I have some AC servo motors that I need to test. I found a great way to test that they work. I simply connect a DC power source to a pair of leads (out of three) at a time. This makes the rotor move nicely to various positions. Just thought that someone may find it useful.
i
I think you could also hook two leads to a variac and a capacitor from one variac lead to the third motor lead.
=20 Dan
AC servos are really just brushless DC motors. So that test would work. It wouldn't show internally shorted windings though. I don't mean shorted to the case, but shorted such that several turns in a particular winding are bypassed. I suppose you could check the resistance of each winding with a good DVM and if all were the same and the motor moves with your test you would be pretty sure the motor was good. AC servos are generally very rugged. Sometimes the insulation can break down under load and so won't show up with a simple ohm check, but this situation usually occurs because the motor was overheated at one time. Fortunately the cooked insulation becomes permanently stinky so if a motor doesn't smell and passes the above quick tests it's most likely good. ERS
Just don't try it with 140vdc or you'll probably let the smoke out...
And a DPDT reversing switch on the 'control' lead to go backwards?
Line Hot _ ---o_/ \o-\ /-----------o-------------------\ | | | | -. ,- | | )|( C| Power | )|( C| Winding | -' '- C| | | | | | | | | | Line Neut | | | | ----------/ o-----------o----------------\ | | | | | | | | | | | | | | Control | | | Winding | | \--o ___ || o--|--o __--o---UUU----||--o--__ | | /--o || o--/ | | DPDT | | | \-------------------------------/
(created by AACircuit v1.28.6 beta 04/19/05
--Winston
snipped-for-privacy@whidbey.com fired this volley in news: snipped-for-privacy@4ax.com:
Uh... no! They are fine pole pitch, high-current, low rotor inertia three- phase AC motors.
They are NOT brushless DC motors. Who ever gave you that idea?
To that end, you can test an AC servo with a 3-phase supply at any voltage less than its rated maximum.
LLoyd
Which brings up a question: for some reason, they do not state maximum voltage on the nameplate!
Three-phase *synchronous* motors.
DC brushless servos are not really DC motors -- they're also three-phase synchronous motors. I've heard it said there's a difference in the construction details of motors touted as "AC servo" vs. "brushless," but I'm not convinced it's anything more than marketing mumbo-jumbo.
But, just as with an induction motor, you need to coordinate voltage and frequency.
could you just hook one up to a VFD and make it go forward and back and change speed? that would be a great checkout
Karl
Ned Simmons fired this volley in news: snipped-for-privacy@4ax.com:
Um... yeah, there's a difference! Brushless DC motors have electronics on- board that convert the incoming DC into multi-phase AC to drive the motor.
AC servos have no such electronics on-board -- all that's in the servo drive assembly.
LLoyd
Greetings Lloyd, You're correct of course. I should have said they are built like BLDC motors. With the coils on the outside and the magnets on the inside. And of course brushed DC motors are really AC machines too with the commutator providing the switching. Thanks for correcting me. It's easy to generalize and give people the wrong information. Cheers, Eric
Karl Townsend fired this volley in news: snipped-for-privacy@4ax.com:
Yeah, you can, but you need to make sure the output voltage of the VFD doesn't exceed the motor's rated input.
The TECO servos I use vary in maximum voltage depending upon the power of the motor. The highest voltage any of them will allow is 148V. The lowest is 77V.
So you'd have to have a VFD that could run at fairly low voltages, or use a 3-phase step-down transformer to assure you didn't exceed maximums.
LLoyd
snipped-for-privacy@whidbey.com fired this volley in news: snipped-for-privacy@4ax.com:
Indeed, they are structurally almost identical, except, as I mentioned to another poster, BLDC motors have the commutation electronics built in, and AC servos have no such; it being in the drive module, instead.
LLoyd
Common sense might lead you that conclusion, but many, perhaps most, motor manufacturers don't stick to common sense when referring to these devices. These are the first four manufacturers that google returns if you search "brushless dc motors."
All V/Hz VFDs modulate their output voltage with frequency, otherwise the motor would overheat at frequencies below nominal. The trick to running a brushless motor on a VFD is translating the brushless motor's characteristics into specs more typical for an induction motor. In other words, an induction motor has a rated voltage, frequency, and base speed. Brushless motors are usually spec'd in volts/KRPM. If the number of motor poles and V/KRPM is known, you should be able to come up with numbers that make sense to the VFD.
I've actually done it, but it was a long time ago, and it took a bit of fussing.
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