DC motor & drives compared to ac
- Thread starter lbernard
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
lbernard said:
Sometimes yes. Sometimes no. It depends on the application.
hockeyoligist2
Senior Member
- Location
- close to greenville sc
What is the application? That will determine which is more cost effective.
lbernard said:
Perhaps the question should be "Are they any less...."
With the advancements in VFD's I believe they are every bit as efficient. Remembering that you first have to rectify your AC power to get DC, and this is the first stage also in your VFD
For more info go to the websites of ABB or Yaskawa
___________________
Wes Gerrans
Electrical Technology Instructor
Northwest Kansas Technical College
Goodland, KS
hockeyoligist2
Senior Member
- Location
- close to greenville sc
Sorry Bob, You beat me to it! I type real sloooooowwwwwwwwwww!
DGrant041
Senior Member
- Location
- Peoria, Illinois
Wes G said:
Wes G said:
For DC motors, you will have commutater & brush maintenance that will always be more time consuming than their AC counterparts. There are some applications that still require low speed/high torque motors like trains and cranes and lathes.
For everything else, there's VFD's & soft starts.
AC: It's everywhere you want to be.
boater bill
Senior Member
- Location
- Cape Coral, Fl.
You can still use a VFD in a low speed, high torque application where in the old days a DC drive would be the choice. With encoder feedback, you can get 100% torque at 0 RPM from several VFD manufacturers. This does require an auxillary cooling fan.
e.g. If you have a printing press application that will require more torque for short moments, size the motor for the peak torque to be applied, and install the appropriate VFD amp rating for the motor you are using. A 20 HP DC motor does not necessarily equal a 20 HP AC motor if the DC is being overtorqued during part of it's process. ALWAYS size the VFD for the motor amps. If you are in a high torque application make sure you use a CT rated VFD instead of a VT. The VT (variable torque) is mainly for fans and pumps.
Hope this helps
e.g. If you have a printing press application that will require more torque for short moments, size the motor for the peak torque to be applied, and install the appropriate VFD amp rating for the motor you are using. A 20 HP DC motor does not necessarily equal a 20 HP AC motor if the DC is being overtorqued during part of it's process. ALWAYS size the VFD for the motor amps. If you are in a high torque application make sure you use a CT rated VFD instead of a VT. The VT (variable torque) is mainly for fans and pumps.
Hope this helps
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
The terminology used for electric motors makes it almost impossible to even answer your question.
1) It is very unlikely that you are actually considering using DC machines. The machines commonly called DC machines use brushes and commutators to internally produce alternating current. They are powered by an external DC supply, but internally they have a considerable number of conductors carrying alternating current.
2) You might also be considering something called a 'Brushless DC machine'. If you look at the way such a machine is wired, what you will see is a three phase AC synchronous machine with a permanent magnet rotor. It is called a DC machine because of the way rotor position feedback is used to determine the inverter output. Because of the way the inverter is controlled, the machine tends to act in a fashion more akin to a brush commutated DC machine with a permanent magnet field. The inverter may be fed from a variable voltage DC source.
The brushless DC machines are among the most efficient for small motors.
-Jon
1) It is very unlikely that you are actually considering using DC machines. The machines commonly called DC machines use brushes and commutators to internally produce alternating current. They are powered by an external DC supply, but internally they have a considerable number of conductors carrying alternating current.
2) You might also be considering something called a 'Brushless DC machine'. If you look at the way such a machine is wired, what you will see is a three phase AC synchronous machine with a permanent magnet rotor. It is called a DC machine because of the way rotor position feedback is used to determine the inverter output. Because of the way the inverter is controlled, the machine tends to act in a fashion more akin to a brush commutated DC machine with a permanent magnet field. The inverter may be fed from a variable voltage DC source.
The brushless DC machines are among the most efficient for small motors.
-Jon
peteo
Senior Member
- Location
- Los Angeles
Members have brought up the important points. Motors and controls are used which are (hopefully) appropriate to the application. It would be rare to find an application which would make sense for either/or, once the costs and energy usage are looked at. As it becomes complicated, best for me to stop.
A very nice page about motors here.
A very nice page about motors here.
DGrant041
Senior Member
- Location
- Peoria, Illinois
boater, winnie & peteo-
Whoa! Thanks for the info. Sometimes the maintenance environment leaves a guy like me in a time-warp. I've never seen these technologies that are very up-to-date.
Imagine my surprise after leaving the Navy and finding-out the rest of the world had been using brushless AC generators for years!
Thanks again for schooling the kid!
Whoa! Thanks for the info. Sometimes the maintenance environment leaves a guy like me in a time-warp. I've never seen these technologies that are very up-to-date.
Imagine my surprise after leaving the Navy and finding-out the rest of the world had been using brushless AC generators for years!
Thanks again for schooling the kid!
My experience with variable speed drives has transcended through several generations of "new technology". The progress in the efficiency and control characteristics of "modern" VS drives has moved along as fast as the "new technology" of solid state electronics and PC control. Compare a desk-top computer of 2006 to one of 1996 and you will get a idea of the progress and changes that have occured. I'm behind now as it's been several years since my last experience.
My history with VS drives started in the 70's with the Original "Ward Leonard" drive.
This drive first became available around 1900 and found wide application. The systems that I was familiar with were still in operation into the 70's. I'm sure that some of the systems are still being used.
This (Ward Leonard) system used a constant speed (prime mover) AC motor to drive a (constant speed) DC generator. Output voltage (DC) from the generator was regulated by increasing or de-creasing the (DC) field voltage supplied to the generator. This generator output was then used to supply power to a DC motor (or several in parallel) that produced the work.
The constant (excitation) voltage for the generator (and work motor) was usually supplied by another (small) DC generator known as the "exciter". It was also driven by the prime mover and usually mounted on the (tail end) of the larger generator.
This was replaced in later years by a "static exciter" using a early model "rectifier bridge". Either way, two seperate sources of DC power were required in order to power a third variable speed DC (work) motor.
Primary speed control (variable voltage) for the work motor(s) was provided (and varied) by using a rheostat to increase or decrease the field voltage supplied to the generator.
If several DC (work) motors were powered (in parallel) by the same generator output, tandem speed control was possible, The generator supplied the same variable DC voltage to the (work) motor armatures, causing each motor to run at basically the same speed. Unison in speed between each individual motor was regulated by individual (manual) rheostats used to increased or decreased (+-10%) the field voltage to each motor.
Believe it or not, after the motors speeds were "synchronized", the system was pretty reliable.
These systems were used a lot in the textile industry on systems that might have up to 12 seperate DC motors. Each motor would power a different process on a continuous output machine, and variable speed control was necessary, along with the need for all (work) motors to run in unison.
It would be considered a "dinosaur" by todays standard, but it was a great advance in technology in those days. The system was really dependable if not efficient.
That progressed to a common (solid state) power rectifier (DC drive) to supply the DC voltage to the tandem (work) motors. Control was basically the same, variable armature voltage with constant (+- 10% controllable) field voltage for each motor. This eliminated the prime mover, generator and exciter.
Then came Individual individual DC drives for each motor and
using "tachometer" (voltage) feedback from a tach (mounted on each motor) to supply a reference "speed" (voltage signal) to a central PC control. This control automatically regulated the speed of the motors. This was a great improvement in regulating speed of tandem motors.
This (IMO) is the best system for a "constant horsepower drive. You need to size the motor HP (and drive) to the maximum torque required at the minimum speed.
Next came the AC "Variable frequency" drives. These control the frequency (Hz) of the supply voltage to a AC motor and thus allow variable speed operation. This is the best system (IMO) for a "constant torque" drive application. Most industrial application are "constant torque".
The motor HP (and drive) needs to be sized to the maximum torque at the maximum motor speed.
The advantage to variable frequency drives is the lower motor costs and higher operating effeciency.
When they first "came out" regular AC motors were all that were available, and they weren't designed for variable speed operation. The torque characteristics of the motors and the generated heat was a problem and led to premature motor failure. This has been vastly improved by the introduction of AC motors specifically designed for variable speed operation.
This is just my opinion based on my experience, others experience may vary.
Feel free to critique.
Another long post, sorry but it's a interesting subject (to me anyway).
steve
My history with VS drives started in the 70's with the Original "Ward Leonard" drive.
This drive first became available around 1900 and found wide application. The systems that I was familiar with were still in operation into the 70's. I'm sure that some of the systems are still being used.
This (Ward Leonard) system used a constant speed (prime mover) AC motor to drive a (constant speed) DC generator. Output voltage (DC) from the generator was regulated by increasing or de-creasing the (DC) field voltage supplied to the generator. This generator output was then used to supply power to a DC motor (or several in parallel) that produced the work.
The constant (excitation) voltage for the generator (and work motor) was usually supplied by another (small) DC generator known as the "exciter". It was also driven by the prime mover and usually mounted on the (tail end) of the larger generator.
This was replaced in later years by a "static exciter" using a early model "rectifier bridge". Either way, two seperate sources of DC power were required in order to power a third variable speed DC (work) motor.
Primary speed control (variable voltage) for the work motor(s) was provided (and varied) by using a rheostat to increase or decrease the field voltage supplied to the generator.
If several DC (work) motors were powered (in parallel) by the same generator output, tandem speed control was possible, The generator supplied the same variable DC voltage to the (work) motor armatures, causing each motor to run at basically the same speed. Unison in speed between each individual motor was regulated by individual (manual) rheostats used to increased or decreased (+-10%) the field voltage to each motor.
Believe it or not, after the motors speeds were "synchronized", the system was pretty reliable.
These systems were used a lot in the textile industry on systems that might have up to 12 seperate DC motors. Each motor would power a different process on a continuous output machine, and variable speed control was necessary, along with the need for all (work) motors to run in unison.
It would be considered a "dinosaur" by todays standard, but it was a great advance in technology in those days. The system was really dependable if not efficient.
That progressed to a common (solid state) power rectifier (DC drive) to supply the DC voltage to the tandem (work) motors. Control was basically the same, variable armature voltage with constant (+- 10% controllable) field voltage for each motor. This eliminated the prime mover, generator and exciter.
Then came Individual individual DC drives for each motor and
using "tachometer" (voltage) feedback from a tach (mounted on each motor) to supply a reference "speed" (voltage signal) to a central PC control. This control automatically regulated the speed of the motors. This was a great improvement in regulating speed of tandem motors.
This (IMO) is the best system for a "constant horsepower drive. You need to size the motor HP (and drive) to the maximum torque required at the minimum speed.
Next came the AC "Variable frequency" drives. These control the frequency (Hz) of the supply voltage to a AC motor and thus allow variable speed operation. This is the best system (IMO) for a "constant torque" drive application. Most industrial application are "constant torque".
The motor HP (and drive) needs to be sized to the maximum torque at the maximum motor speed.
The advantage to variable frequency drives is the lower motor costs and higher operating effeciency.
When they first "came out" regular AC motors were all that were available, and they weren't designed for variable speed operation. The torque characteristics of the motors and the generated heat was a problem and led to premature motor failure. This has been vastly improved by the introduction of AC motors specifically designed for variable speed operation.
This is just my opinion based on my experience, others experience may vary.
Feel free to critique.
Another long post, sorry but it's a interesting subject (to me anyway).
steve
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