Magnetic starter coil test

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nizak

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I need to determine if a 120V coil in a magnetic starter is good or bad.

Input voltage is correct but the coil will not bring in the contractor unless I depress it manually.

What value ohm reading would verify the coil is good or bad.

If it matters, it's a size 0.

Thanks.
 
In general, if the coil is not open it is ok. Does the armature stay pulled in after you manually push it in?
 
If you manually actuate the armature and then it holds, I'd say your issue is the source can't provide enough current to pull it in but can provide enough to hold it. Likely too small of a control transformer.
 
Yes it stays pulled in. C/Xfmr checks fine for line voltage output,122V AC
I don't know what the VA value for this coil is supposed to be.

You can hear a faint buzz like it wants to pull in but it won't.
 
Have heard the term weak coil thrown around before. Can it in fact be "good" from an ohms reading standpoint but still not be strong enough to work properly?
 
nizak said:
Yes it stays pulled in. C/Xfmr checks fine for line voltage output,122V AC
I don't know what the VA value for this coil is supposed to be.

You can hear a faint buzz like it wants to pull in but it won't.
Click to expand...
What is the voltage across the coil when it is trying to pull in?
 
nizak said:
Have heard the term weak coil thrown around before. Can it in fact be "good" from an ohms reading standpoint but still not be strong enough to work properly?
Click to expand...
Yes. The coil may have a "shorted" winding. You usually have to have a new or known good coil to compare ohm reading.
 
nizak said:
I need to determine if a 120V coil in a magnetic starter is good or bad.

Input voltage is correct but the coil will not bring in the contractor unless I depress it manually.

What value ohm reading would verify the coil is good or bad.

If it matters, it's a size 0.

Thanks.
Click to expand...
Okay, nizak, I have a brand new, out of the box Allen-Bradley 120v coil for a size 1 709 starter. My Fluke 177 reads exactly 40 ohms across it. Hope that helps:)
 
nizak said:
Yes it stays pulled in. C/Xfmr checks fine for line voltage output,122V AC
I don't know what the VA value for this coil is supposed to be.

You can hear a faint buzz like it wants to pull in but it won't.
Click to expand...
With the brand name and model of the contactor, you can find coil data from anyone. It won't likely show the ohm reading because that is a little more variable than anyone wants to admit.

But even so, without knowing what you have, comparing it to something else is pointless. Everyone has a different design and the coil strength has to do with the armature design, spring force (or gravity in the case of the AB units), etc. etc., which affects the coil current, which affects the resistance. Some mfrs even now use DC coils that have a built-in AC/DC power supply board embedded inside of it. Any component failure on that board makes it toast.

PS: On the A-B 509 starter coils, there is a built-in thermistor (thermal resistor) embedded in the molded epoxy that can open up if the coil gets overheated. It's there to prevent the coil itself from swelling up and jamming the contactor closed.
 
160406-1637 EDT

nizak:

You have a coil, mechanical binding, or low voltage problem.

With the contactor in its correct machanical position do the following tests:

1. With no power to anything on the contactor (for your safety) try to move the solenoid plunger. This should move freely except for spring force. If it does not move freely, then you have to solve the binding problem or replace the contactor.

2. Use a 7.5 A Variac adjustable voltage supply connected to the contactor solenoid coil directly. Include a series switch if there is no switch on the Variac.

Use a meter to monitor coil voltage.

Set the Variac voltage to 120 V. Apply this 120 V to the contactor coil. Does the contactor pull in? If it does not pull-in and just buzzes, then does the solenoid plunger move at all? If it moves but does not seat look for binding or chips that may prevent seating.

Since you indicated that the plunger could be nudged to seat I don't believe it is chips, but could be binding.

Apply 140 V, but not for long. Does this make it seat without assistance? If so it would seem to imply a "weak" coil.

Measurements on an old A-B 709 #2 starter, 55 years old.

DC coil resistance 40.2 ohms. All starters of the same model probably will be closer than +/-5% of this value. A second identical model I have reads 40.1 ohms.

Pull-in voltage for no buzz (I describe this as a solid pull-in) 72 V.

Drop-out ( not the actual drop-out, but that point where buzzing starts as voltage is lowered) 50 V.

Other measurements at 120 V with a Kill-A-Watt EZ:
Current 0.26 A, meaning an AC impedance of 462 ohms.
Power input 8 W.
VA input 31.6 VA.

If you have shorted turns, then DC resistance will be lower than a good coil.

A sticking plunger with 120 V applied will have a much higher current for an AC coil. If the contactor is built with a DC solenoid and built in full-wave diode rectified then current won't change much with plunger position. This is a design that can reduce coil burnout problems, but does not create the large pull-in force that a shaded pole AC solenoid can produce.

.
 
nizak said:
I need to determine if a 120V coil in a magnetic starter is good or bad.

Input voltage is correct but the coil will not bring in the contractor unless I depress it manually.

What value ohm reading would verify the coil is good or bad.

If it matters, it's a size 0.

Thanks.
Click to expand...

If you have the proper voltage to the coil, but the starter doesn't suck in, you don't need to do any more troubleshooting. The coil is bad.

(Assuming there isn't anything mechanical binding up preventing the starter from pulling in.)

In my experience, the coils that don't work show no continuity. So it is an easy check.
 
A turn to turn short between adjacent turns can disable an AC solenoid but have little effect on a DC solenoid. The resistance difference will be too low to detect reliably.
A turn to turn short between layers can remove enough of the coil to increase the voltage but and not necessarily reduce the field.
A problem with a shading coil on an AC solenoid can keep it from pulling in reliably. And make lots of buzzing.

I don't think shading coils are likely to fail except from mechanical damage.
 
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