Protection Against Single-Phase Operation of Three-Phase Asynchronous Electric Motors

A. I. ZAITSEV and Ya. V. PETROV

(Presented by Prof. I. A. Belantsev, Doctor of Technical Sciences)

For a number of mining installations, prolonged operation is characteristic. These include, for example, ventilating, pumping, and compressor installations, stationary conveyor lines, and certain others. The absence of operating personnel in the immediate vicinity of the indicated installations when their operation is automated, or strong production noise when personnel are present, leads to the fact that a break in a phase circuit of an electric motor may remain undetected for a long time, as a result of which the electric motor fails. As an example one may cite frequent cases of accidents for this reason involving the electric motors of partial-ventilation fans of the “Prokhodka 500-2” type: according to data of TEMZ, which carried out an investigation of the operation of these fans, 70% of the total number of motor failures are attributable to breaks in the stator phase circuit.

Prolonged operation of electric motors with one phase interrupted cannot be tolerated; therefore it becomes necessary to disconnect the motor in the event of a break in a phase circuit. At present there is no unified opinion on the question of protection against single-phase operation of three-phase asynchronous electric motors, as is evidenced by numerous statements in the press [1—4]. The protection scheme recommended by Giprouglemash for certain mining installations, using three minimum-current relays, each of which has a separate normally open contact in the control circuit of the magnetic starter [4], is apparently not the best.

In view of the above, a comparative evaluation of known, as well as other possible, principles of protection against single-phase operation of three-phase asynchronous electric motors is of unquestionable interest.

When the circuit of one stator phase is broken, a three-phase asynchronous electric motor changes over to single-phase operation, for which the following is characteristic. The initial starting torque (at \(s = 1.0\)) is equal to zero, as a result of which the motor cannot accelerate by itself. At rated load the slip increases appreciably, which leads to an increase in the frequency of the positive-sequence current induced in the rotor windings, according to the expression

\[ f_{p_1} = fs, \]

where \(f\) is the frequency of the supply current, and \(s\) is the slip.

With a ratio of maximum torque of 2—2.5, characteristic of mining electric motors, the slip increases by a factor of 1.9—1.2 [5].

In addition, a negative-sequence current is induced in the rotor, approximately equal in magnitude to the positive-sequence current and having the frequency

\[ f_{p_2} = f(2 - s). \]

Under the same conditions, the stator current increases by approximately a factor of 1.8–2.0. The rotor current increases in the same proportion. The consequence of a considerable increase in current is intense heating of the electric motor, aggravated in motors with self-ventilation by deterioration of cooling. Finally, when the circuit of a phase is broken, asymmetry arises in the currents and voltages applied to the phases of the electric motor; in the general case, the asymmetry of the currents is more significant than the asymmetry of the voltages. For example, with star connection of the motor windings and a break in a supply conductor, an asymmetric system of currents is produced with an asymmetry coefficient equal to 1 under all operating conditions of the motor, whereas the line voltages of the motor form an asymmetric system whose asymmetry coefficient is only slightly greater than zero at no load of the motor and is equal to unity only when the motor is stopped \((s = 1)\). At normal loads the latter coefficient is appreciably less than 1 [6].

In principle, protection against single-phase operation can be provided by using the following phenomena:

a) increase in slip (the frequency of current in the rotor);
b) increase in the current drawn from the network;
c) increase in heat losses in the electric motor;
d) disappearance of current in one of the line conductors;
e) voltage asymmetry;
f) current asymmetry.

Effectively operating protection against single-phase operation must satisfy the following requirements:

1) provide selective protection of the electric motor and its supply cable when a phase is broken at any point of the working circuit;

2) operate under different motor loads and winding-connection schemes;

3) ensure a sufficiently short operating time (not more than several minutes);

4) produce the minimum number of false operations;

5) be simple in design and maintenance, have a minimum number of moving parts, small dimensions and weight, and standardized elements for motors of different rated powers.

If the specified requirements are adopted as criteria in a comparative assessment of the quality of various principles of protection against single-phase operation, then the practical expediency of one or another protection principle can be established.

Protection Based on Changes in Slip

(Frequency of Current in the Rotor)

This type of protection can be implemented on the same fundamental principles as the control of an asynchronous or synchronous electric motor as a function of the frequency of current in the rotor during starting. The implementation of protection responding to the frequency of the negative-sequence current is facilitated by the fact that the latter differs substantially from the frequency of the positive-sequence current when the electric motor is operating. With an appropriate design of a frequency relay connected into the rotor circuit, it appears possible to obtain the simplest and most reliable protection against single-phase operation. However, since in the coal industry electric motors with squirrel-cage rotors are predominantly used, and in the future the field of ...

their application will expand, it should be considered that this protection principle has no practical significance.

Maximum-Current Protection

In view of the phenomenon of a considerable increase in the current in the wires feeding the electric motor when the circuit of one phase is opened, in a number of cases the protection under consideration could be implemented by means of fuses or maximum-current relays.

Indeed, if the fuse link is selected according to the rated current of the motor, using time-lag fuses or shunting ordinary fuses during starting, then, when a phase circuit is opened and the electric motor is under rated load or a load greater than rated, the increased current may melt the link within a sufficiently short time; thus, with a link-current multiple equal to 2, the melting time of the link is about 2 min. However, already at a current multiple of 1.5 the melting time increases sharply (to tens of minutes). Taking into account that, at motor loads below rated, the melting time of the link increases impermissibly, as well as the other well-known shortcomings of protection by means of fuses, we must acknowledge that protection against single-phase operation by means of fuses is not reliable and cannot be recommended for practical application.

A maximum-current relay provided with a time-delay mechanism or shunted during motor starting is capable of producing more effective protection over a wider range of loads of a three-phase electric motor, since in this case the relay setting may exceed the rated current of the electric motor, according to the PTEU, by only 1.2 times. The same relay can also protect the electric motor, with a time delay, against prolonged overload in three-phase operation and instantaneously against short circuits. Structurally, relays for low-voltage motors may be made in the form of maximum relays of A-2000N automatic air circuit breakers. For high-voltage motors the indicated protections may be implemented by means, for example, of IT-82 relays according to the circuit shown in Fig. 1. Similar protection can be obtained if the protection is shunted during starting, as is done in RVNO-6 boxes.

Fig. 1

One of the disadvantages of protection by means of a maximum-current relay is the possible failure of its operation at loads below rated, when

a current will flow through the relay that does not reach the relay operating current. However, the heating of the motor in this case will not exceed permissible limits, since the main losses in the motor—in the stator and rotor windings—will be produced only by two phases when the stator windings are connected in star, or by three phases when the stator windings are connected in delta; but in the latter case the current in each phase is less than the rated current. Thus, such a single-phase condition will not be dangerous for the integrity of the insulation of the damaged electric motor; nevertheless, the latter must be disconnected from the network immediately, since the connection of a large single-phase load to a three-phase network will cause a noticeable unbalance of the network voltages, which will lead to rapid overheating of the insulation of the windings of other electric motors.

This shortcoming of the protection applies to a lesser extent to high-voltage electric motors, which, as a rule, are sufficiently loaded, are supplied from a high-power network, and whose protections are maintained in better condition; moreover, the probability of an open circuit in a phase circuit is small here. Therefore, special protection against single-phase operation should not be used for high-voltage electric motors, since such protection is fully provided by overload current protection.

With regard to low-voltage motors, the indicated shortcoming of maximum-current protection is of greater significance, since an open circuit in a phase circuit during operation of an underloaded motor is more probable here; the motors are supplied from transformers of small capacity, and their protections are usually maintained in poorer condition. In addition, it must be borne in mind that the existing mine control apparatus for low-voltage motors in most cases does not have maximum-current relays or, if it does have them, then without a time-delay mechanism. The installation of relays with a time-delay mechanism, or only the latter, into existing control apparatus appears impossible; and the certain complexity of the design of the time-delay mechanism and its expected low reliability under mine conditions make its use generally problematic.

From the foregoing it follows that protection against single-phase operation by means of a maximum-current relay under mine conditions does not provide complete reliability, is sufficiently complicated, and cannot be recommended for low-voltage electric motors.

Thermal (temperature) protection

Protection by means of a properly selected and adjusted thermal relay, with appropriate maintenance in service, satisfies almost all the requirements for protection against single-phase operation set forth above. The scatter of the protective characteristics of the relay, caused by fluctuations in ambient temperature and by the mismatch of the thermal conditions of the relay and the electric motor, apparently has no substantial significance in the present case, since continuous operation of the electric motor is being considered. This shortcoming of thermal relays can, moreover, be reduced if the relay is built under the motor casing (temperature relay). It is only necessary to note that in the latter case the number of conductors in the cable increases by 1–2, and operation of the relay and repair of the electric motor become more complicated.

However, a thermal (temperature) relay, like a maximum-current relay, does not provide protection against single-phase operation at loads constituting a fraction of the rated load, which, as indicated above, leads to overheating of other electric motors.

Thus, a thermal relay, as reliable protection against single-phase operation, can be recommended for electric motors that are continuously and well loaded during operation, provided that careful maintenance of the relay is ensured.

relay. For electric motors capable of operating for a long time underload (on the order of 30–50 percent or more), protection against single-phase operation by thermal relays cannot be considered effective.

Minimum-Current Protection

Such protection is carried out with the aid of three separate minimum-current relays, the coils of which are connected in series in the operating-current circuit either directly or through current transformers. The protection must be set above the no-load current of the electric motor and operates, when the current in a phase circuit disappears, to open the control circuit of the magnetic starter. An example of such a circuit is the Giproglemash circuit [4].

It appears possible to design three relays with one common contact and to use the same relays for a wide range of electric-motor powers, employing generally known methods of changing relay settings. In this form the protection satisfies practically all the requirements indicated above, with the exception, in part, of items 5 and 2, since the protective device has relatively large overall dimensions and weight, many moving parts, and does not operate when there is a break in the winding of a motor connected in delta, when currents flow through the coils of all the relays. At present, in most mine electric motors the stator windings are connected in star; however, with the future operation of electric motors suitable for voltages of 660/380 V, the significance of the latter shortcoming of the protection may increase.

Minimum-current protection, in comparison with the preceding types of protection, is the most reliable; therefore it should be recommended for practical application where this is required. In view of the obvious impossibility of building relays into the enclosures of standard magnetic starters, it is necessary to study variants of placing the protective device in the body of the working machine or in a separate explosion-proof enclosure. It is, however, inexpedient to equip every magnetic starter with such protection during its manufacture.

Protection Based on the Use of Negative-Sequence Filters

When a phase is interrupted, the three-phase system is transformed into a single-phase system, which contains symmetrical components of positive and negative sequence. The zero-sequence component is absent, since there is no neutral conductor.

An asynchronous motor, operating in single-phase mode, becomes a phase converter, and the positive-sequence voltage in the interrupted phase at no load reaches almost its rated value, while at full load it decreases to 0.8–0.9 of rated. With the resulting slight voltage unbalance, the negative-sequence voltage component will also vary within small limits and will depend on the motor load. Therefore it is inexpedient to construct protection against single-phase operation on the basis of variation of the negative-sequence voltage component, since very sensitive relays would be required and the protection would not operate at small loads or when the break occurs beyond the protective device. If sensitive negative-sequence voltage relays are used, false operation due to the existing unbalance under normal operating conditions is also possible.

The situation is different with the symmetrical component of the negative-sequence current. The unbalance factor, equal to the ratio of the symmetrical component of the negative-sequence current to the symmetrical component of the positive sequence, in all operating modes

is equal to unity. This means that both components are always equal to one another. Making use of this circumstance, it is possible to construct a protection system that will respond to a phase break at any point. For this purpose the current relays must be connected through negative-sequence filters, widely used in relay-protection engineering [7].

Figure 2 shows a schematic diagram of electric-motor control with protection against single-phase operation when one of the known negative-sequence current filters is used; in principle, a large number of varieties of such protection can be made. The filter parameters are selected in such a way that only the negative-sequence current, arising when a phase is broken, flows through the protection relay \(P\). In this case the normally closed contact \(P\) opens the control circuit and the motor is disconnected.

In order that only the negative-sequence current flow through the relay, it is necessary to compensate the positive-sequence current. For this purpose the windings of the current transformers are connected according to the circuit as shown in Fig. 2, while the resistances \(Z_a\) and \(Z_c\) rotate the positive-sequence current vectors of phases \(a\) and \(c\) by \(60^\circ\). As a result, these current vectors are subtracted, and the points \(e f\) in the circuit become equipotential with respect to the positive-sequence current, and no current will flow through the relay winding. Thus, a change in the positive-sequence current under any operating condition will not cause the relay to operate.

With respect to the negative-sequence system, the potentials of points \(e\) and \(f\) are not equal to one another, and therefore, when a negative-sequence current appears, the relay operates, and operation of the motor in single-phase mode is terminated.

Let us determine the parameters of the negative-sequence filter possessing the above-described properties. For the circuit of Fig. 2, the current flowing through the relay is

\[ I_p=-\frac{Z_c}{Z_a+Z_c+Z_p}I_c-\frac{Z_a}{Z_a+Z_c+Z_p}I_a . \]

The symmetrical components of the current in the phases are

\[ \begin{aligned} I_a&=I_{a1}+I_{a2},\\ I_b&=a^2 I_{a1}+a I_{a2},\\ I_c&=a I_{a1}+a^2 I_{a2}. \end{aligned} \]

Substituting the current values into the preceding equations,

\[ I_p=-\frac{Z_c(aI_{a1}+a^2I_{a2})+Z_a(I_{a1}+I_{a2})}{Z_a+Z_c+Z_p}. \]

Transforming the equation obtained,

\[ I_p=-\frac{I_{a1}(aZ_c+Z_a)+I_{a2}(a^2Z_c+Z_a)}{Z_a+Z_c+Z_p}. \]

The positive-sequence current will not flow through the relay if the coefficient of \(I_{a1}\) is equal to zero. Then

\[ Z_c=-a^2Z_a=Z_a e^{j60}. \]

In the simplest case, taking the resistance \(Z_a\) to be purely active, we obtain that the resistance \(Z_c\) must include active and inductive components, the vector of the resulting resistance being turned by \(60^\circ\) with respect to the vector \(Z_a\):

\[ Z_c=R_a e^{j60}. \]

Under the specified conditions, the current flowing through the relay will be proportional to the negative-sequence current,

\[ I_a=\frac{Z_a+a^2 Z_c}{Z_a+Z_c+Z_p}\, I_{a2}. \]

Fig. 2

Visible diagram labels: \(a, b, c\); \(Z_a, R_a\); \(Z_c\); \(P\); KL; x; st; \(P\); KL; AD.

Protection using negative-sequence current filters is distinguished by reliable operation, simplicity, a minimal number of relays and contacts, and insensitivity to changes in the magnitude of the three-phase symmetrical current. The short-time flow of current through the relay makes it possible to reduce its dimensions considerably, and also to use one and the same relay for a wide range of electric-motor powers, provided that the parameters of the remaining elements of the protective device are appropriately selected (current transformers, resistances \(Z_c\) and \(R_a\)). The relay operating current must be somewhat greater than the unbalance current under normal three-phase operation, when a certain asymmetry of the voltage system is permitted.

Conclusions

1. Current protection based on the use of negative-sequence currents should be regarded as a reliable means of protecting asynchronous motors against single-phase operation.

2. Protection by means of three minimum-current relays, developed by Giprooglemash, operates reliably, except in the case of an open circuit in a stator winding connected in delta.

3. Low-voltage installations operating in continuous duty should be equipped with protection against single-phase operation. In repeatedly short-time duty, the need for such protection disappears.

Literature

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Noted Misprints