Why Not Use Permanent Magnet Rotor in Motor or Generator?

Because they are not adjustable for voltage regulation.

Thanks for reply. I see that....but I suppose one could switch in and out field coils and so change the voltage that way. I know that would give steps of voltage change rather than smooth control. I realise I dont know much about this topic and I have been trying to look around the internet.

They exist under 'brushless' motors. In the smaller power range. A stepper motor can also one be one. They require some electronics to make them spin.

Yes you are right. I have seen photos of these where the rotor is on the outside of the stationary electromagnets. Thanks for your post. david.

Permanent magnet generators are used in

(a) Small wind generators.

(b) Generators which operate on constant load & therefore voltage regulation is not a criteria.

(c) Any application where electronic control is available downstream for voltage regulation and output of this control device is used to supply load.

As has been mentioned already, power rating is generally upto few kW.

Thanks and regards

Ashok Toshniwal

David,

I have re-read my post and it contains a couple of typos.

1. Is the omission of the indicator ' to DC', after 'rectify'. So, it's "rectified to 240v dc" and the the frequency, Hz, is added.

2. And, it's better quality power 'than' from the grid.

Sorry,

Cheers, Stu.

Thanks for your reply. I saw that your post had a few typos but I had understood your meaning anyway. I will look up the modern inverter types. Thanks. david.

google "magneto"

Yes, I had thought of magneto.. but for some reason I thought these were spinning coil rotors inside stationary magnets...my assumption could well be all wrong and I will go to google to find out. Thankyou for your post.

Most all of my motorcycles use a spinning spinning magnetic rotor with the coils dispersed around it in a radial fashion. Usually not all of the coils are wound allowing different different output voltages depending on the number of coils that are wound.

Google 'stator motor' under their Images heading.

Using PM's in the rotor is common. However, using electro magnets means you can get a much higher magnetic flux hence higher efficiency.

Like many engineering designs there is a tradeoff between initial cost, operating efficiency, reliability, maintenance cost and size/weight.

Yes, there are permanent magnet alternators.

Motorcycle engines typically use permanent magnet generators but have to use a shunt or pass regulator to maintain the correct voltage. You will usually see the regulator with a bunch of cooling fins to dissipate the excess energy produced, especially at higher RPM's. Electrical loads are usually pretty well defined so the designer doesn't usually have to worry about "add-ons" and can size the generator to be just large enough. Admittedly, this is inefficient but the process of adding brushes and slip rings for a field coil doesn't make sense given the reduction in reliability and increase in costs. I'm sure larger touring bikes probably have a more sophisticated alternator to handle all the extra electrical loads.

Motorcycle engines typically use permanent magnet...

This might better read: Small motorcycle engines often use...

Many of the mid-sized motorcycles you see on the road in the US (from about 350cc to 1000cc or so), which tend to be on the large side vs, for example, those in India or China, use alternators in which the fixed field coil magnetizes a rotor (which looks like the rotor in a car alternator, but without the field coil and brushes), which, in turn induces a current in the charging coils. No brushes, can be bathed in oil, can be regulated etc. Neat design, simple, but I'm guessing not as efficient as the car style, because:

Some larger-yet motorcycles use air-cooled alternators like those in a car.

I got 1820cc's and my alternater is just as you described. It is a large touring bike with great gas mileage.

'Permanent' magnets wear down if the polarity isn't periodically reversed.

Something like this would work...

permanent magnet r not used because we can change the voltage by elctro magnet but we cant change with permanent magnet and there is other reason is that there is more hysteresis loss in permanenet magnet