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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

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. The actual load requires approx. 50 Nm, and is therefore geared down to this ~10 Nm.

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?

dc motor versus ac motor

Hi there,

BLAC (Brushless AC) servo motors are the way to go with either a resolver or absolute encoder and a flux vector servo amp. Expensive but will not suffer torque ripple or any other ill effects.

DC Servo Motor could be more cost effective and the control could be simpler compare to AC.

Ripple could be minimized by careful design of the control algorithm.
No doubt DC Servo cause ripple but it is still more cost effective if certain ripple is acceptable for the system.

this sounds like a job for a dc brushless synchronous.

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