Introduction To Electrical Engineering II - Motor Terminology PDF

# APPENDIX 4 ## Motor Terminology ### Fig. A3.2 Magnetising setup Can be generated if the magnetising coil is wound directly on the motor or the magnet to be magnetised. Wind as much as possible on to the workpiece with the remaining wire left as a single loop completing the connection to the rect...

# APPENDIX 4 ## Motor Terminology ### Fig. A3.2 Magnetising setup Can be generated if the magnetising coil is wound directly on the motor or the magnet to be magnetised. Wind as much as possible on to the workpiece with the remaining wire left as a single loop completing the connection to the rectifiers. To complete the magnetic circuit, a U-shaped iron circuit should enclose one side of the coil and finish at least reasonably close to the outside casing of the motor. Any convenient lumps of scrap mild steel can be used but, for small motors, the simplest scheme is to lightly grip the coil and motor assembly in a small machine vice. Figure A3.2 shows a typical set-up. With small motors the field strength is strong enough to both saturate and penetrate the outer soft iron casing to re-magnetise the field magnets contained within the casing. Larger motors may have magnets fitted within heavy cross-section cast or welded soft iron return paths. In this case locate the magnetising winding in two sections, one round each of the field magnets. An alternative technique is to remove the armature and replace it with a slug of mild steel which carries the magnetising winding. The normal domestic 30A ring main is designed to be capable of blowing the 13A fuses fitted to standard 13A plugs but only as occasional infrequent events arising from faulty user equipment. The peak current from such events will cause the ring main voltage to instantaneously drop well below normal voltage limits and this may interrupt the program operating on any computer connected to the same ring main. It is also beyond the rating of many 13A wall switches and may damage switch contacts. For these reasons you should only resort to this technique on those very rare occasions when it is necessary to re-magnetise a motor. Frequent fuse blowing activities are likely to be very unpopular with local computer operators and also, probably, the electricity supply company! Also, use only the standard ceramic cased 13A cartridge fuses that are designed to fail safely and reliably on domestic 240V supplies. Although in the main part of the book every attempt is made to explain things in everyday language, it is impossible to completely avoid specialised terms and jargon. The following list of admittedly rough definitions is provided to help sort out some of the more commonly used terms. * **AC** Alternating Current, current that continuously varies smoothly back and forth from positive to negative values. In domestic AC supplies, this occurs 50 times per second, this frequency is normally called 50c/s or 50Hz. * **240V AC** Used in this book to refer to the normal UK domestic AC supply. Most of Europe uses a 220V nominal supply. In the future, both areas intend to standardise on a rather wide tolerance 230V ± 10 % nominal supply. This range is wide enough to cover the total normal voltage variations of both 240V nominal and 220V nominal supplies. Each area happily continues to distribute its original supply voltage range but is now entitled to call it 230V nominal whenever this is politically more acceptable! * **Armature** The rotating part of a commutator motor or dynamo. * **Autotransformer** A transformer in which the secondary winding is either joined to, or forms part of, the primary winding - see transformer. * **Back EMF** When power is applied to a motor and it rotates, it also acts as a generator and produces within it a voltage that opposes the applied voltage. This voltage is called the back EMF. * **Bridge rectifier** An interconnected assembly of four rectifier diodes. Two terminations are the AC input, the other two are the DC output. * **Brush** A fixed carbon or metal conductor which is spring loaded to bear on, and to make electrical contact with the commutator of a motor or dynamo, or the sliprings of a generator. * **Contactor** A heavy-duty relay (see relay) used for controlling high power circuits. * **Closed** A pair of contacts or a switch in the connected ‘on’ position. * **Capacitor** A device consisting of conducting surfaces separated by an insulator which can store electrical energy see Chapter 8. * **Commutator** A rotating multi-way switch which controls the direction of current flow in the armature of a DC or universal motor. It usually consists of a cylindrical array of copper segments mounted on the rotating part of the motor. Each segment of the array connects to part of the armature windings and power is fed to the windings by fixed carbon or metal brushes which bear on the segments. * **DC** Direct Current, current which always flows in the same direction. Batteries are a source of direct current. Although, strictly speaking, the term defines a current, it is commonly used to mean unidirectional e.g. DC voltages or DC currents are unidirectional voltages or currents. * **Diode** A semiconductor device that only allows current to pass in one direction. Higher power diodes or assemblies of more than one diode are often called rectifiers. There is no fundamental difference between a diode and a rectifier, the terms are just different names for the same device. * **Displacement** The volume of air (at normal air temperature and pressure) displaced by the piston moving from the bottom to the top of its stroke in the cylinder. * **EMF** Electro Motive Force. An almost archaic term for voltage but still commonly used when referring to the back EMF of a motor - see above. * **Energised** When sufficient power is applied to a relay to cause it to operate it is said to be energised. * **Excited** excitation When current is passed through a winding of a motor or generator, a magnetic field is generated and the winding is said to be excited. * **Field** The fixed magnetic field in which the armature of a commutator motor or dynamo rotates. The field can be provided by an electromagnet formed by field coils wound round an iron yoke or by permanent magnets. * **Heatsink** Many power semiconductor devices such as transistors or rectifiers will overheat unless bolted into thermal contact with a larger area of metal that can dissipate the unwanted heat a heatsink. * **Hz** See AC. * **Impedance** In AC circuits, the effective resistance to current flow may differ from the DC resistance because of the presence of inductors or capacitors. Impedance is the AC resistance of such a circuit. * **Inductance** A coil of wire possesses inductance because it can store energy in the magnetic field that it generates when a current is passed through it. On AC supplies, the alternating polarity of this magnetic field causes the apparent resistance to current flow (i.e. the ‘impedance’) to be much higher than the DC resistance. * **LED** Light-Emitting Diode. Several colours available in the visible light region but diodes emitting infra-red are more efficient. * **MOSFET** A type of three-terminal semiconductor that is normally ‘off’ but can be switched partially or fully ‘on’ by a voltage applied to the third (gate) terminal. * **Open** A pair of contacts or a switch in the ‘off’ position. * **Open circuit** Non-conducting, disconnected or without any load connected. * **Parallel** Parallel connection. Components (e.g. resistors, switches, motors etc.) connected across a pair of wires so that the same voltage appears across each component. * **Peak** Alternating current moves smoothly from a maximum value in one direction to a maximum value in the other direction. These maximum values are referred to as the peak value of the voltage or current. The peak voltage of domestic 240V supplies is 1.414 × 240 = 339V * **Pot.** See potentiometer. * **Potentiometer** A three-terminal variable resistor, the slider and both ends of the resistive element are available as connections. * **Reed relay** A relay in which the contacts take the form of two flat metal blades (‘reeds’) hermetically sealed into opposite ends of a small tubular glass enclosure. The reeds are made of magnetic material and this makes them stick together and switch on when current is passed through the relay coil that surrounds the glass tube. * **Relay** A switch or series of switches (usually called contacts) operated by an electromagnet. * **Relay coil** The coil of wire that surrounds the magnetic parts of a relay to form an electromagnet. When sufficient current is passed through this coil, it generates a strong magnetic field which operates the contacts of the relay. * **Rectifier** A semiconductor device, that only allows current to flow in one direction. Used to convert (i.e. rectify) alternating current (AC) to direct current (DC) which is needed by some types of motor. See also diode. * **Resistor** A component that impedes or reduces (i.e. resists) the flow of current round an electrical circuit. The value of a resistor is stated in ohms, kilohms (thousands of ohms) or megohms (millions of ohms). * **RFI** Radio Frequency Interference. Commutator motors and some electronic speed controllers may radiate radio interference, mainly in the medium and long wavebands. Check out your motor rig with a portable radio! * **rms** Root-mean-square. The single figure used to identify the effective value of an alternating current or voltage in spite of the fact that the actual value is changing all the time. Roughly speaking, it is the AC equivalent of the DC value. One amp rms through a one ohm resistor would require one volt rms and dissipate a power of one watt i.e. exactly the same power as 1 volt and 1 amp DC. The peak value of domestic 240V rms AC supplies is a little more than 1.4 X the rms value. * **rpm** Revolutions per minute. * **Rotor** the rotating part of an induction motor or AC generator. * **SCR** Silicon-Controlled Rectifier. Sometimes called a thyristor. A semiconductor switch that works in the same way as a triac but the controlled conduction is in one direction only. In the reverse direction current flow is always blocked, i.e. it acts as a rectifier. See also triac. * **Series** Series connection. Components (e.g. resistors, switches, motors etc.) connected together daisy-chain fashion so that the same current flows through every component. * **Solenoid** A special type of electromagnet in which the moving part (often a cylindrical iron slug) moves within, and is surrounded by, the coil which produces the magnetic field. Used when a relatively long movement is required. * **Stalled** Stationary rotor or armature. Usually when the torque generated by the motor is insufficient to turn the load. * **Stator** The fixed part of an induction motor or AC generator that usually carries the main power windings. * **Tacho generator** A device which generates a voltage or a frequency proportional to shaft speed. * **Torque** Turning or twisting force. May be used to describe the turning force exerted by a motor shaft or to describe the force necessary to turn the device that the motor is driving. * **Transformer** A device for changing the voltage of an AC supply. Consists of a primary (input) winding and one or more secondary (output) windings wound round a laminated iron core. * **Transients** Transient voltages and/or transient currents are extremely short duration excessive voltages or currents. They can occur as a result of switching an inductive load, such as a motor or they may appear at random times on the 240V AC mains. * **Transistor** A three-terminal semiconductor type that is normally ‘off’ but can be switched partially or fully ‘on’ by a current entering the third (base) terminal. * **Triac** A semiconductor switch which can block or allow large currents to flow into a load. When it is switched on by a small current into its gate electrode it conducts in both directions so that it is suitable for use in AC circuits. # APPENDIX 5 ## Suppliers * **Cirkit Distribution Ltd.** Park Lane Broxbourne Herts EN10 7NQ Tel: 01992 441306 Mail-order suppliers of a wide range of electronic components mainly aimed at the electronic constructor market. * **J & N Factors** Pilgrim Works Stairbridge Lane Bolney Sussex RH17 5PA. Tel: 01444 881 554 Retail factors motor start and run capacitors. Battery charger parts and miscellaneous surplus motors. * **Maplin Electronics** P.O. Box 3 Rayleigh Essex SS6 8LR Tel: 01702 554161 Retail distributors of enormous range of electronic equipment, electronic components and related supplies. UK and overseas mail order. Annual catalogue on sale at branches of the W. H. Smith stationers chain. * **Power Capacitors** 30 Redfern Road Tyseley Birmingham B11 2BH Tel: 021 708 2811 Start and run capacitors. * **RS Components** PO Box 99 Corby Northants NN17 9RS Tel: 01536 201201 Major supplier of extremely wide range of electronic, electric and allied components. Mail order plus local outlets in London, Birmingham, Corby and Stockport. * **Rush Industrial Sales** Biggin House 126 Station Rd Tempsford Sandy Beds. SG19 2AY Tel: 01767 640779 Helpful stockist of a very wide range of motor start and run capacitors.