns but fatten the wire. And the following discussion will explain it.

You have a 12 volts motor and say you are running it on 10 nicads of 1000 mah capacity. You series an ammeter and see that you are drawing 0.5amp at the load you have on it. You want it running on 6 volts. if you bring it to me, I will unwind the wires on each pole, halve it and put the two halves in parallel. Wind it back. That is now a 6 volts motor. You can run it on 5 cells but your running time will be halved cuz you doubled the wires and doubled the current draw. What is the solution? parallel the other 5 cells to your pack. You will end up with a 5S,2P battery pack. Did you lighten the pack in your airplane model for example. No! you simply reconfigured your battery! Of course you have now some flexibility cuz you can fly on 5S, land, switch to the another 5S. You did not have to carry a 10S. OBSERVE: You halved the V but you doubled the A. the power in VA or watts remained the same. How did you double the AMP draw? Well, you halved the turns right?thus you halved the R. But why did we have to double the wires. Answer:to carry the doubled amps draw.

If you bring to a rewinder a 110 volts charger, he unwinds the primary and puts in a wire half the size and double the length to make it 220v. How does he know how much thinner wire to buy. Just weight the original wire and buy half the size at the same weight and wind it back. He did not have to count the turns really. Now, if you were going back to the US and brings a 220 charger to be converted to 110V, he does not have to buy new wires. He unwinds the primary, halve the wire then parallel them. Wind it back, that is now 110v. your 110 charger draws double the current than the original 220v charger but don't worry. You are paying in watts which is VA. So the the VA for both remains the same. (110v is not cheaper to run).


This will take us to a relevant topic to our RC: WHICH IS BETTER, HIGHER VOLTAGE LOWER CURRENT OR LOWER VOLTAGE HIGHER CURRENT?

Answer: Utility companies(NAPOCOR ,MERALCO) transmit electricity to as much as 30,000 volts on those bare stranded cables high above. Why? So they can use smaller cables since for the same power(watts), they will be transmitting lower current.

During the vacuum tube era, the tube was a high voltage, low current device. For those of you afficionados and old enough, remember the 500volt power supply? An output transformer had to lower the voltage but with increased current to drive the speakers. The transistor, of which the MOSFET is one type, is a CURRENT device. However, this should nto be confused when designing an ESC. The muscle on an ESC are the MOSFETS which are rated in terms of V, A, and resistance when ON. Look at you ESC. We usually look for the AMP capability. Did you look for the Voltage it can handle? Why? If you want more power, get a higher voltage motor and use 4S or 6S. Power is VA and you can raise V or A to get a higher VA or watts. but , ah, you will say, you will need more cells. True but for the same run time, you need less capacity on the battery since the AMP draw will be less. Remember the above discussion in making a 12V motor into a 6V motor? If you are beyond park flyers and into giant electrics, the voltage used are high, 32 cells in series is usual. That's about 50 volts! So, next time you have a 12 volt motor, just used more cells but don't rewind it to six volts!


Now, how come, you might ask, your Brushless for 3S burned. It wasn't overvoltage. It was over current draw. A stalled motor draws more current than a spinning one. You were "overprop" and your motor was spinning slow. Now, you will see why gearing is good for your motor. You are like in first gear with your real car. The motor spins 4 times for every one for the prop. You have 4 times the torque(speed and torque are inversely proportional). You can put a bigger prop or a higher pitch prop or both without taxing your motor). (The lite Ace is a highly geared small engine. Do you hear its higher RPM?). But can we not use a smaller prop to let the motor spin and since its spins faster, don't we get the same trust? You are darn right if you follow the computation. But why is a bigger prop better? the best answer so far is that it is covering a wider arc of space!

to be continued again....

Of Current, Voltages, Resistance

There are three parameters in an electrical circuit. The CURRENT (I) is like the amount of water flowing. The unit measure is AMPERE. Next we have the EMF(E), or electromagnetic force. The unit measure is VOLTAGE. This is the equivalent of the pressure in a pipe pushing water. The third parameter is RESISTANCE (R) measured in OHMS. This is akin to the resistance of the pipe used. the smaller pipe has more resistance. So, we have E,I, R
. Like any measure, MAN, has to define a unit. The definition agreed upon is what is known as the OHMS LAW and the relatioonship is :

E=IR

How did they come up with this? Well, they said that that One Volt is pushing One Amp through a resistance of One Ohm. It means that if you have a One Volt battery lighting up a bulb, the definition is that the current is One Amp and the resistance is One Ohm. You see, they are not independent. The lower the resistance, the higher the current draw. This is the fundamental relationship. We want to mention an offshoot of these three parameters. The product of V and A is the power used in the circuit. It is called VA or WATTS.


With the above general introduction, we can perhaps now go into making motors or rewinding them.

Let us start with the POWER you want, in WATTS. You have to consider that the battery you want to use. Say you want to use 2S. That is 8.4 volts at peak. If you want 84 watts, you have to draw 10 amps from your battery. To do this, you have to compute for the resistance that you coils should have. Every size of copper wire has a resistance per foot or meter. At 8.4 volts and 10 amperes, R is 8.4/10 which is .84 OHMS. Then compute for the length of wires you need in your motor. You may put different size wires into your motor but the length will differ. The thicker wires has less resistance and thus will need to be more to get .84 OHMS. WE will end up choosing the sizes that will fit into the size motor we built. So much for this fundamental and let us go into rewinding motors.

Let's talk of brushed motors for the RC car. You have a stock motor running on six cells. At peak the voltage per cell is 1.4 x 6 or 8.4volts. Now let us say that this motor is drawing 10 amps. It is an 84 watts motor. If you want to double the power, double the voltage but you have to double the number of turns if you want to retain the 10 amps draw(which means 10 amps running through the wires). Now you have 16.8 volts x 10 amps=168 watts. What happens if you had just doubled the voltage? the R in there remained the same and your AMP draw will double! you will have double the V and double the A but your wires will burn cuz it cannot carry 10 amps. Say, you merely increased the V by 1.4V by adding one more cell, the motor wires might tolerate it but will be hot.

to be continued.........

Many Newbies and even Pros might want to understand the motors now powering their models. I hope this info here will be useful. Before we can explain the Brushless Motors, we have to understand the Brushed Motors.


Brushed Motors


For years, the world has been using brushed motors and naturally, that was what we got into our electrics. And usually, more often than not, the brushed motor is 3-pole. The 3 pole brushed motor is usually DELTA wound. Three coils are all in series and battery energy is inserted at two junctions at a time via commutators. One coil gets fully energized at the battery voltage and two coils are energized in series thus getting half energized at each pole. There are 3 segments in the commutator of which two are in contact to the battery and one idle. Two permanent magnets are used and if you study the magnetic force around the circumference, they are not uniform. The weakest point is the gap between the two magnets. This is where the stronger pole is positioned with the two weaker poles on the stronger area of the magnet. Electricity runs to the coils via the copper contacts which is the weak link in the circuit and where energy is lost. Thus a brushless motor is only about 60-70% efficient. The battery polarities are shifted by the commutators. If we can shift polarities outside of the motor, we do not need the commutator but only slip rings. Better, if we can fix the coils and let the magnets rotate,we do not need slip rings either. Efficiency can jump to 90%. Why wasn't it done before? Simply because fast switcing FETS came later. Now, say we can switch poles around three terminals of 3 coils in series(DELTA), when do we shift? in the beginning, a sensor sense a small magnet on the rotating shaft controlling the timing. Later, a small mirror in the shaft reflected light into a phototransistor to shift. Eventually, if we switch slowly and gradually increasing it to full speed, we do not need the sensor. That is what the ESC is doing today? Why three wires? many ask. Well, aren't there 3 segments in the commutator? If you had fixed the rotor, there will be one wire to each segment to which the three coil junctions are connected. Modify the housing with the PM in it to run on one end to a bushing or bearing, keeping one end open where the three wires are coming out. THAT IS YOUR BRUSHLESS MOTOR.

Now, we see multi poles in our brushless motors today and not 3 poles. Most have 12 poles! Well. it is like the radial piston engine. We can have 3 pistons or 12. In our cars with 4 pistons, they do not fire all at the same time. The timing is usually 1-3-4-2, firing one after another. If you put the firing schematic on a clock, you will have a pistion firing at 12 o' clock, 3, 6, and 9 o'clocks.
In electric motors, the poles all "fire" simultaneously. In a 12 pole motor,poles 12, 3, 6, and 9 become North poles, poles 1,4,7,10 become South poles, and poles 2,5,8, and 11 are North poles. If you look at them in a circle, they become N,S,N,S, etc..The permanent magnets around them are also arrange N,S,N,S, etc. So, if you switch slowly, all the poles with an S will align with the N magnets. All the N poles will align with the S magnets. But just when they are there, the poles are shifted, all the magnets will move to the next poles, thus rotating. Switch fast and the motors spins fast! Inrunners is when the magnets are rotating inside the coils and Outrunners is when the magnets are outside. Outrunners can be made to look as inrunners when a second housing is used, enabling the motor to have bearings on both ends of the shaft. So far, we discussed the DELTA wind since that is the the winding of a brushed motor where we started our discussion. What is the Y wind. The Y wind is where all the three coils start at a common junction. We will still have 3 terminals and the ESC will feed a + + - or a - - + on the three terminals. Two coils end up in series with one coil. The pole shift rotates the magnets.

Now, we have been talking 3 pole models all the time, how do we wind it on 12 poles. Well, we still have 3 coils actually except that we do not wind it on one pole but on four poles in series. We still end up with three terminals. It can be 3 poles, 6 poles, 9 poles, 12 poles, all a multiple of 3. There are motors(align) that have only 4 poles. The more poles the higher the torque. Its like having a 12 cylinder radial versus 4. It fires at every position hour(not necessarily at the same time).

Our next discussion will be current and voltage, number of turns and wire thickness. We will discuss battery discharge rates(10C, 20C, etc) and how it relates to your motor current draw.


BK