Variable Torque Motors - American Made Efficiency

One of the largest down-sides to electric motors is their inherent efficiency range and, in hybrid systems, the drag they can create on the drive train. The unique and innovative Variable Torque Motor eliminates both of those problems and results in a much more efficient, wider-ranged motor for electric and hybrid-electric applications of all sizes.
images courtesy of Variable Torque Motors

by Aaron Turpen


  • Type: Brushless Permanent Magnet
  • Manufacturer: Variable Torque Motors
  • Torque Values: 400lb-ft
  • Wattage and HP: 45kW / 60hp
  • Availability: Now

The manufacturer says

"The VTM patented motor shifts - as RPMs increase the rotor separates from the stator. Think how a transmission helps a car, truck - combining low-end torque (acceleration), higher top-end speed and improved efficiency over a range of speeds. What if your truck had one gear? Not very efficient. Shifting increase mpg - for trucks and VTM users."


The motor's innovation comes from its ability to move in two directions. Normally, an electric motor turns in one direction to convert electricity to physical movement. When turning in the other direction, it generates electricity from physical movement. What the VTM does is also move back and forth, creating varying ranges of torque versus speed.

vtm block diagram

A brushless electric motor has three basic parts: a stator, a permanent magnet, and a shaft. Most designs have the permanent magnet attached to the shaft, all riding inside the stator. When power is applied to the stator, it becomes and electromagnet whose polarity is opposed to the permanent magnets on the shaft. This causes the permanent magnets to "run away" from the stator, creating spin on the shaft. This design creates maximum torque at all times, but is limited in how much speed it can produce.

What the VTM does is move the permanent magnet within and out of the stator, allowing for varying torque:speed ratios so that higher speeds can be achieved by sacrificing torque. Since most of the torque (power) is needed to get a vehicle started rolling and once it is moving, less power is needed to make it go faster, the VTM design allows for much higher speeds directly from the motor.

Speaking With the VTM Inventor - Larry Zepp

Larry Zepp, the inventor of the Variable Torque Motor and owner of the company based in Indiana by the same name, spoke with FutureCars about his motor design and the hybrid system he's built around it. While he's aiming his hybrid system at the medium-duty fleet vehicle market, varying sizes of the motor are also being made for other applications. Models begin at the S1000 for smaller applications (forklifts, ATVs, etc.) and go up to the S2000 and S3000 for larger vehicles.

Speaking of the recently-released medium-duty hybrid retrofit kit which utilizes the S3000, Zepp said:

"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."

That impressive power and efficiency comes from the motor's ability to not only shift itself as the truck speeds up, but to go fully neutral (with no drag) when the truck is above the motor's speed range. This means that it has no effect on the truck's highway mileage, unlike other hybrid systems.


The shift to "neutral" in the motor happens when the permanent magnet fully leaves the stator. In a normal electric motor, even with the power switched off, drag is produced as the magnets spin inside the stator and static magnetism "pulls" against the spin.

When reversed, Zepp's motor design also becomes increasingly efficient at regenerative braking, as the magnets shift back inside the stator in reverse of their exit as the vehicle slows.


This motor can be used in any of a number of electric vehicle applications, from hybrids to all-electrics. Variable Torque Motors' hybrid system is built specifically for medium-duty trucks and buses that see a lot of start-stop, as it utilizes ultracapacitors rather than batteries, which reduces the system weight by over 1,000 pounds compared to similar battery-using hybrid systems. It's currently being used in several Indiana school buses and the company's showcase airport shuttle-type bus.

Innovation is at hand in Indiana.

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