Cross referencing servo motors 1

[itsmoked]

Hi,
I've been struggling with making a comparison between two dissimilar servo motors. I'm working on a large CNC router X/10ft Y/5ft Z/3ft with all the axis' powered by Old Baldor MTE-3363-BLBCN brushed motors.

The Baldor is called a:
180OI,4000RPM,DC,5328P,TENV,F1

That's 180oz-in at stall (continuous).

The spec sheet gives only this info:

and

I'm trying to get into the ball-park for a modern brushless replacement.

Typical Servo-Motor selector

Most these selectors ask things like PEAK torque and continuous torque as their key point.
The Baldor spec states 180 oz-in (stall/cont). How does this get reconciled into these selector sheets as to Peak Torque and Continuos Torque two important values in the finders?

I'm having no success doing the cross.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[cswilson]

Keith:

Here are better specs for what I am quite sure is the same motor from a 2002 Baldor catalog. I think this will give you the info you need.

(Sorry I couldn't convert it quickly to a form I could paste in directly.)

There are some things about the specs you show that don't really make sense, like why they are specifying RMS amps for a DC motor. I would trust the data in my catalog more.

Curt Wilson
Omron Delta Tau

 

[itsmoked]

Curt I'd give you about 5 stars if I could.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[cswilson]

And the young guys mock me for all the old paper I keep in my office...

 

[Compositepro]

I would guess that RMS amps are specified because most DC speed controls use rectified AC that is not filtered.

 

[itsmoked]

That or they're fed with PWM signals that can be RMS'd.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[cswilson]

I'm afraid I can't buy either explanation for the use of RMS amp specs for these motors. These are DC servo motors intended to be operated from a steady DC bus using a servo drive with lots of bus capacitance. While the drives do have a PWM output to the motor, this voltage switching only results in a very slight current ripple.

The specs I found in the 2002 Baldor catalog (4 years before the Reliance purchase) are very standard for DC servo motors, and do not mention "RMS". The "Baldor Reliance" specs Keith had -- which had to have come later -- look to me do have been redone by someone used to simple AC motor specs, who didn't have any idea what servo motors specs were all about.

 

[Compositepro]

I doubt that the spec sheet format was developed for this specific motor. RMS VDC is what is meant when the term volts DC is used in most any context, except for when instananeous voltage is used to differentiate the two terms. And the definition of "instantaneous" is also open to debate.

 

[cswilson]

The puzzle to me is why a perfectly good and useful existing spec (much as you would find for any brushed DC servo motor) was tossed in favor of an inappropriately structured one. As I said above, my best guess is that it was redone by someone used to a different class of motor.

For DC servo motors, when RMS terminology is used, it is not referring to sine wave patterns, but instead to sections of (usually) constant DC current, as for acceleration, constant speed, and rest. I checked several catalogs of data sheets, and none use RMS for these motors.

So if the motor is used in a cycle of +10A for 1 second (accel), +1A for 8 seconds (slew), -8A for 1 second (decel), and 0A for 10 seconds, the RMS current would be calculated as:

I (RMS) = sqrt((10*10*1 + 1*1*8 + [-8]*[-8]*1 + 0*0*10) / 20) = sqrt(172/20) = 2.93 A

This value must come under the continuous current rating for the motor.

When you do this same type of calculations for brushless servo motors, the individual terms themselves are RMS values due to the sinusoidal waveforms, so you are taking RMS of RMS values in these types of calculations. Some years ago, I had to write up instructions to do these calculations, and it was a real bear to keep the terminology clear. It took many drafts.

 

[controlsdude]

My answer for why spec sheets were done that way is the fact who got in the drivers seat in these buy outs. Most company buy outs are about putting the other company out of business. So redid spec sheets are the least of their worries. It just an excuse from the buy out company to say that company's products were shite. So please refer to our line that gives you all the info you need. Seen this too many times.

 

[Compositepro]

"the individual terms themselves are RMS values due to the sinusoidal waveforms".

This is all that I am talking about. Any DC current that is not perfectly invariable over the period of a second or so must be averaged in some way, and the most useful averaging method is RMS. It just is not very common to state that it is an RMS value because every one assumes it without thinking. VDC RMS is scientifically more rigorous.