DC Motor and Driver Troubleshooting Matrix: Symptom, Cause, Measurement, Fix
Motor won't turn, turns one way only, driver overheating, AntiSwitch cutting out early... A 25-row diagnostic table, each row with the measurement to make and its acceptance threshold. The 5 MΩ megger rule, MOSFET diode test, ESR, voltage drop and the DC clamp meter.
The most expensive mistake in troubleshooting is replacing parts without measuring. The matrix below gives, for each symptom, the concrete measurement to make and its acceptance threshold. Where an acceptance threshold cannot be given, this is stated explicitly — a made-up number is worse than not measuring at all.
Before you start: safety
| Topic | What you need to know |
|---|---|
| Battery short-circuit current | A shorted lead-acid battery can deliver 100–1000 times its typical discharge current; on a 12 V automotive battery, the order of 500–1000 A is possible. Lead-acid batteries have no internal short-circuit protection. |
| Jewelry and watches | Occupational safety regulations prohibit wearing rings, bracelets, watch straps and metal keyrings where contact with energized exposed parts is possible — and this rule has no lower voltage limit. |
| DC arc | Because DC current has no zero crossing, an arc is extinguished against the full source voltage; that is why a DC arc is markedly harder to extinguish than an AC arc. On dual-rated products the DC rating is always lower than the AC one. |
Practical consequence: a source capable of 500–1000 A will make a 2.5 mm² cable glow within seconds. The fuse goes at the point on the cable closest to the source.
Main diagnostic matrix
| # | Symptom | Possible cause | Measurement / test | Fix |
|---|---|---|---|---|
| 1 | Motor doesn't turn at all, driver LED lit | Open-circuit winding, worn-out carbon brush, brush spring has lost pressure | Winding resistance across the motor terminals; open circuit = ∞. Open the brush cap and measure brush length | Brush replacement. A carbon brush is replaced when it reaches the wear line or ~6 mm, at the latest before the pigtail lead touches the commutator |
| 2 | Motor doesn't turn, driver LED dark | No supply, blown fuse, loose terminal, reverse-polarity damage | DC voltage at the driver terminal under load; continuity through the fuse | Replace the fuse with the same rating — do not size up before finding why it blew. If there is reverse-connection damage, check the input stage |
| 3 | Turns one way, not the other | MOSFET failure in one leg of the H-bridge; direction signal not arriving | Diode test: a healthy MOSFET shows 0.4–0.7 V across the body diode, open circuit in reverse. A failed one reads <10 Ω between the two terminals and beeps | Replace the failed MOSFET / output stage; if not economical, replace the board |
| 4 | Motor runs / won't stop without a command | MOSFET failed short (permanently conducting); voltage limit exceeded by a back-EMF spike | Continuity across the motor terminals with the supply off; MOSFET diode test | MOSFET replacement plus a review of the snubber / freewheeling diode and the voltage margin |
| 5 | Heavy, bright, continuous sparking at the commutator | Brush worn, spring pressure low; rough commutator; brush jammed in its holder | Visual inspection, free movement of the brush, surface roughness. Recommended spring pressure in industrial practice is 130–300 g/cm² | Brush + spring replacement, commutator grinding, cleaning between segments. Faint pinpoint sparking can be normal; bright continuous arcing is not |
| 6 | Motor weak, overheating; commutator has a hard, black, glossy layer | Patina has become too thick (increased voltage drop) or too thin (excessive brush wear) | Visual: a healthy patina is even and matte brown | Surface cleaning / turning, selecting the correct brush grade |
| 7 | Brushes wear fast, grooving on the commutator | Mica undercut not done; since mica is harder than copper it protrudes and eats the brush | Measure undercut depth; typical range 0.5–1.5 mm, ≈1.6 mm ± 0.4 mm on medium segments, standard width ~3 mm | Mica undercut plus deburring of segment edges. Ideal criterion: depth ≈ mica thickness |
| 8 | Leakage to the housing, chassis sparking, sensation of a shock | Winding insulation breakdown, moisture, carbon dust bridging | Insulation resistance with a megger: 500 V DC, value at the 60th second. Per IEEE 43-2013, a minimum of 5 MΩ for windings below 1 kV | Below 5 MΩ → drying, cleaning, rewinding |
| 9 | Normal at no load, current soars and it heats up under load | Partial short within the winding (turn-to-turn) | Measure winding resistance and compare the difference between symmetric legs; compare no-load current. Suspect it if resistance is noticeably below normal | Rewinding. Resistance measurement alone is not enough; megger and surge testing are complementary |
| 10 | Humming, vibration, shaft heating | Bearing failure, loss of lubrication, axial loading | Shaft play, stethoscope rod, housing temperature, current fluctuation | Bearing replacement plus correction of coupling/shaft misalignment. If misalignment is not corrected, the new bearing will go too |
| 11 | Motor speed low, cable heating up | Insufficient conductor size → voltage drop | Vdrop = 2 × I × ρ × L / A. Verify by measurement: the difference between the battery terminal and the driver terminal under load. Target on critical circuits is 3% → at 12 V only 0.36 V | Increase the conductor size, shorten the run, move the supply point closer, check terminal torque |
| 12 | Voltage collapses when load is applied, driver resets | High battery internal resistance, weak battery, loose battery post | Battery voltage at no load and under load; ΔV/ΔI ≈ internal resistance. A Hall-effect clamp is required for current | Battery replacement / charging, cleaning and tightening the battery posts |
| 13 | Random stops, speed wandering | Common GND fault, loose chassis bond; the GND of the driver and of the control source not tied together | Measure DC voltage between driver GND and control GND — it should be 0 V. Continuity on the order of mΩ on the chassis | Single-point common GND, clear paint/rust from the chassis surface, star grounding |
| 14 | Jitter on the potentiometer signal, speed jumping | PWM-induced EMI, long unshielded analog cable, same duct as the power cable | View the analog input signal with an oscilloscope; test by temporarily shortening the signal cable | Shielded cable; ground the shield at one end only (preferably the source/panel side). Grounding both ends drives current through the shield. Ferrite plus power/signal separation |
| 15 | Driver overheating, protection tripping often, capacitor top bulged | Electrolytic capacitor degradation (rising ESR) | Visual: doming, leakage, discoloration. ESR meter — ESR goes out of spec before capacitance drops by 10–40% | Replace with a low-ESR, high ripple, ≥105 °C type. Every 20 °C reduction in temperature quadruples life |
| 16 | Terminal discolored, smells, intermittent dropouts | Loose terminal / bad crimp → contact resistance → local heating | Scan the terminals with a thermal camera under load; measure the mV drop across the terminal | Re-crimp / retighten, cut back and remake the oxidized end, follow the torque spec |
| 17 | Driver goes into overcurrent protection immediately | Mechanical jam, gearbox seizure, locked bearing, startup current above the limit | Measure startup and continuous current with a DC clamp; compare against the driver's continuous/peak ratings | Fix the mechanics or move up to the correct driver class. KS250: 15 A continuous / 45 A peak (5 s). PT500: 30 A continuous / 40 A peak (15 s). If the start falls outside these windows, the error is in the application selection |
| 18 | AntiSwitch stops too early — cuts out before the motor reaches the target | Current limit threshold too low; voltage drop or cold mechanical friction raising the current | Watch the current throughout the travel, note the peak current | Set the threshold above the peak current and below the current at the mechanical end stop. A1/A2 pots on the PT500, internal adjustment on the KS250. Fix the voltage drop first |
| 19 | AntiSwitch stops too late — the motor strains at the end stop | Current limit threshold too high; the ramp duration is masking the threshold | Measure the current at the moment it seats against the end stop; the threshold must stay below that value | Lower the A1/A2 or AntiSwitch threshold; soften the startup peak with RMP/ramp |
| 20 | Motor starts harshly, shock in the mechanics | Ramp off or too short | Watch the PWM duty cycle rise over time with an oscilloscope; measure the startup current peak | On the PT500, lengthen the ramp with the RMP pot and cap top speed with the SPEED pot; on the KS250, enable the ramp |
| 21 | Button pressed but no response / wrong direction | Digital input wiring incorrect; common terminal not connected | Measure DC voltage at each input terminal in the active and inactive states | KS250: IN1 clockwise start, IN2 reverse start, IN3 clockwise stop, IN4 reverse stop, IN5 analog. PT500: A-B-C-D digital + E analog. Correct the wiring to this map |
| 22 | Driver blew up / smoked on first power-up | Motor leads connected to the supply terminal, or reverse polarity; installation without a fuse | Before applying power: read the terminal labels, and confirm with an ohmmeter the distinction between motor resistance (a few ohms) and supply (∞) | Board replacement. Permanent fix: a fuse in the supply line, reverse-polarity protection, and doing the first power-up with a current-limited supply |
| 23 | Water, moisture, corrosion inside the enclosure | A hole drilled into the IP enclosure instead of a cable gland; cable entering from above (water follows the cable) | Visual plus insulation resistance (item 8) | Proper IP gland, a drip loop in the cable, entry from below. If the box is drilled, the IP rating on the label is void — KS250 IP45, PT500 IP67 |
| 24 | Good in one direction, weak and noisy in reverse | Brush neutral axis shifted, or one brush worn differently | Compare no-load current and speed in both directions; measure brush lengths individually | Set the brush rocker to the neutral axis, replace brushes as a set |
| 25 | Brake ineffective, motor coasts freely | Dynamic braking circuit (winding shorted through the low-side MOSFETs) not working | Watch the trend of resistance across the motor terminals on a brake command; MOSFET diode test | Repair the relevant MOSFET / output stage. The KS250 has a dynamic braking feature |
Measurement methods: the critical details
| Measurement | Instrument | Point not to skip |
|---|---|---|
| Supply voltage | Multimeter, DC V | Measure under load; a no-load measurement hides the voltage drop |
| DC current | Hall-effect clamp meter | An AC clamp (current transformer type) shows no DC at all. Zero the meter before measuring, with no conductor in the jaws — the earth's magnetic field and sensor offset cause deviation |
| Insulation resistance | Megger 500 V DC | Record the value at the 60th second; minimum 5 MΩ below 1 kV (IEEE 43-2013) |
| MOSFET health | Multimeter, diode mode | Healthy: body diode 0.4–0.7 V, open circuit in reverse. Failed: <10 Ω between the two terminals |
| Capacitor | ESR meter | ESR degrades before capacitance drops by 10–40% — a capacitance measurement alone is misleading |
| PWM waveform | Oscilloscope | Duty cycle, switching frequency, back-EMF spikes, ramp profile |
| Temperature | Thermal camera / IR thermometer | Scan terminals and the power stage under load; a hot spot = high contact resistance |
Quick field diagnostic flow
- Look: burnt smell, bulged capacitor, discolored terminal, water marks.
- Measure de-energized: fuse continuity, motor winding resistance, motor-to-housing insulation (≥5 MΩ).
- Measure energized and under load: if the difference between the battery terminal and the driver terminal exceeds 3%, it is a cable problem; current with a DC clamp.
- Check the signals: analog input voltage, digital input levels, PWM on the oscilloscope.
- Thermal scan: terminals and power stage under load.
- AntiSwitch: reset the threshold against the measured peak current — raise it if it stops early, lower it if the mechanics strain.
A note: topics where no acceptance threshold is given
For some values no freely accessible primary source exists, so no number is given in this article on purpose: the numerical insulation resistance and polarization index tables of IEC 60034-27-4 (the standard text is required — this article is based on IEEE 43-2013), the cycle life of a specific capacitor or relay, and the exact short-circuit current of a lead-acid battery (it depends on capacity, temperature and state of charge; the 500–1000 A given is only an order-of-magnitude indication). These values should be taken from the manufacturer's datasheet when needed.