Drive

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The drive system comprises the motor and transmission, and the motor controller.

The UniSA prototype drove the single rear from a motor under the (raised) rear seat, using synchronous belts in two stages:
* a 3:1 reduction from the motor to the swing arm axle
* a 2:1 reduction from the swing arm axle to the rear wheel.

In the prototype car, the motor was inside the tub chassis under the rear seat and the belts passed through a hole in the back of the tub to the rear wheel. A better design would have the motor and rear wheel assembly completely outside the tub.

Team Trev replaced the original drive system with the rear swing-arm assembly from a Vectrix Scooter. This included a motor with 7 kW continuous power and 21 kW peak. It was easy to install and had enough power, but we could not find a low-energy tyre for the small rear wheel.

The process for selecting a motor is:
* determine torque and power requirements for hill climbing, high-speed travel and acceleration
* select a motor and transmission that can deliver the required torque and power to the drive wheel.

You should select your motor before designing the [[Battery|battery]].

== Motor performance requirements ==

The design requirements for the UniSA prototype assumed that the laden mass of the car would be 410 kg. The torque and power required at the road wheel were

{| align="center"
|- style="background: #cccccc;"
!
!align="center"| torque (Nm)
!align="center"| power (W)
!align="center"| duration
|-
| driving slowly up a 20 percent grade
|align="center"| 290
|align="center"| -
|align="center"| 10 seconds
|-
| climbing a 50 mm curb
|align="center"| 230
|align="center"| -
|align="center"| 10 seconds
|-
| driving at 90 km/h up an 8 percent grade
|align="center"| 150
|align="center"| 13000
|align="center"| 10 minutes
|-
| 100 km/h on a level road
|align="center"| 48
|align="center"| 4700
|align="center"| continuous

|}

Accelerating to 100 km/h in under 10 seconds requires even greater torque and power. Furthermore, getting the required power to the road through a single rear wheel is a challenge.


== Selecting a motor ==

Some general principles for selecting a motor are:

* Trev needs high efficiency at city speeds and low loads.

* A high-speed motor will be smaller and lighter than a comparable low-speed motor. The UniSA prototype used a motor spinning at 6000 rpm, with a 6:1 reduction to the rear wheel.

* A motor with permanent magnets will have greater power per mass and power per volume than a motor without permanent magnets.

* Increasing the motor diameter will increase the torque.

* Air-cooled is simpler than liquid-cooled. Trev does not require a lot of power, so air cooling is adequate.

* Brushless motors will give better performance and control, but require a more expensive controller.


=== Perm PMS 150 ===

The [http://www.perm-motor.de/index.php?id=synchron-motoren Perm PMS 150] is a brushless, permanent magnet synchronous motor. In the 96 V version:

* continuous torque drops from 40 Nm at 160 rad s<sup>-1</sup> down to 20 Nm at 630 rad s<sup>-1</sup>
* intermittent duty torque is 1.5 times the continuous torque for 15 seconds
* maximum continuous power is 13 kW
* peak power is 20 kW
* motor mass is 22 kg


=== Perm PMS 156 ===

The [http://www.perm-motor.de/index.php?id=synchron-motoren Perm PMS 156] is a brushless, permanent magnet synchronous motor. In the 96 V version:

* continuous torque drops from 48 Nm at 160 rad s<sup>-1</sup> down to 27 Nm at 630 rad s<sup>-1</sup>
* intermittent duty torque is 1.5 times the continuous torque for 15 seconds
* maximum continuous power is 17 kW
* peak power is 25 kW
* motor mass is 28.5 kg


=== Vectrix scooter ===

Team Trev used a Vectrix Scooter rear end to drive around the world. The motor is a brushless, permanent magnet, radial air-gap motor: 

* motor torque is 22 Nm continuous, 65 Nm peak
* motor power is 7 kW continuous, 21 kW peak.

The motor drives the wheel through an epicyclic reduction gear with a ratio of 4.6:1.

The mass of the rear-wheel assembly (swing arm, motor, reduction, wheel, tyre, brakes) was 28 kg.

'''Advantages:'''

* the torque and power are good
* the mass of the package is low
* it is very easy to install.

'''Disadvantages:'''

* we could not find a low-energy car tyre to fit between the swing-arm forks
* the motor and reduction gear are noisy
* black dust from the dust seal (!) clogged the optical encoder after about 25000 km.


=== Advanced DC X91-4001 ===

The original version of Trev used a brushed DC, series wound motor.

This motor is overpowered&mdash;it can deliver 9 kW continuous at 96 V, and up to 40 kW. We were running at 133 V, which gave us even more power.

The mass of this motor is about 40 kg.


== Motor controller ==

The type of motor controller required depends on the type of motor.

=== Brushed motor controllers ===

With brushed motors, the brushes within the motor switch current between motor windings. The motor controller regulates the torque and speed of the motor by controlling the current through the windings.

The prototype Trev driven from Darwin to Adelaide used a Curtis 1231C series motor controller with an Advanced DC X91 4001 series wound brushed motor. This combination was too powerful, and did not allow enough control over acceleration at low speeds.

Alternative controllers:

* [http://www.alltraxinc.com Altrax] make controllers for 72 V brushed DC motors, and provide some good overview material on control and system design matters.

* [http://www.curtisinstruments.com Curtis] make a variety of controllers.

* [http://ecomodder.com/forum/showthread.php/paul-sabrinas-cheap-144v-motor-controller-6404.html Open Source Motor Controller]  Members of the Ecomodder community are cooperatively developing a 144 volt 500 amp controller.  Estimated cost is around $150-250 unassembled


=== Brushless motor controller ===

With brushless motors, the motor controller must control ''commutation' (the switching of current from one set of windings to the next), and regulate the direction, torque and speed of the motor by controlling the winding current.

For their trip around the world, Team Trev used a  [http://www.semikron.com/ Semikron] SKAI 4001 GD06 1452L (air cooled, 450 V DC link, 245 A continuous) power stage with a custom-built control stage. The Semikron power stage was too large and overpowered (we hacked off half the heat sink!), but was the smallest power stage available at the time that could handle the 140 V battery.

Alternatives:

* [http://ecomodder.com/forum/showthread.php/paul-sabrinas-cheap-144v-motor-controller-6404.html Open Source Motor Controller]  Members of the Ecomodder community are cooperatively developing a 144 volt 500 amp controller.  Estimated cost is around $150-250 unassembled

* [http://www.newkellycontroller.com/ Kelly Controls] make a variety of controllers for brushed and brushless motors.

* [http://www.semikron.com/ Semikron] are now making smaller power stages.
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