Why Will Future Automobiles be Electric? – How to Choose a Motor for Your Electric car

By John Talbot

(Under the Direction of the Lord Jesus, Prov 3:6; Col 3:17)

In this article we will show how space and weight can be saved by the correct choice of electric motor. The desirable characteristics of an electric motor for a car are:

1. High power to weight ratio

2. Small size in relation to power

3. Reliability

4. Suitable torque characteristic

5. Availability

6. Moderate price

7. Established technology

The electric motor was invented in 1828 by the Hungarian, Anlos Jedlik, who used his first motor to propel a model electric car. One of the first practical full-scale electric cars was produced by Thomas Parker in 1884, using a low voltage DC motor. The electric car is therefore older than the internal combustion engined (ice) car. The first in-wheel motor was patented in 1890.

Possible choice of motors include:

11. Conventional DC traction motor

12. In-wheel motor

13. Universal (AC/DC) motor

14. Induction motor (single phase or 3-phase)

15. Synchronous motor

The conventional DC traction motor 11 is still used for some cars but ranks poorly in respect of characteristics1 and 2. It would generally be considered obsolete. Some commercial electric cars have appeared with in-wheel motors. These are somewhat specialized and rank poorly in characteristics 1, 5, 6, and 7. Universal motors 13 are the heaviest for a given power and fail in respect of 1. Induction motors, especially the three-phase version, 14 score high in all 7 characteristics,. Synchronous motors also score high in all 7 characteristics except that of a suitable torque characteristic 4. The three-phase induction motor is a clear winner and would be my first choice.

Most three-phase induction motors available in North America are designed to operate on a frequency of 60 hertz (60Hz). Elsewhere, a frequency of 50Hz is the most common. An induction motor operating on a fixed frequency is essentially a constant speed device. By varying the frequency, it can be turned into a variable speed motor. To avoid damage to the motor, the voltage should be kept proportional to the frequency. Motors designed for 50/60Hz have a power-to-mass ratio of about 250 watts per kilogram (250W/kg). Induction motors designed to operate of 400Hz have a power-to-mass ratio of 1 000W/kg, or better. They are much smaller and lighter than 50/60Hz motors. They are not as readily available but there is a manufacturer in the United States that produces a wide range. Most aircraft use 400Hz electrical systems and aircraft manufacturers may be another source, but their products tend to be on the small side for powering cars. 400Hz systems used to be a standard for ships, but ships have got bigger and and space and weight are less critical, so they have been moving to 50/60Hz. Places that scrap old ships may be another cheap source.

In this article we have seen that the 400Hz three-phase induction motor is the ideal type for powering our electric cars. In the next article in this series we will see how to make a Variable Speed Drive (VSD) for cars.