Permanent Magnet Motors

mullhound; a full wave 1 or 3 phase bridge, of suitable voltage to match motor & line voltage, maybe change the overload settings, or buy a D.C. motor controller. perry

Very good answer edykes!

Unless you get a large energy saving from variable speed, don't change them.

Also consider in the cost of variable speed a reduction in reliability and the possible problems caused by harmonics. You will also need training for the maintenance guys and the eventual service call from a drive expert.

But if your process can be optimized by using variable speed (permanent magnet motors, or induction), it might be worth the trouble. It rarely is for only the motor losses.

I think it almost impossible to say on a general basis. The difference in efficiency varies with the size of the motor and precise details of construction. Without really looking closely at the specific numbers, I'd say it would be very hard to justify a conversion from a largish induction motor (perhaps 50 hp and up), simply because high efficiency induction motors are already very efficient (and large PM motors are relatively hard to find).

In an application in which an induction motor can be started with w/ a simple contactor and runs at relatively high load all the time, I think it would be very hard to cost-justify a conversion. On the other hand, although the industrial trend has been toward VFD drives and induction motors (and away from variable speed DC drives, when variable speed is required) in medium sized motors, there will always be smaller PMAC and BLDC motors in use for both efficiency and control reasons.

When you consider the fundamental differences in the shape of the torque and efficiency curves for PM and induction motors, recommending one over the other becomes even more difficult without actually doing the math. Throw in the variable of an existing installation of a motor which is functioning well, vs ripping that out and going through a new installation, and it becomes even more difficult to generalize.

In my field, the electric vehicle world, there are examples of both induction motors (the Tesla, the GM EV1) and PM motors (most hybrids) and a good case can be made for either alternative (and, in fact, a reasonably good case can be made even for older tech series DC motors, brushes and all). So even when starting with a clean sheet, motor selection can be a complicated process, and that process is further complicated when replacing one motor with another. A justification for an induction motor in an electric vehicle often cited is that one can eliminate a multi-speed gearbox, but this is not really the case in practice, unless one is willing to sacrifice performance. The Tesla is a good example of this, in which an overriding design goal is very high performance, but the car can "only" do 120 mph, which is about 30 mph slower than current fast econocars (like the Subaru WRX). Gear the Tesla for the top speed of comparably expensive sports cars (180 mph or so) and the acceleration is no longer competitive. The torque characteristic of a PM motor is such that eliminating a gearbox is really pretty much out of the question if you want to achieve good low speed grade climbing, good top speed, and good acceleration.

I think the short answer is that each potential motor replacement must be evaluated on the basis of efficiency (though the entire installed operating range of the motor) replacement costs, the cost of money, the cost of energy, the energy policy of the corporation (to what extent does the company want to spend money to be green), recyclability of the new and old motors, etc. You might find that if your plant has a large number of small motors (3hp and less, maybe) then perhaps the conversion would make sense... maybe... and even then, only if the motors were nearing the point at which they might need to be replaced or refurbished.

Re site and starting requirements: typically there is no fundamental difference in site requirements -- all sorts of motors and drives exist side by side in industrial settings, with the vast majority of motors being fed (in the US) by 480 VAC, which is then converted as required to fit the motor and its operating mode. Starting, operating speeds, etc. depend upon specific motor applications. (Very large motors, such as those controlled from switch gear rooms in which the operating voltages are in the thousands are unlikely to become permanent magnet types.)

Thanks guys appreciate your responses, it has given me much insite into the matter.

To clarify my problem more specifically, there are a large number of small 'ish induction motors(below 90KW) that i am looking to replace. At the present time when one of these motors fails it is cheaper to replace the whole motor than to repair it, so this is where my interest in PM motors came in!!

So a study of the application and all round costs of the replacement needs to considered, so will get involved in this research.

Thanks again

Alastair