CNC_B012

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 Maximum Continuous Stall Torque * Maximum Continuous Speed (No Load) = Power

Continuous Stall Torque * RPM / 10⁶ = Horsepower

• If the speed rated speed or 1750 rpm is at 90 VDC at 80 VDC (the maximum for Gecko) the speed will be limited to 80/90 * 1750 = 1555 rpm.
• The motor needs 'headroom' to reliably maintain servo lock in catch-up situations. If the maximum step rate in the CNC software is set to 1/2 or 2/3 of the maximum 1550 rpm available the available rpm will be 800 to 1000.

I just picked up some nice little DC brushed servos and have the same dilemma: no specs. This will help. Thank you. ~ Evodyne

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@Evodyne:
Sorry. It will not work with a brushed DC motor. It will if you can brake it while under test. But a DC motor will have a current consumption largely proportional with load torque. (Very linear if it is a servo motor.) So you would have to control the load instead of the power supply. Not so easy.

In a stepper motor, the same current flows with and without load when it is not rotating. It does not start rotating when you apply a current.

Anyway, given the possibility to control the load, I would:
1. Find the rated voltage by applying an increasing voltage until the motor reaches it's max no-load speed. If this is not given, guess it based on the application the motor was used for. Or plot voltage and speed, increasing until the voltage/speed ratio starts to increase (deviating from linear).

2. Then load the motor down until the 30min. temperature gets above 60C. This is not easy unless you have a brake bench.

You should not try to lock the shaft firmly and make the measurement, as then the current will go through only one winding set instead of distibuted among all. It will probably burn this one winding before the motor housing gets up to 60C. If you can brake down the shaft to almost standstill, that will be OK but it must rotate. 60C may be somewhat on the high side for a brushed DC motor, although it is definately on the safe side for a stepper. The reason being that the brushed DC have it's windings buried in the center of the motor, while the stepper (and BLDC) have them on the periphery. So the transfer of temperature to the outside is a lot better.

Actually in your case you could just check the thickness of the windings if this is possible without taking out the rotor. Then calculate max current based on that.

Warning: Do NOT take out the rotor of any permanent magnet motor! You may end up with seriously lowered torque after having done this. This happens with many brushed DC motors and all modern BLDC and steppers. The reason is that you break the magnetic flux flowing through the rotor and the magnets loose most of their strength and the motor is junk.

Well in your case, I would just hook up the motor and use it. If it burns out, it's too small. And with an unmarked motor it would be no loss. It is actually worth close to nothing anyway. And you can monitor the temperature while using it. If it does not get hot, you're using it below specifications and will not burn it. In Stuart's case it's another story, he must set up a current through it even when not loading it as I mentioned.

~ ESJaavik

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This is some notes I made when I was in the same situation with some DC motors
A Simplified model of a DC motor can be derived from the following:
Assuming the armature inductance to be zero and ignoring the resonance
effect. With these stipulations the equations are:
1. V=Ia R + Ke omega (Ia=armature current, R=armature resistance,
Ke=electr. constant, omega=speed)
2. Tg=Kt Ia (Tg=costant, Kt=torque constant)
3. Tg=J d(omega)/dt (J=inertia, d(omega)/dt=accel.)
The DC motor transfer function is:
Gm(s)=(1/Ke)/(1+s(Rj/KtKe)), which can be written Gm(s)=(1/Ke)/(1+sTm)
where Tm=mechanical time constant.
To measure the parameters you are looking for, I suggests the following:
A. Measure with an ohm-meter the armature resistance, then apply voltage to the motor without load and measure the current and speed. From equation 1. you can easily derive Ke.
B. Apply nominal current to the motor (with the shaft locked) by means
of a variable voltage source. Measure the torque on the shaft. From this you can derive the torque constant Kt=Torque/Amp.
C. You will find that Kt is approx. equal to Ke
D. For the inertia you can obtain it by calculation from the size and
material of the rotor.

Note1: inductance can be ignored- the electrical time constant is
very short compared to the mech time constant so that it can usually be
ignored.
You can measure the mech time constant by running the motor up to
speed at no load, disconnecting the supply and letting it coast down- plot speed vs time and fit to exponential N=No(e^-t/Tm) time to drop to 36.8% of original speed is the time constant.

Note2: If it is a permanent magnet motor, you can determine the internal emf by spinning it at rated speed and measuring the open circuit voltage. The voltage at any other speed will be directly proportional to speed. To measure the winding resistance, lock the rotor so it doesn't turn and measure the current with a small voltage applied (so as not to exceed rated current) Don't bother using a multimeter's ohm range- not worth the effort.
For inductance, you should use a 'scope- apply a voltage, rotor locked and look at the current trace vs time.
This will be of the form i=K[1-e^Rt/L] where i is the current at time t.
In most cases the inductance can be ignored as its effects are generally swamped by the mechanical inertia in transient cases and is of little importance for steady state.
~ Al the Man

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