Servomotors drive green machines

Edited by the Engineeringtalk editorial team Apr 29, 2004

The novel Oxygen Lepton electric scooter is powered by a defluxable permanent magnet motor designed by Lafert Servo Motors as a derivative of its catalogue range of brushless servomotors.

Many different types of electric vehicles (EVs) are available on the market today but few road-going vehicles have made it into the factory production volumes that bring economies of scale and some financial stability to what are usually struggling R and D companies.

One EV emerging not just as an alternative for the "tree hugging" community but now seriously competing with its petrol rivals at all levels is the electric scooter.

Oxygen, of Padova, Italy, has produced and sold more than 4000 Vespa class electric scooters in the past two years - half in Italy and the rest around the globe, to both individual users and for corporate/government fleets.

An illustration of this success is the recent delivery of 120 custom-designed Oxygen Lepton scooters to the Milan traffic patrol department where two of the extra features fitted were flashing lights and a siren.

The obvious benefits of EVs to the inner city environment have resulted in a raft of incentives being offered by European government bodies.

Most involve cash rebates, as in the UK Transport Energy scheme, where environmentally friendly vehicles are listed on the Powershift register and electric scooters can qualify for a GBP 200 rebate from the Government's Energy Saving Trust.

In addition, electric scooters are exempt from road fund tax and congestion charges.

Battery technology has been critical for the development of the EV.

A shareholder and key supplier to Oxygen is Evercel which provides state-of-the-art nickel-zinc (Ni-Zn) battery packs.

Not only are Ni-Zn battery packs supremely reliable (guaranteed for two years or 12,000km) they are almost half the weight of the more traditional lead acid batteries.

This gives the Lepton scooter a range of 50-55km, depending on riding conditions and terrain.

Furthermore, the batteries can be recharged in only one hour to 60% of full charge.

Two factors determine the choice of the drive motor: torque and efficiency: torque, because the more constant the torque curve the better the ability to accelerate and climb hills at low speed through a belt drive transmission that requires no gearbox, cutting out weight and cost; and efficiency - not only for the obvious battery life and hence range of the scooter but for the ability to produce more power when needed.

For this application, Lafert Servo Motors, a Lafert group company, developed a defluxable permanent magnet motor as a derivative of its catalogued range of brushless servomotors.

This motor develops 1.8kW at 7000rev/min and produces high torque at low speeds, maximum torque coming in at 2000rev/min, and reaches a high speed (7000-8000rev/min) keeping an acceptable balance of torque and efficiency across the whole speed range.

In addition to not wasting energy during stationary ticking over, the electric motor also offers the advantage of using the permanent magnet motor's energy recovery capability during deceleration, and so the batteries are recharged during braking.

The control system to convert DC to AC and regulate the speed is a specially developed digital technology, MOSFET output drive.

Its output current is between 60-100A RMS on an intake voltage of 48V DC.

The output voltage to the motor is 35V RMS.

The drive receives a command from the handlebar and exchanges information with a small panel as a reference that changes from "sprint" or "standard" and delivers the output accordingly.

The combination of the Ni-Zn batteries and the efficient motor drive technology from Lafert without the need of a gearbox makes the Oxygen E Lepton model competitive with a petrol scooter of 50cm3.

At GBP 2750, the Lepton is more costly than a petrol alternative at around GBP 1700.

but has far lower running costs.

And with Powershift approval imminent, the gap is closing - particularly when considering that recharging is around 8p per go, the equivalent to around 0.16 litre per 100km, against the petrol model of 2.35 litre per 100km.