Design a DC Equivalent AC Motor

Something must be suffering in this translation....

There are attributes of a synchronous induction motor that a series or servo DC motor cannot perform. Similarly there are attributes of a DC motor that a synchronous induction motor cannot perform. I recommend that you take an electric motor design class before trying to design any electric motor. I warn you, there are a few prerequisite classes you will need first.

In order for you to replace a DC motor with an AC induction motor the following minimum standards must be true:

(Stated in layman's terms.)

AC Motor Horsepower/KW = DC Motor Horsepower/KW

AC Motor Torque = DC Motor Torque

AC Motor Torque Development Curve = DC Motor Torque Development Curve

AC Motor Maximum Speed = DC Motor Maximum Speed (RPM Max)

AC Motor Minimum Speed = DC Motor Minimum Speed (RPM Min)

AC Motor Service Factor = DC Motor Service Factor (SF)

AC Motor Bearings Load Rating and Shaft Strength = DC Motor Bearings Load Rating and Shaft Strength

AC Motor Winding Insulation Temperature Rating and Rise = DC Motor Winding Insulation Temperature Rating and Rise (K,L, ?)

AC Motor Enclosure Type = DC Motor Enclosure Type (TEC, TEFC, ?)

AC Motor Frame = DC Motor Frame (Footprint)

If the application requires variable motor speed control:

The AC Motor Design must be rated for use with/on a Variable Frequency Drive (VFD)

The selected VFD must be capable of mimicking the DC Motor Torque Development Curve throughout the motor application duty cycle.

Most motor manufacturers have application DATA sheets available that provide a list of all pertinent questions that must be answered when replacing a DC motor with an AC motor.

I suggest you obtain several of these questionnaires from several different motor manufacturers and compare them.

Good luck!

A very good approach for comparison of existing AC versus DC motors. There is one thing a DC motor will do that an AC motor cannot, hold a static position.

Also I thought the OP wants to design a DC motor to replace a 3 phase AC motor. This comparison method will not at all inform anyone how to design either an AC or DC motor.

I love finding about technology that's new to me. This sounds almost like a hybridization of stepping motor driver technology with three phase motors.

Thank You.

I will certainly look into this.

Elevator ac motor and drive set up is one good example of this.

Agreed.

My thought is to give the OP a better idea of what they are faced with so that they seek professional guidance.

It amazes me how many do not take advantage of the free services and information manufacturers offer to potential customers.

I wonder if the OP is attempting design for DC motor production or investigating replacing in-service DC motors with AC units?

Or maybe it is homework?

Having worked for GE Service Electric in a motor winding shop in the past I know enough to know when to get competent help with any motor redesign or modifications.

I really miss those days when I could go to one or more of my mentors and get "real" answers to my questions and straight forward directions on how to proceed.

In today's world I keep running into those "experts" that know less than I do and it really concerns me because I know my limitations. (Downright scary at times.)

Thanks for the reply and good information.

Merry Christmas and Happy New Year!

With the internet and smart phones providing so much information with little comprehension I so often get the idea that just because they see an episode of "How To Do It" they feel they can out perform Jean Pierre Rampal.

They can't even though he's dead.

"I want to design"

You want to design it. Well, go ahead and let us know how it works out.

This does not require a special induction motor, a standard one will do. The trick is to have a drive which will control the quadrature and in-phase current components separately. This allows the control of HP and torque separately. The great thing about DC motors was always the ability to control horsepower and torque separately which always made them great for industrial applications, especially in the paper and metals industries. But then came modern AC drives. I remember in the late 70's and early 80's when my company came out with such an AC drive. Up till then we had specific design motors and drives to do this but then an AC drive was designed to do this with standard AC induction motors. The most impressive thing was when a standard 5 horsepower motor was started with a normal full voltage motor starter and the motor was just setting on the floor. It started fine and didn't move although we had one person put their foot on it. The current pegged the meter. Then the instructor started it with the drive set to maximum acceleration (torque) and limited the current to rated current. He told us to have 4 people try to hold it with their feet, but they were not able to. The motor moved several inches, the rate of acceleration was nearly straight up and it never exceeded normal rated current. I never got over my surprise and am not surprised now to see very few DC motors and drives on new industrial applications.

If you design a motor specifically for this, and it can be done, you will be repeating what was done in the 1970's. I question why you would do it. Most of those that are still running are usually above 10K HP and 13K voltage and are still operating mainly because of the large investment and the fact that they haven't failed yet. .

Some people here appear to surprised to hear about DC 3 phase motors, but they have been around for many years now.

I worked with some for the first time in 1986, and I do believe it was nothing new then (only new to me!)

The ones we used had hall effect sensors embedded in them to allow the electronics to "sense" where the rotor is at any one time.

Whereby, the motors used in most modern (if not all!) electric bikes today are also DC 3 phase motors with no brushes, commutators or slip rings. Sometimes with Hall effect sensors.

The only difference being that modern DC 3 phase electronics does not HAVE to have the Hall effect sensors anymore (though I cannot say when that happened!), in fact a good modern Bike controller can work with or without them!!! Which means that a further point of failure can be removed/not installed.

The motors have few "wear" points when properly designed and are often possessed of a high torque and very easy/simple to understand electronic speed control. They are also usually (in my limited experience) very easy to repair for relatively untrained staff, if well designed of course!

Here are some simple schematics for anyone interested:-

3 phase DC motor Schematics

The motor and drives I was refering to did not have any sensors. Basically the drive sent a short pulse towards the motor and then read it's reflected wave. Based on the reflexion, a corrected pulse was delivered to the motor and this was continued throughout the load and acceleration cycle. The wave shape looked like giberish until you set up the scope to look at inphase and quadrature wave shapes separately. You could see each one but they were constantly changing angle and potential. This is the way all new drives of high quality are made. One interesting newer thing is the use of a motor that starts out as a brushless DC (3 phase equivalent) for smooth starting and low speed control and then changes to an AC synchronous motor for efficiency at about 6% rated speed. The motor of course does not change, the drive just commutates for the DC at first and then changes to AC after 6% speed.

You have to be clear about which DC motor equivalent u want to replicate in AC .Series,shunt or compound ,separately excited,series compound etc. each gives a different characteristic and meant for different application

Don't rely on the fact that all previous designs of DC motor can be used with DC 3 phase motors, they simply cannot....!

So your comment of:-

You have to be clear about which DC motor equivalent u want to replicate in AC .Series,shunt or compound ,separately excited,series compound etc. each gives a different characteristic and meant for different application

was simply all wrong and does not apply......

True Shunt DC motors for example have a commutator in parallel with the field winding, series DC motors have a commutator in series with the field windings. Due to the fact that such DC 3 phase motors as we are talking about here have no physical commutator, slip rings or similar, its rather difficult to use it in parallel with the field windings, when the commutator simply does not even exist in them per se!!!!

If we ignore completely the possible usage of Hall effect sensors or similar in some designs, such motors can often appear to be very similar electrically to a normal 3 phase AC motor in design internally!!!

I posted a link in my previous post which also showed Brushless DC motor internals, if you had looked at that and taken it in, you might not have posted at all.

My personal take is that such motors have only been practical since high(er) power electronic switches and the like have been available.....but I am always ready to learn different from someone who really knows better......

On this website:-

Electronic commutator motor

I found the following:-

Some of the problems of the brushed DC motor are eliminated in the BLDC design. In this motor, the mechanical "rotating switch" or commutator is replaced by an external electronic switch synchronised to the rotor's position. BLDC motors are typically 85-90% efficient or more. Efficiency for a BLDC motor of up to 96.5% have been reported, whereas DC motors with brushgear are typically 75-80% efficient.

I hope this clears up such major differences in design for you between true old fashioned DC motors and (relatively) new 3 phase DC motors......which may or may not be in use where you live.....

You may find this link also informative:-

Brushless DC electric motors

Best wishes for the new year.

The query states DC equivalent of ac motor.So I don't find anything wrong in my surmise.I am aware of BLDC motors which we have designed too.But they are economical up to 15 KW.Large DC motors are not made in BLDC version.Yes BLDC motors,They are very efficient .Even a small induction motor in frame 112M 3.7 kw 4 P eff are below 90% where as BLDC can give much higher eff.It is a good idea for lower ratings.The query does not state rating or speed .I hope u understand and leave it at that.

My post was intended to show you what you obviously did not understand and provide you with online information to back that up.

Your quote:-

I hope u understand and leave it at that.

Have a great New Year.

I fully Understand the query and also the DC motors .I also know about BLDC and we use it where we live.My post is relevent and appropriate. Thanks for the on line ref.Have a great year.

Please explain the relevance online here for me, I simply cannot see it myself.

I also have worked with BLDC, three and single phase motors, but I am simply not understanding the comments you made with regard to shunt/series motors and BLDC motors.....

Maybe I am simply missing something important. So Educate me please?

Remember, I am always prepared to be wrong and I really like to learn something new and relevant to such a subject as I find it extremely interesting as well.

Many thanks in advance for your time and trouble.....