Friday, November 19, 2010

Thermal overload relays for Motors

A. Thermal Overload Relays
The most common over current protective device is the thermal overload relay associated with motor starting contactors.

In both low-voltage and medium-voltage motor circuits, thermal overload relays detect motor over currents by converting the current to heat via a resistive element. Thermal overload relays are simple, rugged, inexpensive, and provide very effective motor running over current protection. Also, if the motor and overload element are located in the same ambient, the thermal overload relay is responsive to changes in ambient temperature. The relay trip current is reduced in a high ambient and increased in a low ambient. 

The curves level off at about 10 to 20 times full-load current, since an upstream short-circuit device, such as a fuse or circuit breaker, will protect the motor circuit above these magnitudes of current. The thermal overload relay, therefore, combines with the short-circuit device to provide total over current protection (overload and short-circuit) for the motor circuit. The various categories of thermal over current relays are

(1) Melting alloy type overload relays, as the name implies, upon the circuit when heat is sufficient to melt a metallic alloy. These devices may be reset manually after a few minutes is allowed for the motor to cool and the alloy to solidify.

(2) Bimetallic type overload relays open the circuit when heat is sufficient to cause a bimetallic element to bend out of shape, thus parting a set of contacts. Bimetallic relays are normally used on automatic reset, although they can be used either manually or automatically.

(3) Standard, slow, and quick-trip (fast) relays are available. Standard units should be used for motor starting times up to about 7 seconds. Slow units should be used for motor starting times in the 8-12 second range, and fast units should be used on special-purpose motors, such as hermetically sealed and submersible pump motors which have very fast starting times.

(4) Ambient temperature compensated overload relays should be used when the motor is located in a nearly-constant ambient and the thermal overload device is located in a varying ambient.

B. Magnetic Current Overload Relays

Basically, magnetic current relays are solenoids. These relays operate magnetically in response to an over current. When the relay operates, a plunger is pulled upward into the coil until it is stopped by an insulated trip pin which operates a set of contacts. Magnetic relays are unaffected by changes in ambient temperature. Magnetic current relays may be used to protect motors with long starting times or unusual duty cycles, but are not an alternative for thermal relays.

C. Information required for coordination.

The following motor and relay information is required for a coordination study.
(1) Motor full-load ampere rating from the motor nameplate.
(2) Overload relay ampere rating selected in accordance with NFPA 70.
(3) Overload relay time-current characteristic curves.
(4) Motor locked rotor amperes and starting time.
(5) Locked rotor ampere damage time for medium-voltage motors.

Because of their spread between hot and cold characteristics, these relays allow a tripping time of less than the starting time when a hot motor stalls, so a separate stalling protection is normally not necessary. They detect the rms value of the current and thus account for the effects of harmonics, present in the current, drawn by the motor. They also take into account the heating, due to previous running of the motor as they are also heated along with the motor. This feature is known as thermal memory. These relays thus possess tripping characteristics almost matching the thermal withstand capacity of the motor. The operating mechanism of a thermal overload relay is given in Fig 2.

These have three heaters in series with the circuit. One or more bi-metallic strips are mounted above these heaters, which act as latches for the tripping mechanism or to give an alarm signal if desired. The heaters may be heated directly for small motors or through current transformers (CTs) for medium-sized motors. Bending of the bimetallic strips by heating, pushes a common trip bar in the direction of tripping to actuate a micro-switch to trip the relay or contactor. The rate of heating determines the rate of movement and hence the tripping time, and provides an inverse time characteristic. The power consumption of the bimetal heating strips varies from 2 to 2.5 watts/phase, i.e. a total of nearly 7.5 watts.

The latest practice of manufacturers is to introduce a very sensitive differential system in the tripping mechanism to achieve protection even against single-phasing and severe voltage unbalances. In the relays with single-phasing protection a double-slide mechanism is provided. Under single phasing or a severe voltage unbalance, the two slides of the relay undergo a differential deflection. One slide senses the movement of the bi-metal that has deflected to the maximum. while the other senses the minimum. These slides are linked so that the cumulative effect of their movement actuates a micro-switch to trip the relay. Figure 2 illustrates the tripping mechanism of an over current-cum-single-phasing thermal relay. Because of differential movement it possesses dual characteristics, one for an ordinary over current protection during three-phase normal operation and the other with differential movement for over current protection during a single phasing or severe unbalance. For instance, for a setting at the rated current (100% I,) under normal conditions the relay would stay inoperative, while during a single phasing it will actuate in about 200 seconds and provide positive protection against single phasing.

Characteristics of a bi-metallic thermal relay

The thermal characteristics are almost the same as those of an induction motor. This makes them suitable for protecting a motor by making a judicious choice of the right range for the required duty. A typical characteristic is shown in Figure 3.  Ambient temperature compensation is achieved through an additional strip in the overload relay, which operates the tripping lever in the other direction than the main relay to achieve a differential effect and is so arranged that it is independent of the main relay. Operation of the relay may not necessarily start at the preset value due to certain allowable tolerances. As in IEC 60947-4-1, the relay must not trip within two hours at 105% of FLC but it must trip within the next two hours when the current rises to 120% of FLC. Also, it should trip in two hours in the event of single phasing when the line current in the healthy phases is 115%, but it should not trip in less than two hours during a healthy condition, when two of the phases carry 100%, while the third carries 90% of FLC (a case of voltage unbalance). A good thermal relay should be able to detect these operating conditions and provide the required protection. The thermal curve of a relay is thus in the form of a band as shown in Figure 4. With the introduction of single-phasing detection and protection feature in the conventional thermal relays the tripping current-time (I2 versus t ) characteristics of the relay traverse almost the same thermal curve as may be prevailing in the most vulnerable phase of the motor winding during a single phasing. The characteristic curve of the relay is chosen so that it falls just below the motor thermal curve and has an adequate band formation, somewhat similar to the curves of Figure 4.

Relays for heavy duty

Such relays may be required for motors driving heavy-duty loads with large inertias or for motors that employ reduced voltage starting and require longer to accelerate. Consequently, a relay which can allow this prolonged starting period without causing a trip during the start will be desirable. CT-operated relays can be used for such duties. They comprise three saturated current transformers (CTs) associated with the ordinary bi-metal over current relay. These saturated current transformers linearly transform the motor line or phase currents up to a maximum of twice the CT primary current. Above this ratio, the cores of the CTs become saturated and prevent the secondary circuit reflecting the starting current in the primary and thus prevent the relay from tripping during a permissible prolonged start. For example, a CT of 150/5A will have a saturation at approximately 300A, irrespective of the magnitude of the starting current.

Over current setting of relays

These can be adjusted by varying the contact traverse. The mechanism’s design is such that an increase or a difference in the line currents, due to voltage unbalance or single phasing, drives the mechanism towards the tripping lever. These relays operate at 100% of their setting and are therefore set at

Relay setting (% of FLC) = (Operating current %) x Ir /[ CT Ratio x Relay Rating]


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