AC Motor

why we don't use DC Motor on heavy load like 700 hp compressor ?

Where do you use DC motor ? why ?

The electrical motor is used as primoover.

The selection of motor is considered with respect to the task. i.e type of equipment to drive ., whether it is required to start in un load ? OR with load. If with load what is type of load ? ie. whether it is pump ? compressor ? centrifuge ? agitator ? textile ring frame ? looms ? sizing m/c ? Warping ( In warping and ring frame there is increase in load gradually with respect to winding of yarn or Bobbin OR on warping drum ? etc.

Some time on load also the application like blower, ID Fan etc.

All above has to study torque - speed characteristic for the selection of motor and than the starter / drive etc.

The AC motors are available for most of type of application, which is simple in design for operation and maintenance.

nascon

The simple answer is that your power source is usually ac.

And if the power has to travel any distance DC is not efficient.

High voltage DC trasmissions are used for distant transmissions , due to non skin effect on dc small conductors also lesser can carry equal ammounts of power to long distant but at extremly high voltage 20,000kv or so

Thanks vikas, But I can't let you off so easy teasing me with new knowledge.

What distance?

What insulators at 20,000,000 volts?

Besides a Flyback transformer (AC) how do you get 20,000Kv?

What type of Amperage are we talking about?

And how do you keep the terminals, brushes, windings, etc. from arcing?

Thanks again

Brad

Hello UV

This is not related to dc generation and transmission so brushing ,arcing ,winding ,losses cannot be considered, it is basically AC generation but under certain circumstances it becomes desired to trasmit power over DC transmission line this is quiet economical ,were large amount of power is to be transmitted over long distance from remote generation plant to load centre . this breakeven distance for HVDC overhead transmission lines lies somewhere between 300-400 miles and lesser for underwater cable transmissions

Here you get improved transient stability also dynamic damping for electrial sytems oscillations apart from power losses , again it is possible to connect two AC systems with two different frequency , or not synchronized by means of HVDC system.

It is also done with rating range of 69 -230KV , it is done by first transformed to transmission level then rectified and applied to HVDC line , at the other end DC power is inverted and matched the AC voltage and than distributed towards application, to be more specific some utilise 12pulse converter at transmission end for rectification ,also filters for reduction of current harmonics generated by converters preventing from entering ac system , also power factor correcting capacitors to supply lagging reactive power at both ends .

This is not new knowledge , but also not used widely due to complex controls and setup also distance governing , most power generations and distributions networks are shorter in distance , amperage is just same that you are transfering from one side to other , insulation is also issue and can be disadvantage for this side of transmission, you require expensive cables with insulation built , but transmission efficiency over rides this expense in long run

Little correction it is not 20,000KV as i stated earlier since i studied it as a topic in university 14 years back and also busy answering other threads in the forum simulteneously , sorry for that , it is definetly too dangerous voltage, that too DC

Pardon me for butting in, but I agree with vikas, on the DC transmission statement.Many long-distance high power lines are DC.It simplifies interconnection of two or more grids, and has less total losses. The reason A/C has become the primary standard is because it is easily transformed from one voltage level to another,not because it was more efficient than DC. Hi temperature superconductors are also now being used to carry large amounts of power, and Long Island is one of many examples of it's use.There are even underwater superconductors in use for high power transmission.A lot of new technology has been implemented in power distribution in the last few years. If interested,you can subscribe to Transmission and Distribution Technology and be updated frequently by email on the industry and trends. HTRN

Forgot to add a link to subscribe.Here it is tdworld@pbinews.com

Not too long ago, some fairly large motors were powered by DC drives, in the interests of variable speed control. (Now, variable frequency AC drives are more frequently used). But for single-speed use, it would make little sense to rectify AC to DC just so you could deal with the hassle of brushes.

I have found that eddy current a.c. drives have filled my need for speed control of large (hp) drives. The variable frequency drives tend to overheat and fail in the applications we use. D.C. will not fit our needs as we need high torque starts and would have to distribute the power to many machines. The eddy current drives fills that niche and works well without the problems of D.C. drives and current distribution to 100 to 125 machines in our shop. Most of our problems with motors is wear of bearings. We have such loading that bearing seem to be wearing out on us. But we are solving that with newer motors that have larger bearings and longer service lives. Some of our motors are 30 plus years old. 30 to 150 hp. Some of the ships I was on in the past (1960's) were D.C. powered. Short transmission lines and continual variable control. Also they were powered by large steam plants or huge diesel motors coupled to generators. Most of the D.C. motors have been replaced by 3 phase electric motors to do the driving.

Wow... eddy drives. If it works, that's good, I suppose, although it would bug me to have so much energy going off as heat. But maybe in your application you are running the motors with little slip much of the time and using something close to their synchronous speed most of the time?

Actually we loose very little in the way of heat when you consider that the motor part is turning at rated speed and the 90 volt eddy current coil is not really getting hot. The eddy current clutch is not touching anything and the magnetic flux is what makes the drive transfer torque and motion. Really a slick idea and works well to control the speed of the main shaft. We run at speeds of 5% to 100%, most of the time we are working about 50%-60%. This also lets us determine the speed of acceleration when starting machinery from cold. Slow spin up to speed with everything normalizing is better than just going from 0 to 300 rpm on a 20 ton mass. Allows everything to come up without breaking things or just making that burned belt smell we would get from trying to go to rotational speed with a normal motor.

Pretty neat. I've run into zillions of AC and DC variable speed drives, but only one or two eddy current drives -- and even those were used as shock absorbing slip clutches, mainly for start up. I've been thinking of them as somewhat outdated, more or less in line with articles like this one. I'll have to revise my thinking regarding them -- it sounds like they work very well in your application. Thanks for the update.

Check this out http://www.dynamatic.com/default.asp?id=3

We use them in many applications and the process works well in startup were you need a lot of torque.

Funny -- this has to be proof that deranged minds think alike! Just yesterday I spent some time looking over that site.

I've been casually looking for a clutch for a project I have underway, and hadn't really considered an eddy current clutch. But the lack of mechanical contact and wear is really appealing. In this particular application, the 2% (typical) slip is an issue, as is the fact that energy is required constantly to keep the clutch "engaged". However, yesterday I came across a patent for an eddy current clutch that mechanically locks, which seems like the best of both worlds. Given you'd only lock it when slip was low, the mechanical wear would be virtually zero. Although I haven't read through the patent, I'd assume that energy required to keep the thing locked is very close to zero too.

Thanks again for opening my eyes.

Compressor need not high torque even though its 700hp, because DC motor apply in used when high torque loads are required.

The construction of 700hp DC motor will be quite bigger than AC motor, take a lot of spaces.

Cost and efficiency view, 700hp connected to 690V or 1000V. Utilities supply power in AC, it then need to convert AC to DC, I think this will be more complicated than to directly choose AC voltage system.