Should I use a DC motor for a variable speed mechanism or an AC motor with an inverter?

Question

I'm building this machine which will enable user to change the speed of the motor, rather frequently. The motor may have to work in speeds much lower than its maximum speed for long periods or even always. Should I use a DC motor with electronics to control the speed? Or an AC motor coupled with an inverter?

Answer 1

Usually the driver in making the choice between an AC machine powered by some sort of inverter drive (typically a Pulse-Width Modulation system for varying frequency) and a DC machine (operating on rectified AC power) is the lowest speed required AND the torque requirements at the various speeds.

If the design is basically required to have constant torque to zero speed - and/or requires direction reversal - the most effective choice is a DC machine and drive combination. If the design requires constant torque over a fairly narrow range (like 50-100 percent rated speed or less), or varying torque through the speed range ... and the torque requirement isn't all that high ... then the most common choice is the AC machine and the PWM drive.

Most PWM-type drive have a lower limit on the frequency at which they can maintain good control - this is often around 12 Hertz or so. In part this is because of the increased harmonics when the line frequency is chopped to produce lower operating frequencies, and partly to stay away from inducing critical resonances in drivetrains (which often happen below 10 hertz).

Since the rectified DC doesn't have a 'frequency', this isn't a restriction on operating - providing excellent control right down to zero speed.

Another possible factor is cost; the AC machine is (generally) cheaper to obtain than the comparable DC machine - but the electronics for control are usually more costly.

Finally, the absolute maximum top speed might be an issue as well. Since the DC machine relies on the mechanical contact between the brush and the commutator to 'switch' polarities in the rotor, there is a limit to how fast a machine of a certain physical size can operate. In small machines (a few horsepower), the AC machine can often achieve higher operating speeds because it doesn't have the same mechanical contact restriction.

Answer 2

Hi. AC motors are designed and built differently than DC motors. AC motors take advantage of the 60 hertz clock of a power line (or 50Hz in other countries) - this means the power line changes phases 60 times a second - to rotate, while DC motors are designed with the voltage level applied. You can control them both, but with different means. The easiest to control is the DC motor (change the potential (the voltage) and it turns slower or faster.

Answer 3

I would consider using a DC motor with Pulse width modulation control. There are some inexpensive motor driver IC's out there if your motor is small. DC motors have a lot of torque for their size compaired to a synchronous (AC) motor.

Answer 4

Use DC and stepper motors, especially for anything moving slow for long periods of time. You don't need A/C arc over etc.
Anything robotics traditionally uses DC stepper motors.

Answer 5

depends on where you wanna use it.induction motors are much lighter and their speed control is more accurate .and if you use line voltage for its power it is better because you dont need to rectify the power

Answer 6

ac with the inverter ,dude.. thats the way.....aay..