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DC vs. AC servo motor in a high acceleration application

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Junior Member level 2
Mar 9, 2008
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ac servo motor

Hi all,

I am designing a positioning system using a servo motor to move my load.

The motor is to drive approximately I_load = 0.07 kg*m^2 load from a stop position, only 1 degree increment in 3.5ms.
This will be a stepping system in that sense it will step 1 degree every 3.5 ms.
The motor should accelerate (linearly) the load from 0 to 11.3 rad/s in the first 1.75ms and then decelerate from this 11.3 rad/s to 0 in 1.75ms.

I have found out that I need a motor that can deliver approximately 20-25 Nm continuous torque, where the max velocity of the motor will be bit more than 100 RPM. I really only need 10 Nm motor, but I want to be on the safe side, only driving the motor at half its capabilities.

I don't want a stepper motor since it has various drawbacks such as vibration of the load while stepping (i need smooth movement), and also I want feedback possibility to control the position precisely, and not micro-stepping.

The question is, do I need DC servo motor or an AC servo motor? and why?

I have heard that AC motors have little torque at start, where as DC do not and therefor DC would suit me better, is this true?

Do you suggest any specific motors that would be suitable?

Thank you in advance, I am a novice when it comes to motors.

ac servo

Hello jonnasi,

I think that you must use some gearing. Your intertia is orders of magnitude greater than the typical inertia of a suitable servomotor.
Given (just as example) a 10:1 transmission ratio, you´ll need a motor with 2 Nm torque and not 20 Nm. With such ratio, the inertia "seen" by the motor will be 1/100 the original, giving better power coupling. Of course, you will have to increase the speed of the motor, and you must achieve a balance between greater gearing ratio, speed and acceleration.
BTW, to specify a servo motor you also have to state cycle time. As start point you can select the servo based on the RMS torque, rather than static estimation of torque.
I dont think that AC servos are less responsive than DC servos.


dc motor vs ac high starting torque

Hi jorgito,

These torque numbers i have written have gearing considered. I have found optimum gearing ratio between 6 and 15, depending on the moment of inertia of the motor rotor.

So additional gearing is not possible. The actual torque required to move my load is closer to 50 Nm, but with gearing it need a motor to drive 10 Nm, so a 20-25 Nm motor is required.

The cycle time would be 5 ms, where as 3.5 ms of these would be the time the motor is rotating.

I have just calculated the required torque from the acceleration, time, motor rotor inertia, gearing + load inertia.

I have the book: Mechatronics, An integrated approach, 2004, by Clarence W. de Silva and he states on page 845 that synchronous AC motors needs a starter to start the motor and that the starting torque is virtually zero........ i assume he knows what he is talking about?

And on induction AC motors he states one of the drawbacks to be Low starting torque?

Help please.

servo vs stepper motor acceleration 2009

Hello Jonnasi,

Thanks for your feedback.
The author is right, regarding conventional AC syncronous motors. But your application, given the dynamics you metion, requires a servo. Any servo, AC or DC, have the rated torque as starting torque. There is not derating to be applied.
The AC servos tend to be more responsive than DC because the mass in the rotor uses to be less than in a comparable DC servo.

Hope this helps.


time acceleration vs torque

Hi jorgito (and everyone else reading this),

Thank you very much for using your time in helping little old me :)

As I understand from you, from a (starting) torque perspective both the AC and DC servo motors would be suitable. So other factors such as noise, brushed vs brushless should be used to further eliminate my choice of either AC or DC motor?

Thank you for your response.

As much as I value your opinion, jorgito, I want to be careful and ask if anybody else has a second opinion, either confirming or not confirming the statements already made here?

Always better to have more gifted people than few to put their opinions forward, so I know for sure that this is the case ;)


rms torque rms speed

Hello jonnasi,

Regarding your question, is clear that you know the criteria to select between an AC or DC servo. I would also add: price, availability, support from manufacturer (now and near the end of life of your product).
In my personal opinion, I would favor the AC servo.

Regarding the search for more opinions, I think you must ask until you have no doubt about you are going to do. Errors or misconceptions in an early stage of any design uses to be expensive at least, but can be catastrophic also.
I'm also interested in reading more opinions.

servo motors argentina

Thank you very much for your reply.

Do you know manufacturers that provides solutions for a motor with the specifications that I have given here, along with motor controller/driver solution which is (hopefully) connected to the 230Vac grid for power and has come communication bus (CAN, USB...etc) in order to interface with a PC for controls???

I must add that the motor has to be able to rotate in both directions.

ac servo motor application

in an older MTS motion system that ran draw roll profiles, we had problems with the DC brushed motor. It had excellent power/response but low life. comm dressing with brush reseating was frequent as was motor replacement.

acceleration time servomotor

Hello jonnasi,

Here in my country, Yaskawa servos uses to be the least expensive solution. They have AC servos, drives and motion controllers. Having a programmable drive or motion controller relieves you of having to close the position loop in your hardware.

Hope this helps and regards!


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