How can stepper 3V, 3A = 9W is powerful enough to turn mini table?? Any comment?

[cjchands]

How can stepper 3V, 3A = 9W is powerful enough to turn mini table?? Any comment?

Recently, I just purchased 200oz-in 3V, 3A unipolar stepper motor and drive for cnchobby and thru my understanding that 3V and 3A doesn't stay constant when you run the mini-mill table X, Y, Z....but for an average of constant movement you get 9W (VxI) for the power...

I have bigger size CNC servo (FANUC10M) motor that has 1.2KW and 12Nm of Torque to drive knee mill type table.

My point is that for the power of 9W, it just make little sense how it could move table load. If it comes out like 100W, ok, that's might sound reasonable.

Any comment?

[pastera]

At a stand still the stepper consumes 9W (per coil). ONce you start moving, the voltage needed to push the same 3 amps goes up therefore the power goes up also. At high RPM you may need 10x the nominal voltage to get the full 3 amps of current, so the power would also be 10x or 90W.

Aaron

[cjchands]

Aaron,
Are you suggesting that voltage of nominal 3V can go up to 30V at time?
If so, is there relationship then for the motor of 200oz-in torque with changes in Voltage output?
Also, is the current of nominal 3A stays pretty much the same most of the time machine is cutting then.
Thank you
J

[cjchands]

I was told one time that Power = (Torque x RPM)/9.55
So for hobbycnc motor 200oz.in = 1.4N.m
And although I found no RPM vs Torque curve on this motor, I assume that you go low with 150RPM.
So the max power come out to be P = (1.4 x 150)/9.55 = 22W????

Any advice on formula relate to V, I, and Torque to get Power?

[gar]

cjchands:

Run these experiments.

1. Connect an AC voltmeter, for example a Simpson 260, across one coil of you stepper. Nothing else connected to the stepper. Use a variable speed driver to the motor, maybe 0 to 30 RPM. Do not rotate the stepper too fast. Plot output voltage vs RPM. Do not go over 100 V.

2. Take your servo motor and lock the rotor. Apply maximum rated DC current thru the armature and measure the voltage across the armature.

3. Provide a torque load on the servo motor. Apply different armature voltages, measure the armature current, load torque, and RPM for different input voltages and load torques.

4. Drive the servo motor with a variable speed drive and measure and plot the output voltage vs speed with no electrical load except the meter.

Study the basic relationships of conservation of energy, torque vs magnetic field intensity, induced voltage vs velocity, power loss in a resistive element, sum of the voltages around a closed loop, what e = i * r, and p = e * i really mean.

See if this will help you understand your question.

.

[bunalmis]

You see the single phase of stepper motor at this picture.

Pe: Electrical power (instantaneous power)

Pm: Mechanical power

e: electro motor force (This is proportinal with rpm. e=k*n, k motor constant )

n: rpm

Pm = efficency * Pe

T: Torque



P is not equal to V * i

(V, e and i are AC signals.)

[diecutter]

My HobbyCNC controller runs off a 36 volt dc supply which is what HobbyCNC recommend. My stepper motors are rated 1.7 volts which means I am running them at about 20 times the rated stepper voltage which is the high end of recommended overvoltage. This setup results in a max of about 80 ipm through 1/2 by 10 acme rods. The steppers only warm up slightly.

[pastera]

Torque is directly related to the coil current. The stepper driver tries to force the current to the set value (depends on step position when microstepping).

As the motor rotates the movment of the magnitized rotor induces a voltage in the coils. This voltage is called BEMF (Back ElectroMotive Force) and opposes the applied voltage. In order to force the the same current through the coil the stepper driver increases the applied voltage to overcome BEMF.

Aaron