Motor protection

[rm2]

Hi,
I have a DC motor which has the following specs:
CI, jump start and reverse polarity
(+26 +/- 0.5) V
(-13.5 +1/-.5) V
Maximum amplitude for conducted transients
positive polarity +100 V
Negative polarity -150

Nominal working voltaje 12 V (range of 10-16 V)
The issue here is that I need some suggestions from any of you to protect the motor after stall to avoid peaks (the range is a maximum of 30 volts and a min of -5 volts)

 

[Skogsgurra]

Is this a "pure" motor (copper and iron)? Or is there a lot of electronics involved? Is it a 1 W or a 1 kW motor? Or something in-between?

Gunnar Englund

http://www.gke.org

 

[rm2]

Pure motor. 7.5 W. No electronics involved.

 

[Skogsgurra]

I do not think that you have to worry about transients then. Especially not with the rather broad input range and transient tolerance given. Just use it. No transient prtection needed.

Or, maybe, you need some protection to protect the rest of your equipment from transients emanating from the motor? Is that what you are aiming at? And is that what the transient specs are telling us?

Gunnar Englund

http://www.gke.org

 

[rm2]

Big mistake.
Maximum amplitude for conducted transients
positive polarity +30 V
Negative polarity -5
the +100 and -150 were before, now we need this new specs. Big problem trying to control this range using cheap components (no more than 50 cents).

 

[Skogsgurra]

Is this motor fed from a car battery? Or from someting else? If so, describe.

It is only when connected in a car that those motors may need some protection - mainly against load dumps (when battery is disconnected while motor runs).

There are very few other situations where a little DC motor needs protection - and I doubt if it is necessary even in the load dump case.

Gunnar Englund

http://www.gke.org

 

[fsmyth]

If I understand your problem correctly, a zener and
resistor snubber combination would probably do it.
Bypass the zener.
<als>

 

[rm2]

This motor is used on a door latch. The latch basically has a housing and a cover that protects all the mechanism and motor, over the cover a connector provides with its different pins the energy to every device. This connector feds from a module, but we can assume its connected to the battery of the car (which is the worst case).
The client ask for this specs.
I've being trying with some chip varistors and other diode arrangements but still can't control the range they want (+30 V / -5V). I've seen transients of 45 V and -20 V.

 

[Skogsgurra]

I think this is a situation where you two (you and your customer) needs to sit down an discuss what you really need. Nothing has been said about source impedance (which determines 'punch' in the transients) and nothing has been said about half-width or waveshape (which determines energy in the transients).

It is futile to try to find a solution without knowing these details. I wouldn't be the least surprised if it turns out that there is some misunderstanding somewhere down the line. Never, ever heard about the need to protect a copper-and-iron motor from transients before. And I have done truck and bus electronics to and fro for almost thirty years. Protected electronics, yes. But 7.5 W motors, no. Never had any problem with them.

Are you sure that the customer isn't asking for EMI protection so that the motor doesn't kick back when switched? That would make a lot more sense.



Gunnar Englund

http://www.gke.org

 

[rm2]

Our intention is not to protect the motor, what we want is to protect the electrical system in the car from the transients generated by the motor. Currently we are using a rin varistor that is mounted inside to the motor, but with this feature we met the current EMC spec that is 100/-150 Volts, but this is in the process to be change to 30/-5 Volts therefore we need a powerfull varistor or diodes array to handle this new requirement.
I apoligize for the other incorrect explanations.

 

[Skogsgurra]

Thanks a lot!

That's what i asked in my second and third posting...

All you have to do is to go to the filter guys and buy a decent filter. They will know what you need.

Gunnar Englund

http://www.gke.org

 

[rm2]

with the spec I have, could you suggest something?

 

[fsmyth]

If the motor does not reverse, a simple back-biased
diode across the terminals should be ample for driver
protection. A forward diode would add even more
isolation. For most small D.C. motors, a single
capacitor across the terminals will suffice.
MOV's are bi-directional, so the +/- values would
be the same. Plus the fact that MOV tolerances
leave a bit to be desired, and fail shorted (mostly).
To achieve the goal you stated, one polarity will
have to be shunted by a different device than the other.
So, add a diode/resistor combination in both directions,
values appropriate to the limits desired.
If it does reverse, you would have to
use a bridge, or back-to-back diodes with a current
limiter (resistor/capacitor network). In which case
the offsets make no sense.
For absolute transient protection, a choke/capacitor
network on the supply leads is appropriate, but the
cost and size may be prohibitive.
I agree with skoggs. If it is that crucial, get a
filter from a reputable vendor. For devices, Motorola
and ONSemi have comprehensive lists of automotive parts.
All this sounds a bit odd, since most decent driver
equipment will have protection built into the _driver_
unit, especially in a consumer automotive environment.
<als>