# Thread: Electro-Craft 703

1. ## Electro-Craft 703

Looking for some help. I have an EC Model 703. Part Number 71319002
It is 4" Diameter and 7" long and has a 1/2 shaft.
Does anyone know what the specs of this motor are?
It seems to be a well built beefy motor.

I also have two more that are very similar, but on the EC sticker it doesn't list the model or part number just the customer part No. Which is 200389001

Does anyone have a clue to the specs of these motors?

If not, is there any way I can derive the specs?

Bob

2. If these are brushed motors, they would have a top rpm of between 2000~3000 rpm, you can find out the applied voltage by rotating them at a known rpm and measure the DC output, the applied voltage will be directly proportional.
For the rest this is some notes I put together when I was looking for similar information and found them in various sources:
A simplified model of a DC motor can be derived 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)

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.

Note 1: 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 not use a multimeter's ohm range.
If you want to find the 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.
.
Al.

3. ## Motor Specs

Al,
Thanks for the helpful info.
I'm trying to make sense of the first Equation:

V= (Ia * R) + (Ke * RPM)

I assume that Ia is the current draw when the motor is spinning with no load
at a given voltage (V)?

and Omega is the RPM @ that Voltage (V)?

So, can't I find the winding resistance of a free running motor (no load)
by using ohms law R = E/I. for example, If I apply 50 volts to the motor with no load on it and measure a current of 1 amp does that give me 50Ohms of resistance. Or do I really need to lock the shaft and measure the current with a known voltage applied to get R?

Thanks Again!
Bob

4. If you run a DC motor off load, There exist a Back EMF that opposes the applied voltage, practically equal to the applied voltage.
So the practical way is to apply a voltage to a locked rotor and measure the current, there will be some brush resistance also that will affect it slightly, a meter on resistance range is less accurate.
I find that the motor rotate method to find applied voltage max is the best way.
This and the Inch-pound/amp (torque/amp) I find are the most usefull, and maybe rotor inertia
Al.

5. Al,
That makes sense to me. I will try it that way. Let me ask, how do you know that the max RPM for these motors are around 2000-30000, I don't doubt it, but how do you make that determination.
Also, If I know my torque per amp, that will give some idea of how powerful these motors are at a given current, but how does one go about determining Max current that the motors can handle without burning up? I mean if I stall the motors with 60 volts applied to them, I assume they are going to draw some pretty good current and will cook themselves?

THanks again!
Bob

6. Traditionally DC Brushed motors of this size are usually limited to this max. rpm because of mass etc, you can get 6000 rpm brushed motors but are generally much smaller.
I have the specs on the ElectroCraft 644 and 720, I will scan in the page and you can get an idea, as your motors may be close to one of these.
Al.