From an _OLD_ yahoo Insight group post:
(probably more than you want to know about electric motors
)
From: John Wayland <
[email protected]>
Date: Tue Jun 25, 2002 5:22pm
Subject: IMA Motor-Generator 101 (was 12 v. battery life)
Hello hybrid fans,
RRiemersma wrote:
> All excellent points, John.
>
> I forgot all about the 3 phase AC, are the same wires used to take
> generated juice back to the controller for battery charging too?
Yes, same three 4 gauge wires. These are the only high current wires to and from the motor/generator, and they go straight to the power stage of the inverter/speed controller mounted midship in the IMA box.
> So the IMA motor runs as an alternator?
Yes, almost identical to the 12v system alternator of the average car, with two exceptions:
(1) The car alternator has a 'wound rotor' with slip rings. Brushes pass variable DC current into the rotor windings, making the rotor a variable intensity electro magnet... by varying its magnetic strength, the voltage and thus charging rate, can be controlled by either an external or internal voltage regulator circuit.
(2) Because it is never called on to serve double duty as a motor, the car alternator's 'stator' (the coils around the perimeter of the alternator that surround the spinning rotor), are fed through beefy high current internal diodes that turn the 3 phase AC coming out of the coils, into DC that is used to charge and run the 12v system of the car.
The Insight's IMA motor also has a rotor surrounded by stator coils, but the rotor of this motor-generator is comprised of permanent magnets (Honda refers to the Insight's motor as a 'permanent magnet' type), so the rotor has no windings, no slip rings, and there are no brushes to make contact with, thus, the rotor's magnetic field is fixed. The outer perimeter stator coils are not fed through diodes at all, and instead, they are always either sending out, or being fed, 3 phase AC juice. Three hefty 4 gauge wires are connected directly to the stator coils, and run from the 10kw motor-generator straight back to the power stage of the Insight's inverter-speed controller. The inverter-speed controller either sends high current variable voltage and frequency AC juice up to the motor-generator, causing it to run as a motor to either spin over the ICE or to help propell the car, or it receives high current AC juice from the motor-generator when it is acting as a generator and converts that 3 phase AC into DC to charge the 144vdc battery pack.
> How come they keep calling it a "brushless DC motor?" Or is this what such a beast is,
> a 3-phase motor with a controller?
These are great questions, and I'm glad you've asked them. Before I answer, here's a quick motor lessen that will set a backdrop for my explanation of things....
ALL motors, all of them, internally, run on AC. Yes, it's true, even permanent magnet (PM) 'DC' motors from small hobbyist motors, to larger ones that are used as car window lift motors, to even bigger ones used as trolling motors for fishermen, turn DC into AC 'inside' the motor. Even non-PM type motors, like the super high torque series-wound starter motors of cars or the even BIGGER series-wound extreme torque motors that power most backyard-converted electric cars, run by converting DC into AC inside the motor. Conventional DC motors, whether PM field type or wound field type, have a mechanical DC-AC converter system, and it's comprised of a 'commutator' and a set of 'brushes'. The commutator is part of the spinning mass of the motor, the 'armature'. The armature then, is made up of laminated steel insulated and wrapped with copper windings, and a commutator, and it has the motor shaft running through its center. The cylindrical shaped commutator has many copper bars that are thinly separated by insulation and pairs of these 'com bars' are connected to a respective winding of the armature. DC goes through the brushes and into the commutator bars, and this DC feeds into the armature's windings, turning it into an electro-magnet. The magnetic field of the armature attracts and repulses against the magnetic filed of either permanent magnets of electro-magnets (wound field) that surround it, causing the armature to rotate. As the armature rotates, the next set of com bars that line up with the fixed brushes, but this set of com bars is wired in reverse to the set that preceded it, reversing the direction of the DC current through the windings...and so it goes, as the armature spins, the DC current is constantly being switched back and forth in its direction through the armature's windings, making it AC. Brushes made up primarily of carbon both conduct and lubricate as they are pressed against the copper commutator. This is how the mechanical DC-AC converter of all DC motors works.
I've now illustrated that all DC motors actually run on the inside as AC motors, but because they are fed pure DC to make them run, they are still known as DC motors....feed them DC current, and they run, but feed them AC, and they'll blow up, with the exception of the series-wound motor, which can run either on DC or AC juice! Because the series-wound motor can do this, it's also called a 'universal' type motor. Most plug-in hand drills and vacuum cleaners, have 'series-wound' or 'universal' type motors, and this is why, if you connected them up to 120vdc, they will run just fine. Ever notice that the older vacuum cleaners had 'AC/DC' on their model name plates? As a weird little side note, this is where the band AC/DC got the idea for their name...that's right, off the model name plate of a vacuum cleaner! If you had a super high current 12 volt AC power supply that could make 200-400 amps like a car battery has the power to do (DC though), you could connect it up to the starter motor in your car and the motor would run fine....I digress...
Motors are then, identified as either AC types, or DC types, by what kind of juice you connect to them, to make them run. If you feed it DC, it's a DC motor....if you feed it AC, it's an AC motor....Let's look at DC motors:
All DC motors have armatures and brushes, except for one that was developed because of transistor technology, the brushless DC type, or BDC...more on this a bit later. For now, picture a more typical 'brushed' PM motor...it has permanent magnets around the perimeter of the motor that act as the magnetic field, and an armature and brushes for the rotating part. Mechanical commutation makes AC flow through the armature, causing it to spin. This old design works, but it still has many drawbacks that make it inefficient:
(1) The armature gets hot with all that current flowing through the windings, but it's hard to cool it off with it in being in the center of the motor and spinning.....you can't easily get air to all of the windings, and you can't pump coolant through it (at least not practically). To cool off the brushes, commutator, and armature windings, most higher powered DC motors have a built-in cooling fan, with the blades fixed to the armature shaft. This cooling fan makes noise and robs the motor of some of its power. Because of this internal fan, there are parasitic losses of delivered motor power.
(2) There is inherent inefficiency due to the electrical resistance of the carbon-based brush material...remember, resistors are made of the same stuff. Because the brushes have to stay pressed against the spinning commutator, they need to be made of an electrically conducting material that is at the same time, a lubricant....this a why they are made of high conductivity carbon. There are electrical losses through the brushes.
(3) Because the brushes are always pressed against the commutator, there are frictional losses...more inefficiency.
(4) As voltage and current are raised, the mechanical limits of the commutator/brushes begin to get in the way, and arc-over becomes a limiting factor. It becomes difficult to get higher power by raising the voltage and current when the brushes and the commutator begin arcing and flashing-over. A brushed motor has a certain limit as to how high a voltage can be sent to it, due to arcing.
(5) Because of the way the armature has to be made, there are 'windage losses' where the armature catches the air, like a fan, so the faster it spins, the more drag it creates. Some PM type DC motors are only 50-60% efficient, though better designed ones approach 90%. The same inefficiency goes for wound filed types as well.
(6) Mechanically commutated motors are limited in the rpm they can safely spin at, because of the centrifugal forces that tend to cause the windings and com bars to come loose after the safe rpm high end has been hit.
Pretty much all of the above in reference to PM type DC motors, applies to wound-field type DC motors, too. Now, let's look at AC motors:
AC motors never have an armature, because there is no need to convert incoming power from DC to AC. Instead, they all have a far less complex 'rotor'. There are many types of AC motors, from squirrel cage induction to PM types and many other sub categories. The Only AC motor that has windings on the rotor and brushes to deal with, would be the wound Rotor type that is exactly like a car alternator, minus diodes on the stator windings. If you were to take a car alternator with the one mod of removing the diodes from the stator coils, and feed the brushes with rectified DC from an incoming AC power source (an AC-DC converter), then feed the stator coils or 'field' with the same AC power source, the alternator would spin and become a nice AC motor.
Most AC motors are not of the wound rotor type type and instead, have rotors that have no windings. The squirrel cage induction motor has an aluminum rotor with a sparse number of solid copper rods passing through it that are capped off on the ends...these are very primitive 'windings', but for all intents and purposes, they are not considered 'windings'. The other kind of AC motor has permanent magnets that make up its rotor.
Because of the simplicity of the rotor (as opposed to the very complex armature), the typical AC motor rotor is free of things that can fly apart from extreme rpms, like windings, loose steel laminations, and com bars. Because of this, the aluminum rotors and PM rotors of AC motors are smooth and densely made and can be spun up to very high rpms, extending their usable power band far above that of DC motors. Because there are no brushes, no commutator, and no arcing and flash-over, AC motors can be run at very high currents and voltages without the physical breakdown that occurs with DC motors. There is also no brush drag or windage losses like in a DC motor.
Of course, until modern technology brought us transistors, there was really no way to use AC motors off of battery power. With this technology though, and with the ability to convert pure DC from battery packs into AC, AC motors can and are used in all types of DC power applications, especially in electric and hybrid vehicles.
Back to DC motors.....
Enter the brushless DC motor. The BDC motor is considered a DC motor, because external to the motor, you feed it with DC to make it run. Just like the mechanically commutated DC motors, inside the BDC motor, DC is converted to AC, but this time it's all done with electronics. An electronic commutator replaces the mechanical commutator-brush assembly, with transistors and related circuitry doing it to convert DC into AC. Since a moving source is no longer needed for mechanical commutation, the BDC motor can be made to be a lot more efficient, and in fact, it's made just like a PM rotor AC motor! Remember my example of a typical PM type brushed DC motor? In that type, the permanent magnets were placed on the outside perimeter to serve as the field, and the windings were on the 'armature' that spun in the center. Now, take this idea, but reverse it, and put the magnets on the inside and let them spin, while the coils that tend to get hot, are placed on the outer perimeter, where they can be easily cooled with convection fins, or liquid cooling, neat, huh? Since transistors are doing the power switching, there is no longer a mandatory placement of the coils on an armature with its commutator. This type of DC motor has all of the advantages of an AC type....a smooth, non-wound PM rotor that can spin at very high rpms, field coils mounted to the outside of the motor where they are easily cooled, and no arcing limitations from a commutator and brushes.
Here's a cool example of the efficiency gains a BDC has over its elder, the brushed motor.....I have two electric scooters with the identical amount of battery weight and battery power on board. One has a single12v, 20ahr battery and the other has two smaller 12v, 10ahr batteries wired in series for 24v operation. Both battery sets though, are from the same manufacturer and are both AGM types. 12v X 20ahr =240whrs, and 24v X 10ahr=240whrs, so as I said, both power supplies in these two scooters are the same amount of power, each at 240 watt hours. The 12v scooter has essentially, a motor designed to be used in Ford trucks as a radiator fan motor, where efficiency took a back seat to low cost. As such, it is a run of the mill brushed PM motor. Used to power the scooter, it comes in at a poor 57% efficiency, and can take the scooter 8 miles per charge at 13 mph with a 200 lb. dude riding it. After a few miles of operation, you can see just where the other 43% of the battery power went....heat! The motor gets very hot to the touch. The 24v scooter is essentially the same size and weight, and as I stated, has the same amount of battery power on board, though configured for higher voltage and less current. The big difference, is that this more expensive scooter has a trick BDC motor. The motor is nearly identical in size to the other scooter's motor, but it is 97% efficient! The commutation circuitry is on a thin round heat sink at the face of the motor. This scooter runs 15 miles at 21 mph! Even after 10 miles of high speed scooting, the motor feels tepid to the touch.
So now.....back to the original question,"Is the Insight's motor a BDC type or is it an AC type?"
Answer...the Insight has an AC motor, more specifically, a PM rotor AC motor. It has no internal circuitry that converts DC to AC, though it does have commutation sensors inside. It has three wires that feed 3 phase AC power into the motor. If you took the motor inverter from the IMA box and built it into the motor, so that there were two wires that brought in DC, then it would be a brushless DC type.
>Or is (it) a 3-phase motor with a controller?
Yes, you've got it correct, this is exactly what it is. Where it gets a bit more complicated, is the fact that electric cars and hybrid cars need two things on board to make an AC motor work in their application:
(1) They need a DC-AC inverter to change the power source from pure DC out of the battery pack, into AC that can run the motor.
(2) In addition to the inverter, they need a motor controller that can send variable amounts of AC power to the motor to change it's speed and torque characteristics.
Because of these two 'needed items', using an AC motor in a DC powered car (the batteries), can get expensive and complex....it's all in the combination inverter/controller package. EVs and hybrids that use DC motors, only need a motor controller, a box that is a high voltage, high current speed control that costs about $1500. AC motors need a 'box' that first converts all battery power into 3 phase AC power...this by itself, takes three times the silicon and related components. Also inside this box, is the rest of the goodies...the speed controller portion. In limited quantities to the backyard EV converter, these AC inverter/controllers can cost up to $20,000 at the same high power levels we can get from a good old series-wound DC motor and speed controller, about $3000 for both the motor and the controller. The AC motor itself, is actually less expensive and far less complex that the DC motor.
When you build things in mass quantity, as a car factory can, the cost of things drop dramatically, of course. Also, in a hybrid car, the motor and controller package is far lower in power output, when compared to the motor and controller package of a pure EV. Compare the Insight's modest 10kw rating, to the 137kw power level of the EV1's electric power train! The cost of the parts needed to make a 10kw power train is understandably, far less than the cost of the parts to make a 137kw system.
I hope the above was informative and helpful.
See Ya......John Wayland
