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Technical Explanations - The Full Story
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Pdelcast
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Technical Explanations - The Full Story - 08.19.2008, 03:42 AM

How a motor can become demagnetized

Demag happens when the magnetic force applied to the magnet exceeds the coercive strength of the magnet. In other words, if the strength of an external magnetic field (like the field winding of an electric motor) exceeds a certain level, the magnet will be demagnetized (or remagnetized in a different orientation.) So any field of sufficient strength will demag a magnet.

It just so happens that the field strength at which demag happens is also affected by temperature. Look at this picture:



This is for an N44SH magnet (fairly common magnet type.)

If the field strength exceeds the coercive force of the magnet (at any temperature) the magnet will fail. It's just MUCH more sensitive to demag at high temperature (see the sharp downward part of the diagonal curve (at 140C)? That's the demag point. And the higher the temp, the lower the power required to demag.) On the chart, B is the field strength of the magnet itself, and H is the applied field trying to demag the magnet (field from windings in a motor...)

Curie temperature is the temperature where the magnet will fail, just by being at that temperature (think about the atoms moving enough that they start jumping around -- and out of alignment with each other.)

SO -- you can still demag a magnet if the applied field is very very strong, even at low temperatures (for this type of magnet, you would have to apply a field so strong that it crosses the knee on the -B side of the graph, which isn't shown.) And at high temperatures, it takes much less field strength to demag a magnet. This is why BOTH the temperature grade of the magnet, AND the field strength of the magnet are important. The higher the field strength of the magnet, the more resistant it is to demag at lower than max operating temperatures with very strong fields. The higher the temperature grade of the magnet, the more resistant it is to demag at elevated temperatures. IF you operate above the "knee" in the diagonal line on the graph, the demag is temporary, below the "knee" is a permanent demag.


Patrick del Castillo
President, Principle Engineer
Castle Creations

Last edited by BrianG; 08.25.2008 at 10:51 AM.
   
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08.22.2008, 11:51 PM

Slotted vs Slotless motor explanation

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Originally Posted by BrianG View Post
Speaking about motor efficiency, I've heard it said that a slotless stator design (basically an air core) is generally more efficient, but the Neus are slotted. I would think because of the slotted nature (windings wrapped around a "core") that the magnetic field would be much more focused, and wouldn't have as much flux loss. I can see that the width of the magnetic field would be a lot less in a slotted vs slotless. What are your thoughts on this?
Depends on who you talk to -- there are those who think that slotless is better, and those who think that slotted is better. I just think you should use whatever motor design fits your application...

Slotless motors have a larger air gap, and so, are generally less torquey than slotted motors. But they also have more space for copper, so typically have lower copper losses. Slotless motors have less inductance, so they switch at high frequency (higher RPM) better than slotted motors. Slotless motors also have virtually no torque ripple, so deliver power smoother than a slotted motor. The CM20 and CM36 motors (our standard Mamba and Mamba Max motors) are slotless because we wanted maximum spool-up speed, fast response, and moderate torque. IMO, Slotted motors don't do as well as slotless in 1/10 scale applications because the buggies and cars are lightweight, and accelerate very quickly without high torque -- and the high RPM performance and quick response make up for the lack of low-end torque (although our slotless motors generate a LOT more torque than our competitor's slotted 1/10 scale motors -- -but that's due to poor rotor design, rather than a slotted vrs slotless tradeoff.)

Slotted motors can have very small air gaps, and so are very torquey. They can generate much higher peak torques as well, but have the advantage of higher inductance at low RPM, which helps to limit peak currents and keep temperatures lower at low RPM. Our 1515/1Y and 1512/1Y Monster motors are slotted because we wanted maximum low RPM grunt and shaft twisting torque. And IMO, slotless motors just don't do as well in a Monster Truck or big buggy as a slotted motor. Once you get above about 3 pounds or so, the slotted motors have a slight advantage.

So, for smaller, higher RPM motors (low torque, high horsepower), slotless motors usually have the advantage. For larger, lower RPM motors, slotted motors usually have the advantage.

So you can think of it this way: Slotless motors are like motorcycle engines -- high RPM, lower torque, high horsepower. Slotted motors are like automobile engines, low to medium RPM, high torque, high horsepower. Outrunners are like Diesel engines... lower RPM, lower horsepower, very high torque.

All of these comparisons are relative -- the difference in practice is fairly small. We have run Monster trucks on slotless, slotted, and outrunner motors with good results. We really went with the Neu slotted design simply because it had the highest efficiency of any motor in that size that we had tried. And because of the high efficiency, it really performed extremely well.

There are trade offs for every type of motor design. You can build a good slotted motor and it will compare very well with a good slotless motor. Or you can build a bad slotted motor, and it will compare very well with a bad slotless motor. I've seen both good and bad motors of every type.


So the real answer is: Efficiency is the single most important thing. Efficiency is directly related to power to weight ratio. And power to weight ratio is performance. So the better the efficiency, the better the performance. Whatever motor type gives you the best efficiency in your application is the one you should use...

I'm sorry for the long ramble...


Patrick del Castillo
President, Principle Engineer
Castle Creations

Last edited by BrianG; 08.25.2008 at 10:52 AM.
   
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12.23.2008, 02:47 PM

MM and MMM settings explained


The following explanation is an excerpt from a different post originally by Pdelcast, edited slightly:


Timing Advance --

Let's see. Timing advance is just like the timing on a 1:1 car -- In an internal combustion engine, it takes some finite amount of time for the flame front to burn through the gasoline/air mixture in the combustion chamber, so the spark plug is fired early -- to make sure that by the time the piston is at top dead center, the fuel is mostly burned, and cylinder pressure is near maximum -- making the most power.
In an electric motor, there is an effect called inductance -- inductance is a resistance to change in current in a circuit -- so current ramps up and down, and doesn't change instantaneously. At low timing, the controller is actually centering the current ramp on the switch of coils, so that the efficiency of the motor is highest. But-- it is also possible to advance the timing even more, which increases the amount of current drawn by the motor (and therefore torque) -- however, this also increases (significantly) the amount of current drawn by the motor when the rotor is in a position where it doesn't generate torque efficiently, lowering efficiency.

So timing is a trade-off of torque generation (power) and efficiency. Above a certain amount of timing advance, and the rotor actually starts generating REVERSE torque for a short period, and then efficiency drops very quickly.

-------------------------------

Start Power --

Sensorless: Start power is the maximum amount of power that the STARTUP algorithm is allowed to apply to the motor PRIOR to detecting that the motor has started (or the position of the rotor.) The higher the start power, usually the quicker the startup algorithm can successfully start the motor, but the more power might be wasted (read "extra motor heat") in starting the motor.

Remember, these are sensorless motors and controllers- - prior to startup the controller doesn't know the position of the rotor, and must "tickle" the rotor to detect rotation and position of the rotor. The higher the power in the "tickle" the quicker rotor position can be detected.

Sensored: In sensored mode, start power limits the amount of current allowed at startup -- startup is the first few times the motor turns. This allows the motor to develop some EMF before the throttle is allowed to ramp up quickly.

If you use a little throttle at the start, you probably won't notice any difference.

Where it really makes a difference is in drag racing -- high start power will allow a faster spool-up of the motor. BUT -- it's dangerous as it is more likely to damage a motor.


-------------------------------

Punch Control --

Punch control is the maximum RATE of change on the throttle. So, if you go instantly from zero to full throttle, the punch control will integrate the throttle over a very short amount of time (much less than a second) to limit the huge surge currents drawn by the motor at very very low RPM. So at very low punch control settings, the throttle follows the stick as closely as possible. At high punch control settings, the throttle is "smoothed" a little more, and driveability is usually improved.

Hope that helps!!!!!

Patrick

Last edited by BrianG; 01.21.2010 at 02:27 PM.
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What makes a "good" motor?
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BrianG
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What makes a "good" motor? - 01.13.2011, 05:38 PM

More great info from Pdelcast where he explains what makes a good motor.

The no-load current tells you the approximate magnetic efficiency of the motor. Lower is better.

The steel for laminations comes in many varieties. Most of the cheap motors use either .35mm or .5mm laminations. The thinner laminations are more efficient. The cost for the steel goes up significantly with thinner steel (because it needs a LOT more processing to make thinner steels.)

The other variable in steel is the amount of silicon (sand) that they add to the steel. The silicon increases the electrical resistance of the steel, increasing the magnetic efficiency by lowering electrical losses (you want the steel to be good at conducting magnetism, but poor at conducting electricity.) The higher the silicon content, the more brittle the steel and the longer it takes to process (and therefore, the more expensive it is.)

So, the cheap steels are thick with a low silicon content, and the expensive steels are thin with a high silicon content.

We use .2mm thick, high silicon content steel. Most of our competitor use .35mm or .5mm low silicon content steel. The .2mm high silicon steel is about four to five times more expensive than .35mm high silicon steel, and about ten times more expensive than .5mm low silicon steel.

But the difference is large: A 1415-2400Kv motor from Castle has a no-load current of about 2.4A.

To figure the magnetic losses, you multiply the no-load current by the battery voltage.

So, for example:

Hobbyking motor: 5A * 24V = 120 watts of magnetic loss
Castle 1415-2400Kv motor: 2.4A * 24V = 57.6 watts of magnetic loss

--------------------------------------

The second type of loss is resistive loss. This is the loss caused by the current flowing through the copper.

The Hobbyking motor lists a 5.8 milliohm resistance. The Castle 1415-2400kV motor has a resistance of 4.2 milliohms.

The formula for resistive losses is (Current in amps) ^2 * resistance (amps squared times resistance)

So, at 125 Amps (the "rating" from the hobbyking motor) the losses would be:

Hobbyking motor: (125)^2 * .0058 (ohms) = ~90 watts
Castle 1415-2400: (125)^2 * .0042 (ohms) = ~65 watts

---------------------------

These two types of loss (magnetic and resistive) add up in the motor, and get turned into heat.

So to make a good motor, you need both low resistance (for low copper losses) and low no-load current (for low magnetic losses.)

To compare the two motors:

Losses at 24V battery voltage, 120A current:

HobbyKing motor: 90 watts (resistive) + 120 watts (magnetic) = 210 watts of loss
Castle 1415-2400Kv: 65 watts (resistive) + 58 watts (magnetic) = 123 watts of loss

Because the "loss" watts ALL TURN INTO HEAT, the Castle motor will run much cooler in the same setup -- it's efficiency is much higher.

It all comes down to this: It's expensive to make a good quality motor. And, you get what you pay for.

Last edited by BrianG; 01.13.2011 at 05:40 PM.
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