Unit Selection - Electric Motor

SEED Guides Unit Selection - Electric Motor <

4. Motor Selection and Specification

By considering the above factors information relating to the requirements of the motor and the conditions in which it is to operate is established. By comparing this information with the motors available from a range of manufacturers a suitable motor may be found. It must then be specified completely. A selection of manufacturers is given in Appendix 8 and an example of motor specification is shown below.

Manufacturer's Name:	Crompton Parkinson Ltd.
Catalogue reference number:	MD 132 S
Motor Type:	AC Squirrel Cage Induction
Power Rating:	5.5 kW
Duty:	Continuous
Nominal Speed:	2855 rev/min
Enclosure Type:	Totally- enclosed, Fan-cooled
Insulation Class:	E
Starting Equipment:	Star-Delta; one start per hour
Overload Protection:	120% overload before max. current limit

APPENDIX 1: Relationships between Load and Speed

Figure 3 shows typical relationships between load and speed for three different machine types. They are idealised diagrams and real systems will approximate to these characteristics.

APPENDIX 2: Some Common Electric Motor Characteristics

It should be noted that universal motors can be supplied with AC or DC power. Other single phase motors are only self-starting by virtue of an artificially created 'split phase'. The most common methods of achieving this are shown.

3-phase induction motors are the most common in industrial applications of power transmission. Different characteristics can be achieved by varying the design of the windings. The most common variations are shown. Small variations of speed can be achieved by varying supply voltage or rotor resistance.

DC motors may be designed to have constant power, constant torque or a combination of the two.

APPENDIX 3: Service Factors

TYPES OF DRIVEN MACHINE		TYPES OF ELECTRIC MOTOR High Starting Torque Normal Starting Torque e.g.: DC Series wound e.g.: DC Shunt wound OPERATIONAL HOURS PER DAY <10 10-16 >16 <10 10-16 >16
Light Duty	fans; light conveyors;	1.0 1 .1 1.2 1.1 1.2 1.3
Medium Duty	pumps; machine tools;	1 .1 1.2 1.3 1.2 1.3 1.4
Heavy Duty	compressors; mills;	1.2 1.3 1.4 1.4 1.5 1.6
Extra Heavy Duty	crushers; hoists;	1.3 1.4 1.5 1.5 1.6 1.8

For high inertia and reversing applications the above should be multiplied by 1.25.

APPENDIX 5: Efficiencies of Drive Systems

TYPE OF DRIVE EFFICIENCY (%)	TYPE OF DRIVE EFFICIENCY (%)
Vee Belt 95 Flat Belt 96 - 98 Chain 97 Spur Gear (per pair) 96	Worm Gear (5:1 reduction) 96 Worm Gear (40:1 reduction) 86 Hydraulic Coupling 98

APPENDIX 6: Selected Enclosures for Electric Motors

Drip-Proof casings have ventilation openings in the end shield which are screened so that accidental contact with moving parts is impossible. They are protected against the ingress of failing water (e.g.: condensation) and are suitable for use in areas where cooling air is uncontaminated. They are the lightest and least expensive of the options quoted in this Guide.

Totally-enclosed, Fan-cooled casings have all working parts completely enclosed. Cooling air is forced over the casing by a fan mounted on the non-driving end of the shaft. They are suitable for use in contaminated environments which do not contain inflammable substances.

Flameproof casings are similar to TEFC but of heavier construction and are designed to withstand the explosion of a prescribed inflammable gas within it and not to transmit the flame to the same gas in the surrounding atmosphere.

Other enclosures covered by BS 4999 include open frame, screen protected, marine.

APPENDIX 7: Comparative Cost of some Electric Motors

MOTOR TYPE	LOW COST MED COST HIGH COST
Split Phase Capacitor Start Cap Start/Cap Run Permanent Split Cap Shaded Pole Repulsion Start Basic 3-Phase Induction DC with speed control	X X X X X X X X

APPENDIX 8: Selected Manufacturers of Electric Motors

The addresses and telephone numbers of the above and the product ranges from other manufacturers may be found in a good quality engineer's Buyers' Guide.

APPENDIX 9: Selected relevant British Standards

BS 4941 BS 4999	Motor Starters: Direct on-line; reduced voltage; rheostatic. General Requirements: classification and dimensions of casings; cooling methods; methods of operation; limits to temperature rise; Characteristics and performance; Noise; Thermal protection.
BS 5000 BS 5856	Requirements for particular applications; general; special protection; flameproof. Motor Starters for high line voltage.

Further information may be found in the following reference:

Small Motor, Gearmotor and Control Handbook (4th Edition) Bodine Electric Company (1978)

NOTE: Whilst every effort has been made to ensure that the information given is correct and up-to-date, SEED the publishers and authors cannot be held responsible for any errors or omissions that might occur in this Guide. Use of the methods or data on projects for application outside the educational environment should be justified and validated during the course of the designer's normal professional duties.