TECHNICAL AND ECONOMIC ASSESSMENT OF AN ELECTRIC MACHINE AMPLIFIER WITH A SMOOTH ARMATURE (EMUG-12P)

A. S. BATURIN, A. I. SKOROSPELKIN, G. G. KONSTANTINOV

(Recommended by the seminar of the departments of electrical machines and general electrical engineering)

The shortcomings of commutator electric-machine amplifiers, like those of all commutator machines, are due chiefly to unsatisfactory commutation. Sparking impairs the operation of the brushes and commutator and, ultimately, reduces the reliability of the amplifier as a whole.

Studies of the reliability of EMU-5P electric-machine amplifiers [2] showed that the main element reducing the reliability of the unit is brush wear. It was established that the motor and generator brushes wear unevenly over the same period of time. Table 1 gives data on the wear of the motor and generator brushes.

Table 1

Average brush wear of the EMU-12P electric-machine amplifier

From Table 1 it is evident that the wear of the amplifier generator brushes is more than three times greater than that of the motor.

Table 2 gives the upper and lower limits of brush reliability as a function of time.

The data of Table 2 confirm the fact that the reliability of the generator brushes is considerably lower than the reliability of the motor brushes. Consequently, the weakest element in the electric-machine amplifier is the generator brushes. They reduce the reliability of the unit as a whole. The nonlinear change in the resistance of the brush contact and sparking are the cause of instability of the output voltage and a source of radio interference, which adversely affects the quality of operation of the automatic control system and electronic apparatus.

In this connection, there arose a practical need to develop such a design of a commutator electric-machine amplifier in which sparkless commutation would be achieved. Work on the creation of such amplifiers is being conducted at the Department of Electrical Machines of the Tomsk Polytechnic Institute.

Upper and lower reliability limits of EMU-5P electric drives

Table 2

One of the real ways of improving commutation is to reduce the reactive emf, which creates an additional current in the section being commutated. Proceeding from this, a design was proposed for a cross-field EMU with a smooth armature (EMUG), making it possible to reduce the inductance of the commutated sections and, consequently, the magnitude of the reactive emf by a factor of 5–10 [5]. A very important advantage of the new design is the identity of the commutation conditions for all sections of the armature, as well as the absence of pulsations of the magnetic flux in the commutation zone.

In the amplifier with a smooth armature, the cooling conditions are significantly improved, which made it possible to increase the current density from 3 A/mm² to 21.3 A/mm².

Experimental verification of the new design under laboratory conditions at rated load showed sparkless operation of the generator commutator-brush assembly. The main objective was achieved.

However, in the new design the nonmagnetic gap between the armature and the stator is increased, which reduces the amplification factor of the unit. This shortcoming is to a significant extent compensated by the introduction of a longitudinal magnetizing winding.

For a technical and economic assessment of the new amplifier, it is necessary to carry out a comparison. The EMU-12P amplifier is taken as the baseline model for comparison, since the amplifier with a smooth armature (EMUG-12P) was developed within the same overall dimensions, has a direct-current drive motor, and has the same functional purpose. The technical and economic indicators for the designs being compared are given in Table 3.

Table 3

Technical and economic indicators of EMU-12P and EMUG-12P
(electric-machine amplifier with a smooth armature)

B. Economic indicators

I. In manufacture

II. In operation

Comparing the technical indicators of the amplifiers (Table 3), we find that the new model (EMUG-12P) has a lower power (1000 W < 3250 W), amplification factor (1340 < 3250), and lower efficiency (54 < 59). Owing to the absence of slots on the armature, the new amplifier has a somewhat greater steel weight (71 kg > 69 kg).

At the same time, the new amplifier has very important qualitative advantages: sparkless commutation and better speed of response (0.150 sec < 0.266 sec). These indicators also determine the progressive nature of the new design.

The economic indicators in manufacture (Table 3, section B. I) of the EMUG-12P are better than those of the EMU-12P. Table 4 gives numerical data on the cost of both amplifiers.

Table 4

Manufacturing cost of the EMU-12P and EMUG-12P
(electric-machine amplifier with a smooth armature)

The manufacturing cost (Table 4) of the new amplifier is reduced by 5.22 rubles due to the reduction in material costs (a smaller amount of armature winding copper, as well as the absence of armature slot insulation). This changed the relative distribution of cost items in the manufacturing cost of the EMUG-12P. One of the advantages of the new amplifier is the reduction in winding-copper consumption (2.46 kg per amplifier).

The economic indicators of the EMUG-12P in operation (Table 3, section B. II) have identical values. The exception is the lower cost of electrical-energy losses, due to the lower power of the EMUG-12P.

Since both amplifiers have identical quantitative and qualitative indicators, for comparison they must be reduced to a comparable form [4]. The power of the EMUG-12P must be conventionally brought up to 1.3 kW. In this case the manufacturing cost, the cost of electrical-energy losses, and depreciation expenses must increase.

On the basis of prices for electric-machine amplifiers with a direct-current motor [6], we established the following relationship:

\[ C_p = 214 \cdot p^{0.133} \tag{1} \]

where \(C_p\) is the calculated price of the amplifier, rubles;
214 is the constant coefficient of the relationship;
\(p\) is the amplifier power, in kW;
0.133 is the exponent.

Taking into account the relationship found, the capital expenditures for the amplifiers (\(K\)) will have the following values:

EMU-12P — \(K_1 = 265.72 \cdot 1.15 = 306\) rubles.

EMUG-12P — \(K_2 = 260.37 \cdot 1.15 \cdot 1.3^{0.133} = 312\) rubles, where 1.15 is the coefficient for converting cost into capital expenditures (enterprise profit, transportation costs, and installation costs at the consumer’s site).

The operating costs for the amplifiers \((C)\), according to the data of Table 3, with allowance for the correction, will be:

EMU-12P — \(C_1 = 119 + 13.18 + 30.80 = 162.98\) rubles/year;

EMUG-12P — \(C_2 = 119 + 12.64 \cdot 1.3 + 30.30 \cdot 1.3^{0.133} = 166.9\) rubles/year.

The obtained indicators are summarized in Table 5.

Table 5

Capital investments \((K)\) and operating costs \((C)\), reduced to a comparable form

From the latter (Table 5) it follows that an electric-machine amplifier with a smooth armature (EMUG), in comparison with conventional EMUs, has an insignificant increase in capital costs and operating costs (2–2.45).

The loss to the consumer from operating one EMUG-12P will be:

\[ U = (C_2^1 + E_n^1 \cdot K_2^1) - C_1^1 + E_n^1 \cdot K_1^1 = \]

\[ = (166.90 + 0.15 \cdot 312) - (162.98 + 0.15 \cdot 306) = 4.52 \ \text{rubles/year}, \]

where \(U\) is the amount of loss to the consumer, rubles/year;

\(C_2^1\) — operating costs for EMUG-12P, rubles/year;

\(C_1^1\) — the same for EMU-12P, rubles/year;

\(E_n^1\) — normative coefficient of comparative economic efficiency, \(1/\text{year}\). \(E_n^1 = 0.15\) [4];

\(K_2^1\) — capital costs for EMUG-12P, rubles;

\(K_1^1\) — the same for EMU-12P, rubles.

Thus, when operating the EMUG-12P in comparison with the EMU-12P, the consumer will incur a loss of 4.62 rubles/year per amplifier. However, it should be borne in mind that the new amplifier model has sparkless commutation, increased reliability, and better response speed, which was not reflected in the calculations. In a number of cases these indicators are the most important for particular operating conditions.

Conclusions

1. An electric-machine amplifier with a smooth armature has sparkless commutation and high response speed. Good commutation conditions make it possible to reduce wear of the brushes and commutator [1] and increase the reliability of the amplifier as a whole.

2. The advantage of the EMUG is a reduction in the consumption of winding copper due to the higher current density in the generator armature winding.

3. The capital investments for the EMUG, reduced to comparability, are 2% higher, and the operating costs are 2.45% higher than for conventional EMUs. However, this does not take into account the reduction in operating costs of the EMUG due to lower wear of the brushes and increased reliability.

4. The developed amplifier with a smooth armature is not the optimal variant. Further studies should identify the optimal design and determine the reduction in brush wear due to sparkless commutation.

References

1. R. Holm. Electrical Contacts. Foreign Literature Publishing House, Moscow, 1961.

2. Study of the reliability of electrical machines. Report No. 4, 1964. Department of Electrical Machines, Tomsk Polytechnic Institute.

3. Unified system of planned-preventive repair and rational operation of technological equipment at machine-building enterprises. Moscow, 1964.

4. Standard methodology for determining the economic efficiency of capital investments. Moscow, 1966.

5. A. I. Skorospelkin, G. G. Konstantinov. Some results of a study of the characteristics and commutation of a transverse-field EMU with a smooth armature. Izvestiya TPI, vol. 160, 1966.

6. Price List No. 15-01. Wholesale prices for low- and medium-power electric machines. Moscow, 1963.