View Full Version : How much A, and how much V

10-12-2010, 09:39 PM
I need answer for probably simple question (I am a layman, but I am ready to learn).
My motors are with this specification: 5,1Nm, 1,9V, 3,5A, 0.53Ω, 6.5mH.
What is the optimal power supply for them?

11-12-2010, 01:10 AM
From the Gecko G251 manual -

A very accurate calculation of power supply voltage is to find your motorís inductance, and put it into the following equation.

32 * (√mH inductance) = Power Supply Voltage

If your motor has 2mH of inductance, the equation would look as follows.
32 * (√2) = 45.12V

So, using the 6.5mH of your motor would give an optimum voltage of 81.58V.

11-12-2010, 07:00 PM
Goran, do you really need such big motors? I guess those are NEMA34 units...

11-12-2010, 09:20 PM
Hi and thank you for answer.
There is one thing that make me confused. I found on lot of sites that voltage of power supply should be 10-20 multiplied voltage of motor. So if my motor have
voltage 1,9V, my power supply should be 19V-38V?
Reason for the this motors are their price (93€), motor 4,4Nm is 83€ and motor 2,1Nm is 65€. I thought that 10€ is not to much for 0,7Nm stronger motor.
I want to do 3d, and I thought stronger is better.
I have once more question. What to by:driver SD4350 (voltage 24-50V optimal 36V) 76€ or driver SD5980 (voltage 80V) 98€?
What is your advice for motors? My intention is to do 3D carving. Is NEMA 23 enough?
Sorry for my English. I hope you understand what I havew to say.

11-12-2010, 11:28 PM
If this formula is correct, then the motor NEMA 23 1,6Nm 7,5V 1,5A 10mH need power 101.19V.

12-12-2010, 12:37 AM

That is technically correct... the inductance of the motor affects the rate at which its torque drops of with increasing revs. The Nm rating of the motor is when it is stalled, a condition that rarely exists. A stepper motors torque is relatively constant, but below the stall torque, at low speeds until a point called the corner speed. Above those revs the torque (and therefore the ability of the motor to accelerate the load) drops rapidly. The corner speed is dictated by the inductance and the voltage.

Without knowing anything about your mechanical arrangements it is impossible to say what torque and revs you need. It is possible your mechanics are such that the motor never gets above a certain revs and therefore a higher voltage is not needed. Therefore advising you on driver selection is near impossible. It is also possible that the motors you have selected, despite their high torque, are unable to run fast enough for your requirements - larger motors do not rev as high as smaller ones. In either case you need to tell us a lot more about the mechanical arrangements.

There is a spreadsheet and tutorial here (http://www.mycncuk.com/forums/showthread.php/1524-What-size-stepper-motor-do-I-need.)that you may find useful.

The approach that uses a multiplier of 10 - 20 x the motor voltage rating is a very rough rule of thumb and doesnt really apply to chopper based bipolar drivers but to the earlier unipolar L/R drivers. Like many 'rules of thumb' it is often quoted with no understanding of why (or if) it works. I would ignore it in favour of the other approach.

12-12-2010, 06:17 PM
Hi again.
I would like to thank you for your help, but I still can not belive that for Nema 23 (http://www.alco.co.rs/index.php?option=com_content&view=article&id=206:step-motori-nema-23&catid=82:step-motori&Itemid=1316&lang=sr) 1.6Nm, 7.5V, 1.5A, 10mH I need power suplly 101.19V DC.
My linear guides will be this (http://www.alco.co.rs/index.php?option=com_content&view=article&id=219:support-cilinder-linear-rail&catid=79:linearni-sistemi-za-voenje&Itemid=1286&lang=sr),and my ball screw will be this (http://www.alco.co.rs/index.php?option=com_content&view=article&id=219:support-cilinder-linear-rail&catid=79:linearni-sistemi-za-voenje&Itemid=1286&lang=sr).
Thank you for your time and your help.
Regards from Serbia by Goran.

12-12-2010, 06:58 PM
I forgot link for ball screw (http://www.alco.co.rs/index.php?option=com_content&view=article&id=188:sfu-sistemi&catid=80:kuglina-navojna-vretena&Itemid=1231&lang=sr#twoj_fragment1-2)