Response to Overload on PM-25MV

erikmannie

erikmannie

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This morning I was foolishly using a HSS end mill to mill some hardened steel (a lathe chuck key). What is more, this was a VERY tired import end mill.

Upon trying a .004” z-axis cut, the mill powered off. I was afraid that I killed it! Fortunately, when I started the mill again, it came right back on. That was a very happy moment!

This phenomenon happened one more time before I retired that end mill & managed the job very easily with a carbide end mill.

So is this how a PM-25MV is supposed to work? It senses an overload & kicks off to save itself?

I believe that this same phenomenon occurred 1 or 2 times on my PM-1030V when I was parting steel, but in that case it is easy to sense it coming & the operator just backs out the tool before an overload occurs.
 
alot of motors will have overload protection on them. some automatically reset others are manual reset
 
My EE friend (who is a member of this forum) said “When the motor comes under heavy load and slows down, the back EMF is reduced which causes the motor current to rise considerably and typically there is a thermally triggered breaker that trips”.

This makes perfect sense, but I do wonder what exactly this “thermally trigger breaker” is. My 36V DC eBike batteries have a 100A circuit breaker in them. If this breaker is tripped, I reset it by unplugging and plugging in the battery again.

In any case, I’m glad that the PM-25MV & PM-1030V motors are able to protect themselves.
 
I am not very good with electronics, but I would be interested to know what is happening here.

My guess is that everything is going along fine, but when the tool cannot clear the work, the motor ends up generating heat instead of work. At some point, this heat reaches a high enough level to trip the aforementioned breaker in order to prevent damage to the motor.
 
Higher demand=higher current=higher temp in the breaker.
Most autos had self-resetting breakers for the headlights, if a short occured, the breaker would trip then re-set, etc., so you still had lights, albeit briefly.
 
I have overloaded & burnt up cheap power tools (drill, angle grinder, die grinder). You smell the burning plastic of the windings, & the power tool is dead.
 
I don't know how common it is, but my LMS 5500 has circuitry that shuts it down when it stalls. I know this because I use a donut style spindle lock to change tools and if/when I forget to remove it before I hit the go button, the electronics turns it off and shows "ERR" on the tach display. All I need to do then is remove the donut (duh), hit "stop" to remove the error message and hit start again and watch the chips fly. I'm not aware of any thermal overload device that will reset that quickly, either manually or automatically. Erik's machine may have similar circuitry to mine since his came right back on. I would imagine it's pretty common on modern bench mills, but I don't know that for a fact on mills other than mine.

Tom
 
When a motor slows the back EMF is reduced. This increases the current flow in the motor, and heat is current squared times resistance. So the current rises and the windings get hot. Detecting the increased current or temperature triggers the trip mechanism. Some need to cool off (thermal), some don't (magnetic), depending on the design.
 
AlanB said:
When a motor slows the back EMF is reduced. This increases the current flow in the motor, and heat is current squared times resistance. So the current rises and the windings get hot. Detecting the increased current or temperature triggers the trip mechanism. Some need to cool off (thermal), some don't (magnetic), depending on the design.
Click to expand...

Ah, the motor slowing down was what I failed to consider. The motor functions better in it’s intended RPM range, unless you are looking for an electric heater, which we are not.

A vicious cycle of increasing temperatures leading to ever increasing resistance, culminating with a burnt up motor save for the trip mechanism.

Thanks for the clear explanation. I will make sure to take reasonable cuts, using sharp cutting tools of an appropriate material.

A little more about back EMF, which I do not find to be intuitive (I would have guessed that really low motor RPMs is P=iv with a fixed P and increasing i cause v to decrease):

Counter-electromotive force - Wikipedia

en.m.wikipedia.org en.m.wikipedia.org

From:

“This is why a DC motor that is running should never be stopped with the supply connected. If this is done the back EMF will fall to zero, the current will become very large and the coil may burn out.”
 
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