reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

Introductio n

Types and performance of motors

2 l Cage lnduction motors

2 2 Sill:31ing induction motors

2 3 AC commutator motors

2 4 Variabie-speed AC converter drives

2 5 DC motors

3 Design and construction

3 6 Bearings

37 Terminal boxes

4 Technical requirements

5 Power station auxiliary drives

5.1Boiler feed pumps

5.2Coaland oil-fired boiler units

5.2.1Draught plant

5.2.2Milling plant

5.3Nuclear reactors — AGR

5.3.1Gas circulators

5.4Nuclear reactors — PWR

5.4.1Reactor coolant pumps

5.4.2Safety-related drives

5.5Circulating water pumps

6Testing

7Future trends

8References

9Additional references

9.1ESI Standards

9.2CEGB Standards

9.3British Standards

9.4IEEE Standards

9.5IEC Recommendations

1Introduction

i'tc modern power station requires a wide range of motors ranging from small power motors up

motors as large as 15 MW. There are about 2000

. 'lots in a typical power station. Their total installed

.. T.L..Liv is between 5 070 and 10 07o of the station MW depending on the type of station, e.g., nu-

. c.ir. coalor oil-fired, and on the type and number motor-driven auxiliaries. Whilst most motors in modern power station are of the cage induction •.;, c. others are also used when there is a technical

'r economic need. Examples include variable-speed -prir42 induction motors for boiler feedpump drives,

;)( ,

'uolors for turbine-generator standby lubricating pumps, two-speed cage induction and converter-fed , i .iriable-speed AC motors for drives such as boiler

- -, fiqtht fans, barring and the low speed facility for circulators on advanced gas cooled (AGR) nuclear

FhiS chapter examines typical auxiliary drive require- and the selection of the motors used. It considers

.,e lunctional requirements and the effect of these on moor design, constructional features and technical

performance. Many textbooks and technical papers are available to the reader on detailed motor design, theory, insulation, etc., and such theory is not repeated in this chapter. Some of these aspects are, however, dealt with insofar as they are necessary to describe particular features of the motors, to explain the functional needs and to portray modern practice.

2 Types and performance of motors

There are large numbers of small power (formerly known as fractional horsepower) motors used, e.g., sootblowers, servo motors, instrument drives, etc. These types are not described here. Their technical requirements are specified in CEGB Standard 44011 Electric motors — small power and in BS5000, Part 11, Small power electric motors and generators.

Five types of motor are considered:

•Cage induction motors.

•Slipring induction motors.

•AC commutator motors.

•DC motors.

•Variable-speed AC converter drives.

Other types of motor not dealt with in detail include li near motors which have, for example, been used on cranes and sliding doors, synchronous motors and mo- [ors for glandless pumps of the wet-stator winding or of Me canned typo. Tcelmi,:al requirements of motors for glandless pumps are given in CEGB Standard 620106, Glandless pump/motor units.

2.1 Cage induction motors

Cage induction motors are very reliable, since the rotors are of robust construction and have no sliprings, commutators or brushes. They are relatively low in cost and have high operating efficiency; their simplicity and reliability has led to their extensive use for power station auxiliary drives.

A disadvantage of the cage induction motor is the large starting current, which is about 5 to 7 times normal full-load current. This presents voltage drop problems to the electrical supply system to which it is connected and can create some difficulties in providing adequate electrical protection, e.g., overcurrent and short-circuit protection. Whereas many industrial users have to restrict the power rating of cage induction

motors because of limitations on starting current j _

posed by their electrical power supply system, powerrn stations have auxiliary power systems backed by hj oh

MVA infeeds and even the largest cage inductio'n motors used in these stations can usually be started direct-on-line. High values of starting current also present problems in that the stator windings must be designed to withstand the electromechanical forces produced by the starting current. The windings m ust

also be designed to meet the temperature rise durino starting, which may be considerable, particularly f or

the rotor cage windings if a high number of starts per hour is required or the driven load has high inertia.

Table 7.1 gives maximum values of starting current permitted with CEGB practice. These values compl y with BS4999, Part 41.

The motor torque-speed characteristics must be d e . signed to meet the requirements of the driven l oad under the most arduous conditions of service. Th e torque requirements may in some cases present diffi- culties to the motor designer since the maximum permitted value of starting current (see Table 7.1) affe cts the starting and maximum torque values obtainal from a given design. From a study of the equivalt circuit theory of induction motors (see Alger 1951 [IL 1970 [2], and Say 1983 [3]) it will be apparent th,it the starting torque and current characteristics can be controlled by varying the values of rotor resistance and

2

100 ,JOLTS A

80% VOLTS

2

LOAD TORQUE Ot (SPEED)

05 SPEED p.u.

Fic. 7.2 Torque-speed curves of cage induction motor at reduced voltage

The main application on modern power stations has been for variable-speed drives for starting and standby boiler feed pumps, where its low operating efficiency has not been a disadvantage, due to the relatively low running hours per annum, and the capital cost has been relatively low compared to other variable-speed drives. However, the tendency is now to use direct-on-line cage induction motors driving through variable-speed hydraulic couplings (see Section 5.1 of this chapter).

The starting current is much lower than for cage induction motors, a typical value being 120 07o fullload current with 100 07o starting torque. The combined starter and speed controller for these large feed pump motors is of the liquid type. The slip energy to be dissipated in the controller at reduced speed is considerable and the liquid controller is therefore invariably water-cooled in order to dissipate this energy. Methods for recovering this slip energy are available, such as the Kramer scheme where the low frequency slip power from the rotor is converted by an inverter to 50 Hz and recovered by feeding it back to the power supply. However, this scheme is generally only economic for drives with high load factors because it has the disadvantage of a high wear rate on brushes and sliprings which results in a heavy maintenance burden. This scheme has not been used much by the CEGB, although it has found favour with several Western European supply companies. One or two examples do however exist in CEGB, including the boiler feed pumps at West Thurrock power station.

2.3 AC commutator motors

The most commonly used variable-speed AC commutator motor has been the stator-fed type, in which both the stator and armature are insulated windings, the armature winding being brought out to a commutator.

A typical diagram of connections for a stator-fed shunt-connected motor (which is more often used than the series connection), is shown in Fig 7.3.

The action of the commutator is to convert the slip frequency generated in the armature windings back to supply frequency (i.e., normally 50 Hz) across the brushes. The brushes are connected to the main pow er supply through an induction regulator which provides the link between the 'variable 'voltage at the commu_ tator brushes and the constant %..oltage supply. This regulator is then used to inject a voltage into the armature winding via the brushes and causes the speed to be varied in relation to the injected voltage. A t speeds below synchronous, power is drawn from zhe armature and returned to the supply; at super-synchro- nous speeds power is drawn from the supply and fed into the armature. Further infomation on commutator motors and induction regulators is given by Adkins and Gibbs, 1951 [4j.

One of the main applications has in the past been on exhauster fans associated with pulveriser mills on boiler plant, where the variable-speed feature has been used to control the boiler fuel/air flow. The commutator and brushes associated with this type are an obvious disadvantage representing a considerable maintenance task.

2.4 Variable-speed AC converter drives

With an increased emphasis now being placed on energy saving methods, coupled with the need to reduce overall costs, increasing attention has been given to

INDUCTION

REGULATOR

FIG. 7.3 Stator-fed AC variable-speed commutator motor