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Author Topic: Replacing a Resolver of an AC Servo with an Encoder  (Read 13303 times)

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Offline Dan13

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Replacing a Resolver of an AC Servo with an Encoder
« on: August 27, 2009, 02:25:35 PM »
Hi,

I am considering to buy an AC servo motor. This one I found satisfies all my requirements (the main of which is top speed of about 8,000RPM) but one - it has a resolver. I found that it's really hard to find motors in the above speed range, so there is no much choice really. So I am considering to buy it and fit a commutating encoder in place of the resolver (the other choice is to find a servo drive that will support servo commutation, but it seems even harder).

I have two questions:

1. Has anybody done this before and is it worth the effort? Any special difficulties?
2. Any sources for reasonably priced commutating encoders? I found some at www.encoderprod.com , just called them but their prices are much more higher than I am willing to pay for an encoder. They are selling them for $250 for a single encoder!

Thanks,
Daniel

Offline Hood

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Re: Replacing a Resolver of an AC Servo with an Encoder
« Reply #1 on: August 27, 2009, 02:55:54 PM »
I have done that on a couple of motors and it was easy enough but another motor I had the resolver seemed to be fixed to the motors shaft and no way it was coming off.
 The encoder sounds about right and is cheaper than the quote I got from Renco by about $100
What I ended up doing was buying a small motor with the same amount of poles and using the encoder from it, had to machine special mount plate and an extension shaft as the motors shaft was 1/2inch and the encoder was 12mm :(

The drives I use have auto sensing for the commutation but its not the best as on power up it moves a bit while it senses the commutation, fine for a spindle I suppose but not great for an axis. Then again if the aux power is kept on the drive it will not need to do this every time the main power comes on.

What size motor you after? Could you not get a bigger motor and gear it to get the speed?

Hood
 

Offline Dan13

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Re: Replacing a Resolver of an AC Servo with an Encoder
« Reply #2 on: August 27, 2009, 03:17:29 PM »
Hood,

I need the motor for my Emco F1 spindle. The original one is a DC motor 4000RPM, 440W, 1Nm, geared down 2:1, so at the spindle I get max speed of only 2000RPM (not really enough for most of my work) and torque of 2Nm. The torque is a bit insufficient at times, but I think an AC servo with the same continuous stall torque rating would do much better, since it can ramp up to it's peak torque much faster and stay there for longer times.

Getting a bigger motor - that was what I was thinking to do before I took apart the F1's head. The timing pulley mounts on a 35mm shaft, setting the limit to how small a pulley I can use, and the head housing limits the max pulley diameter I can put on the motor - about 70mm. So gearing down 1:1.5 seems to be the best I can do. Thus using a 8,000RPM motor I should be able to get 12,000RPM at the spindle - which is what I am aiming for.

But may be you're right, I'll need to compromise on a lower speed. May be I can find a 6,000RPM and get 9,000RPM at the spindle.

Thought using a High Speed VFD spindle, but it would limit me to a lower speed of 4,000RPM (if geared down 2:1).

Daniel

Offline Britt

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Re: Replacing a Resolver of an AC Servo with an Encoder
« Reply #3 on: August 27, 2009, 08:15:06 PM »
How about http://usdigital.com/   -- much lower cost?

Offline Hood

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Re: Replacing a Resolver of an AC Servo with an Encoder
« Reply #4 on: August 27, 2009, 08:18:34 PM »
How about http://usdigital.com/   -- much lower cost?

Do they have commutation signals?
Hood

Offline Britt

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Re: Replacing a Resolver of an AC Servo with an Encoder
« Reply #5 on: August 27, 2009, 08:27:41 PM »
Wait a sec -- that needs an absolute encoder doesn't it? That's not so cheap...  <slaps forehead> doh! :-[

Offline Hood

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Re: Replacing a Resolver of an AC Servo with an Encoder
« Reply #6 on: August 27, 2009, 08:34:42 PM »
 Abs may be needed on certain drives but incremental is more common I would say. I would imagine Daniel will be looking for incremental  but they also have to have commutation signals in addition to the A, B and I channels. It is also preferrable if they are differential for A, B and I although you could get a line driver from US Digital to convert.
 He will also have to make sure that the commutation signals match the pole pairs in the motor he gets.


Hood
Re: Replacing a Resolver of an AC Servo with an Encoder
« Reply #7 on: August 27, 2009, 11:23:26 PM »
Renco are dumping commutation encoders on ebay right now, for $20.00 and $90.00 for the differential bearing model.
Essentially you are converting an AC sinusoidal - resolver motor to BLDC. I have converted a few including the proprietary Fanuc Red caps.
As mentioned, you have to find out the number of pole pairs and align the commutation up when installing.
Renco have the pole pair designation after the resolution, 1000/4 will be for an 8 pole motor.
Nosmo.
 

Offline Hood

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Re: Replacing a Resolver of an AC Servo with an Encoder
« Reply #8 on: August 28, 2009, 04:04:02 AM »
Quote
Essentially you are converting an AC sinusoidal - resolver motor to BLDC

What is the difference between AC Servo and BLDC? I have searched and not really come up with anything conclusive, the best I can make out they are one in the same and its the drive that actually determines it.
 I know that the motors I have are certainly described by my drives setup parameters as having a sinusoidal commutation so I would presume they would be what you are calling AC sinusoidal rather than BLDC?
Hood
« Last Edit: August 28, 2009, 04:05:57 AM by Hood »

Offline Dan13

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Re: Replacing a Resolver of an AC Servo with an Encoder
« Reply #9 on: August 28, 2009, 08:01:42 AM »
Hood,

Once read somewhere (can't recall where exactly) :

AC servo and BLDC are basically two types of motors operating on the same principle - electronic commutation. The only difference between the two is that an AC servo doesn't have magnets at all, while a BLDC has magnets on the rotor. AC servo has windings on both the rotor and stator.

Daniel