Electric Motors

Posted by starstuff on Thursday January 01, 2009 5:14pm,UTC
Article Source: http://EzineArticles.com/expert=George_Christodoulou

"There are basically two types of motors: the AC conversion and the DC conversion. The AC conversion is more expensive but gives higher acceleration and can make the car carry more load. It is easily charged and will sustain the charge for long. The AC conversion would also last longer. The DC system is cheaper but will carry less load. It is not as durable as the AC system and has a short life span.

"The AC system would fit larger cars and heavy trucks that are been converted to electric. They provide better traction and can withstand heavy usage. It is best for converted cars that need to do a lot of out of state traveling. The DC systems would however suit a light truck mostly used for city driving. It cannot provide the high speed that the AC system offers but would be strong enough to move smaller sedans."

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"The DC motor offers 96 to 192 volts. This is the type you will find most in many electric cars. Originally this is electrical system used by forklifts. DC systems are easy to install and do not cost so much. It may give limited acceleration and will heat too quickly. You may not be able to drive this kind of motor for a long time. The DC system may not be able to give you top speed. It is specially suited for small cars and light trucks.

"The AC system is usually a three-phase AC motor running at 240 volts with a 300 volt battery pack. An AC system is also easy to install. You would be able to use any industrial type motor. Whatever the power rating, you may be able to fix this type of motor in your electric car. Many a time, this type of motors have something called regenerative braking which will convert the motor into a generator to recharge the batteries when the car stops. The AC system will give you more power and better acceleration. It is well suited for larger cars and trucks."

There are a lot of electric motors out there, but just a few make up the majority of the motors being used in electric cars. When people choose a motor for their car, they're balancing factors like cost, availability, and the "do it yourself factor" - the confidence that they can get all these expensive gizmos installed and not just wind up with a project scattered all over the garage...a well-known cause of marital discord; ) - more

DC Motors

There are four main types of DC motor, namely permanent magnet, series, shunt and seperately excited. The latter three all use field coils in the stator (the part which doesn't move) to generate a magnetic field for the rotor to spin in, and their name simply refers to the way the field coils are wired with respect to the rotor coils. All four types use a commutator to control which rotor coils are energised at any given time in order to maintain rotation, and it is enough just to apply a DC voltage across their terminals to get the motor to spin, so they are relatively easy to control.

Currently series DC are the most economical and commonly used type of motor in electric vehicles. Being a tried-and-tested technology, they are actually quite good with efficiencies up to 90% and only needing servicing every 100,000kms or so. However using a commutator is restrictive and a source of inefficiency. Also, with series DC motors regenerative braking is very difficult to do (in fact, you basically have to operate the motor as a sepex DC motor). Regen can increase your range by 10-20%, so is quite valuable.

The most popular brands of DC motors for EVs are Advanced DC and NetGain.

Permanent Magnet DC Motor

DC Motor with wound stator

Brushless DC Motors

In a brushless DC motor (BLDC), the rotor has permanent magnets and the stator has an electronically-controlled rotating field, using sensors (rotary encoders or back-EMF) to detect rotor position. As such they have no commutator, and tend to be more efficient and more powerful than commutated motors. They do require a more complicated motor controller, although as the technology matures and costs come down they are becoming increasingly popular, particularly for smaller motors.

The main disadvantage for EV use is the cost of the large permanent magnet(s) required for the rotor, and the added expense of the speed controller. Unfortunately, at present there are no economically viable BLDC options available for EV use. ZEVA is hoping to change this in the near future.

Two example manufacturers currently producing good brushless motors are UQM and Aveox.

Brushless DC Motor ("in-runner" type)

AC Motors

Although there are a variety of motors which will run on AC power, for EV use it usually refers to AC induction.

The operation of induction motors is a somewhat difficult concept to grasp at first. Basically they use a rotating magnetic field in the stator to induce a magnetic field in the rotor and hence a current to flow in the rotor's coils. The rotor coils actually just loop around on themself - they are not explicitly powered. The induced field in the rotor tried to stay aligned with the rotating field of the stator, so it turns to chase the stator's field. Due to loads on the motor, the rotor's field is forced to rotate slightly slower than the stator's field (if it kept up exactly, there would be no difference in the fields and hence no torque).

Three phase induction motors are very common for industrial use because they are highly efficient and reliable. These same advantages apply for electric vehicle use, except for the added complication that a variable-speed inverter is required to control the AC motor from a DC power supply (the battery). These are a relatively expensive piece of hardware. Although they do include regenerative braking and are generally more efficient, AC systems currently cost about twice as much as series DC.

The most popular brands for AC induction motors suitable for EVs are Siemens and Azure Dynamics.

AC Induction Motor

The Future

Currently, the most economical (and indeed common) option for electric vehicles is series DC technology. However, it is likely that all commutated motors will be phased out over the next decade or two, since a commutator's functionality can be replaced by clever electronics (which is getting cheaper every day), and with improved reliability and efficiency. At present, AC induction and permanent magnet brushless DC are the best technologies available, with efficiencies up to 98%, silent operation, and almost never requiring any servicing. They each have various advantages and disadvantages over one another. It will be interesting to see which one becomes the new standard in the years to come.

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