Motor/controller inductance

[jonescg]

Hmm, looks like my query was moved.
Still, at the EVWA meeting we had a very good discussion about induction, and how this is a common reason for controller failure. Most PMDC motors have a given inductance of a few miliHenry, or amps per second. Doesn anyone know how to determine the inductance value of a motor, and how to match controllers with motors?

Cheers,
CHRIS

 

[fechter]

At work, I have a fancy LCR meter that can measure something like that directly. That's about the only accurate way that's easy.

The thing that can really blow up controllers is if the motor has a low inductance and the controller has a slow switching frequency. This can allow the windings to saturate and the current in the FETs can go into the destruction zone. Motors with low inductance need to run at a higher switching frequency.

If a motor has a higher inductance, then you want to switch at a lower frequency to avoid excessive core losses and heating and get lower switching losses in the controller.

Most of the brushed axial motors like a Perm or Agni have very low inductance. In some cases I've see an external inductor added to try and keep the controller happy.

 

[Miles]

Hmm, looks like my query was moved.

Hi Chris,

I moved it to a separate topic - I thought it deserved it (& might get more attention)... I meant to send you a PM but I forgot... sorry.

 

[jonescg]

No worries Miles,

So, given inductance of an Agni is low, that means the rate at which the current soars towards 2000 A is within the realm of the Kelly 1200 A controller? It switches at 50kHz, so this is a good thing, but fast switching comes at a cost of ineffieicncy? I can add an induction coil to slow it down, but I don't know whether it will work out as is.

 

[olaf-lampe]

So, given inductance of an Agni is low, that means the rate at which the current soars towards 2000 A is within the realm of the Kelly 1200 A controller? It switches at 50kHz, so this is a good thing, but fast switching comes at a cost of ineffieicncy? I can add an induction coil to slow it down, but I don't know whether it will work out as is.

Adding a coil woudn't slow down the controller.
With a second inductance in series to the motor, the controller could switch at a lower frequency.
Don't forget, the added coil must be capable of the same current as the motor. And usually they are quite big and heavy.
The best way to run a low inductance motor is using an oversized controller with a good working current limit. Especially for dragsters, because starting ful throttle from zero RPM is the biggest problem.
For a race-bike the RPM will almost never be in the low range.
-Olaf