Variable Torque Motors – Hybrid Retrofits and Innovations
Filed Under: green automotive technology, GreenBigTruck on June 23, 2010
by Aaron Turpen, GreenBigTruck
Variable Torque Motors is based in Fort Wayne, Indiana and has designed a very unique, highly innovative diesel electric hybrid system that can be installed on new trucks or retrofitted to vehicles already in service. The VTM system is mounted between the transmission and the drive axle and utilizes super-capacitors rather than batteries. This greatly lowers its weight, requires no new EPA testing, and gives the benefits of a diesel electric hybrid truck: lower emissions, lower fuel usage, and better power and performance.
Larry Zepp, the inventor of the Variable Torque Motor, contacted me through Green Big Truck and introduced his system to me.
“This system produces a 400 ft-lb. launch torque to the driveshaft and is designed for class 3 to the lighter class 7 trucks and buses. The hybrid can be installed as a retrofit to existing vehicles or on new vehicles. In many start / stop applications like an airport shuttle bus, the VTM hybrid improves MPG by 20-35%. It also installs on any type of engine – gas, diesel CNG, propane.
“After 2 1/2 years of BETA testing, the hybrid system went into production this March. We were approved for retrofit into Indiana school buses in Jan. 2008 and have converted (4) demo school buses. Most of the hybrids are going into new class 4 buses.”
The specifications and information he sent are fascinating. Let’s look at this innovative system and how it works.
Basic block diagram illustrating the components of the VTM system.
The Innovative Variable Torque Motors’ Design
The motor that Zepp has designed is a brushless permanent magnet motor at it’s base. These are the most common type of electric vehicle motors used because they are lower in maintenance. The motor is a 45kW (60hp) unit. What Zepp did was change how the motor operates by giving it two ranges of motion.
So rather than just turning a shaft, which all electric motors do, he also allows the permanent magnet (the core of the motor, which is attached to the shaft to make it spin) to move back and forth within (and out of) the stator coils (the outer, unmoving portion of the motor to which power is applied).
Zepp’s motor design (in a cutaway for illustration).
In most electric motors for vehicles, the stator receives power from an outside source, which changes its magnetic polarity. This causes the permanent magnets which are attached to the shaft to turn to “run away” from the stator’s magnetic field. That causes the shaft to turn. In Zepp’s design, one of the down sides to this fixed design is eliminated, as the magnet can move back and forth inside the stator, allowing more or less torque to be produced and for the motor to become “neutral.” This eliminates drag when the motor is not in use, but the drive shaft is still spinning. It also allows the motor to be optimized for maximum torque at all speeds it’s operating under. Most motors have a specific speed range (rotations per minute / RPM) and anything slower or faster is sub-optimal. Zepp’s design eliminates that problem.
Here’s a comparison to give you an idea of what this means: a standard, commonly-available 400lb-ft torque motor that would see use in any heavy application has a top speed of about 8mph. By moving the magnet within the stator as Zepp’s design does, that top speed increases to 35mph while retaining 90% or higher efficiency and full torque.
There are two system sizes made by Variable Torque Motors, one for lighter medium-duty applications and one for heavier medium- and heavy-duty applications. Because the motor operates under less stress, it becomes more reliable, is more efficient in power usage, gives better output, and is also more efficient with regenerative braking.
For the VTM system’s design, this is important because all of the vehicle’s electric charging for hybrid operations comes from regenerative braking. And that energy is stored in ultra-capacitors.
The Ultracapacitors in the VTM System
Rather than using batteries, such as expensive lithium-ions or heavy lead-acid types, the VTM system uses ultra-capacitors. These are capable of storing large amounts of energy very quickly, for short periods, and of disharging that energy just as quickly. They are lighter, smaller, and of simpler design than chemical batteries and have a much longer life span as well. The Flux Capacitor of Back to the Future fame was an ultra-capacitor. Though the VTM capacitors aren’t made to hold the massive 1.21 gigawatts of the DeLorean in the movie.
The ultra-capacitor box on a VTM S-3000 system.
Those that Variable Torque Motors are using are 73Farads (F) units putting out 96 volts in DC. For lighter applications, VTM installs one ultra-capacitor. For heavier applications, they use two. To get an idea of what that means, a 1F capacitor can be as small as about 2 centimeters (about 3/4 inch) in diameter and about 1/3 that in height. A Farad is a number of amps per second which makes one Volt of change in a storage device. The VTM ultra-capacitors are capable of storing 16 seconds of power at 300 amps. The ultra-capacitors weigh 31 pounds as compared to the equivalent in deep-cycle lead-acid batteries, which would weigh 500 pounds.
Ultra-capacitors have a lot of other advantages in the medium- and heavy-duty diesel electric hybrid truck as well. They are safer than batteries (no toxic chemicals, no risk of bursting or leakage), hold more power faster, and discharge more power faster than chemical batteries. They’re much cheaper and have a lifespan measured in millions of charge/discharge cycles rather than the 1,000s of c/d cycle rating of high-energy battery technologies like lithium.
Their main disadvantage is that they do not store power for very long. For this reason, VTM does not have a “plug-in” system to charge up the truck overnight as do battery-based diesel electric hybrid units. Of course, this also means that no one will forget to charge the truck overnight and lose the extra efficiency the next day.
How The Variable Torque Motors System is Mounted
The motor is mounted behind the transmission and before the drive axle, along the drive shaft. The shaft is replaced with shorter ends and couples using commercially-available end yokes and universal joints. It can be mounted on virtually any medium- or heavy-duty vehicle, new or used. The other electronics, including a controller box, a small cooling system, and the ultra-capacitors are mounted under the vehicle chassis wherever they can be accommodated.
Once installed, the VTM system is fully self-contained. Emergency shutoff switches are clearly marked and easily located (including one at the driver’s side) in case of an accident or for maintenance safety. These switches isolate the ultra-capacitors to ensure that no power can be discharged to the vehicle and simultaneously inform emergency responders that the vehicle is a hybrid, which can change their response tactics.
The VTM cooling system.
The cooling system is a small, two-motor setup that cools the motor during operation using standard anti-freeze coolant. This means the maintenance shop is not required to carry extra supplies and this cooling system is one of the few things requiring maintenance on the hybrid system: a technician should check the level and cleanliness of the coolant periodically and flush and replace it, as you would for the combustion engine.
The entire hybrid system weighs only 630 pounds, which is more than 1,000 pounds lighter than similar (battery) hybrid systems currently on the market.
How the Variable Torque Motors System Performs
During testing, the VTM system was put on the road for over three years in buses. These typically operated in start-stop driving cycles with 4-7 stops per mile. During operation, they saw a 25-40% increase in miles per gallon (MPG).
The system works seamlessly for the driver, with the motor acting as a generator (similar to engine brakes, but without the noise) whenever the driver slows to 35mph or less and applies pressure to the brake pedal. As the brake pedal is depressed, the hybrid braking system increases its pressure (thanks to Zapp’s motor design), going up to 100% before engaging the physical brakes. Because of this, mechanics saw an 800% extension in brake service intervals, another maintenance and dollar saver. For safety reasons, the hybrid regeneration is automatically shut off if an ABS event occurs on the rear axles.
The regenerated energy is stored in the ultra-capacitors and then used to power the motor to boost acceleration up to 35mph. This significantly reduces engine noise and fuel usage when starting from low (or zero) miles per hour, such as when leaving a bus stop or stop light. In a tested Ford E-450 shuttle bus, the VTM system provided 50% faster acceleration from 0-25mph.
The acceleration and deceleration properties of the VTM system were tested under winter conditions in 2009 by Fort Wayne Citilink, who found the smooth coasting and braking afforded by the system reduced stopping distances on glare ice by half and prevented ABS engagement in the rear wheels. They concluded that this made the diesel electric hybrid system from VTM a significant safety advantage.
The VTM motor, installed.
Once the vehicle reaches 30mph in speed, the VTM system puts the motor in “neutral,” moving the magnets from the stator coils entirely. This removes all motor drag from the drive line, which means the hybrid system does not negatively effect highway mpg (as do most others).
Conclusions
The Variable Torque Motors system is one of the more innovative hybrid designs I’ve seen. The motor is where most of the real action is, as far as I’m concerned, and the total system is both unique and extremely effective for its target market. Several vehicle types could greatly benefit from VTM’s technology: transit buses, school buses, shuttles, trailer/yard/dock hostlers, refuse collection trucks, and similar duty-cycle vehicles would all see big benefits from using a hybrid system of this type.
Nearly every hybrid and many battery electric vehicles could benefit from the Zepp motor design as well.
