# Low Inductance Motor



## ken will (Dec 19, 2009)

Have you tried using extra long cables from the controller to the motor and wrapping each of them in a loop to add inductance?


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## Tesseract (Sep 27, 2008)

PZigouras said:


> An older fella in my EV group has a low inductance 9-inch DC motor on in fairly heavy car. Pack voltage is 192.
> 
> We tried three different 1000-amp IGBT-based controllers on it so far, and none seem to want to move it. It appears that the desaturation detection kicks in almost right away on every controller. We also tried a Kelly controller, but that just caught fire.
> 
> I can't get a really good reading on my inductance meter, but the motor seems to be between 4.3 and 5.1 uH. Is there any way to drive this on a 4000-pound car? Does anyone know if an older SCR-based controller may do the trick?


Plug these numbers into the standard inductor equation rearranged to calculate change in current and it doesn't make sense that a modern 1000A IGBT controller would fault off with a desaturation error in any reasonable amount of minimum on time. To wit:

dI = V * (dt / L)

where V is 192V, dt (i.e. - on time) is, say, 10us, and L is 5uH then current will ramp up to 384A. 

Thyristor/SCR controllers tend to operate at much lower switching frequency because they have much longer minimum on (and commutation/off) times, so that won't help you.

That said, it is much more likely that the reason the controllers are desaturating is because of a turn-to-turn short in one (or more) of the field coils. Ie - the motor is damaged.

Finally, most LCR meters give nonsensical or too-low readings when trying to measure the inductance of a motor. The way I like to do it is by measuring the peak to peak ripple current through the motor vs. the on time of the IGBT vs. the applied voltage, with the motor shaft stalled. Our esteemed motor guru, major, frequently suggests applying a low 60Hz AC voltage across the motor in series with a known resistance and then using the output of the divider thus formed to determine the reactance of the motor (and therefore its inductance).


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## PZigouras (Jun 5, 2010)

Tesseract said:


> That said, it is much more likely that the reason the controllers are desaturating is because of a turn-to-turn short in one (or more) of the field coils. Ie - the motor is damaged.


Now that you mentioned that, I took a better look at the car and noticed that the motor is a SepEx. So the controllers that he tried were attempting to drive the armature directly (which explains the extremely low inductance). The field is apparently powered by the vehicle's 12 volts battery. This is how the car was wired originally, but I don't know if the previous owner has ever driven it this way.

I'm assuming that no series-wound controller can run this motor on a car this heavy, so we will probably have to go with a sepex controller. The only issue is that I don't know of any sepex controller big enough to push 4000+ pounds around. Does anyone know of one?


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## Russco (Dec 23, 2008)

PZigouras said:


> Now that you mentioned that, I took a better look at the car and noticed that the motor is a SepEx. So the controllers that he tried were attempting to drive the armature directly (which explains the extremely low inductance). The field is apparently powered by the vehicle's 12 volts battery. This is how the car was wired originally, but I don't know if the previous owner has ever driven it this way.
> 
> I'm assuming that no series-wound controller can run this motor on a car this heavy, so we will probably have to go with a sepex controller. The only issue is that I don't know of any sepex controller big enough to push 4000+ pounds around. Does anyone know of one?


Back in the early 80's when low inductance aircraft motors were the norm and controllers operated at 4000 hz., it was necessary to add a series inductor in the armature loop. Willey provided an inductor about 5 x 5 inches wound with something like 1/16 inch copper bar. 

The 9 inch Advance motor has low inductance even as a series motor, which forced Curtis to redesign the 1221 and 1231 controller to reduce the operating frequency a low duty cycles so the pulse-by-pulse current limit would operate. 

A series inductor or perhaps using the series fields for a spare motor for testing purposes will show if the low inductance of the motor is your problem. Using a scope across a DC CT's output will show the duty cycle and change of current during operation.


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## PZigouras (Jun 5, 2010)

ken will said:


> Have you tried using extra long cables from the controller to the motor and wrapping each of them in a loop to add inductance?



Adding a massive inductor was going to be our last resort. But to step up from the neighborhood of 5 uH to 30+ may take a rather large inductor. Possibly almost as large as the motor, from what I have been told.

I did manage to scope the controller’s output side with a light load on the motor. It was horrifying. 

How do sepex controllers manage the spikes and ripple so well? Does it have anything to do with good field control? Do they even make IGBT-based sepex controllers, or are they strictly Mosfets?


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## gunnarhs (Apr 24, 2012)

> How do sepex controllers manage the spikes and ripple so well? Does it have anything to do with good field control? Do they even make IGBT-based sepex controllers, or are they strictly Mosfets?


 Sepex-controllers are exactly the same as other DC-controllers with the difference that they have an additional field control connection.
If the field/armature current is rightly set up, there are no big spikes/ripple (the point of the Sepex is exactly to avoid that).
The first to do is to get the right motor specs (max/min armature current under max/min field current. armature resistance and field resistance).
Later you can worry about the inductance.
Then find an adequate controller, this could be a problem. You can also use two serial controllers, one smaller for the field and one bigger for the armature. 
But you need to control them together.

Here an example of a typical sepex-motor of a car.
It has a field current from 20A to 5A ( field resistance of 5 ohm).
It has an armature current from 0-300 A (armature resistance of 0,5 Ohm).
Note the difference of current in field and armature, both in terms of size AND direction

1) When the car is started field current is maximal (in this case near 20A, field voltage 100 V). 
2) Then armature voltage (0 -150V in this case) is applied resulting armature current and the car starts rolling. If the field current is steady 20A the motor will spin up to a certain value (usually near base speed)
3) To increase speed field current/voltage is lowered as armature current/voltage is increased more
If this is done somewhat right you have a constant torque first (up to motor base speed depending on pole, often 1500 rpm for 4 pole motor), then the torque decreases slowly as speed increases over base speed.

Note the motor behaviour is different to serial DC , it is more like AC-motor. meaning you have little torque in upper speeds.
However your efficiency values will be better than in DC-series.


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## madderscience (Jun 28, 2008)

New motor?

Sounds like its going to take just about as much cost and effort to make the current one work. A series rewinding job may do the trick depending on what it is.


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