MechMate CNC Router Forum

Go Back   MechMate CNC Router Forum > Electrical & Electronic > 702. Power Supplies
Register Options Profile Last 1 | 3 | 7 Days Search Today's Posts Mark Forums Read

Reply
 
Thread Tools
  #1  
Old Wed 18 August 2010, 13:20
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Confusion on power supplies for Pk296A2A-SG7.2 motors . . . . .

One of our recently assembled MM's was prone to loosing steps when pushed hard. So we set out to find the reason. Our first assumption was that we had 3 identical MM's and that we could swop parts between them until the problem swopped as well. After some extensive swopping, and no changes being noticed, we then looked at the power supplies.

It was then discovered that the longest running machine (2 years service) had a 50VAC transformer (75VDC to Gecko) and 18kOhm resistors on the Gecko's (2 Amp). The problem MM had a 30VAC transformer (42VDC to Gecko) and 36kOhm resistors (3 Amp). Another recently assembled MM, running without problems, also had this 30VAC transformer and 36kOhm resistors.

In theory, the 75VDC is way to high for half-coiled Pk296A2A-SG7.2 motors, but the 2 Amp current limit kept them cool enough in their 2 year's of use.

Today the transformer on the problem MM was upped to 50VAC (75VDC) and the resistors dropped to 18kOhm......and the problems are now gone.

I don't know what to make of all this yet, and I don't suggest that others follow what we have done, but it may be interesting for the adventurous folk to hear of this experience.
Reply With Quote
  #2  
Old Wed 18 August 2010, 13:53
bradm
Just call me: Brad #10
 
Somerville(MA)
United States of America
Gerald, I assume you must have already triple checked that the motors are, in fact, wired half-coil on the problem child? You have a very interesting data point there.
Reply With Quote
  #3  
Old Wed 18 August 2010, 14:20
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
My "elves" did assure me that the machines were identical........and then we discovered the differences in transformer and resistors! Yeah, I don't think we have reached the bottom of this issue yet.
Reply With Quote
  #4  
Old Thu 19 August 2010, 08:25
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Checked today that all motors on all MM's have red,blue,green,black connected to their cables (yellow & white insulated off). Thus, all motors are half-coil wired.
Reply With Quote
  #5  
Old Thu 19 August 2010, 08:34
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Gerald,

The numbers actually seem to fit within the expected range.

75V X 2A = 150 WATTS

42V X 3A = 126 WATTS

39V X 4.5A = 175.5 WATTS

A stepper motor / stepper driver is NOT a resistive load, so the Watts is not exactly accurate, but it is usable for comparisons between configurations.

The 75V @ 2A is what you're using.
The 42V @ 3A is what works on another machine.
The 39V @ 4.5A is the maximum of a PK296-03AA motor which has the same electrical characteristics as the PK296A2A-SGxx motors. (Remember that Oriental Motor derates the geared motor, probably to keep the gearbox from being driven too hard.)

The 39V @ 4.5A configuration is the maximum voltage and maximum current according to Mariss' formula, SQRT(1.5mH) X 32. It generates 175W. So, any combination that generates less than 175W would probably be acceptable.

Last weekend, I ran some heat related tests using a PK299-F4.5 motor, a PK296-F4.5 motor and a PK296B2A-SG3.6 motor. With the PK296-F4.5 wired half-coil and current limited to 4.5A, its temperature hit 75C while the other two motors stayed at 50C. (Power supply was set at 35VDC.) That was the third test in a series and I only waited until the case temperature was 45C before starting the test. Because the temperature was higher than expected, I suspect that the core temperature of the motor was already hot at the start of the test. Also, I realized after running the tests that the case temperature varied almost 15C depending on where I measured the temperature. The hottest point was at the junction of the "black" body and the "silver" end-cap at the rear of the motor directly over the shaft. The coolest point was at the junction of the "black" body and the "silver" end-cap at the front of the motor directly over the shaft.
Reply With Quote
  #6  
Old Thu 19 August 2010, 08:44
bradm
Just call me: Brad #10
 
Somerville(MA)
United States of America
Drat. That was my favorite theory. Rather than clinging to the theory and accusing OM of a manufacturing error, here's another: Higher voltage might allow a more severe acceleration ramp, right? Suppose your accel settings are right at the limit - and frictional variations between machines would show up here.

This one is a serious stumper
Reply With Quote
  #7  
Old Thu 19 August 2010, 09:54
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Mike and Brad, I wish had more time to dig deeper for the reasons of these anomalies. The one area where I still need to find/test a definitive answer is the possibility of bad capacitors. (Apparently the caps were also changed when the higher voltage supply was put in yesterday).
Reply With Quote
  #8  
Old Thu 19 August 2010, 14:25
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Gerald,

Check the bridge diode also. I've had one of the four diodes in a bridge burn out which turned the full-wave bridge rectifier into a half-wave bridge rectifier. A half-wave bridge rectifier only "charges" the capacitors at the line frequency (60hZ or 50hZ) instead of at 2X the line frequency (120hZ or 100hZ).
Reply With Quote
  #9  
Old Thu 19 August 2010, 17:34
smreish
Just call me: Sean - #5, 28, 58 and others
 
Orlando, Florida
United States of America
Mike....good point. Never would have even thought of that one.
Sean
Reply With Quote
Reply

Register Options Profile Last 1 | 3 | 7 Days Search Today's Posts Mark Forums Read

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Wiring the PK296A2A-SG7.2 motors & power supply voltage selection Richards Motors & their mountings 97 Fri 29 July 2011 11:46
Pk296a2a-sg7.2 albertoslayer General - MM Build 1 Tue 08 June 2010 01:19
Resistors for G504 and wiring for PK296A2A-SG7.2 motors (unipolar) jehayes 701. Motor Drives 7 Sun 15 November 2009 19:16
Are these the same: Pk296ae-sg7.2 (metric) = Pk296a2a-sg7.2 (inch)? gixi Motors & their mountings 15 Wed 26 November 2008 14:39


All times are GMT -6. The time now is 11:59.


Powered by vBulletin® Version 3.8.3
Copyright ©2000 - 2018, Jelsoft Enterprises Ltd.