Electric Actuators - Burning Up Motors

Gentleman,

Any one have a good suggestion to specify # of cycles/min for an electric actuator with a servo positioner? I've come across this in the past, but being the 'controls' guy, wasn't privy to what the exact problem/answer was.

Read the nameplate data for service factor.
Consult the factory
Send us crystal ball

The question gives us no data to use to make informed guidance.

Dan Bentler

I'm guessing your company doesn't let you talk to customers much?

"An electric actuator with a servo positioner" is too vague a description to give any meaningful answer. There are many brands and types of those and each one should normally have its own ratings for maximum duty cycle, service factor and such.

If you were to provide the particular brand and model of the unit in question, someone familiar with it may respond.

tvich said:

I'm guessing your company doesn't let you talk to customers much?

Click to expand...

I allow myself to talk to customers. But I do not read minds well and crystal ball is broke.

Give us full description of process make model s/n of actuators voltage amperage how often started run duration and stop duration.

Dan Bentler

For each cycle of the actuator you're pumping a little more energy into the actuator than you're getting out of it. You can calculate that energy difference from the efficiency data published in the actuator's specifications, but to do so you'll need to know the details of the load the actuator has to move and the time cycle under which it has to be moved. The excess energy will show up as heat and cause the actuator's temperature to rise.

The higher the actuator's temperature relative to its environment, the more heat it can shed to the environment. As you continue to cycle the actuator, you reach a temperature at which the excess energy absorbed by the actuator each cycle equals the amount of energy shed to the environment due to the temperature difference. The faster you cycle the actuator, the higher that equilibrium temperature. If the equilibrium temperature is higher than the actuator's operational limit, the actuator will eventually fail. That's true regardless of whether the device that supplies motive power to the actuator is a servo motor or a chipmunk on a treadmill.

Edit:
I just noticed you titled your thread "Burning up motors". To me that implies that the person who specified the motor may not have done a thorough enough analysis of the load profile.

Servo motors typically carry two torque ratings, a continuous torque and a peak torque. The peak torque is the absolute limit of loading, beyond which there be dragons. The continuous rating is a load below which the motor will happily operate 24/7. During periods of acceleration/decelaration you can operate the motor between the continuous and peak torque levels so long as the average (RMS) torque remains below the continuous duty level.

Nice explanation Steve.........

tvich said:

I'm guessing your company doesn't let you talk to customers much?

Click to expand...

Dont take everyones answer to heart. Some people on this forum like to help out as much as possible, others just complain about every question asked. Read through some of the posts offered by members with high post counts and you will see what I mean.

If your actuator is single phase and it burns out, in most cases you will find you told the motor to go, it turns the power onto the motor, and just sits there quite happily humming its own tune, but not moving and never reaching its limit switch so it can turn off, during all this time it make a nice heater, that may eventually burn out the motor, now its very normal in these cases to find the motor is a capacitor run motor, and its got a faulty capacitor.

Really apprecaite your response Steve.

From the little information I know, and and last second trying to find an answer for spec's going out, for a continuous positioner? (not on/off, meaning modulating, if I'm killing the terminology), one particular vendor labelled the term in # starts/hour. That seems simple for an on/off valve, but a modulating valve (which will be on a PID loop), that's completely out of my hands as a controls engineer how often that valve will start/stop. This paticular vendor recommended 60 starts/hr continuous, and 200 peak. However, for a modulating application, they've listed a spec close to 1200 starts/hr.

In the end, basically what one could do, I'm THINKING, is just slow down the deadband, or response of the positioner? In my application, this valve is requiring VERY little accuracy, or speed response.

Again, I truly appreciate everyone's attempt to help me, including Mr. Personalities above. I reach my 5 year old to not talk to people like that.

Don't set me up as a paragon of civility. Your question just happened to catch me in a mellow mood and willing to guess at your meaning. The information you provided was (and remains) woefully inadequate. I'm still not sure what your specific question is. Are you trying to interpret the vendor's specs? Are you trying to select the proper size motor for the application? Are you second guessing the person who selected the valve for this application?

Because your first post was so vague, my initial response was pretty generic. Once you've determined that the device can handle the loads involved and the duty cycle it needs to operate under, the next thing you have to concern yourself with is whether your design can deal with the heat that's going to be generated as a result of mechanical inefficiencies.

After I posted my response, I noticed that you had included the phrase "Burning up motors" in the title. That made me wonder if you were troubleshooting an existing system with a history of destroying motors, so I added the part about analyzing the loading over the entire duty cycle.

Today it begins to sound more you have an issue with applying the manufacturer's specs of number of starts per hour to your application. Dan's comment about our lack of a crystal ball is right on the money. Nobody here knows anything more about your system than you tell us. We can't see what you see. Nor can we read your mind. The better the information you can provide, the better will be the responses. The more we have to guess, the less likely that our guesses will apply to the specifics of your system.

tvich,

by modulating operation, I guess you're assuming that actuator is controlled by digital signals (from digital output card on PLC or similar). If that is the case, than modulating operation assumes PWM (Pulse Width Modulated) signal which means that duration of OPEN or CLOSE pulses will depend on control error in your loop (set point - actual value).

For this purpose, a so called Step controllers are used since control is done in steps (pulses). Most step controllers have various paramters such as minimal duration of pulse and minimal time break between pulses, as well as a deadband zone.

For example, these actuators are made for so called modulating operation and can be controlled via continuous signal or via discrete signals (pulses ON or OFF).
I don't know if this information can help you, since you didn't provide sufficient data.
Do you need to set up PID loop with your electric actuator?
What kind of controller/PLC you're going to use?

If accuracy and speed is not a problem, then controlling with PID and modulating are unnecessary. Get rid of PID. Do your own control in ladder.
Control response by controlling read times. Read once per second, determine whether error is positive or negative, move motor a little.
For faster response, read 10 times per second, every 5 seconds for slower, etc.