Gecko 201 current set question

[stirling]

Firstly Hi to all as this is my first post on the zone. Done a search and can't find this issue covered anywhere so...

motors are RHT23-260 as http://www.homeshopcnc.com/page5.html with a rated current of 2.8A per phase.

So according to gecko's instructions I should limit the current sourced from the drive to 2.8A by a current limit resistor with R=(47000*2.8)/(7-2.8) i.e. 31.333 or in practice a 33K resistor.

But elsewhere in the gecko docs is the statement: "The drive will always draw 1/3 of a motor's rated current when it is full wire connected" which of course this particular type of (4-wire) motor is.

So does this mean that although the motor is rated at 2.8A it will only ever be allowed (by the drive) to draw 2.8/3 i.e. 0.93A? in which case:

surely the motor will never achieve its potential power and secondly what was the point of limiting the current with the resistor?

I'm sure my reasoning must be wrong but I can't figure out why.

[ger21]

The 1/3 of the rated current is for determining power supply requirements. Set the Geckos to deliver 2.8A as you have figured it. The motors will get 2.8a, but your power supply only needs to provide about 1/3 of that. Although a little extra won't hurt.

[tauntdesigns]

Howdy,

My pdf says:
*"The maximum power supply current required is 67% of the
motor’s rated phase current."*
or 2/3rds

???
For the 2.8amp motors (31k ohms), do you round up to 33k ohms or down to 30k ohms resistor, or does it matter?
???

Thanks,
Jack

[stirling]

Hi ger21 - Now I'm really confused... if the motors will get their 2.8A but only a 1/3 of it is coming from the power supply, where's the other 2/3 coming from?

Hi tauntdesigns - agreed. the pdf states 67% but in the stepper motor basics pdf it expands on this and differentiates between parallel and series connection. (in that paper it uses 1/3 and 2/3). I'm taking that to mean the 67% is for parallel connection which of course is not applicable to 4 wire motors.

33k v 30k, I thought - what the heck - let's give it a little more juice rather than a little less. Also the other .pdf modifies the 47000 multiplier to 47500 so I'm guessing it's not too critical

[ger21]

Originally Posted by stirling Hi ger21 - Now I'm really confused... if the motors will get their 2.8A but only a 1/3 of it is coming from the power supply, where's the other 2/3 coming from?

I don't really know how to explain it properly, but it's a function of the way the drive works.