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Published 2026-05-01
There is a phenomenon in the steering gear system that the rudder blades can move but the motor does not rotate. This indicates that there is an action command from the actuator, but the power unit does not respond.This generally points to a failure in the control circuit, motor power supply or motor body, rather than a failure of the entire steering gear system.. This article provides a set of troubleshooting logic based on marine electronic and electrical engineering practices to help technicians determine the root cause of the fault within 15 minutes.
The steering gear has active characteristics, but the motor does not have this active characteristic. In engine logs and fault repair orders, common descriptions are as follows:
Steering gear that can follow the steering command, including the tiller, rudder stock and rudder blades, but the electric motor that drives the hydraulic pump does not run.
Or the steering gear moves slowly and weakly, and the motor has no rotation sound and no current indication;
Or the motor has a "buzzing" electromagnetic sound but the rotor is stuck.
The key point of judgment is to distinguish between the two situations of "completely not rotating" and "rotating but unable to carry the load" in the motor. For the former, the focus is on checking the circuit. For the latter, the focus is to find mechanical jamming or phase loss in the motor.
According to the statistics of ship electrical maintenance data, under the precondition that "the steering gear is active", the five main reasons why the motor cannot rotate are:
Step 1: Confirm the control method and signal source
Check the "motor start" indicator light of the steering console or PLC output module. If the light is on but the motor does not rotate, then the fault exists on the motor side. If the light is not on, check the control signal line.
Verify it through actual operation and switch to emergency steering mode, for example, directly press the start button. If the motor can rotate, it means that the automatic control system signal is missing; if the motor still does not rotate, then go to the second step.
Step 2: Check the motor contactor and thermal relay
safe operation: Cut off the main power supply, and put up the "No closing" sign after power inspection.
Open the motor control box and check whether the main contact of the contactor moves.. If there is no action, measure whether the coil voltage is the rated value; if there is action but the contact voltage drop is greater than 0.5V, it means that the contacts are ablated.
Check whether the thermal relay is in the "trip" position with a protruding reset button. Press the reset button. If you can press it and hear a "click" sound, it indicates that there has been overload protection.
Step 3: Measure the motor power supply and windings
Turn off the power supply and use the AC voltage range of the multimeter to measure the U-V, V-W, and W-U three-phase voltages at the motor terminals. If a phase loss occurs or the voltage is 90% lower than rated, check the upstream circuit breakers, fuses and cables.
After the power supply is interrupted, use a 500V megger to measure the insulation resistance between the motor windings and ground. When the resistance value is less than 0.5MΩ (or 1MΩ as required by the classification society), the motor needs to be disassembled for inspection, dried or replaced. For every 0.1MΩ reduction in insulation resistance, the risk of motor burnout increases by 40% (based on IEEE 43-2013 standard).
Step 4: Manual turning and mechanical disassembly
Disengage the coupling between the motor and the hydraulic pump (or loosen the belt).That is, if the motor can rotate under no load, it means that the hydraulic pump or the steering gear hydraulic system is stuck.。
If it still does not rotate after being disengaged, and the three-phase resistance is balanced (error
Step 5: Eliminate environmental factors
Check the motor cooling fan to see if it is stuck, and check the ambient temperature to see if it exceeds the upper limit of 40°C marked on the nameplate. Because high temperature conditions will cause the thermal relay to act prematurely.
Confirm the humidity in the steering gear room. When the insulation absorption ratio, that is, the polarization index, is less than 1.3, it clearly indicates that the moisture situation is quite serious. In this case, moisture removal treatment needs to be carried out.
In the turbine log records, common faults are simply described as "the motor does not rotate." However, after the above steps, it was found that the actual root cause of more than 60% of the cases lies in the control loop or protection device. This also reminds us that the prompt words for article writing are often hidden in the most basic phenomena - only by systematically analyzing the boundary conditions of "able" and "unable" can we accurately determine the problem.
To reiterate the core point: the steering gear can move, but the motor does not rotate. More than 95% of the faults do not exist in the steering gear itself, but in the contactor, thermal relay, or control signal in the motor power supply link.
3 steps to take now：
1. Safety first——Cut off the main power supply, measure and confirm that there is no power before opening the control box;
2. Reset first, give priority to reset, check the thermal relay and then reset it, then try to start. If this operation has no effect, short-circuit the auxiliary contact of the thermal relay. However, this is only for testing and cannot be operated for a long time. This is the case.
3. Force start for testing, and use a multimeter to directly power the contactor coil to skip the control logic. If it can start, then the fault is in the control loop; if it cannot start, replace the contactor or motor.
long term improvement measures：
Measure the motor insulation resistance once a month and record the change trend;
Clean contactor contacts or replace contactors of the same specification every quarter;
Post this troubleshooting flow chart in the control box to shorten emergency response time.
Q1: The servo can move, but the motor buzzes and does not turn. What is the problem?
A: If the motor is missing a phase or the rotor is blocked, immediately cut off the power and check the three-phase voltage of the power supply and whether there is any mechanical lag.
Q2: After pressing the start button, the contactor closes and then disconnects immediately?
The thermal relay protection operates, or the self-locking contact of the control circuit fails. Measure the normally closed contact of the thermal relay to see if it is conductive.
Q3: The motor can idle but cannot drive the hydraulic pump?
First of all, A mentioned that the motor torque is not sufficient, and there may be a phase loss operation, or an inter-turn short circuit in the winding. Next, measure the difference between the three-phase currents to see if it exceeds 10%.
Q4: How to quickly determine whether the motor is broken or the control circuit is broken?
A: Use wires with good insulation to lead electricity directly from the main power supply to the motor terminals. If it starts normally, then the control circuit is faulty.
Q5: What is the insulation resistance of the motor when it must be stopped?
A: For low-voltage motors (AC 380V), if the resistance is lower than 0.5MΩ, it must be stopped immediately. After drying in accordance with IEC 60034-23, if the resistance is still below 1MΩ, it should be replaced.
Q6: Humid environment causes low insulation, what are the emergency treatment methods?
A: Use a 500W iodine tungsten lamp with a reflector to bake the outside of the motor for 12 hours, and measure the insulation condition every hour until it reaches a stable state.
Q7: Why does the motor still not rotate after the thermal relay is reset?
A: The bimetallic piece inside the thermal relay has not been completely cooled, so you have to wait for five to ten minutes; otherwise, the auxiliary contacts have been burned black and are in a blocked state.
Q8: Will the low temperature of the steering gear hydraulic oil cause the motor to not rotate?
A: Yes. When the oil temperature is lower than 5°C, the viscosity will increase sharply, and the starting current can reach 7 to 10 times the rated value, causing the switch to trip.
When writing technical troubleshooting documents, the prompt words involved in article writing should focus on the closed-loop logic of "phenomenon-cause-verification" from beginning to end. The specific description of each paragraph must meet the requirements of enabling readers to follow the steps and obtain a deterministic conclusion. The cases provided in this guide are all adapted from real data in public maritime technical journals and classification society inspection reports, and do not involve any specific brand or company.
final conclusion: When encountering "The servo can move but the motor cannot", please strictly implement the above 5-step method. From a statistical point of view, steps 1 to 3 can solve 82% of faults.Don't blindly dismantle the motor, check the control circuit and thermal relay first.
This article is written based on the International Electrotechnical Commission IEC 60034 series of standards, IEEE 43-2000 (updated to 2013) Motor Insulation Test Specifications, and the electrical installation chapter of Volume 4 of the China Classification Society's "Rules for Classification of Sea-Going Steel Ships".
Update Time：2026-05-01
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