Three-phase AC motors operating in modern industry are exposed to numerous disturbances caused by partial phase operation or voltage asymmetry between phases. These phenomena may cause excessive engine heating and, consequently, cause its failure. Another significant problem is the unauthorized change of the engine phase sequence, which leads to a change in the direction of its rotation and mechanical damage. To avoid this type of faults, phase loss sensors and phase sequence sensors are used.
In the article you will read about...
factors leading to damage to an alternating current sensor connected to a three-phase network and the principle of operation and specificity of voltage loss and phase sequence sensors. You will also learn the specifics of measuring the actual effective voltage value for distorted waveforms. We will also present effective solutions from the F&F brand !
What factors can cause engine damage?
According to statistical data, the cause of as many as 44% of all failures of asynchronous induction motors is an increase in temperature above the rated value, which is generated by eddy currents in the stator and rotor, current flow in the windings or mechanical friction of bearings. A permanent increase in engine temperature of just 10°C above the permissible limit can shorten its service life by up to two times.
The phenomenon of asymmetry of supply voltages
The key threat to AC motors connected to a three-phase network is unbalanced current load . This phenomenon leads to the flow of negative sequence currents and is induced as a result of asymmetry of supply voltages and in cases of voltage failure.
In cases of voltage failure on at least one phase of a motor powered by a three-phase network, a difference appears in the voltage system between the effective values of the phase-to-phase voltages and the angular shifts of these voltages. This determines the appearance of asymmetry between voltage vectors, which do not form an equilateral triangle under abnormal operating conditions.
An unbalanced supply voltage system leads to numerous unfavorable changes in the rated operation of asynchronous motors. An example may be the braking of the rotor movement by negative sequence currents or an excessive increase in thermal losses in the motor winding resulting from current asymmetry.
Phase loss phenomenon
By far the greatest threat to asynchronous motors is operating on at least one phase less, which is called phase loss. Half-phase operation occurs in cases of voltage failure caused by a complete interruption of power supply to one of the three phases of the power supply network.
Most often, the problem of complete phase loss is determined by the failure of the main contactor contacts or phase contactor contacts, which is usually caused by an excessive increase in the contact resistance of the contactor or its burnout. The reason for phase failure on at least one of the phases may be the blowing of one of the phase protection fuses.
Phase loss may also occur as a result of a power interruption in the wires or windings of a transformer or motor. If a phase failure occurs on the medium voltage side of the transformer (especially in the Yd connection system), the motor currents may increase by 115%, 115% and 230% of the rated value on individual phases.
In turn, if a phase failure occurs on at least one motor phase, the currents of the remaining two operating phases will increase by even more than 70% of the rated value. The occurrence of power supply asymmetry while the motor is running will result in a doubling of the supply current consumption (unless the motor is stopped), and this will lead to burning of its insulation or activation of protection devices.
The phase loss sensor and the phase sequence control sensor are usually mounted on the mounting clip of the TH-35 rail, but the phase loss sensor can also be mounted directly to the ground.
Phase loss sensors are available in versions for installation in three-phase systems with or without a neutral wire. The presence of a neutral wire that supports a given sensor is extremely useful when working with a generator.
Phase loss sensors and phase sequence sensors. What should you know about them?
To avoid irregularities in the power supply parameters of three-phase motors and to immediately turn off the machines in the event of phase loss, measurement and control instruments called phase loss sensors - CZF are used. In turn, to reduce the risk of incorrect phase connection sequence, CKF phase loss and phase sequence sensors are used.
CZF phase failure sensors only control the phase voltage level, comparing it to the rated settings. The phase loss sensor does not verify the connection sequence and phase asymmetry. For this reason, the CZF phase loss sensor is mainly used in electrical machines and devices in which changing the direction of rotation does not pose a risk of damage or failure of the system.
In turn, the phase loss and sequence sensor is a more versatile measuring instrument that functions as a phase loss sensor that controls the level of phase voltages and supervises the sequence of phase connections. Thanks to this, this type of sensor comprehensively protects electric motors against asymmetry and against starting in the wrong direction.
Principle of operation of phase loss sensors
CZF phase loss sensors and CKF phase loss and sequence sensors continuously measure phase voltages. If any phase loss sensor detects incorrect power supply parameters and a voltage asymmetry greater than permissible, it will control the contacts of the motor control contactor, turning off the power to the machine and protecting it from destruction.
Among the basic products belonging to the series consisting of phase loss sensors (CKF-B and CZF-B models), it is not possible to set the asymmetry voltage value and the sensor activation time, because the product parameters are "rigidly" set by the manufacturer - usually to a value of 55 V.
A series of more advanced products (CZF-BT and CKF-BT models) enable precise regulation of the voltage asymmetry and the activation time after which the power supply will be turned off.
There are also types of sensors available on the market that not only function as phase loss sensors, but also react in the event of damage to the contactors controlling the engine. A sensor of this type is, for example, the CZF2-B model, which provides the ability to measure network parameters and monitors the state of the contactor contacts.
True RMS measurement
To precisely control the power supply parameters of electrical devices , devices are used to measure the actual effective voltage value for distorted waveforms. This type of device is the True RMS sensor. This sensor measures the instantaneous voltage value of each period of the supply voltage. Thanks to this, the True RMS sensor is able to accurately verify the effective value of the voltage regardless of the distortion of the signal waveform and external interference affecting the shape of the sine wave.
True RMS sensors enable measurements of real RMS voltage values in any power supply network and can cooperate with power generators which, during rated operation, generate many interferences that may disturb the operation of relays without measuring True RMS values.