# How to Test a Power Tool Armature: Multimeter & Visual Inspection Guide
Nothing kills productivity faster than a power tool that won’t run. Before you order a replacement, you need to confirm the armature is actually the problem — and not the stator, brushes, switch, or power cord.
Learning **how to test a power tool armature** is an essential skill for any repair technician, distributor, or workshop manager. A few minutes with a multimeter can save you from ordering the wrong part and keep your tools in service.
In this guide, we’ll cover the three most common armature faults — shorts, opens, and grounds — plus visual inspection techniques for commutator and shaft damage.
## What You’ll Need
– **Digital multimeter** with continuity/ohms function
– **Armature growler** (recommended for professional shops)
– **Screwdriver set** for disassembly
– **Clean rag** and electrical contact cleaner
– **Vernier caliper** for shaft wear measurement
## Step 1: Visual Inspection of the Armature
Before touching a multimeter, inspect the armature visually. Many problems are obvious to the naked eye.
### Commutator Condition
The commutator is the segmented copper ring at one end of the armature where the brushes make contact. Check for:
– **Dark or burnt segments** — indicates arcing from worn brushes or a shorted winding
– **Grooves or pitting** — worn from abrasive brush dust over time
– **Copper slivers between segments** — conductive debris creating a short circuit
– **Mica undercutting worn flush** — mica should sit slightly below the copper surface; flush mica causes brush chatter
A commutator with surface damage can sometimes be cleaned or turned on a lathe. But if the damage is deep or multiple segments are burnt, the armature likely has a winding fault.
### Windings Inspection
Look at the copper windings around the laminated core:
– **Burnt or blackened windings** — clear sign of overheating and insulation failure
– **Broken or loose wires** — obvious open circuit
– **Burned smell** — confirms internal winding damage even if not visibly charred
### Shaft and Bearings
Check the shaft for:
– **Scoring or wear marks** — bearing failure can damage the shaft surface
– **Bent shaft** — spin the armature and watch for wobble at the commutator end
– **Bearing fit** — the bearing should slide on smoothly without play
If the shaft is bent or bearing surface damaged, the armature is typically not worth repairing.
## Step 2: Multimeter Testing for Shorts and Opens
Now let’s test electrically. **How to test a power tool armature** properly means checking three things.
### Test 1: Continuity Between Commutator Segments
Set your multimeter to ohms (Ω). Place one probe on one commutator segment and the other probe on the next segment. You should get a low resistance reading — typically **0.5 to 2 ohms** depending on the motor design.
Work your way around the entire commutator, testing every adjacent pair of segments.
– **Reading too high or infinite** — open circuit in that coil (broken wire)
– **Reading identical across all pairs** — armature coils are likely healthy
– **One or two pairs reading much lower** — possible shorted turn in that winding
### Test 2: Ground Test (Armature to Shaft)
This test checks if any winding is shorting to the laminated core or shaft — a common failure mode.
Set your multimeter to the highest ohms range. Place one probe on a commutator segment and the other on the **armature shaft** (bare metal). The reading should be **infinite (OL)** — no continuity.
Test all commutator segments.
– **Any reading other than OL** — winding is shorted to ground, armature is faulty
– **Multiple segments showing ground** — likely overall insulation breakdown from overheating
### Test 3: Coil-to-Coil Resistance Balance (Advanced)
For higher-precision diagnosis, measure resistance between every commutator segment and an arbitrary reference segment (e.g., segment #1). Write down all readings.
In a healthy armature, **all readings should be very close** — within 5-10% of each other.
A significant variation indicates:
– Uneven winding tension from manufacturing defect
– Partially shorted turn in one coil
– Corrosion or contamination in one coil group
This is especially important when evaluating **armature price** versus quality from a supplier. A lower-priced armature may have less consistent winding resistance, which means shorter service life.
## Step 3: Using an Armature Growler
A growler is a specialized testing device for armatures. It uses an AC electromagnet to induce current in the armature windings.
**How to use a growler:**
1. Place the armature in the growler’s V-notch
2. Turn on the growler — it should hum (the “growl” gives it its name)
3. Hold a thin metal strip (like a hacksaw blade) over the commutator
4. Slowly rotate the armature
If the blade **vibrates or sticks** over a specific segment, that coil has a shorted turn. The magnetic field induces heavier current in the shorted winding, pulling the blade.
Growler testing is the gold standard for armature inspection. It catches shorted turns that a multimeter alone might miss.
## Step 4: Stator Check (Don’t Forget)
When you’re testing an armature, check the stator at the same time. A bad **stator replacement** is often confused with a bad armature.
To test the stator:
1. Measure resistance across both stator winding leads — typically 2-10 ohms
2. Test continuity from each stator winding to the iron core — should be OL
3. For universal motors, check both field winding pairs individually
If you need a new stator, work with a reliable **stator manufacturer** that provides documented specifications and quality certifications.
## Common Test Results and What They Mean
| Test Result | Diagnosis | Action |
|—|—|—|
| All continuity readings consistent, no ground fault | Armature is healthy | Check stator, brushes, switch |
| One or two segments high resistance | Open winding in that coil | Replace armature |
| Ground fault (continuity to shaft) | Insulation failure | Replace armature |
| Growler blade sticks at one segment | Shorted turn | Replace armature |
| Visual burn marks + ground fault | Overheating damage | Replace armature and check ventilation |
| Grooved commutator, coils fine | Brush wear | Clean or turn commutator, replace brushes |
## When to Replace vs. Repair
Armature repair — rewinding — is rarely cost-effective for power tools. High-volume rewinding shops can do it, but the cost often rivals a new OEM-quality armature.
Consider **armature price** when deciding:
– **$6–12** — Standard armatures for common tools (angle grinders, circular saws)
– **$12–18** — Larger armatures for demolition hammers, rotary hammers
– **$18+** — Specialty armatures for industrial tools
Weigh replacement cost against the cost of rewinding labor (often $15–30) and the risk of inconsistent quality from a rewound coil. For most power tools, **replacement is the smart move.**
Browse our range of [armatures](/product/armature-gws20-180/) for common replacement models, or check our [armature price guide](/armature-price-guide/) for wholesale rates.
## Why Testing Matters for Distributors
If you’re a **power tool spare parts distributor**, accurate armature testing serves two purposes:
1. **Reduces returns** — Customers who test before ordering get the right part the first time
2. **Builds trust** — When you can help customers diagnose the problem, they come back
Inventory management also gets easier when you track common failure patterns. For example, if a particular tool model keeps returning with ground faults, you may need to recommend better dust protection or verify that your **armature supplier** is meeting OEM specs.
## Frequently Asked Questions
### Can I test an armature without removing it from the tool?
Partial testing is possible. You can check for ground faults from the brush holders (remove brushes first, test from commutator to armature shaft). But accurate multimeter testing and growler testing require removal.
### What resistance should a good armature show?
Most power tool armatures show 0.5 to 2.0 ohms between adjacent commutator segments. The precise value depends on wire gauge, number of turns, and motor design. Consistency between segments matters more than a specific number.
### My multimeter shows 0.0 ohms — is that a short?
It depends. Some high-quality multimeters can read below 1 ohm. If all segments read 0.0 consistently, your meter may lack the resolution. Try a lower range setting or test with a known good armature to verify.
### How do I know if I need an armature or a stator replacement?
If only the armature has ground faults or winding damage, replace the armature. If the stator also shows faults (winding-to-core continuity or open windings), replace both. A matched **stator replacement** ensures proper magnetic field alignment.
### Can a bad commutator be fixed?
Mild commutator surface damage — grooves, light burns — can sometimes be turned on a lathe. Once mica depth drops below 0.5mm or segments are heavily burnt, the armature should be replaced.
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*Need help diagnosing a specific tool? Email us at sales@primearmature.com with the tool model and test results — we’ll help you find the right part.*