In the daily work of maintenance services, few situations cause as much consternation as the sight of a machine that theoretically works perfectly, but in reality, is balancing on the edge of failure. The HMI operator panel indicates a drive load at a safe level of 60-70%, the status LED on the servo amplifier in the control cabinet glows a reassuring green, yet the body of the servo motor is hot to the touch. Pyrometer measurements leave no doubt - the temperature is dangerously close to the thermal protection trip limit or insulation degradation.
A cold reading on the screen and a hot motor in reality is one of the most interesting issues in the diagnostics of precision drives. This is due to the fact that the concept of nominal load is much more complex than a simple percentage displayed on the screen. A servo motor is a complicated energy converter, in which heat is generated not only by the current flowing through the windings, but also by magnetic phenomena, parametrization errors, and control dynamics. To understand why the motor heats up, we need to look deeper than just at amperes.
Does the operator panel tell the whole truth about the load?
The first and most common reason for an incorrect diagnosis is reliance on load indicators visible on the machine's panel. Visualization systems very often present an averaged value of torque or current. Meanwhile, in servo drive applications, operation is impulsive.
During rapid acceleration, the motor can draw current two or three times its rated value. This lasts for fractions of a second, followed by a phase of uniform motion or standstill. Although the arithmetic mean of such a cycle looks safe, the physics of heating are governed by the law of the effective (RMS) value. The amount of heat dissipated in the windings increases proportionally to the square of the current intensity. This means that a twofold increase in current generates four times more heat. Short but aggressive current peaks can quickly heat up the windings, while the monitoring system, averaging these values, still shows a safe level. If the intervals between cycles are too short, the motor does not have time to dissipate this heat into the environment, which leads to the accumulation of thermal energy inside the stator.
When does excessive precision turn into excessive heat?
Another problem, invisible to the eye, is excessive ambition in configuring control parameters, known as PID loop detuning. In the pursuit of achieving the highest possible system stiffness and positioning precision, engineers often set very high gains for speed and position controllers.
In such a situation, the servo motor behaves like a sprinter in starting blocks, with all muscles tensed, even though he is not yet running. The motor shaft, even when stationary or moving at a constant speed, constantly vibrates. The control system tries to correct the position thousands of times per second, reacting nervously to every encoder noise. This phenomenon, often manifested by a characteristic squeal or metallic hum, means that continuous high-frequency currents flow through the windings. These currents do not drive the machine but are entirely converted into heat. In extreme cases, a poorly tuned motor can overheat even at a standstill, without any external load.
Why can a servo motor's standstill be more strenuous than its operation?
Many servo drive applications require position maintenance under load. This particularly applies to vertical axes (counteracting gravity) or tensioning systems (maintaining film or paper tension). In such a scenario, the motor generates what is called holding torque.
From a thermal perspective, this is a very unfavorable situation. Firstly, current flows continuously through the same sections of the windings, creating local hot spots, instead of distributing heat evenly throughout the entire stator circuit. Secondly, if the motor is not equipped with an independent fan, and cools only through the movement of its own rotor, its ability to dissipate heat drops drastically at a standstill. We are thus dealing with heat generation with almost zero ventilation. Engineers often forget to include this state in the energy balance, assuming that since the machine is not performing work, the motor is resting. In reality, it may be working harder then than during movement.
Is the feedback sensor setting precise?
A servo motor is a machine that requires precise synchronization between the rotor's position and the magnetic field generated by the stator. Information about the position is provided by an encoder or resolver. The most important parameter here is the so-called commutation angle or encoder offset.
If, as a result of servicing, motor disassembly, or loosening of the encoder coupling, there is a minimal shift in this angle, the controller starts to operate inefficiently. The current vector is not applied perpendicularly to the rotor's magnetic field. Figuratively, this can be compared to pushing a door near its hinges instead of by the handle. To achieve the same effect, we must use much more force. As a result, the motor draws a much higher current than would result from the mechanical load, and the excess energy is lost as heat.
Does mechanics pose hidden resistance?
Finally, it is worth returning to the basics, i.e., mechanics. It often happens that an electric motor is fully functional, yet the nominal load on the screen is false, because the control system compensates for the increase in mechanical resistance.
Worn bearings, warped linear guides, or, above all, an overly tensioned drive belt, all generate a constant load that the motor has to fight against. Particularly insidious are failures of electromagnetic brakes built into the motor. It sometimes happens that the brake does not fully release, and the friction lining gently rubs against the disc during operation. The servo controller, striving to maintain the set rotational speed, will simply increase the current to overcome this resistance. The machine will operate, the cycle will be maintained, but the motor will turn into a heater, drawing heat directly from the source of friction inside its own casing.
Are the environmental conditions in line with the design assumptions?
The last piece of the puzzle is the working environment. The motor dissipates heat into the surroundings mainly through convection. In industrial conditions, a layer of dust, dirt, or oil mist often settles on the motor casing. This creates a kind of thermal blanket, which drastically reduces cooling efficiency.
Equally important is the altitude above sea level. If the plant is located in mountainous terrain, thinner air has a lower ability to dissipate heat. In such conditions, a nominally loaded motor is actually thermally overloaded, which manufacturers describe in the technical documentation as the necessity of so-called derating, i.e., a deliberate reduction of the permissible servo drive power.
In summary, overheating of a servo motor with theoretically correct operating parameters is a signal that energy losses are occurring in the system, the source of which lies beyond simple mechanical loading. The key to solving the problem is not to replace the motor with a larger one, but to thoroughly analyze the control parameters, power supply quality, and the mechanical condition of the machine.
The key to solving the problem is not to replace the motor with a larger one, but to thoroughly analyze the control parameters, power supply quality, and the mechanical condition of the machine. At PLE Service, we verify these parameters, checking the encoder's operation and performing load tests that simulate the real operation of machines.
