AC or DC Motor?

(1) DC motors are not cheaper. Polyphase squirrel cage motors are much cheaper. 3 phase motors are most commonly of the integral HP variety.

(2) VFDs ramp motor speed up and down just as fast as DC drives.

(3) SCR controls for single phase motors work just as well as DC drives. Single phase motors are most commonly of the fractional HP type.

Modern elevators use 3 phase motors instead of the old DC units.

DC motors, except for very specific applications, are on their way out in the integral HP sizes in most industies.

One manufacturer even has a line of small integral HP motors with the VFD built right on top of the motor instead of a terminal box.

Ac motors are cheaper.

They don't have commutators and brushes to maintain. DC motors are high maintenance.

AC motors generally have lower inertia than DC.

AC VFDs are generally more expensive than DC drives. Especially if regenerative power is required.

DC volt amps and torque are easy to measure and predict.

VFD volt amp torque is close to magic to measure.

VFD is far more complex than DC drive.

VFD generally has close to unity power factor. DC power factor is approximately equal to the output voltage to the motor divided by the rated output voltage, so running at 50% speed (constant flux) it only has about 0.5 PF.

AC VFD drive system can have much faster response than DC phase controlled rectifier.

For constant hp operation as in a center winder I still prefer DC with a field range. AC drive system does not match the ease of design or operation of DC drive system.

For high response speed control AC is superior, but the drive train has to be stiff without backlash to realise the potential (experience is integral and large hp sections for Paper machine drives).

Personally I am still partial to DC for many applications, but AC is rapidly replacing it.

I am still waiting to see the life expectancy of the large electrolytic capacitors used in AC drives. There should start to be a large failure rate shortly in the Pulp and Paper industries as the service hours are exceeded. These caps are not used in DC. I am predicting DC drives will outlast AC VFDs. Ac motors will outlast DC motors. Time will tell.

Your question is very broad. It is like asking which is better - A car or a truck?

It all depends what you want to do with it.

Three phase AC induction motors using new technology and windings have managed to improve efficiency dramatically in the last decade and because of the power grid supplying AC voltage they are the first choice motor for general applications.

However this does not mean that DC motors cannot be superior to AC in some applications.

Also there are servo motors which are usually best as the brushless AC servomotor type.

Hope this helps.

I agree with the other comments made and add :-

DC brush type motors can be quite electricaly dirty, there are EMC considerations.

They are mechanicaly dirtier too.

Like for like AC motors are speed stable.

Dynamic braking on DC motors is easy.

DC motors allow easy regeneration, some energy recovery.

Stating these extra points may lead the reader to think I favour AC motors, that is not true, although I use lots of single phase to 3 phase inverter driven motors, this will allow a motor an greatly extended specification, other that that is written on it's plate.

There are applications that suit both, it is the application that should determine choice.

Nobody mentioned brushless DC motors, 3 phase with electronic speed control, not cheap, but good torque and stable runing and / or speed contol - whatever.

In some applications they are great, but for "normal" use today, most AC motors have very good qualities and are also cheaper to produce for a given HP.....

Please leave out "universal" AC motors as they suffer in all directions it would appear and are only really good enough for hand tools such as electric drills etc, with short usage and plenty of cooling time....

To answer the original question, it depends on the application.

Does any one still use brush DC motors?

You will find the highest power density in brushless DC motors, and efficiencies can be in the low 90% range.

For near and around zero speed, it's hard to beat brushless DC's torque.

I think the electronics prices are comparable for both, I'm sure the brushless motors are more expensive, due to the permanent magnets.

DC motors are still widely used because batteries are a main source for portable power. I tell you it's much simpler to implement a small brushed DC motor for power tools, etc. where mechanical and electrical cleanliness aren't that much of a concern.

I think we are getting answers from 2 different groups.

Larger HP (3 to 1200+HP), and another group that are using them in fractional to several HP.

Brushless in higher HP is not generally seen, but in fractional it is more common.

In industrial Large HP group we see AC replacing DC, but as I previously mentioned I still have a preference for DC for some applications.

Application drives the choice!

If you need very high torque at very low RPM need an DC motor. Or an AC motor most bigger. DC motors have an zone of constant torque from 0 to nominal RPM. Over this have an constant power zone where torque decreases as RPM increases. In some cases (lamination trains, test benches, cranes) this is an advantage.

Hi there thankyou for your views. But tell me, I have seen the torque curve of a DC motor it dips at lower RPM. It reaches to about 60% of the full load. If we have to maintain a constant torque, then I understand you may have to resort to forced cooling methods as in the case of AC motors. Even then the torque I understand cannot reach the rated level! Is it so?

In general I am interested in low speed high torque applications. Hence this question. I realised I need to be more specific. By the way thank you all for all of your comments. I learnt a lot.

For most wide speed range applications forced cooling is required for any appreciable sized motor. Example, Center winder for paper machines, 300Hp.

Most industrial DC machines will develop 150% rated torque up to base speed for 1 minute.

Some motors are designed for up to 200% rated torque for 1 minute.

You will find that in general the torque per amp above 100% will not be exactly linear. It may require 160% current to develop 150% torque.

Above base speed (in the constant HP range) the torque drops off as the flux is weakened. Torque in field weakened range is (base speed)/(running speed) x rated torque.

You may also find the commutation limit in field range is not so good. 150% OL current up to base speed, tapering to 100% with a 2 to 1 field range is common.

All motors of appreciable size require forced cooling at low speeds, with the exception of TENV motors for both AC and DC machines. Ventilated motors with a shaft mounted fan lose their cooling very quickly with low speed operation, so if you need a wide speed range operation you require separate ventilation as with drip proof force vent motors.

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Dear Guest, why are placing an advertisement for petrol engines on a blog from CR4 to do with AC or DC motors. Is it just to get free advertising?

I personally will now never buy any products on principle that would appear to be produced by you and or your company. I will also mention this to other people and state my case as to why I feel the way I do.

I'm lost when it comes to electric motors. I do have a question you folks may have answered already and I just missed it. The question is, I have a drill press that is about 1 horse power & using pulleys to change RPM with you know just like you would buy at Sears and the motor has died. So when I replace it, what should I buy to replace it, I mean most of the work I do with it is deburring at a low rpm but not always and it has never been strong enough. Thanks for any help you may have,,, Chuck

For a drill press the usual choice is AC. 1 HP is already fairly hefty and is the size in small machine shops. (unless it is a Sears 1 HP, and then it is a funny number that represents the peak power as the breakers trip and you let a little smoke out of the motor. Sears does not put much smoke in their machines so after you let a little out a couple times it is no longer functional. You can't put the smoke back in.)

The belt changes allow you to slow the chuck down and increase the torque.

In my shop I have a JET 14. I originally ran it with 1/4 HP and was disappointed. Switched to 1/2 HP and have been happy since. Holes up to 1/2 inch in mild steel. Only occasionally used, and not in production.

The size of holes you are working with, RPM and feed rate determine the HP.

I also have a small Mill / Drill machine and it has 2HP. Much more capacity and it is unstoppable up to 3/4 inch or a 3" fly cutter.

Condition of tooling has a big impact and a dull drill can easily take 1.5 times the power.

I don't know your skill level, but many people think drills should last forever. The home mechanic box of dull drills. At production rates some tool life is measured in minutes of operation. The tooling is essentially a consumable item.

"Machinery's Handbook" from Industrial Press, LC card number 72-622276 provides the information on how to calculate the power required for optimum production rates. I highly recommend you use your local library, or if you feel rich, buy a copy.

Thanks GW, it is just one of those questions I've always wanted to ask someone who new better than I do. As far my skill level goes, I am the owner of Hi-tec Tool, Inc. here in South Carolina. Our shop has 2 CNC Mills and 2 CNC lathes along with all the normal manual support equipment such as manual Lathes & Mills and a full Fabrication shop on the other side. Anyway thanks for your answer,,,,,,,

Chuck H.

Green with envy over your shop tools.

I am the owner of Advanced Systems Integrators in Surrey, British Columbia. We specialize in industrial variable speed drives and the MMI and mill wide information interfaces. Mainly in the Pulp and Paper and Lumber industries. We have also designed controls for container cranes at the ports, Rolling mills, automatic guage for a rolling mill, extrusion machines, and printing presses. We have also done digital drive upgrades for municipal water treatment plants, 1200HP DC machines.

One of my interesting challenges years ago was to replace Russian made drives for X,Y,Z axis on a large milling machine. There was 30HP on each axis. Salesman thought it was some low response (Russian) equipment and sold standard DC drives. We had to upgrade to our best drives to get the response required! The Russian drives had outstanding performance, just no parts or service in North America.

Great GW, Hell I love to get into new things and all the bigger companies around here have gone the china way so work is slow, send me some work and I'll go to the field for ya,,,,,but don't be envious of my shop tools as I got all them the same way you got what you have, spilled blood,,,,,,lol,,,,I'm an ole'fart now so I do really get to enjoy my machines much more as there is hardly ever a push anymore.

Years ago I along with 4 other guys opened up and started the first plant in the USA for Eurodrive, or you may know them as SEW power transmissions. I was over the production CNC mills and Lathes along with any support equiment. That was back when I was much younger and 36 hour work days happened a lot.

There is something good for the soul working on the tools. I drive a desk 10 to 12 a day out of a home office. The occassional tinker in the shop really clears the mind and is very calming. The satisfaction of actually producing something is really nice.

Home office kills the commute, but you end up chained to your work.

Like a slave you are never far from the work. You have to plan to get away from it.

Hello all!

I have new doubt and a new question. Thought it would make more sense if it is asked in here! Does anybody know anything about Hybrid permanent magnet motors? I have some perceptions about it! I understand they are fast coming into low speed applications!

i have twisted , mangled and beat the hell out of just about every motor out there testing shit for the navy. i can tell you the best all around motors that stood thru heat, and i mean smoke coming off the wires, starting and stopping a hundred thousand times, and being dropped, bolted and unbolted, taken apart and ressambled, sometimes without all the screws: were the single phase washing machine motor( those are some tough little bastards) and 3 phase motors ac induction type. hell we even figured out how to use large hp 3 phase motors that were rated way over our requirements, run them on 1 quarter of the voltage, use flywheels and get 3 or 4 hp for 120 volts and 7 or 8 amps. in the end if you can afford the weight increase, the size increase, the price, and the extra figuring and riggering, the 3 phase is the best even on single phase power. it runs cooler, has bigger bearings, is overated for the work load so it will out last them all. example: you need 2 hp, 1700 rpm, 60 inlb torque. use a 3 phase 8-9 hp motor wired for 460vac, put about 60 uf on the third to second leg and 120 vac on the first and 2nd, kick it off with a temporary start cap and you got about 2 hp for about 700 watts. with the flywheel of dense mass you can torque it on up. this is the navy telling you this. trust me we know big bulky heavy shit, just ask my lower back.....troyboy76127@ayhoo.com