Newbie- AC Servo voltage

[HuckMilton]

Now, I understand that if I run my servos at a voltage that is less then recomended they will run slower. Is this bad for them? Is the speed decrese linear with voltage? I have some Pacific scientific servos that are 240v and rated at 6250 rpm, which is way faster than I need. If I run them at say 80v, will thay run at 80/240*6250=2083?

[Al_The_Man]

They will run slower and you may not be able to reach full torque.
BTW many of the PWM servo drive manuf. recommend a power supply voltage of between 10% and 50% higher than motor rated voltage.
http://www.a-m-c.com/download/docume...l/engnotes.pdf
Al.

[HuckMilton]

Yes but? Will it work? Am I right about he speed in general? The odd fact is that they are custom and the only indication I could get about wich model they are is to cross reference the continuous stall torque and current. Is there some way of knowing what the correct voltage is without taking one appart and counting the windings?

[Al_The_Man]

The voltage rating of a servo is spec'd by the manufacturer that is referenced to the maximum rpm the motor is capable of, as rpm is proportional to voltage.
As these are AC servo's, that rpm would be normal.
If you wanted to roughly confirm the operating voltage, rig up a 3 phase bridge with 6 cheap diodes off the three windings and with may be a 10µf cap on the resultant DC. back drive the motor at a known rpm, and extrapolate the voltage for 6000rpm, from the voltage you acquire.
Al.

[HuckMilton]

You rock man, I have seen you all over the place on here giving good advice. Much appreciated. So, as its after 5 and I dont have any diodes handy, could I use a bridge rectifier and measure the voltage of one phase?

Thanks, Huck

[Al_The_Man]

It may give you the peak across 2 phases, which may be enough, if you have another bridge, you could use one side, take the 3rd phase into one AC connection and connect the + & - of each bridge together.
Al.

[HuckMilton]

So, if a synchronous, permanent magnet, servo's speed is dependent on voltage, can I assume that the stated voltage for the motor is the voltage at top speed? If so, is there any reason that a drive with a high enough current rating but lower voltage should not work? It seems it would be just like having the right drive but running the motor a lower speed.

[Al_The_Man]

Originally Posted by HuckMilton So, if a synchronous, permanent magnet, servo's speed is dependent on voltage, can I assume that the stated voltage for the motor is the voltage at top speed?

See post #2.
Yes the rated voltage will give you the rated rpm, anything less will be not be able to achieve this, it will work, just the general performance will be lower.
But you won't damage anything.
Al.

[HuckMilton]

Forgive me. Hold my hand. Speed in these motors is a function of voltage? Is this how the drive controls the speed of the motor? This is new ground for me. If speed is controlled by voltage, which aspects of performance would be degraded?

[Al_The_Man]

Depending on the level of voltage, you may well reach synchronism at high rpm, but you will have much lower usable/practical torque at this speed.
You can lower it to the point where the torque required to rotate the armature is insufficient.
I have a couple on my test set up right now, and with 50% rated voltage, I cannot reach anywhere near rated rpm.
Al.

[HuckMilton]

Synchronism as in the rotor being synchronous with the rotating magnetic field? So I made myself a 3 phase bridge rectifier and measured the voltage and I was happy. However, as I dont know which leads are the phases and which the ground I tried various combinations and they all produced the same voltage at the same RPM. and if I dissconected any lead, the voltage dropped the same for all of them. Why is this? Isnt one the ground and shouldnt behave like the others?

Thanks

[Al_The_Man]

You can find the three phase by simple ohm test, you should have very low resistance between all three, the ground does not come into it, leave that disconnected.
any combination of the three will produce the same reading at a given speed.
Al.