Well, this topic is a pleasant departure from the typical ("Why Does My LED Array......?") post. It's also been a confusing thread because it contains statements that seem to contradict themselves,... all in the same sentence. I've found myself reading the same sentence repeatedly because of this. Admittedly, sentence structure can cause this.

Let's face it, unless someone can give a plausible explanation as to what's popping these diodes.. Mr. Back EMF keeps jumping out of his seat, waving his hands and yelling "It's me!!!".

Tina's 'Tech Support' will be getting an email from me regarding their lame (generic) DC Motor Model. I was hoping to spice this but Tina's motor model contains only two properties, Voltage and Wattage. There's no reactance to introduce and there are no poles to introduce commutator hash.

This is probably asking a lot but this is my 'Properties' wish list for a reasonably accurate Spice Motor:
(1) Voltage
(2) Current
(3) Reactance
(4) Number of poles
(5) RPM, relative to input voltage
(6) Ability to introduce a Stall condition
(7) Ability to introduce degrees of motor loading
(8 Series Wound - Parallel Wound - Permanent magnet

 

The brush noise is in the nS range and will be missed by the diode, hence the need for a capacitor to slow and suppress these fast pulses.

Would someone like to calculate what the commutator hash period would be with the following conditions?

Assume a motor with the following.

DC motor with the most common number of poles.
RPM = 2000 RPM
Commutator gap = .015' or .008'

I will readily admit that I'm not a motor expert but I find the nanosecond region to be unlikely for anything mechanical. If someone can prove me wrong I will glady eat crow.

 

Hi CDRIVE,

Welcome to the thread.

Fire up almost any DC brushed motor and then hold a radio next to it set to almost any band you like. Let me know what you hear.

Most appliances and tools with motors require noise suppression (EMI filter) built in so as to meet FCC requirements. However, if you hold the radio close enough you can still hear it.

I too have longed for a good motor model and have ended up making my own most of the time. The inductor model in post #6 would be good for modelling the stalled, or locked rotor situation (item 6 in your list), but not much else

Regards,
Ifixit

 

Hi CDRIVE,

Welcome to the thread.

Fire up almost any DC brushed motor and then hold a radio next to it set to almost any band you like. Let me know what you hear.

Most appliances and tools with motors require noise suppression (EMI filter) built in so as to meet FCC requirements. However, if you hold the radio close enough you can still hear it.

I'm not questioning that. Spark noise is filthy with harmonics. I would expect a single spark to extend far into the RF spectrum.

 

Hi All.

Thank you all for your responses.

I will keep you posted once I hear back from my friend on how it has gone with the large, fast diode.
I do have a scope and did meter the voltages in operation and saw a pulse of the opposite polarity which spiked to -60V after the mosfets were switching off. I did not measure the current and no longer have the device in my posession so I cannot test that.
Next stop may well be a cap and/ or a transorb if it fails again.

The motor in question drives a propellor of approx 8inch diameter direct so it is quite slow (I would say well under 1000rpm) I do not know what effect this has on the factors you have mentioned (who knew dc motor characteristics were so complicated)

Once again many thanks

Bruce