Sparrow / NmG
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A motor controller is a fancy electronic gadget that modulates the amount of energy flowing through the motor and consequently the amount of torque the rear wheel produces. Since the motor in a Sparrow is a DC series-wound motor, it can be controlled quite easily by regulating the amount of time voltage is applied to it. The more time voltage is applied, more current flows and thus more torque produced. A VOM (voltmeter) attached to the motor will appear to show an increasing voltage due to averaging the ON time. At least that's the simple way of thinking about it.
A more in depth description would be that a series-wound motor has, at any RPM, a native "back EMF" which amounts to the voltage that must be applied to keep it spinning at that speed. If you apply more voltage than the back EMF, then you get a flow of current and produce torque. Applying a lot more voltage results in a lot more current flow and torque produced.
Since back EMF is linearly related to motor speed, "flooring" the accelerator gets you more torque when the motor is spinning slowly than it does when the motor is spinning quickly. This is why a Sparrow has lots and lots of pick-up between 25mph and 50mph, but less above 50mph and little above 70mph when the speed governor kicks in. Below 25mph, the current required to accelerate a Sparrow quickly is very large and most motor controllers end up in "current limit" where they can't produce any more.
The power delivered by the controller does not directly track your throttle pedal, to do so would be dangerous. This is why a Sparrow seems to start moving slowly, gradually accelerating to 5 or 10mph and then seems to suddenly have more power. The power is being incremented on a time fixed basis, to provide a controllable throttle pedal. Note that this effect is not a current limit function (in the Kilovac controllers). Other controllers depending their construction may act differently, the details will be revealed by careful testing. In most all controllers the throttle will track your foot position as you release the throttle request.
These motor controllers all work by a technique called "pulse width modulation" where the voltage is turned ON and OFF at some fixed frequency, usually around 15Khz, with some as high as 20Khz, since that's mostly out of human hearing. When you apply little pressure to the accelerator, the pulse width are a very short part of the 15,000 pulses per second. When you apply more throttle, the pulse width is increased, "wider" and they last a longer part of the reoccurring pulse period.
Most of these motor controllers (and all of them used by Corbin) were air-cooled.
There were three motor controllers used during the Sparrow production run and two used in NmGs (to my knowledge.) Your vehicle could have any one of them or something else yet, particularly if yours has been heavily modified, as so many of them have. They are:
The Curtis is the most widely available motor controller on the market, though pretty much everybody seems to hate it. It's a scaled-up version of a golf cart motor controller and it barely produces enough power to keep most vehicles moving. It's notable for having no built-in safety features, meaning that everything has to provided externally. It's also notable for having a relatively low current limit. It can sustain 500A for a few seconds before it goes into thermal limit, at which point it could only sustain 150-200A. At low motor speeds, this is very little.
The version installed in the Sparrow was a prototype 156V version of their 144V design. It tended to fail quickly, particularly on vehicles where the motor hadn't been advanced. Many of the Sparrows that were fitted with this controller were refitted by Corbin or others with other controllers. There is at least one company successfully rebuilding these, so if you have a Sparrow with a failed Curtis, you should consider that route.
This motor controller was built by the company that eventually became Alltrax [http://www.alltraxinc.com], which currently deals only in low-voltage controllers. They are rare at this point, as most of them failed in motor fireballs. Again, see the section on motor advancement for fixing issue.
You can get a new Raptor 1200 built for you these days by Peter Senkowsky. The 600A current limit was better than the 500 from the Curtis and thermal issues weren't as bad, so they could actually produce that 600A. If you have a 1200A version, you have very quick car indeed.
Kilovac is still in business building relays and contactors, they are no longer building DC motor controllers. These controllers were used in both hatchback ("PizzaButt") and ("Jellybean") Sparrows.
They had a better reliability record than the others used by the factory, but they still failed pretty frequently, usually due to excessive heat and lack of motor advancement. Ron Anderson at Black Sheep Technology, LLC now rebuilds these controllers and provides spare parts.
Located at www.black-sheep.us
The OEM Kilovac CPU does not limit motor current until the PWM is 50% or higher. At 50% PWM duty cycle the Motor Current Limit is approximately 300 motor average amps. The OEM version of the controller uses 4 IGBT and Diodes in the Power Section. The Power section can be upgraded to 5 IGBT's and Diodes. The implementation of forced air cooling with ductwork, greatly improves reliability.
Due to some recent failures of the original Kilovac, a new CPU board has been made by Black Sheep to provide this critical spare part in case of a severe internal meltdown. This new CPU incorporates new features many not found in other controllers. See Black Sheep Technology website for details as they are released.
Pictures of one of Ron's rebuilds are here Kilovac Rebuild.
This is the gold standard of DC traction motor controllers. They can be purchased today from http://www.cafeelectric.com/. The version of the Zilla in an NmG is the 1000A 156V (Z1K-LV) model. There has only ever been one failure of a Zilla in the field and it was in a dragster pulling 2000A at 360V.
Please be aware that running a Z1k at high motor current limits will tend to fail the contactors used for foward-reverse direction control. The tattletale will be a brown discoloration around the power posts on top. As the plastic heats up due to motor amps circulating they allow the posts to move out of plane and lose proper internal contact.
(See http://www.plasmaboyracing.com/whitezombie.php.) If you have one of these, you have a lot of power and tremendous features and reliability. One of the notable reasons for its reliability is that it is water cooled, so any installation in a Sparrow must involve a water tank, small radiator, a pump and some plumbing.