0
\$\begingroup\$

I do electronics for fun and need help understanding the principles when coupling stepper motors and drivers!

I have this type of stepper motor: kh56km2u027 and a drv8825 stepper driver, and was wondering if they would run safely together.

In the motor label, it's written 1.76V 2A and the driver can operate on 8.2V-45V 2.5A(peak)

There are few things that get me confused, like:

  • Labeled amps -> does it mean that it draws 2A in total whatever the case? (so I assume the driver can run it)
  • amps-per-phase -> what's the difference when it's labeled: 2A and 2A per-phase?

Or is there anything else I'm missing here?

Thank you!

Edit: Forgot to mention that I power the driver using 12V/10A supply and the logic through Arduino board.

\$\endgroup\$
1
\$\begingroup\$

First of all, the big question for stepper motors: it's bipolar, unipolar or configurable?

It depends on how many wires it has. A 5-wire motor can't be used with that driver. A 4-, 6- or (rarely)8- wire motor can. The KH56 is a 6-wire motor so it can be used in bipolar configuration.

As for the current rating: the stepper motor (especially in bipolar mode) is usually driven with a constant current, not with a given voltage. For that kind of motor you could either:

  • Simply apply the rated voltage (1.76V) to the winding, in which case it will consume about 2A, or

  • Apply a current of 2A and keep that regulated, which is the approach followed by drivers.

This is the per phase current rating.

The motor has two coils in quadrature so you will have a total of maximum 4A flowing in the windings. This is the whole motor current rating.

However this doesn't mean that you need 4A on you 12V power rail.

The motor driver is more or less a switching converter. It only gives pulses of current to keep the magnetic field charged; you can see the current ripple in figure 9 in the datasheet, for example.

In short, you program the driver for a given current (with a resistor, in this case) and it regulates it but it's better if you think of it as power. About 2A at 2A are 4W, so it's an 8W motor. At 12V it's somewhat less of 700mA.

By the way circuit layout with the TI stepper drivers is quite critical; do it exactly as it is shown in the datasheet. Obey the recommendations on capacitors, they are quite important, too.

\$\endgroup\$
4
  • \$\begingroup\$ Thanks for your time Lorenzo! As I understand, the 2A rating stands for per-phase, but even tho it's a 2-phase motor, it never draws 2x2A at the same time, and so it's possible for the drv8825 to run the motor, right? As for wiring, I followed this diagram and set the Vref on driver using Vref = I(max) / 2 so Vref = 2A / 2 = 1. I tried this, it spins.. but driver gets hot after few minuets.. I jus don't know if it's safe to keep it spining like that or will it burn stuff!!! D: \$\endgroup\$ – Fisnik Tahiri Feb 14 at 21:29
  • 1
    \$\begingroup\$ It will draw 2A per phase but the driver has 2A available per phase so in the end it works. Quoting the datasheet "The DRV8825 is capable of driving up to 2.5 A of current from each output (with proper heat sinking, at 24 V and 25°C).". See figure 4, 24V is its sweet point'' for Rdson, as 12V is slightly degraded. Proper heat sinking" is the key word here. You should do some thermal calculation (section 7.4), maybe adding some heat sink or even lower the current if you are worried. Your board datasheet could have info on that. The motor will work at reduced current but with less torque \$\endgroup\$ – Lorenzo Marcantonio Feb 15 at 7:21
  • \$\begingroup\$ I very much appreciate the info, as I couldn't wrap my head around all this by just combining bits from research! Thank you mate! I attached the heatsink (the small one that came with driver) and reduced the current draw a bit so now it doesn't get very hot and the torque is sufficient for my needs.. now only the speed is the issue :p.. have to figure that one out. Thanks again. \$\endgroup\$ – Fisnik Tahiri Feb 16 at 8:41
  • \$\begingroup\$ I'm already using these drivers for positioning so I'm sharing the experience. Speed in steppers is a difficult issue because 1) they are simply not designed for speed 2) they have so called 'resonance point' where torque lowers a lot. If you can rise the supply voltage it will helps (due to inductance di/dt limiting), it's not uncommon to see steppers powered at even 72V \$\endgroup\$ – Lorenzo Marcantonio Feb 17 at 8:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.