Big Red Buttons of Doom
In the electric vehicle community, there is probably no topic discussed more than the comparative advantages between different types of electric drive and control systems. Off the top of my head I can think of about 5 or 6 different types. Some motors can use different types of motor controllers, others are custom-fitted to only one drive system, still others are a hodge-podge of leftover scraps jerry-rigged just enough to get the rig rolling a bit at a time. Still, all of these systems can be very generally categorized as either direct current or alternating current, DC or ac.
Both motor and drive styles use the same basic principle to turn the shaft: A magnet attracts something, sometimes another magnet, sometimes a piece of magnetic material, causing rotation. Something else then changes the placement of the attracting field. The principle is so simple, in fact, one can make a very simple electric motor in a couple of minutes.
A Very Simple Motor!
Technically, the spinning wire and green base pictured above is a direct current permanent magnet motor. The magnet, called the "stator," lies in the green base. The spinning wire is called the "armature." This is a direct current motor; as long as direct current electricity flows from the power supply, the motor's design will activate the electromagnets automatically through the simple brushed axle. The brush assembly is called the "commutator." Essentially, any motor with brushes works best with direct current.
As the name implies, alternating current motors require a little more assistance. Some system must switch the polarity of the electromagnets at exactly the right time for the motor to spin at all. These motors don't spin automatically. Many large home appliance and tool motors rely upon the household current changing polarity sixty times per second (60 Hertz, or cycles). That's why many table saws, for example, must be started carefully lest they blow a breaker or fuse; until the motor's spin matches the household frequency (60 Hz. X 60 seconds in a minute = 3600 rpm), the motor will draw gobs of current, often three times the peak current for the motor when cutting!
So what's the debate? Essentially, it is one of cost. Both alternating and direct current motors have been around for over a century. They very seldom "burn out" or break down, if maintained properly. That leaves a lot of old motors in surplus yards just waiting for a new installation. DC motors tend to be a bit more expensive, but the controllers that drive them, built to operate mostly forklifts and other precision industrial electrics, are relatively cheap. By contrast, the ac motors are cheaper, but the controllers that drive them require more expensive control electronics to handle the same power as a comparable dc system. Since power electronics are today more expensive than factory machining differences between motors, dc systems currently win on the issue of price, though the cost gap between the two is closing fast as silicon gets cheaper.
Before we declare dc the victor, though, we should more closely examine how the systems that control the motors work.
It's important to remember that most all dc motors will spin faster as one increases the voltage. At the turn of the century, 1/3 of the automobiles were powered by gas, 1/3 by steam, and the remaining 1/3 by electricity. There were several manufacturers of electrics, Baker and Detroit being probably the most common. The electrics had quite a few advantages over the gassers of the time, foremost being the fact that early speed limits seldom considered vehicles traveling more than 15 mph to be safe. The road was, after all, still quite filled with horse-drawn conveyances, and horses spooked quite easily. The gassers were also quite a bit louder, spooking more than their share of horses.
Thomas Edison in his Baker Electric
These early vehicles raised and lowered the voltage to their motors through series-parallel control actuated by mechanical contactors, a complex wiring and switching of the batteries into voltage different configurations, or through much simpler but highly wasteful rheostat control. With a rheostat, extra voltage is simply diverted from the motor and dumped as heat. This last system meant one used exactly the same amount of power at both low and high speed travel. So much for slowing down to save juice!
Today these early systems are rare and, when used, very expensive. Instead, as they do for much of our lives today, semiconductors do the heavy power regulation in electric drive systems. Called gate transistors, these little marvels stop current from flowing very, very fast. At a signal, they can open and close the circuit, allowing current to flow in short bursts. Chop this current up fast enough -- say, thousands or millions of times per second -- and the effect is very similar to fine rheostat or series-parallel contactor controls but at a fraction of the hardware cost.
Both dc and ac motor controllers use these transistors. However, remember that the brushes guide the power to the proper electromagnet in a dc system, not the controller. With ac, the logic control must know the position of the armature and actuate the proper stator electromagnet to maintain torque and spin. This means, as noted above, a more complex logic system and at least six times more power transistors to achieve the same power as a similarly sized dc motor/controller, meaning ac systems are always going to be more expensive on the controller side.
More features can be worked into that price, however. It's relatively easy, when the system is still in the design phase, to allow software to do the heavy lifting in an ac system. This makes regenerative braking, coasting, power and battery management much simpler to achieve. Even so, the cost conscious will continue to gravitate toward the dc systems, often spouting the evangelic rhetoric of Green Movement True Believers: If those old forklift motors aren't put into cars, they'll be wasted! Why buy lavish systems when tried and true utilitarian systems of old have proven their worth? We can't afford not to build every electric possible! Our economy, our atmosphere, our society, our very souls teeter on the brink!
While I do agree with many of these cited concerns, there is one overriding consideration that is simply not appreciated -- safety. Consider the name of the transistor I pictured above: a "gate" transistor. It is designed to stop the flow of power. What, then, happens when it fails? The circuit closes, allowing the power to flow.
And there's the rub.
Let's say you're cruising in an ac-powered Toyota RAV-EV or other electric from the major manufacturers. Something happens. Perhaps the controller overheats, or a single weak transistor fails. Since individual transistors cannot stop much power on their own, these transistors are aligned in arrays. Consider these a row of micro-gates, each stopping a tiny bit of flow that adds up to a torrent of current. Therefore, should a single transistor pop, not much could happen. The others in the array could be able to handle the power, to stop the flow safely.
Ah, but the most likely time for a single transistor failure is during a heavy push of power followed by a rapid deceleration. This is essentially like asking all the gate keepers to open wide, allowing a good, steady stream of power to build from the batteries, then asking them to slam everything shut. Before, the battery pack voltage potential was akin to the still waters of a mill pond; afterward, like a rushing river. Meaning if just one transistor fails, the rest are likely to fail in a cascade.
Now you will see why I cited the RAV-EV as an example. Should that car's ac drive lose a string of MOSfets, only one of the six electromagnets would be disabled wide open. It would tend to fully power one magnet out of turn, which would just grab the armature and not let go. The motor would come to a stop. It would suck, but would not be overly dramatic. All major automobile manufacturers produce only ac drive systems for this reason.
Not so with a dc system. Remember -- and I cannot emphasize this enough -- the brushes determine exactly where the power should go.
Should the controller fail on a dc system, should the gate transistors all crap out at once and open the floodgates, full power from the batteries would connect to the motor without any control whatsoever.
This is not the simple "stuck accelerator" we all learned about in Driver's Education. Simply turning the key will likely not cut the power. Oh, no. You see, most hobbyist built EVs do have a contactor to supply power from the pack to the drive controller, but it is undersized, not designed to handle a catastrophic surge of current. Twisting the key to off will try to separate the contactor plates, sure, but the surge flowing through that beast will likely weld these plates solidly together. Turning the key can be worse than doing nothing.
For such eventualities, most well-built dc powered EVs have Big Red Buttons, an emergency physical power disconnect. Hit that sucker and a spring-loaded Frankenstein knife switch will pop, killing the power to the motor no matter what the current. It's a last resort.
Okay, I'd like you to pause, Dear Reader, and consider what you would do when you first encounter a Big Red Button in a car. In all likelihood, you would ask what it was for. Right?
What if the owner of the car were not present to answer? Would you press it? Keep in mind that, to be truly effective, BRBs tend to disconnect the power in Very Effective Ways, often requiring a trip to the garage to be reset. Ah, but you didn't know that, did you? For that reason, many BRBs today are being mounted not on the dash where convenient, but sometimes in places that many hands may not know to reach, like under the driver's seat. (More on that in a later post.)
Worse, let's say you didn't press it. Let's say you twisted the key, heard no motor, pressed the accelerator to give it some "gas," and launched. I should also add that, since the torque of many electric motors can handle the full range of speed, many cars are built with no transmission and therefore no clutch. We're talking about a direct connection between the motor and the wheels with no mechanical disconnect. Have I got your attention yet?
Let's say this launch surprised you. You very quickly unfloor the accelerator . . . yet continued to accelerate. You stomp on the brakes, which slows you a bit; but the torque from most dc motors powerful enough to drive a car at freeway speeds will easily overwhelm the wimpy stopping power of most automotive brakes. You have just experienced the worst-case scenario. Training in Driver's Ed says Turn the Key! You do. Other than a "zorch" sound, nothing happens.
Would you then think of the BRB? Would you?
In truth, most people don't get that far. Cascading dc controller failures often happen in constrained driving conditions like parking lots. You likely wouldn't have time even to turn the key. Your only option would be to stand on the brake and crank that wheel until you ran out of options, and hope no one gets killed.
I admit this is an unlikely scenario. Very few would have the opportunity to hop behind the wheel of an electric without some introduction of BRB presence and use, let alone without any. Still, one would be surprised at how many of us lose even the knowledge of basic disaster avoidance when the Worst Case Scenarios of life make themselves known.
Years ago, tacit introduced me to a concept I had suspected to be important, but had not known by name -- graceful degradation. It is an engineering design term referring to the way systems fail. When they fail -- not if, but when -- what happens? Will system failure lead to small problems, or large?
That, folks, is why my electric vehicle money rests firmly on ac drive systems rather than dc. Sure, the cost is higher. I'll indulge. If I can't afford today's ac offerings (and I can't, but more on that later), I'll wait, and gladly, for affordable graceful degradation to meet my budget.
I have but one life to use playing with toys. I very much like toys designed not to kill.
Both motor and drive styles use the same basic principle to turn the shaft: A magnet attracts something, sometimes another magnet, sometimes a piece of magnetic material, causing rotation. Something else then changes the placement of the attracting field. The principle is so simple, in fact, one can make a very simple electric motor in a couple of minutes.
A Very Simple Motor!
Technically, the spinning wire and green base pictured above is a direct current permanent magnet motor. The magnet, called the "stator," lies in the green base. The spinning wire is called the "armature." This is a direct current motor; as long as direct current electricity flows from the power supply, the motor's design will activate the electromagnets automatically through the simple brushed axle. The brush assembly is called the "commutator." Essentially, any motor with brushes works best with direct current.
As the name implies, alternating current motors require a little more assistance. Some system must switch the polarity of the electromagnets at exactly the right time for the motor to spin at all. These motors don't spin automatically. Many large home appliance and tool motors rely upon the household current changing polarity sixty times per second (60 Hertz, or cycles). That's why many table saws, for example, must be started carefully lest they blow a breaker or fuse; until the motor's spin matches the household frequency (60 Hz. X 60 seconds in a minute = 3600 rpm), the motor will draw gobs of current, often three times the peak current for the motor when cutting!
So what's the debate? Essentially, it is one of cost. Both alternating and direct current motors have been around for over a century. They very seldom "burn out" or break down, if maintained properly. That leaves a lot of old motors in surplus yards just waiting for a new installation. DC motors tend to be a bit more expensive, but the controllers that drive them, built to operate mostly forklifts and other precision industrial electrics, are relatively cheap. By contrast, the ac motors are cheaper, but the controllers that drive them require more expensive control electronics to handle the same power as a comparable dc system. Since power electronics are today more expensive than factory machining differences between motors, dc systems currently win on the issue of price, though the cost gap between the two is closing fast as silicon gets cheaper.
Before we declare dc the victor, though, we should more closely examine how the systems that control the motors work.
It's important to remember that most all dc motors will spin faster as one increases the voltage. At the turn of the century, 1/3 of the automobiles were powered by gas, 1/3 by steam, and the remaining 1/3 by electricity. There were several manufacturers of electrics, Baker and Detroit being probably the most common. The electrics had quite a few advantages over the gassers of the time, foremost being the fact that early speed limits seldom considered vehicles traveling more than 15 mph to be safe. The road was, after all, still quite filled with horse-drawn conveyances, and horses spooked quite easily. The gassers were also quite a bit louder, spooking more than their share of horses.
Thomas Edison in his Baker Electric
These early vehicles raised and lowered the voltage to their motors through series-parallel control actuated by mechanical contactors, a complex wiring and switching of the batteries into voltage different configurations, or through much simpler but highly wasteful rheostat control. With a rheostat, extra voltage is simply diverted from the motor and dumped as heat. This last system meant one used exactly the same amount of power at both low and high speed travel. So much for slowing down to save juice!
Today these early systems are rare and, when used, very expensive. Instead, as they do for much of our lives today, semiconductors do the heavy power regulation in electric drive systems. Called gate transistors, these little marvels stop current from flowing very, very fast. At a signal, they can open and close the circuit, allowing current to flow in short bursts. Chop this current up fast enough -- say, thousands or millions of times per second -- and the effect is very similar to fine rheostat or series-parallel contactor controls but at a fraction of the hardware cost.
Both dc and ac motor controllers use these transistors. However, remember that the brushes guide the power to the proper electromagnet in a dc system, not the controller. With ac, the logic control must know the position of the armature and actuate the proper stator electromagnet to maintain torque and spin. This means, as noted above, a more complex logic system and at least six times more power transistors to achieve the same power as a similarly sized dc motor/controller, meaning ac systems are always going to be more expensive on the controller side.
More features can be worked into that price, however. It's relatively easy, when the system is still in the design phase, to allow software to do the heavy lifting in an ac system. This makes regenerative braking, coasting, power and battery management much simpler to achieve. Even so, the cost conscious will continue to gravitate toward the dc systems, often spouting the evangelic rhetoric of Green Movement True Believers: If those old forklift motors aren't put into cars, they'll be wasted! Why buy lavish systems when tried and true utilitarian systems of old have proven their worth? We can't afford not to build every electric possible! Our economy, our atmosphere, our society, our very souls teeter on the brink!
While I do agree with many of these cited concerns, there is one overriding consideration that is simply not appreciated -- safety. Consider the name of the transistor I pictured above: a "gate" transistor. It is designed to stop the flow of power. What, then, happens when it fails? The circuit closes, allowing the power to flow.
And there's the rub.
Let's say you're cruising in an ac-powered Toyota RAV-EV or other electric from the major manufacturers. Something happens. Perhaps the controller overheats, or a single weak transistor fails. Since individual transistors cannot stop much power on their own, these transistors are aligned in arrays. Consider these a row of micro-gates, each stopping a tiny bit of flow that adds up to a torrent of current. Therefore, should a single transistor pop, not much could happen. The others in the array could be able to handle the power, to stop the flow safely.
Ah, but the most likely time for a single transistor failure is during a heavy push of power followed by a rapid deceleration. This is essentially like asking all the gate keepers to open wide, allowing a good, steady stream of power to build from the batteries, then asking them to slam everything shut. Before, the battery pack voltage potential was akin to the still waters of a mill pond; afterward, like a rushing river. Meaning if just one transistor fails, the rest are likely to fail in a cascade.
Now you will see why I cited the RAV-EV as an example. Should that car's ac drive lose a string of MOSfets, only one of the six electromagnets would be disabled wide open. It would tend to fully power one magnet out of turn, which would just grab the armature and not let go. The motor would come to a stop. It would suck, but would not be overly dramatic. All major automobile manufacturers produce only ac drive systems for this reason.
Not so with a dc system. Remember -- and I cannot emphasize this enough -- the brushes determine exactly where the power should go.
Should the controller fail on a dc system, should the gate transistors all crap out at once and open the floodgates, full power from the batteries would connect to the motor without any control whatsoever.
This is not the simple "stuck accelerator" we all learned about in Driver's Education. Simply turning the key will likely not cut the power. Oh, no. You see, most hobbyist built EVs do have a contactor to supply power from the pack to the drive controller, but it is undersized, not designed to handle a catastrophic surge of current. Twisting the key to off will try to separate the contactor plates, sure, but the surge flowing through that beast will likely weld these plates solidly together. Turning the key can be worse than doing nothing.
For such eventualities, most well-built dc powered EVs have Big Red Buttons, an emergency physical power disconnect. Hit that sucker and a spring-loaded Frankenstein knife switch will pop, killing the power to the motor no matter what the current. It's a last resort.
Okay, I'd like you to pause, Dear Reader, and consider what you would do when you first encounter a Big Red Button in a car. In all likelihood, you would ask what it was for. Right?
What if the owner of the car were not present to answer? Would you press it? Keep in mind that, to be truly effective, BRBs tend to disconnect the power in Very Effective Ways, often requiring a trip to the garage to be reset. Ah, but you didn't know that, did you? For that reason, many BRBs today are being mounted not on the dash where convenient, but sometimes in places that many hands may not know to reach, like under the driver's seat. (More on that in a later post.)
Worse, let's say you didn't press it. Let's say you twisted the key, heard no motor, pressed the accelerator to give it some "gas," and launched. I should also add that, since the torque of many electric motors can handle the full range of speed, many cars are built with no transmission and therefore no clutch. We're talking about a direct connection between the motor and the wheels with no mechanical disconnect. Have I got your attention yet?
Let's say this launch surprised you. You very quickly unfloor the accelerator . . . yet continued to accelerate. You stomp on the brakes, which slows you a bit; but the torque from most dc motors powerful enough to drive a car at freeway speeds will easily overwhelm the wimpy stopping power of most automotive brakes. You have just experienced the worst-case scenario. Training in Driver's Ed says Turn the Key! You do. Other than a "zorch" sound, nothing happens.
Would you then think of the BRB? Would you?
In truth, most people don't get that far. Cascading dc controller failures often happen in constrained driving conditions like parking lots. You likely wouldn't have time even to turn the key. Your only option would be to stand on the brake and crank that wheel until you ran out of options, and hope no one gets killed.
I admit this is an unlikely scenario. Very few would have the opportunity to hop behind the wheel of an electric without some introduction of BRB presence and use, let alone without any. Still, one would be surprised at how many of us lose even the knowledge of basic disaster avoidance when the Worst Case Scenarios of life make themselves known.
Years ago, tacit introduced me to a concept I had suspected to be important, but had not known by name -- graceful degradation. It is an engineering design term referring to the way systems fail. When they fail -- not if, but when -- what happens? Will system failure lead to small problems, or large?
That, folks, is why my electric vehicle money rests firmly on ac drive systems rather than dc. Sure, the cost is higher. I'll indulge. If I can't afford today's ac offerings (and I can't, but more on that later), I'll wait, and gladly, for affordable graceful degradation to meet my budget.
I have but one life to use playing with toys. I very much like toys designed not to kill.