# The Roles of Magnets, Voltage and Amps



## Grandcheapskate (Jan 5, 2006)

Electricity has never been my strong suit. No matter how hard I study, it just doesn't seem to sink in. I understand why pure DC power is better for the armatures and why they would get hot when there's a lot of AC ripple. What I'd like to have explained is the roles that magnet strength, voltage amd amperage play in the working of an electric motor.

Why does a lack of sufficient amps cause an armature to run hotter?

When are strong magnets a bad thing?

Does voltage give you speed and amps give you torque?

I know the answers are going to give me a headache, so I've got the aspirin handy.

Thanks...Joe


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## rc_jeff (Jun 18, 2009)

lack of amps causes heat build up due to the straining to move the mass.
Strong magnets could cause a drag, lower magnets strength easier roll.
think of amps as nailing the gas pedel and voltage as nos

simple yet effective


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## Dyno Dom (May 26, 2007)

Amperage is amount of available electrical strength. ( current )
As Jeff stated, insufficient amperage is a problem. Excessive amperage isn't.
If a slot car only needs to draw 1/2 an amp, having a 10 power supply does not affect performance. The car will only draw 1/2 amp. Raising voltage will
make the car faster because potential is increased.


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## AfxToo (Aug 29, 2003)

*Why does a lack of sufficient amps cause an armature to run hotter?*

The DC motors that we are dealing with in slot cars are self speed regulating. When you set the voltage to a particular value, the motor will assume a particular speed. When the motor is at a fixed speed there is very little current flow in the armature. Very little current flow in the arm means less heat. 

When motors are turning they generate a voltage proportional to the speed of the motor. The generated voltage is the opposite polarity of the applied voltage from the power supply and cancels it out. Therefore the only current that is flowing in the arm when it is running is what is needed to overcome the electrical and mechanical losses in the motor and the load placed on the motor. 

So what happens when an additional load is placed on the motor after it is running at steady state? First, the motor slows down due to the load. Second, because it is running slower it is generating less voltage to offset the applied voltage. This causes more current to flow in the armature. More current equals more heat. However, as long as there is additional current available, the current increase causes and increase in motor torque and the motor goes back to operating at the same speed it was at before the load increased. Remember, these motors are self speed regulating.

Once back to the set speed, the generated voltage again helps counter the applied voltage so the current flow is again decreased. However since the load has increased there will still be an increase in current flow and power consumption in the motor due to due the additional load. 

So what happens if the power supply cannot supply the required current to overcome the increased load? The motor slows down as more load is applied. Does this equate to the statement that insufficient current capacity from the power supply will cause a motor to run hotter? I would not draw that conclusion. I'd say that too little current will cause the car not to run at all, or run too slowly. Too little current will place a hard limit on the maximum current the motor can draw, which will limit power consumption and heat generation. If you want to maintain speed under high loads, you'll need plenty of current and a motor that can handle all that current.

*When are strong magnets a bad thing?*

I assume you mean strong motor magnets. At the extreme, I suppose the motor magnets can be so strong the arm cannot generate sufficient torque to rotate. Moving to a lower resistance winding to get more current flow through the arm, thus more torque, would solve the problem. In general, for regular ranges of magnetic strength and motor operation, strong magnets would be a bad thing if they result in an operational pattern that you are not comfortable with. Stronger magnets give greater acceleration and stronger braking. If this is something you do not want for a particular track or racing situation, then this would be a bad thing.

*Does voltage give you speed and amps give you torque?*

Yes. But the amount of torque will only be what is needed to maintain the desired speed with a given load.


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## bemoore (Dec 23, 2008)

I've never done an investigation to see exactly what happens with insufficient current, but my guess is that when you don't have enough current, the AC ripple goes way up, and we all know that AC ripple will cause excessive heat.


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## AfxToo (Aug 29, 2003)

AC ripple is a function of the rectifier design and filtering on the output of the power supply, not the current output. These tiny little slot car motors have very little inductance and torque. They also have no provisions for addressing armature reactance and reaction so the commutation efficiency is not all that great to begin with. I suspect the effects of AC ripple on HO motors is far less pronounced than some people would want you to believe.


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## Grandcheapskate (Jan 5, 2006)

I knew this topic was going to give me a headache....


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## bemoore (Dec 23, 2008)

AfxToo said:


> AC ripple is a function of the rectifier design and filtering on the output of the power supply, not the current output. These tiny little slot car motors have very little inductance and torque. They also have no provisions for addressing armature reactance and reaction so the commutation efficiency is not all that great to begin with. I suspect the effects of AC ripple on HO motors is far less pronounced than some people would want you to believe.


You're right about the ripple being "a function of the rectifier design and filtering on the output of the power supply", but you're wrong about the effect of current. When a car doesn't have enough current, you're saying that you are exceeding the current output capacity of the supply, at which point the supply is no longer properly regulating the voltage. You will then get significant ripple from the input rectifier.

As for the effect of ripple, I must admit that I have never really verified the effects of AC on motor temps myself, but I have seen credible evidence from trustworthy people that the effect is real.


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## NTxSlotCars (May 27, 2008)

All this ripple talk is making me think of Ruffles for some reason, which is making me extremely hungry!


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## Grandcheapskate (Jan 5, 2006)

All right, let me ask this question. I've just taken two aspirin.

Why would you want an armature with a higher resistance? For example, the old 440-x2 Tyco armatures were what, 12 ohms? The later Mattel battery chassis were about 4-6 ohms, if I remember correctly. On the same voltage, the lower ohm armature will go faster, but could/will burn out on higher voltages. So, if a low arm armature gets the same speed as a high ohm armature, yet can do it on less voltage, what is the advantage of the higher ohm arm?


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## AfxToo (Aug 29, 2003)

> When a car doesn't have enough current, you're saying that you are exceeding the current output capacity of the supply, at which point the supply is no longer properly regulating the voltage. You will then get significant ripple from the input rectifier.


I don't see any mention of loss of regulation. 

What happens to a power supply when you exceed its maximum current rating depends on the design of the power supply. There are a number of different design strategies employed to deal with excessive current draw including fold-back current limiting, constant current limiting, voltage clamping, etc. These are primarily intended to deal with dead shorts or near dead shorts across the output terminals. The behavior of the voltage regulation circuitry under heavy loads is also dependent on the design of the voltage regulator. There is no blanket statement that could be made that would apply to all regulated power supply designs.


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## rc_jeff (Jun 18, 2009)

AfxToo said:


> I don't see any mention of loss of regulation.
> 
> What happens to a power supply when you exceed its maximum current rating depends on the design of the power supply. There are a number of different design strategies employed to deal with excessive current draw including fold-back current limiting, constant current limiting, voltage clamping, etc. These are primarily intended to deal with dead shorts or near dead shorts across the output terminals. The behavior of the voltage regulation circuitry under heavy loads is also dependent on the design of the voltage regulator. There is no blanket statement that could be made that would apply to all regulated power supply designs.


Also, some have used un-regulated power supplies. The ripple current could be more pronounced, but the current has "no limit" within the reasonable voltage range of the supply. But when you over drive a Un-regulated supply the current doesn't seem to tail off as much as on my regulated supply. Which is why you see more unregulated PS units at a drag meet.


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## rogerhd1 (Oct 23, 2009)

*UNrelated to this topic*

Hey GrandCheapSkate,
Do you still have access to the Lifelike renegade Racer sprint cars or have any available for sale? You can reach me at rogermmm1 then at then ya then hoo if you'd like.
Thanks,
Rog


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## ParkRNDL (Mar 20, 2002)

Grandcheapskate said:


> All right, let me ask this question. I've just taken two aspirin.
> 
> Why would you want an armature with a higher resistance? For example, the old 440-x2 Tyco armatures were what, 12 ohms? The later Mattel battery chassis were about 4-6 ohms, if I remember correctly. On the same voltage, the lower ohm armature will go faster, but could/will burn out on higher voltages. So, if a low arm armature gets the same speed as a high ohm armature, yet can do it on less voltage, what is the advantage of the higher ohm arm?


my guess: durability/reliability/drivability. using what i know from pancake chassis: low-ohm Mean Greens (7-8 ohms or less, I think) in original Aurora Tuff Ones ran hot (see Nuclear Meltdown thread). higher-ohm "stock" gray lam/red wire arms (more like 15 ohms) run all week and never get more than warm, as long as you put a drop of oil on the bottom of the arm shaft every so often.

also, with the right controller, you can "drive" the higher-ohm arm more...more part-throttle control thru the twisties. with a low ohm arm and a high-ohm controller, you have to use like the last 1/4 of your trigger/throttle travel to keep the car moving. you can make this better with a lower-ohm controller, but then the car seems kinda "twitchy". i find it hard to get a nice smooth drive with Aurora TOs (low ohm arm), but Tjets (high ohm arm) are no problem.

i acknowledge that maybe that's because i have the wrong equipment or i'm doing it wrong, feel free to set me straight. MOO. YMMV.

--rick


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## AfxToo (Aug 29, 2003)

> what is the advantage of the higher ohm arm?


That's not a correct assumption, but I think I can explain this in non aspirin inducing terms...

This is kind of a trick question because we are usually not simply comparing the difference in armature resistances for the types of motors you are probably thinking about, say a 15 ohm standard TJet arm versus a 6.5 ohm TuffOnes or A/FX arm. If the only difference between two otherwise identical motors is the armature resistance, there would be absolutely no benefit to the higher resistance armature unless you were trying to go slower with a less efficient armature.

To understand what's happening you need to look at the torque equation for a permanent magnet DC motor. A simplified version of this equation is:

T = Ka x I

where T is torque, I is armature current, and Ka is something called the "armature constant." Putting Ka aside for now, this equation tells us that the motor torque is proportional to current. You already know that the armature current is inversely proportional to armature resistance (Ohm's Law), so a motor with lower resistance will have higher current and therefore greater torque. That's why I said there would be no benefit and only a detriment to having "two otherwise identical motors" where one had a higher armature resistance. The higher resistance arm would suck. If you were to plot the torque of each motor for a given armature voltage, the higher resistance arm would always be significantly lower. The shape of the curves would be the same, but the magnitude of the torque would always be lower for the high resistance arm.

But, and there's always a but, what about Ka, the armature constant? That's where we have to be careful about what we are comparing because that's that's where things get interesting. The two arms I mentioned earlier, the TJet 15.0 ohm and the Tuffy 6.5 ohm, well, they are not exactly the same. What's different between the two is much more than the resistance, it's also the Ka. These two motors have different armature constants. Because they have different armature constants the shape of the torque versus voltage curves between the two motors would be different because the Ka factors are different. The magnitudes would also be different, but because of the different resistances.

So what is the armature constant? Without getting into too much detail, it's the electromagnetic force constant, or something that describes how good of an electromagnet the armature pole is. What's the biggest variable in figuring out the electromagnetic force constant, or Ka? The number of turns of wire on the armature pole. The more turns, the better the Ka. 

So... when you are comparing a 15 ohm TJet arm to a 6.5 ohm Tuffy arm, you are comparing two arms that were designed with two different strategies in mind. The TJet arm is designed for a higher armature constant Ka, which yields greater torque at lower controller voltages, and the Tuffy arm was designed to give up some of the armature constant Ka (fewer turns of wire) but deliver more torque by upping the armature current by reducing the armature resistance. Why did the motor designers choose the strategies they chose? Perhaps the power supplies available for use in toys at the time of the TJets inception were expected to be run with less current available or maybe they just hit on a formula that worked well and they liked the results. They chose wisely, at least for home race set use. If you really wanted to see the difference between a TJet arm and a Tuffy arm you could develop some graphs that show the torque curves. Or you could slap the arms in a car and see how it all plays out in the real world. The empirical results would obviously support the calculated ones.

The ideal case of course would be to have an armature with a high armature constant and a low resistance. This sounds trivial enough, but in reality the mechanical design of the armature and the amount of space available on the armature poles presents a hard limit. Lower resistance windings are physically larger so stuffing a lot of turns on to the pole is more difficult. Using lower resistance winding conductors, say silver wire, to keep the size small would theoretically help, but the thermal properties and cost issues would come into play. Engineering is all about solving real problems and providing real solutions that apply to the real world. The design of motors both large and small must be amenable to engineered solutions and all engineered solutions represent a compromise between multiple possibly conflicting requirements and concerns.


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## bemoore (Dec 23, 2008)

AfxToo said:


> I don't see any mention of loss of regulation.
> 
> What happens to a power supply when you exceed its maximum current rating depends on the design of the power supply. There are a number of different design strategies employed to deal with excessive current draw including fold-back current limiting, constant current limiting, voltage clamping, etc. These are primarily intended to deal with dead shorts or near dead shorts across the output terminals. The behavior of the voltage regulation circuitry under heavy loads is also dependent on the design of the voltage regulator. There is no blanket statement that could be made that would apply to all regulated power supply designs.


When the current demand of the car exceeds the limit of the supply, the voltage drops. By definition, it is no longer in regulation. Most well regulated, current clamped supplies provide plenty of power for HO cars. The problem of lack of current capacity is more typical of a cheap regulated supply, or an unregulated supply. Every power supply is designed to a set of specs. When you exceed the current limit, the mfr will no longer guarantee the other specs. Usually, ripple is the first spec to go.

And again, I make no claims to how much heat is generated from additional ripple.


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## Slott V (Feb 3, 2005)

*resistance*

Plenty of good technical information here, but for the average layman it can be confusing.

Lets try this:
DC voltage uses a positive and negative connection. I think all of us have seen what happens if you cross those connections; you get a 'short'. With a slot car you are basically shorting those connections to make the motor turn. The amount of wire inside the motor is very long so the short is spread out over all that wire. Any time you cause a short you create heat from the resistance. The ‘short’ is resisting being controlled by all the wire. If you shorten that wire you get much more resistance, therefore much more heat. Ohms is a term for measuring resistance. That is how an electric armature is measured.

As the electric motor is energized it rotates and releases and regains the resistance numerous times. If you hold the car from moving, all that resistance builds up in the wire of the motor, thereby creating heat. The same happens in your controller.

Magnets:
If you take any 2 magnets and place them near each other they will move to join poles. Opposite poles on each end of the magnets cause the magnets to orient themselves to attract. It’s the same function in an electric motor except one magnet is fixed. When the coils of wire on the armature are energized they also create magnetism, with one end being the positive pole and the other becoming negative. This forces the motor to turn due to the fixed magnets next to the armature. The opposing poles force the motor to turn briefly until it loses power to that coil of wire. Then the next coil of wires gets energized. The commutator is what determines this ‘timing’ and causes your electricity to jump from coil to coil as the armature turns, creating a continuous rotating movement.

Everything creates resistance in electrical paths. The greater the resistance the lesser the voltage at the end of the path. The longer your track is, the more resistance you create in the electrical path. That’s why long tracks get slow at the far end. The principle of resistance is how you control your slot cars. A controller is a device for controlling how much resistance is in the electrical path to the car/motor.


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## AfxToo (Aug 29, 2003)

> The principle of resistance is how you control your slot cars. A controller is a device for controlling how much resistance is in the electrical path to the car/motor.


This is only true for resistor style controllers, i.e., set controllers and Parma and the net result to the motor is a variable current source. The downfall with this style controller is the fact that controller reacts to the current draw of the motor. This causes the controller to drop more voltage as current increases. This is why you then have to pull the trigger more to get a high current car (lower ohm arm and/or heavy magnets) moving with a resistor controller or go to a lower ohm controller.

Diode style controllers like PM and Omni control _voltage_ directly using additive semiconductor junctions to create discrete, linear voltage steps. Each junction drops a fixed voltage. As you pull the throttle you remove more semiconductor junctions from the circuit which delivers more voltage to the motor. The big difference here is that the controller itself is no longer part of the current equation because the semiconductor junction voltage drops are independent of current (within the operating range of the junction which is why these controllers have junctions rated for tens of amps or hundreds of amps). The fact that the controller is current independent allows the same controller to be used with a much wider range of cars and motors. 

Transistor amplifier style controllers, Difalco, Lucky Bob, etc., control voltage by using a simple voltage amplifier. The amplification, or gain, is determined by bias resistors, one for each band and are additive. The output voltage steps are typically linear since the bias resistors are all the same for each band. Fancier units like the Difalco Fanatic use potentiometers for the bias resistance for each band, so the steps can be adjusted independently. These controllers also contain a sensitivity potentiometer which changes the base level gain of the entire amplifier. This moves the starting or reference point up or down. With sensitivity the net gain is a result of the gain affected by the sensitivity pot plus the gain affected by the bias resistors for each band. 

While not seen in HO very often, the other class of electronic controller is the pulse train or pulse width modulated (PWM) controller. This style of controller controls voltage by putting out a series of voltage pulses. The relationship of ON time to OFF time, or duty cycle, of the pulse train determines the average voltage seen by the motor. For you ripple freaks this would seem counterintuitive because sharp pulses would tend to generate high frequency harmonics. However, different pulse shapes can be used to reduce the effect, but more importantly, motors are inductors and inductors are low pass filters. The inductance of the motor helps to smooth the PWM into something closer to DC.


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## Grandcheapskate (Jan 5, 2006)

I was taught that electricity is like water in a pipe or a hose.  Amps is equivilant to the water going into one end of the pipe, and volts is the pressure pushing the water through the pipe. The water (amps) into one end of a pipe, must equal the water (amps) which comes out the other end. The water pressure at the outlet side depends upon the size of the pipe. I guess the one major difference is that water has weight whereas electrons are almost weightless.

Using this analogy, let's say we wanted to turn a wheel using the water coming out of the pipe. I'm guessing there's two ways to do that. One is to hit the wheel with a large volume of water at low pressure, or hit it with a low volume of water at high pressure. Both ways would move the wheel.

However, I'm guessing that the large volume/low pressure method would turn the wheel slowly, but it would have more torque (be harder to stop) than the low volume/high pressure method which would turn the wheel more quickly, yet be easier to stop. Is this assumption correct?

If all the above is true, I should be able to take this analogy and apply it to a pancake armature. The biggest difference here is there are no magnets involved in the water example and I'm guessing it is the magnetic attraction/repulsion which turns a pancake armature.

Now, if someone could piece this together for me, I might be able to get it through my head how each of these components contribute to moving our little cars.

Thanks...Joe


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## rc_jeff (Jun 18, 2009)

Grandcheapskate said:


> I was taught that electricity is like water in a pipe or a hose. Amps is equivilant to the water going into one end of the pipe, and volts is the pressure pushing the water through the pipe. The water (amps) into one end of a pipe, must equal the water (amps) which comes out the other end. The water pressure at the outlet side depends upon the size of the pipe. I guess the one major difference is that water has weight whereas electrons are almost weightless.
> 
> Using this analogy, let's say we wanted to turn a wheel using the water coming out of the pipe. I'm guessing there's two ways to do that. One is to hit the wheel with a large volume of water at low pressure, or hit it with a low volume of water at high pressure. Both ways would move the wheel.
> 
> ...


I was always taught water and electricity don't mix.
No, you are correct, with the exception, resistance causes the water pipe to collapse, with no gain in flow.


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## slotnewbie69 (Dec 3, 2008)

hey cheapskate!check out slotmonsters,and read jets articles on pancake arms.they are also a wealth of info,as can be found here aswell,but he has a neat way of explaining things you may enjoy.other than that, i am also reaching for the aspirin reading this thread!


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