You are here:

Electric Motors/growler testing AC and DC electric motors


Hi Will
I sometimes collect old electric motors from trash en treasure markets etc and ofcourse they are often defective.  With the brushed motors it seems impossible to tell if the rotor is ok without a growler.  I wonder if you could use an multipole stator from a much bigger motor and turn the rotor by hand to see if all the segmnents are ok?

well first I am not understanding the description of a brushed motor with segments,   are your referring to a wound commutator type armature?

These wound "segmented" armatures are often used in DC only motors,   or in tool type motors and are called series or universal motors because the rotating segmented armature is in series with the stationary fields and can be powered by either AC or DC.

Caution on series motors, without a load   they will tend to keep accelerating until they reach a speed where they will self destruct,  

And yes  on both a bar type ROTOR you really need a growler also because the human eye cannot detect hairline cracks or cracks under what appears to be solid bars,  or loose connections to the end rings/resistance rings,   they don't all have a "ring"   but they are all shorted together on each end in some fashion,    

You need to find some sort of induction method to introduce voltage into the rotor or armature,  

On a bar rotor      when the growler is of a size adequate for the diameter,   which is important too small may not introduce enough power into the rotating component to give you a valid test,    

We have maybe 6 different size growlers,        some that have to be hung by a hoist

With a segmented  armature,  [the segments are parts of the commutator]  where the windings are wound around the laminations and connected to the segments by soldering or welding into the individual segments,          

To test an armature,  first use a piece of solder or other bare conductor,  and wrap it around the commutator on the brush riding surface of the commutator,       then take a piece of something  that is flat and straight,  [used hack saw blades make great test tools]

When the comm is shorted out completely the blade should stick or vibrate all the way around the laminations,       

Next   set the growler on the armature or armature on the growler,   and without shorting it, find a spot where two bars next to each other will arc a bit with a piece of solder shorting the two out at the very end of the bar,    this will tell you that you are introducing from the growler at the correct area on the laminations,    each armature is wound in a different manner and SPAN  and if you have the growler too far away   the windings may check good by the saw blade NOT vibrating,     but actually it has no electromagnetic properties in that area and is a false positive,    

ONE Thing to be careful of   on larger armatures,     sometimes they have a winding that goes from bar to bar to bar in a pattern  called equalizers,     these are used in larger machines to control the arcing       [much more complex but if there are equalizers it will tend to growl all the way around and can be falsely condemned as a shorted armature]

If you can see the front of the risers,  where the windings are soldered in,    you might see the pattern,    where bar one has  4 wires  bar 2   4    bar 3 four   bar 5  4  wires  but segment 6 will have 5 FIVE wires, and will repeat in a pattern,     that is one visual way to see if a larger armature has equalizers,  you will find equalizers in armatures as small as 10 HP  but more often   in much larger machines,   but always look if the armature growls all the way around,    and change to a smaller growler      

An induction motor with bars,    either casted in the laminations or actual bars,   should growl  when the growler is in the correct position,       an open bar   will cause the motor to have problems with torque, starting,  so on,     many times they open at the end rings, and can be soldered or welded back      other cracks are centrifugally created, and not always visual,   bars in rotors, especially casted type rotors where the lams are simply poured full of a conducting mixture of copper, brass, bronze,aluminum,    can look like they are maybe a 1/16th inch wide  but in reality  under what is on top might be a void filled that is triangle shaped and may extend out,  quite a distance underneath,     so the bar must growl all the way up and down the length  to test for a solid connection        and that is not always full proof because     hairline cracks can close in a static test, but when under power  open by centrifugal force,  

It is not a simple as using an open stator for a growler,    because an open stator of any size is likely to create a very high current   without a rotor or keeper inside,      I put some links on how to build a growler,  but depending on the types and sizes of motors you buy  there a lot of growlers that can be bought for next to nothing off ebay,         both the hand held type and the external type,       there are also  real INTERNAL growlers which are hand held,    the growler has a flexing part like the saw blade tool fixed to the end,  you insert the growler into a stator and if the tab vibrates  in just one slot or around just one slot   you found a shorted stator,          if it growls all the way around  but looks good it is probably a parallel connected motor causing the false negative          the better way to test a stator is with reduced voltage and an amp meter,      or a surge tester,  which can also be purchased used,    not fancy but they work,     and those produce a read out in the form of waves like a scope,    one winding over the next,         

You should also own a good megger,   which is an non destructive test  that is really a glorified ohm meter,  and reads the insulation value to ground    [or between windings in multiple speed motors, or multiple pole windings found in Dc motors,         it reads out in megohms,    rule of thumb  one megohm for 460 volts ac is considered good to test  but should be much higher when cleaned or reconditioned as needed,    we like to see 5000 or more megohms on a clean stator to ground,        

MEAR Services Inc in Kansas City,    816-650-4030  if you run into a special looking or motor you don't understand or want an opinion or value,  drop me a note  and with a few photos and we will be glad to try and help,  there are so many types and varieties of motors,   you will run into something odd along the way,       you can use my personal email  also     Will Babbitt  

Electric Motors

All Answers

Answers by Expert:

Ask Experts




Three phase/ AC DC single phase motors, controls, any problems or failures, motor installation, performance issues, connections. All other electric motors/gearboxes/apparatus. Specialty repair concerns, obsolete motors and solutions. Other mechanical or specialty equipment. See my profile under Home/electrical at this site


30 plus years in the electrical motor and apparatus repair industry. VP level management of repair facilities, current owner of my own specialty repair and consulting firm.

EASA, IBEW [retired], other specialty organizations, Lubrication, Vibration EDI, Tribo-electric Councils

Currently fielding concerns at this site under "Home Electrical"

4 year technical, College level specific courses, EASA repair courses, vibration analysis electronic and electrical trade school.

©2017 All rights reserved.

[an error occurred while processing this directive]