Motor Starter Voltage Drop

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jim dungar

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coulter said:
I'm not seeing that - here's why:

Once the armature is pulled in, and the pole pieces are touching, no further work (physics definition of work) is being done. So the analogy to a motor doesn't fit - no energy output.

I don't agree, the armature needs to constantly work against the springs that want to open the starter contacts, if not the starter would not open simply by disconnecting voltage to the coil.

And of course a low enough voltage will not be able to generate enough flux to keep the aramture sealed in. Most starters are designed to pull in at 85% nominal voltage and stay sealed in until 65% nominal voltage. And there are some starter designs that utilize economizer circuitry to reduce the current drawn by a closed starter coil, but I seem to recall that these employed a half wave rectified DC rather than simply a lower AC voltage.

I have never seen any measurements of the relationship of current to voltage when a starter is sealed in.
 

coulter

Senior Member
jim dungar said:
...the armature needs to constantly work against the springs that want to open the starter contacts....
Okay, I'm seeing one of the communications issues. "Work" takes both pressure and motion. Pressure alone is not "Work". I would suggest, that once the coil is pulled in, it is not doing any "Work".

carl
 

rattus

Senior Member
Work:

Work:

coulter said:
Okay, I'm seeing one of the communications issues. "Work" takes both pressure and motion. Pressure alone is not "Work". I would suggest, that once the coil is pulled in, it is not doing any "Work".

carl

Mechanical work is defined as force x distance--not pressure. For example, foot-pounds, and 550 foot-pounds per second is defined as one horsepower.

Work is performed in pulling in the armature, so we have a motor of sorts, then once the plunger is seated, the force remains, but with no more motion, there is no more work. To be strictly correct we should say that force is exerted against the spring. Loosely speaking, we can say "work against the springs".
 

coulter

Senior Member
rattus said:
Mechanical work is defined as force x distance--not pressure. ...
You're correct. For this discussion, "Force" is a better word.

rattus said:
Loosely speaking, we can say "work against the springs". ...
No, I wouldn't say that - not even loosely.

carl
 

coulter

Senior Member
rattus said:
I didn't say it. Jim said it, but I knew what he meant.
Perhaps you missed posts 18 - 21.

As I understand, Jim is basing his premise of changing coil impedance on analogies to motors and constant VA. So, I suspect he meant "Work" when he said " Work".

However, I also think Jim is pretty sharp - I'm certain he can explain his position better than I can.

carl
 

rattus

Senior Member
coulter said:
Perhaps you missed posts 18 - 21.

As I understand, Jim is basing his premise of changing coil impedance on analogies to motors and constant VA. So, I suspect he meant "Work" when he said " Work".

However, I also think Jim is pretty sharp - I'm certain he can explain his position better than I can.

carl

I would not use the motor analogy at all. I would simply say that the inductance drops as the armature is pulled out of the coil. I doubt though that this is the problem.

A defective MOV might be responsible for insulation breakdown which could lead to a shorted turn which would lead to excessive current which would smoke the coil. Voltage spikes are also death to solid state devices which drive inductive devices.
 

jim dungar

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It appears my motor analogy was a bad idea.

Some day I will need to set up a test circuit.

I know that once the starter is pulled in the coil must continue to create a magnetic field to counter the action of the "opening" springs. This is why the magnetic circuit includes a shading coil to reduce the "60 cycle hum" caused by the armature constantly dropping out and pulling in.

But, overall I do not see the OP sealed coil voltage as a primary factor in the failures. I would spend my time looking at other issues.
 

radioeng

Member
Location
Portland
The current in an AC coil is limited by its self-inductance. The inductance is determined by the reluctance of the magnetic path formed by its steel core, frame and armature. So if the armature does not pull-in for some reason, say low voltage, there is an airgap, the reluctance is high, the inductance is therefore low and the coil current will be high.........
 

Bob NH

Senior Member
I suspect that the failing coil may be in some way connected to the motor winding. When you open the contactor to stop the motor, the inductance in the motor winding will produce a large voltage on the circuit as the current tries to find a place to go. If there is some point where the motor winding is common to the coil, or a transformer that drives the coil, then the current going through the large inductance of the motor winding will want to keep going, and could be finding the path of least resistance to return through the coil winding.

It may only happen occasionally when the current is near a peak value in the motor winding when the contactor is opened.

As a civil engineer I'm at the end of my knowledge after V=L*di/dt, but I'll bet Winnie could explain it if that is the case.
 

ezeke

New member
What if you install a small relay in your starter enclosure energize it via your start command and use the dry contacts to energize your starter coil.
 
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