Is my stepper motor wired correctly?

Using Arduino Motors, Mechanics, Power and CNC
3

Hi all,

Tonight I managed to wire up the stepper motor using a 20V power supply (old notebook adapter) and once starting up the motor did some humming.
As I knew I fumbled with the wiring so I'd line up all phases correctly but then one transistor (rated 800mA) burned up.
Shortly after the motor got very hot because the transistor shortened the collector and emitter.

At first I thought I might have shortened something but after some investigation I knew for sure I didn't.
So I trashed my transistor and did some measurements on the wiring.
I found out the middle wire is common and all other wires measured 25 Ohms to common (center coil taps) and around 50 Ohms when I measure the full coil.
So far so good, somehow I must have overdone it using the 20V supply, but strangely enough the motor is rated for 24V if I interpret the datasheet correctly.

I found another 12V adapter which I wired up and I used BD139 transistors rated for up to 1.5A.
I did the math: current = V(12 Volt) / R (25 Ohm) , so current must be around 500mA theoretically.
Again I started up and the humming appeared but no transistor went up in smoke so I finally managed to get the wiring correctly and the Arduino stepper example (which is 2-2 Excitation as far as I know) gave me a nice spin on the stepper!

Again the motor got very hot in a very short amount of time, so I measured the current on the common wire (which is wired to +V) which gave me 900-1000mA's.
So my questions are:

  1. Am I misinterpreting the datasheet? It clearly states rated V=24, Current/Phase=513mA, 50 Ohms/Phase, 4 Phases
  2. 2-2 Excitation should have 2 phases energized at any moment which would sum up to around 1000mA, which I am measuring as well. Is this correct?
  3. If the sheet says 50 Ohms per phase would that be the resistance over the full coil? The label on the stepper says 20 Ohms, and I measured 25 Ohms for a half coil.
  4. Does anyone know why the motor gets so hot in say 1 minute even though I am running the motor below it's rated voltage?
  5. Why did my earlier transistors in the 20V case went up in smoke since current = V(20 Volt) / R (25 Ohm) would result in 800mA which they are rated for.

I'd really appreciate any help on this matter :slight_smile: