Testing a motor comes down to checking three things: whether electricity can flow through the windings properly, whether the mechanical components spin freely, and whether supporting parts like capacitors are still good. You can diagnose most motor problems with a multimeter, your hands, and your eyes. Here’s how to work through each test systematically.
Safety First: Disconnect and Discharge
Before touching anything, cut power to the motor and verify it’s fully disconnected. If the motor has a capacitor (common in single-phase motors like those in HVAC systems, well pumps, and shop tools), that capacitor can hold a dangerous charge even after the power is off. You’ll need to drain it before handling it, which is covered in the capacitor section below.
Check the Windings With a Multimeter
The windings are coils of wire inside the motor that create the magnetic field. When a winding breaks internally (an “open circuit”) or its insulation fails and wires touch each other (a “short”), the motor won’t run correctly or won’t run at all. A simple resistance test with a multimeter tells you which problem you’re dealing with.
Set your multimeter to the ohms (Ω) setting. For a three-phase motor, you’ll test between the three terminal pairs: T1 to T2, T2 to T3, and T1 to T3. Each pair should read roughly the same resistance, typically in the range of 0.3 to 2 ohms. What matters most is that the three readings are close to each other. If one pair reads significantly higher than the others, that winding has a problem.
Here’s how to interpret the numbers:
- Balanced readings (0.3 to 2 ohms): Windings are healthy.
- A reading of zero: You have a short between phases, meaning the insulation between windings has failed and current is flowing where it shouldn’t.
- Infinite reading or well above 2 ohms: The winding is open, meaning the wire inside has broken and current can’t complete the circuit.
For single-phase motors, you’ll typically find two windings: a start winding and a run winding. Test each one for continuity (the meter should show some resistance, not infinite). Then test from each winding terminal to the motor’s metal frame. If you get any continuity to the frame, the insulation has broken down and the motor is grounding out. That’s a safety hazard, and the motor needs repair or replacement.
Test the Bearings by Hand
With the motor disconnected, grab the shaft and try to spin it. A healthy motor turns smoothly with minimal resistance. You’re feeling for three things: grinding, roughness, or excessive looseness.
Grinding or a crunchy feeling as the shaft rotates means the bearings are failing. Worn bearings create friction, generate heat, and will eventually seize the motor entirely. You can sometimes hear this too: a motor that hums loudly but barely turns, or makes a scraping noise at startup, often has a bearing problem.
Next, try to push and pull the shaft in and out (axial play) and wiggle it side to side (radial play). A small amount of axial movement is normal in ball-bearing motors, since the shaft needs room to expand as the motor heats up. A common engineering guideline allows at least 0.010 inches of clearance per foot of shaft length between bearings. But if the shaft wobbles noticeably from side to side, the bearings are worn and need replacement. Excessive play causes the rotor to contact the stator, which damages both.
Inspect Brushes and the Commutator (DC Motors)
If you’re working with a brushed DC motor (common in power tools, treadmills, automotive starters, and older industrial equipment), the brushes and commutator are the most common failure points. Brushes are small carbon blocks that press against the spinning commutator to transfer electricity to the rotor.
Pull the brushes out and look at them closely. A healthy brush has a smooth, shiny contact surface with no chips or cracks. Its shape should conform to the curved surface of the commutator. If a brush is worn down to about one quarter of its original length, it’s time to replace it. Also check the pigtail, the small braided wire that connects the brush to the motor’s circuit. It should be intact and firmly attached.
Now look at the commutator itself, the copper ring the brushes ride against. You’re looking for several warning signs:
- Threading: Fine scratches on the copper surface, caused by copper particles embedding in the brush face and scoring the commutator as it spins.
- Grooving: Smooth, worn slots in the commutator surface where the brushes have dug in unevenly.
- Copper drag: Copper particles smeared across the edges of the commutator segments, which can bridge gaps between segments.
- Dark buildup between segments: Dirt, carbon dust, and copper particles packed between segments can cause flashover, a short circuit between the brushes that produces visible arcing and can damage the motor quickly.
Light threading or surface discoloration can often be cleaned up. Heavy grooving, copper drag, or evidence of flashover usually means the commutator needs to be professionally resurfaced or the motor replaced.
Test the Capacitor
Single-phase motors use capacitors to help them start, maintain speed, or both. A failed capacitor is one of the most common reasons a motor hums but won’t spin, or starts sluggishly. There are two types: start capacitors (which give the motor a brief torque boost at startup) and run capacitors (which stay in the circuit continuously to improve efficiency).
Start by removing power and verifying the motor is disconnected. Remove the capacitor cover, then photograph the label and wire connections before disconnecting anything. This saves you from guessing which wire goes where during reassembly. Use insulated pliers to pull the spade connectors off the terminals, and don’t touch the terminals with your fingers. Discharge the capacitor by bridging both terminals with a 20,000-ohm resistor. If the capacitor has a bleed-off resistor soldered between its terminals, unsolder one leg before testing.
Visual Inspection
Before you pull out the multimeter, look at the capacitor. If the relief port on the end has blown open, the case is bulging, split, or corroded, the capacitor is dead. Replace it without further testing.
Continuity Test
Set your multimeter to the continuity setting (the one that beeps). Touch one lead to each terminal. If the meter beeps, current is flowing straight through the capacitor’s internal insulation (the dielectric layers), which means it’s failed. Replace it.
Short-to-Case Test
Skip this if the capacitor has a plastic housing. For metal-cased capacitors, place one multimeter lead on a terminal and the other on the metal case. Test both terminals this way. A beep on either one means current is leaking from the internal components to the case. Replace it.
Capacitance Measurement
If your multimeter has a capacitance setting (marked with the μF symbol), measure the actual capacitance and compare it to the rating printed on the capacitor’s label. A start capacitor should read within 20 percent of its labeled value. A run capacitor has a tighter tolerance and should be within 10 percent. If the reading falls at the edge of or outside these ranges, the capacitor has degraded and should be replaced.
What Each Symptom Points To
Once you’ve run through these tests, the results usually point clearly to the problem. A motor that won’t start at all but has good windings likely has a bad capacitor or a seized bearing. A motor that trips its breaker or blows fuses probably has a winding short or a ground fault (winding touching the frame). A motor that runs but vibrates excessively or makes noise typically has worn bearings or, in a DC motor, damaged brushes.
If every electrical test comes back clean and the shaft spins freely, the problem may be upstream of the motor: a bad switch, a broken wire in the supply circuit, or a failed controller. Work outward from the motor until you find the break.