The motor turns one way but not the other: what could be the cause?
The fault is almost never in the motor: check the input signal for that direction, its current limit, mechanical binding and the relevant half of the bridge.
If the motor runs in one direction and not in the other, the problem is almost never in the motor. The motor winding and brushes are common to both directions; if one direction works, the mechanism itself is sound. The fault lies in the parts specific to that direction: the input signal, that direction's current limit, mechanical binding and, last of all, the relevant half of the bridge.
The systematic order of diagnosis
- Verify the input signal. On the KS250, IN1 is clockwise start, IN2 is reverse start, and IN3 and IN4 are the stop inputs for the respective directions; on the PT500 the A, B, C, D digital inputs are used. Measure whether a signal appears at the input terminal of the direction that does not work when the command is given. A broken wire, an oxidised button contact and the relevant RF receiver output not working are the most common causes. When you measure, connect the black lead of the multimeter to the driver's own negative terminal; a reading taken against some other chassis point can mislead.
- Check the current limit. On the PT500 each direction has its own limit: clockwise is set with pot A1 and reverse with pot A2. If the pot for one direction is set too low, the movement is cut off as soon as it starts; from the outside it looks as if it does not work at all. How a threshold that cuts too early is adjusted is a separate topic.
- Check the mechanics. Remove the load or try moving it by hand in that direction. A slide binding in one direction, a tangled cable or a mechanism resting against its end stop are typical causes. If a limit switch is wired into the system, verify as well that the switch for that direction is not being held pressed.
- Power the motor directly. Connect a battery directly to the motor terminals briefly and see that it turns in the reverse direction; if it turns, the motor is sound.
- Assess the driver. If everything above is clear, one half of the bridge may be faulty; in that case the module has to be taken in for evaluation.
From measurement to conclusion
| Finding | What it means | Next step |
|---|---|---|
| No signal at the input | The fault is upstream of the driver | Follow the button, the cable and the receiver output |
| Input present, no voltage at the motor | The driver is not applying the command | Rule out the current limit and the mechanics |
| The motor starts briefly and cuts | Current threshold or binding | Try it unloaded, measure the current |
| Both directions turn straight off the battery | The motor is sound | Focus on the driver and the control side |
Common mistakes
- Removing the motor first. Taking the most expensive step first wastes time; measuring the input takes a few minutes.
- Turning the pot at random. A current limit pot turned without marking its old position creates a second fault.
- Treating a relay system like a driver. If there is a relay at the output the chain is different; the motor not turning although the relay pulls in means welded contacts or a break on the contact side.
- Enlarging the fuse. If only one direction fails, the problem is not current capacity; enlarging the fuse only enlarges the risk.
- Trying both directions at once. Apply the inputs one at a time and in order; two direction commands arriving together create an undefined state in the driver and corrupt the symptom you are diagnosing.
Safety while measuring
Secure the load mechanically and assume the motor may move suddenly during the test. In the test where you connect the motor directly to a battery, fuse the line and keep the contact to a matter of seconds. If you work through the order without breaking it, the location of the fault usually becomes clear in the first two steps. Write the steps and the values you measure down: if the module has to be replaced, you will have a record showing what was ruled out.
The full topic: DC motor and driver fault diagnosis matrix