Model of a Synchronous Generator with a Smooth Armature
Abstract Generated abstract
This paper describes the design and experimental investigation of a model synchronous generator whose armature winding is placed in the air gap rather than in stator slots. It outlines the expected advantages of a smooth armature, including reduced stator steel and insulation, lower tooth-pulsation losses and noise, and reduced transient reactance, while noting associated problems such as added losses from the main magnetic flux and increased short-circuit currents. The authors present the construction of a three-phase, four-pole model generator with a single-layer concentric winding, a salient-pole rotor, damper winding, and controlled excitation, and report no-load, short-circuit, and sudden short-circuit test results. The measured voltage and current behavior, including nearly sinusoidal line emf and absence of tooth pulsations, is reported to agree well with calculation.
Full Text
MODEL OF A SYNCHRONOUS GENERATOR WITH A SMOOTH ARMATURE
G. A. Sipailov, K. A. Khor'kov, V. F. Kulakov,
V. Z. Khor'kova, V. I. Andreev, V. S. Baklin
(Presented by the scientific seminar of the Departments of Electrical Machines and Apparatus and General Electrical Engineering)
The use in synchronous generators of an armature winding placed wholly or partly in the air gap promises a number of advantages. The elimination of teeth and the compact filling of the active zone of the stator with copper lead to an approximately twofold reduction in the weight of stator steel and insulation. Losses and noise associated with tooth pulsations disappear. The required interpole gap ensures short-circuit protection and stable operation in the system. Transient and subtransient inductances decrease owing to the reduction of the leakage inductance of the stator [1, 2].
A rational type of three-phase winding is a single-layer concentric winding with a 60-degree phase zone. Here no additional interphase insulation is required, nor are powerful wedge fastenings needed to restrain radial electrodynamic forces. The end connections are small and are made in two layers.
There are, however, difficulties that require resolution. If in a toothed armature the winding was shielded by the tooth steel from the main magnetic flux, then in a winding on a smooth armature additional electrical losses appear, proportional to the main flux and not to the leakage flux. The elimination of tooth steel substantially reduces the leakage inductance of the active part of the armature winding and causes an increase in short-circuit currents.
Of considerable interest is the use of a generator with a smooth armature as a pulsed (impact) generator. The low subtransient reactance and the possibility of forcing the excitation field create the prerequisites for efficient conversion of the kinetic energy of the rotor into powerful current pulses.
To investigate the parameters, losses, electrodynamic forces, and a number of other questions connected with the design of synchronous generators with a smooth armature, an experimental installation was created, consisting of a generator model, a load, and a control unit.
Brief Description of the Model Generator with a Smooth Armature
Figure 1 shows the wound stator of a model synchronous generator with a winding placed in the air gap of the machine. The stator core is assembled from sheets of E-21 electrical steel 0.5 mm thick. The outer diameter and the length of the active part of the core are determined
Fig. 1. Model stator
Fig. 2. Model rotor
with the dimensions of the frame of an AR-7 motor, within whose overall dimensions the model was made: diameter \(D_i = 368\) mm, length \(l_i = 273\) mm. The inner diameter of the stator core is \(D = 242.7\) mm. The stator core is assembled without radial ventilation ducts. Cotton-paper insulation, impregnated with ED-5 epoxy resin, is laid in the stator bore. A three-phase, four-pole, single-layer winding with subdivided end portions is placed in the stator bore. The number of turns per phase is 10. The dimensions of the copper strip of the winding are \(2.63 \times 10\) mm. The end portions of the winding are bent at an angle of 90° to the axis of the ma-
bars in two planes. From the end surface of the stator, the end portions are shielded by copper disks with a total thickness of 12 mm on each side. On the outside, the end portions are also enclosed in copper disks which, in addition to serving to fasten the end portions, also shield the end-leakage fluxes toward the bearing shields.
The calculated air gap of the machine, i.e., the gap between the ferromagnetic surfaces of the stator and rotor, is 7 mm. The one-sided air gap between the surface of the insulated copper of the stator winding and the rotor is 2 mm.
The rotor of the generator model is made with four salient poles. The rotor stack is laminated from sheets of E-21 electrical steel 0.5 mm thick. Slots for the rods of the damper winding are punched in the pole shoes; the diameter of the brass rods is 6 mm, and the number of rods per pole is eight. The rods are short-circuited by copper segments. The field winding is wound with round wire 1.81 mm in diameter with PEV insulation. The outside diameter of the rotor is \(D_p = 228.7\) mm. A general view of the wound rotor is shown in Fig. 2.
The generator rotor is driven by an asynchronous motor of 7 kW power. The generator field winding is supplied from a rectifier unit with adjustable field voltage.
Some test results
Since the salient-pole rotor is made with a uniform gap under the poles, the field pattern in the machine gap is trapezoidal. The phase emf contains higher harmonics, of which the third harmonic has the greatest value. The line emfs are practically sinusoidal. The current and voltage curves have no tooth pulsations. Data on the no-load characteristics and on single-, two-, and three-phase short circuits of the stator winding of the generator model are presented in Table 1. The rated line voltage of the generator is 65 V, the rated current 92 A, and the apparent power 10.5 kW. The principal parameters of the generator model, found experimentally according to the generally accepted procedure [3], are given in Table 2.
Table 1
| \(i_{\mathrm{в}}\) (A) | 0 | 2 | 3 | 5 | 10 | 15 | 20 |
|---|---|---|---|---|---|---|---|
| \(U_{\mathrm{ф}}\) (V) | 0 | 8 | 15 | 24 | 37 | 41 | 43.5 |
| \(I_{к1}\) (A) | 0 | 260 | 400 | ||||
| \(I_{к2}\) (A) | 0 | 160 | 240 | 400 | |||
| \(I_{к3}\) (A) | 0 | 120 | 180 | 300 |
Table 2
| \(r\) | \(x_s\) | \(x_q\) | \(x_d''\) | \(x_q''\) | \(T_0\) | \(T_{г0}\) |
|---|---|---|---|---|---|---|
| % | % | % | % | % | sec | sec |
| 2.0 | 12.5 | 5.16 | 4.12 | 3.75 | 0.38 | 0.06 |
In investigating the sudden short-circuit condition, the stator winding was short-circuited by means of a thyristor. The moment of applica-
... the signal to fire the thyristor is coordinated with the position of the generator rotor by means of a photoelectric rotor-position sensor. Figure 3 presents oscillograms of the emf and current of a sudden short circuit of the generator. The short circuit was made at the moment when the generator emf passed through zero.
Fig. 3. Voltage and current curves of a generator short circuit
Experimental studies of the model generator with a smooth armature showed good agreement with the calculations performed.
REFERENCES
- Large generators with air gap windings. Technical survey, Electrical Times, 14 March, 1968.
- Towards better machines. Technical survey, Electrical Times, 7 March, 1968.
- G. K. Zherve. Industrial Tests of Electrical Machines. GEI, 1959.