# SepEx Field Curve



## major (Apr 4, 2008)

I have a 7 inch diameter Separately Excited (SepEx) wound DC motor which I will eventually run with a SepEx controller. So today I ran a No Load Generated Voltage curve on it. I belt drove it with another motor intending for a constant RPM. Started out at 929 RPM. But as I increased field current, the RPM dropped. So I corrected the measured armature voltage by the RPM ratio to get a "corrected Eg" as shown on the attached plot. From this I should be able to intelligently program in the Field Map.

I suspect I will also be required to enter the field resistance and inductance. Resistance is easy for me to get, even though it is less than an ohm. However inductance is a bit tougher. I can excite it with a variac and get a scope picture of the V and I at 60 Hz. Anyone remember the formula to get inductance from the phase angle, frequency and resistance?

Been too long since Electronics 101 

major


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## Tesseract (Sep 27, 2008)

Good grief - no engineer remembers those formulas (formulae?) - we look them up!

But, you will need to insert a known resistance in series with the field and then excite both with a known ac voltage/frequency to calculate the inductance from the phase angle. 

Cutting to the chase: Θ = tan^-1(XL/R)

E.g. - if R = 100Ω and XL = 100Ω then Θ = 45°

You can also calculate phase angle by dividing the voltage drop across the inductance by that across the resistance.

Or you can measure the resonant frequency with a known capacitance.... my personal favorite.


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## GerhardRP (Nov 17, 2009)

Tesseract said:


> Good grief - no engineer remembers those formulas (formulae?) - we look them up!
> .....
> Or you can measure the resonant frequency with a known capacitance.... my personal favorite.


Am I right that the resonant technique has the advantage that it can be done in parallel to the DC drive, so you can make the measurement of inductance as a funtion of DC excitation?
Gerhard


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## Tesseract (Sep 27, 2008)

GerhardRP said:


> Am I right that the resonant technique has the advantage that it can be done in parallel to the DC drive, so you can make the measurement of inductance as a funtion of DC excitation?
> Gerhard


Nope. Has nothing to do with the DC drive. What I was getting at is to make the winding part of a resonant circuit with a known capacitor and then measure the frequency and the voltage drop across a known (non-inductive) resistor.


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## GerhardRP (Nov 17, 2009)

Tesseract said:


> Nope. Has nothing to do with the DC drive. What I was getting at is to make the winding part of a resonant circuit with a known capacitor and then measure the frequency and the voltage drop across a known (non-inductive) resistor.


Sorry, I should have explained better: One problem with any magnetic system that carries a DC component is that when you go into the saturation region, the inductance of the system drops because the differential permeability curve flattens out. My comment was the resonance technique can be carried out even when the DC current is flowing, allowing the analysis of the inductance vs. DC excitation.
Gerhard


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## Tesseract (Sep 27, 2008)

Ah, ok. Yeah, you definitely needed to be clearer, but, yes, you could use a dc controller to bias the field during a resonance test. Should see the frequency go up as the field saturates. I suspect saturation won't be very deep even with extreme overcurrent because of the air gap (the magnetic circuit of the field is completed through the armature).

Also, you might want to insert an additional inductance in between the controller and the field to prevent the controller from affecting the resonant frequency.


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## GerhardRP (Nov 17, 2009)

Tesseract said:


> Ah, ok. Yeah, you definitely needed to be clearer, but, yes, you could use a dc controller to bias the field during a resonance test. Should see the frequency go up as the field saturates. I suspect saturation won't be very deep even with extreme overcurrent because of the air gap (the magnetic circuit of the field is completed through the armature).
> 
> Also, you might want to insert an additional inductance in between the controller and the field to prevent the controller from affecting the resonant frequency.


Actually, if you compare the initial and final slopes on major's curve at #1 of this thread, you will see that the they differ by a factor of ten.


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## Tesseract (Sep 27, 2008)

GerhardRP said:


> Actually, if you compare the initial and final slopes on major's curve at #1 of this thread, you will see that the they differ by a factor of ten.


Ever seen the BH curve of an ungapped ferrite transformer core? Permeability drops by a factor of 2000-2000 when it saturates. THAT'S hard saturation, not what occurs in a motor.


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## GerhardRP (Nov 17, 2009)

Tesseract said:


> Ever seen the BH curve of an ungapped ferrite transformer core? Permeability drops by a factor of 2000-2000 when it saturates. THAT'S hard saturation, not what occurs in a motor.


Only in book larnin'.... never seen one in person.


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## major (Apr 4, 2008)

major said:


> I suspect I will also be required to enter the field resistance and inductance..........Anyone remember the formula to get inductance from the phase angle, frequency and resistance?
> 
> Been too long since Electronics 101


Hey Tesseract,

If you're still watching, I finally got around to doing an inductance. Found another way. Seemed easier. Maybe you can check my logic 

Used a variac at 60 Hz on the field. Measured 10.14 Vac and 0.460 Aac. Gives me an impedance of 22.03 Ohms.

Used DC supply and measured 5.155 Vdc and 5.012 Adc. Gives me resistance of 1.03 Ohms.

I know that Z² = R² + X². And X = 2*pi*f*L. So using:

Z = 22.03
R = 1.03

I get X = 21.38. Then, with:

2 = 2
pi = 3.14
f = 60

I get L = 0.0567 Henry or 56.7 mH.

Sound right?

major

ps. I remembered how to make that tiny 2


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## GerhardRP (Nov 17, 2009)

major said:


> Hey Tesseract,
> 
> If you're still watching, I finally got around to doing an inductance. Found another way. Seemed easier. Maybe you can check my logic
> 
> ...


I wonder if the inductance of the Variac confuses the measurement. Do you have a low inductance 1 ohm resistor to compare?
Gerhard


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## major (Apr 4, 2008)

GerhardRP said:


> I wonder if the inductance of the Variac confuses the measurement. Do you have a low inductance 1 ohm resistor to compare?
> Gerhard


Hmmm. O.K. How do I measure the inductance of my one Ohm resistor 

I guess those carbon resistors would be like zero inductance. I have 5 watt 5.6 ohm leftovers from gate drive resistors. Maybe I'll throw one of those on it. Or better yet, 5 of them in parallel.


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## GerhardRP (Nov 17, 2009)

major said:


> Hmmm. O.K. How do I measure the inductance of my one Ohm resistor
> 
> I guess those carbon resistors would be like zero inductance. I have 5 watt 5.6 ohm leftovers from gate drive resistors. Maybe I'll throw one of those on it. Or better yet, 5 of them in parallel.


5 in parallel sounds like a plan.
Curious to hear if it actually looks like the pure resistance that it actually is. I was tempted to do the experiment myself
Gerhard


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## Tesseract (Sep 27, 2008)

major said:


> Hey Tesseract,
> 
> If you're still watching, I finally got around to doing an inductance. Found another way. Seemed easier. Maybe you can check my logic
> 
> Used a variac at 60 Hz on the field. Measured 10.14 Vac and 0.460 Aac. Gives me an impedance of 22.03 Ohms.


Yep - this is perfectly reasonable and had I realized the inductance you were trying to measure would have a significant impedance at 60Hz I would have suggested this method in the first place.

GerhardRP - by measuring the actual voltage across the "unknown" inductance, along with the current passing through it, the impedance of the variac is rendered irrelevant.


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