Motor protection (electrical engineering)
The motor protection (e.g. a motor protection switch ) protects electric motors (mostly asynchronous motors ) from thermal overload due to mechanical overload or in the event of failure of a single or two outer conductors.
Basics
Electric motors are designed for a specific rated operation in which the critical parts of the motor remain within the permissible temperature range and short-term overloads are not endangering. On the one hand, the motor protection device must enable economical operation of the motor with full utilization of the characteristic values, but on the other hand react sufficiently quickly to overloads. Due to typically 4 different insulation classes for the winding and special designs of the motors for special applications, there are different maximum permissible continuous temperatures and different requirements for the motor protection device, which must be met through professional project planning and execution.
Motor protection in general is divided into the subject areas of protection requirements (of the motor and the system) and protection methods (current-dependent protection and / or temperature-dependent protection).
Motor protection
There are two basic ways of protecting an electric motor from overload during operation: on the one hand, monitoring its current consumption, and on the other hand, direct monitoring of the temperature in the motor windings. A combination of both methods is often used (e.g. in the case of difficult load profiles). The first category includes the motor protection switch and the overload relay (also motor protection relay ), the second includes self-resetting bimetal switches and PTC thermistors as well as full motor protection.
The tripping time of the overload protection is determined by the current strength or temperature and the setting of the tripping range. For all set current values, it must be below the endangerment time of the motor insulation. According to EN 60947, this results in the requirement to specify maximum times for the overload. In order to avoid unnecessary tripping, limit values for the limit current and motor standstill are specified here.
Motor protection switch
Motor protection switches are z. B. manufactured for three-phase motors . With this version of the motor protection, an OR- linked triggering takes place by monitoring the currents in the three supply lines (current-dependent protective device ). The monitoring can be implemented thermo-mechanically ( bimetal ), thermo-electrical (PTC) or electronically (current measurement).
Three-phase motors should only be connected to the power supply via suitable motor protection switches or motor protection relays in order to avoid damage due to overload or failure of an external conductor. This type of motor protection is usually set according to the rated motor current I n . Switching on again after tripping can be done either automatically or manually by pressing an unlock button.
Undervoltage releases are partially integrated into the motor protection switch. Motor protection switches often also use a short-circuit release to protect the supply network from short-circuits, but this is not necessary to be considered a motor protection switch. Motor protection switches, which are used for short-circuit and overload protection, must be installed at the beginning of the motor supply line in accordance with DIN VDE 0113.
The short-circuit protection function can also be taken over by a coupled miniature circuit breaker or a fuse at the beginning of the supply line.
Auxiliary contacts can also be added to e.g. B. to monitor the state of the motor protection switch, e.g. B. via a PLC or a signal lamp. The auxiliary contacts are not switched when switched off (tripped). When the motor protection switch is switched on, the auxiliary contacts are activated - in contrast to the motor protection relay.
Overload relay
Overload or motor protection relays work on the same principle as the motor protection switch, but they do not switch the motor off directly. When an MSR ( M otor s rotection r elay ) triggers one or more smaller contacts are actuated (power contacts), but at least one normally closed contact. A contactor is usually controlled via this normally closed contact , which then switches off the equipment. Many motor protection and overload relays are designed in such a way that they can be attached directly to a contactor and connected to it without additional cables. In contrast to the motor protection switch, an MSR does not have a short-circuit release, but only the thermal release (bimetal), which might react too slowly in the event of a short-circuit. Therefore, fuses must be installed in the supply line for one or more motors that are protected by motor protection relays.
Triggering by hand (for a function test) should only be done using the special "test" actuator (in some cases can only be operated with a screwdriver). The 'stop button' only activates the normally closed contact and not the normally open contact. This prevents the system from malfunctioning during operation (because the MSR is monitored), but the motor is still off for the duration of the actuation - via the switched-off contactor, which is attached to the NC contact. The test button switches the auxiliary contacts permanently - just like a real trip. A reset takes place via the reset button. Some motor protection relays have an automatic reset option. As a rule, the reset button is pressed permanently and when the bimetals have cooled down, the auxiliary contacts return to the rest position and the system continues to run until the next trip. However, this should only be used very carefully, as it could possibly lead to the destruction of the system.
Overload relays are also available at higher voltage levels in the power supply network . They are connected there via appropriately insulated current transformers and trip a circuit breaker .
Motor monitoring with thermistors (full motor protection)
Many motors today have built-in thermistors that are used to monitor the temperature of the windings. Exceeding the permitted temperature then leads to a message or to the motor being switched off. The thermistors are PTC thermistors , which have a resistance increase to, for example, 1 kΩ at the nominal switch-off temperature. They are standardized in DIN 44081. Since the temperature of the motor windings is monitored directly here, this protective device is suitable for all types of electric motors, regardless of the type of current and connection. Switching on again is only possible after the motor has cooled down sufficiently, either manually or automatically, depending on the external wiring. As a rule, 3 PTC thermistors are installed in series. The evaluation takes place via an external device, which in the event of an error then switches off the contactor for the monitored motor via an auxiliary contact (NC).
Trigger mechanism
Overload protection
The thermal tripping protection is provided by bimetals that are heated by heating windings (string resistors) through which the motor current flows. A separate bimetal with an associated heating coil is provided for each current-carrying line to the motor. If the current consumption of even one winding of the motor exceeds the specified value for several seconds, the bimetal, deformed by the heat, triggers the switching mechanism of the motor protection and interrupts the circuit to the motor. Likewise, if an outer conductor fails (uneven heating of the bimetal strips), it is switched off after a short time (OR link). In the event of thermal release, the switch can only be switched on again after the bimetals have cooled down. The tripping currents of thermally tripping motor protection switches can be adjusted within certain limits (up to a factor of 1.6) and must be set to the rated current of the motor. The type of connection (star or delta) of the windings must be observed, especially for three-phase motors. The reset after triggering can only take place after the bimetal has cooled down sufficiently.
Electronic overload protection
Electronic releases measure the current of each external conductor with current transformers and imitate the behavior of a thermal motor protection switch. The advantages of electronic devices are a larger range of adjustable motor rated current and a greater variety of functions. For example, the tripping characteristics of many electronic motor protection switches can be set (e.g. delayed tripping in motors with heavy starting ) and the switch can be reset remotely after a fault.
Test button / test slider
The test is a purely mechanical test. The release mechanism is operated using this button, slide, etc. The bimetals and / or the electromagnetic quick release act on the key switch exactly on the way .
Phase failure monitoring
There are relays for phase monitoring to monitor the equality of all three voltages of the three-phase line conductors. They protect three-phase consumers (motors, rectifiers, transformers) from the increased loads associated with phase asymmetries. Such monitoring relays often have an adjustable tolerance threshold.
Single-phase motors and consumers
If three-pole overload protection is used for single-phase motors , the three current paths must be connected in series. If only one current path was used, many switches would switch off prematurely because they often also monitor the even load or voltage of the three outer conductors . Whether this circuit with electronic motor protection switches is permissible must be taken from the product data sheets of the respective manufacturer in each individual case.
Small single-phase motors (such as refrigerator compressors) are often protected with a simple, self-resetting bimetal switch.
For devices and control panels, there are also toggle switches with thermal, pressure-dependent or overcurrent release.
literature
- Günter Boy, Horst Flachmann, Otto Mai: The master's examination in electrical machines and control technology. 4th edition. Vogel Buchverlag, Würzburg 1983, ISBN 3-8023-0725-9 .
- Günter Springer: Expertise in electrical engineering. 18th edition. Verlag Europa-Lehrmittel, Wuppertal 1989, ISBN 3-8085-3018-9 .
- Theodor Schmelcher: Handbook of Low Voltage, project planning information for switchgear, switchgear and distribution boards. 1st edition. Siemens Aktiengesellschaft (Publishing Department ), Berlin / Munich 1982, ISBN 3-8009-1358-5 .
- A. Senner: Electrical engineering. 4th edition. Verlag Europa-Lehrmittel, 1965.
Web links
- Rockwell: Basics for Practical Engine Protection. (PDF), accessed on January 31, 2012