Electric motor failure modes?

[halleffector]

I tried with a new diode. It works fine. Reversed the power and the magic smoke escaped.

I've used a IN4742A zener diode with a maximum current rating of 21 mA.

When running the motor at 12 volts and letting it get up to some RPM, is it possible it generates more than that (21 mA) when the power is suddenly turned off?

But even with the new diode destroyed, the motor works with polarity reversed and not reversed, just very poorly. I'm guessing there is some resistance in the diode in both ways after it has burned.

 

[RODALCO]

If the motor has to run forward or backward you can not have a protection diode fitted. The switching may have to be done via relay instead of a semiconductor.
Unless you run it from a bridge driver cct.

 

[halleffector]

Actually, I think what killed the motor (or the diode I should say) was not just running it on 12 volts, but running it on 12 volts with PWM at 50% duty cycle or so.

 

[halleffector]

If the motor has to run forward or backward you can not have a protection diode fitted. The switching may have to be done via relay instead of a semiconductor.
Unless you run it from a bridge driver cct.

I don't run it both ways. This is just for testing.

 

[RODALCO]

That explains it all, when you changed the polarity, the diode conducts and emits smoke, and will get destroyed.
Depends how strong your power supply is.
The motor may run at 0.6 Volts while the diode is being fried up.

 

[RODALCO]

PWM shouldn't kill a good dc motor.

 

[halleffector]

I tried with a new diode. It works fine. Reversed the power and the magic smoke escaped.

I've used a IN4742A zener diode with a maximum current rating of 21 mA.

When running the motor at 12 volts and letting it get up to some RPM, is it possible it generates more than that (21 mA) when the power is suddenly turned off?

But even with the new diode destroyed, the motor works with polarity reversed and not reversed, just very poorly. I'm guessing there is some resistance in the diode in both ways after it has burned.

 

[halleffector]

That explains it all, when you changed the polarity, the diode conducts and emits smoke, and will get destroyed.
Depends how strong your power supply is.
The motor may run at 0.6 Volts while the diode is being fried up.

How does it explain it?

I didn't change the polarity while it was in the original circuit, i.e. when it stopped working.

The motor was working fine at 12 volts, it thought it sounded too loud for its own good so I decided to change it down to 5 volts. It then stopped working. But I never reversed the polarity at any time.

 

[RODALCO]

I tried with a new diode. It works fine. Reversed the power and the magic smoke escaped.

I've used a IN4742A zener diode with a maximum current rating of 21 mA.

When running the motor at 12 volts and letting it get up to some RPM, is it possible it generates more than that (21 mA) when the power is suddenly turned off?

But even with the new diode destroyed, the motor works with polarity reversed and not reversed, just very poorly. I'm guessing there is some resistance in the diode in both ways after it has burned.

You said that here.

 

[RODALCO]

You said that here.

 

[halleffector]

You said that here.

Sure but that was after removing it from the original circuit, which was where it failed.

Or maybe I'm not reading you correctly, do you mean that the motor itself destroyed the diode while it was still in the circuit?

 

[RODALCO]

A Zener diode will fail when exposed to excess voltage. What voltage is the Zener rated at ?

 

[halleffector]

A Zener diode will fail when exposed to excess voltage. What voltage is the Zener rated at ?

12 volts, but only 21 mA.

 

[Les Jones]

Hi halleffector,
If the original diode was a 12 volt zener (Which the 1N4742A is) Then applying a solid 12 volts across it in either diraction can kill it. With the positive connected to the anode it will try to clamp the voltage to about 0.6 volts (The forward voltage of( the diode.)which is almost a short circuit across the 12 volt power supply causing the current to only be limited by the power supply. With the positive connected to the cathode the current depends on which end of the tolerance the diode is, The slope resistance of the zener diode and the actual voltage of the nominal 12 volt supply. The lower tolerance voltage of the 1N4742A is 11.4 volts. It's slope resistance is 9 ohms.
So if the 12 volt supply is exactly 12 volts then there will be 0.6 volts (12.0 - 11.4) across the slope resistance of 9 ohms. So the current through the diode will be 0.6/9 = 0.067 amps = 67 mA This will be a dissipation of 0.8 watts which is less that the 1 watt rating of the diode but the 12 volts only needs to a little bit higher to exceed the rating. If it was 12.5 volts then the dissipation would be 1.53 watts which exceeds the disipation rating. The 21 mA is not the current rating as you said. It is the test current at which the zener voltage is measured. I think this explains what RODALCO was thinking in post #32. Thanks for your comments on the pump that I have ordered being different to the one you have.

Les.

 

[MrAl]

I tried with a new diode. It works fine. Reversed the power and the magic smoke escaped.

I've used a IN4742A zener diode with a maximum current rating of 21 mA.

When running the motor at 12 volts and letting it get up to some RPM, is it possible it generates more than that (21 mA) when the power is suddenly turned off?

But even with the new diode destroyed, the motor works with polarity reversed and not reversed, just very poorly. I'm guessing there is some resistance in the diode in both ways after it has burned.

Hi,

Oh, so it was the diode. So the motor itself is ok then.

The diode will see whatever current the motor runs at when run at 12v. If it is 50ma, then 50ma, if 1000ma, then it will see 1000ma. If it is 50 percent duty cycle then it could see a 50 percent duty cycle at 50ma or 1000ma as above but the average would be 1/2 of that, or 25 to 500ma. So the diode could EASILY have been overpowered. The diode wuold get hot, so you could try a new diode and see if it gets hot.

HOWEVER, you dont use a 12v zener to clamp something that runs at 12v anyway. You use a higher voltage zener such as 14v or 16v or even 20v. Also, you cant just use just one zener if you reverse the current to run the motor backwards, you'd have to use two zeners in series, back to back, with the two cathodes together or the two anodes together.
Alternately, you might be able ot use a full wave bridge and clamp the motor voltage right to the power supply voltage and ground, depending on how you are driving the motor and what your power supply looks like.

So what you should do is measure the current of the motor when it runs at the normal 12v and we can then calculate the average diode current, or come up with another method to clamp such as the bridge rectifier if you specify what kind of power supply you use to run the motor.
If the motor draws more than about 160ma then the diodes could blow again, so something else has to be done.
It would also help to know where you are getting the PWM signal from.

 

[dr pepper]

The brushes on little motors are usually held in place by the plastic endplate, if this has melted which is possible the brushes could be well out of line and shorted, I've had this happen.

 

[MrAl]

The brushes on little motors are usually held in place by the plastic endplate, if this has melted which is possible the brushes could be well out of line and shorted, I've had this happen.

Hi,

Didnt he say the diode shorted out?

 

[dr pepper]

Didnt see that, but it would explain things.