Stalling the Motor on a Mazzer Super Jolly

Grinders are one of the keys to exceptional espresso. Discuss them here.
Bezman
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Joined: Nov 24, 2011, 2:00 pm

Postby Bezman » Dec 09, 2014, 1:52 am

I have a Mazzer SJ clone. Yesterday, while cleaning, I accidentally stalled the motor for about 5 seconds while grinding some rice through it with a grind setting which is too fine.
Everything seems to be working fine, but what could I have damaged?

ira
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Joined: Dec 24, 2007, 5:03 pm

Postby ira » Dec 09, 2014, 3:38 am

I can't imagine you could have damaged anything.

Ira

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HB
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Postby HB » Dec 09, 2014, 9:46 am

Conventional wisdom I've read in past forum posts says a second or two will heat up the windings, but won't damage the motor. Prompted by your question to learn a bit more, I searched on damage electric motor by stalling. That led me to forums on power tools and Wikipedia. The excerpt from A Solar Car Primer referenced by Stall Torque sums it up nicely:

Eric F. Thacher wrote:Stall Torque. Suppose the torque loading the shaft increases until the motor stalls. In this condition the shaft power is zero and the current may be large. When stalled, all electrical power supplied to the motor is converted to heat. Consequently the temperature is high, and damage may result. The motor may stall at different torque loads depending on the current available to it. The largest stall torque the motor can endure without damage is called the maximum continuous stall torque.

References aside, if the motor still works, the damage clearly wasn't fatal. I've stalled motors myself, but only for a second or two. I'm with Ira: You're fine.
Dan Kehn

Bezman
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Joined: Nov 24, 2011, 2:00 pm

Postby Bezman » Dec 09, 2014, 9:49 am

Makes sense, I should be more careful in the future.
Thanks for the insightful read :)

OldNuc
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Postby OldNuc » Dec 09, 2014, 10:06 am

If you get away with it once, don't do it again as the damage to the insulation is cumulative.

Bezman
Posts: 77
Joined: Nov 24, 2011, 2:00 pm

Postby Bezman » Dec 09, 2014, 10:11 am

What would happen if the insulation was damaged considerably? Would the motor lose torque, or just stop working completely?

OldNuc
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Joined: Jan 02, 2014, 12:02 am

Postby OldNuc » Dec 09, 2014, 10:29 am

If the winding insulation fails then the motor quits running. The usual fault path for a winding insulation failure is a winding short to ground followed by a winding open circuit. The number of variables involved make it impossible to accurately predict how many stalled rotor events will lead to failure.

 
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