how would you know at what frequency to operate your motor?

[maruben]

i'm tasked to make a control system for a linear motor. and so i've looked at the data sheets of the linear motor but can't seem to find any information about at what frequency to operate the motor (frequency of the commutation waveforms)...

i do remember reading somewhere that the motor acts as a low-pass filter. the data sheets show that the ratio R/L is constant for all variants of one type of motor. and i read about passive integrator circuits that the cut-off frequency is 2pif=R/L...

so my question is...would it be correct if I assume that the maximum operating frequency of the motor i'm using would be the cut-off frequency of the RL motor?i'm sorry, if my question is kind of vague, please let me know so i can try to re-phrase it.

thank you.

 

[MrAl]

If i understand you right, you are driving a motor with a set of pulses but im not sure what kind of motor you are using. Is it a brush motor or stepper?

If it is a brush motor, the average DC will get to the motor somehow.

If it is a stepper, the max frequency will depend partly on the required torque and the voltage rating of the windings, as it takes a given time to get up to a certain current given the max voltage of the winding. At max voltage you'll get max current response which will give you max torque at some frequency F. As you increase F the time it takes to get to full current starts to get more and more significant until you find that the current may only reach max for 50 percent of the time (instead of almost 100 percent of the time) and this will reduce torgue. As you increase F even more, the time it takes to get to full current equals the time of the pulse width and so the torque is much less now, until finally you increase F even more yet and the current never reaches max which gives you very low torque.
So it's partly a matter of how much torque you need.
You can test for this by increasing the frequency and noting the reduction in torque at some frequency F, and then increasing F and noting the further decrease in torque.

 

[cr0sh]

He's trying to control a linear motor - think of it as a motor with the coils/stator/etc "unrolled" into a straight line. It sounds like the motor may be 3-phase controlled - or some other multi-phase control scheme; maruben, can you point us to a datasheet for the motor - it would help with the answer (and I can't say I can give the answer; I have zero experience with the devices, other than knowing what they are and rough knowledge of how they work). They will have similar specs like a 3-phase or stepper motor; things like starting vs. running frequencies, step rates, etc - from coil-to-coil....

 

[MrAl]

Hello,


Ok thanks for that. I was thinking maybe it was a linear motor the type where we have a rotating motor and some sort of mechanical means to provide the actual linear motion such as a lead screw.

 

[cr0sh]

Well - now that you mention it, I suppose he could be talking about what you are describing; those are typically referenced as "linear actuators", whereas "linear motors" are more like what I was mentioning (an "unwound" motor); linear actuators are typically used where accuracy and power (for lifting or moving larger loads) is needed, whereas a linear motor is more used for high-speed (and depending on the motor) accurate positioning (generally with lighter loads - although, in certain cases, like amusement park roller coasters - let's just say linear motors come in all sizes, k?)...

Maybe the OP needs to better explain what he is trying to use - pics, model numbers, manufacturer, etc...

 

[cr0sh]

linear actuator is a stepper
linear motor is linear

No. See: Linear motor - Wikipedia, the free encyclopedia

Notice how in the discussion it mentions that a linear motor is an AC motor; in an actual linear motor, it works something like a 3-phase AC motor, usually, but it could (in theory at least - I don't have any experience with them, as I have already said) be designed and controlled like a stepper motor (a stepper motor is inherently an AC motor - even if the coils are driven by DC voltages; in fact, with the right controller, you can drive the coils of a stepper with an AC waveform to in effect get nearly -infinite- "micro-steps" in-between each true rotor position; however, torque is compromised).

Now see: Linear actuator - Wikipedia, the free encyclopedia

Notice how such an actuator can be driven by an AC or DC motor? Such a motor can be a typical AC or DC motor, or it could be a stepper motor as well. This motor is connected to a lead-screw (sometimes via a gear reduction transmission) that rotates a nut which actuates the linear drive mechanism. Other layouts are possible, but this is the most common form you see.

Just for completeness sake: Stepper motor - Wikipedia, the free encyclopedia

Notice in the discussion where it reads "A stepper motor is a polyphase AC synchronous motor..."?

 

[maruben]

yes actually i'm trying to control a linear synchronous motor using 3-phase sinusoidal commutations. the problem is, i don't know at what frequency i should operate them (frequency of the sinusoids).

am i allowed to post links to datasheets here?if I am, then here's a link to the datasheets. The motors i'm using are the LEA and LEB Brushless Linear Motors.

https://www.electro-tech-online.com/custompdfs/2010/08/linear_motors-4.pdf

thank you.

 

[cr0sh]

Have you contacted Rockwell Automation about this? They might be the best source of information, or can at least point you in a good direction...

Looking at the datasheets, I get the feeling that "frequency" is more a matter of what the upper and lower bounds could be, based on the mass being moved, and the acceleration needed. I am sure all the information needed to calculate these upper and lower bounds are in the datasheet, but I don't know how you calculate it. Anything in-between those bounds would cause the moving coil assembly to move (for that mass - things change if the mass changes, of course). Raise the frequency, and the coil assembly and mass moves faster (while likely consuming more current - these things appear to take a LOT of current!).

If you note in the datasheet, though, it says that you need to provide an active feedback servo control to these motors to get them to operate properly. Think of it this way: In order to be able to move from point A to point B along the track, your controller must do the following:

1) Accelerate (ramp up) the frequency to get the coil and mass moving.
2) Hold that frequency until you approach point B.
3) Decelerate (ramp down) the frequency as the coil and mass approached point B.
4) This step is potentially optional: Apply a direct current (or some out-of-phase) holding current to the coils to keep it stationary at point B.

Step 4 could be automatically accomplished with a proper servo control mechanism (likely PID based - if the mass overshoots, it would be moved back; if the mass is moved by an external force, it would be moved back). This servo control feedback loop could be accomplished in numerous ways, depending on the accuracy and quickness of response needed (and how much money you're willing to spend); basically, it is a sensor that can somehow measure where along the track the coil and mass are. This information would be fed back to the controller, which using its PID loop would determine what the frequency is needed to apply to the coil to move it to the position commanded.

You're talking about a fairly complex system here, it seems; the linear motor is one part, you need a controller with a PID loop, a servomechanism feedback sensor system, and to top it all off, you need a controller that can handle a lot of current at fairly high voltages (meaning if you design it wrong, you might get an impromptu fireworks show instead!).

Once again, though, I want to state that I have -zero- experience with these devices; the above is pure speculation on my part as to how these things work based on my little knowledge of them in general, and the datasheet you have provided for your particular motor, coupled with what I know of servomechanisms (which isn't that great, either)...

Seek the advice of experts...hopefully some will post here (but since this thread has been running a while, and none have showed up - contacting RA and asking them for advice or outside services might be your best bet).