| FAQ MOTORS |
FAQ's for PMDC & SINGLE PHASE Motors 1. PERMANENT MAGNET DC MOTOR This motor runs off a DC supply - either Pure DC which is from a Battery or rectified DC from a AC to DC Converter. The motor consists of an armature - usually the rotating part, and
The Permanent Magnet Field assembly - usually the stationery part.
However in case of small Brushless motors the magnet assembly could
be the rotor and the winding with its electronic commutator circuit
may be the stator. Various kinds of Magnets are used. ALNICO, FERRITE, RARE EARTH, Neodymium etc. Commercially the Ferrite Magnets are the most Popular due to the overall economy. SHANCO 'ROHINI' PMDC Motors uses Strontium Ferrite of Grade MMPA 8 or equivalent. This type of magnet has a relatively high energy and requires relatively high energy to magnetize and demagnetize. The magnets are in ARC shape and are radially magnetized to form the 'pole' of the motor. 1.3 Characteristics OF PMDC MOTOR. The PMDC motor has a constant flux field due to the Permanent Magnets. Thus the motor has a special speed torque characteristics of almost a straight line. That is as the load increases the speed drops in a linear proportion. Thus they are extensively used for servo applications. The SERVO motor however needs more expensive construction to ensure a accurate speed torque relationship unlike a commercial PMDC motor, which need not have a precise torque - speed tolerance level. The PMDC motor has a very high starting torque.Thus it is ideal for applications which need high starting torques such as traction ( cars etc), treadmills, conveyors, pulverizes, beaters and crushes and such applications. 1.4 SPEED CONTROL The torque developed by the motor varies with the voltage applied. Thus at lower voltage the motor has a lower speed. Thus by controlling the voltage applied to the motor, the speed can be adjusted. This can be done manually or through electronic speed controllers. 1.5 PMDC vs. WOUND FIELD MOTORS In the wound field motor, the flux is generated by the current passing through the field winding. This winding is placed around a 'pole'. As the current passes through the field winding , it generates Copper loss in the winding. In case of PMDC Motor, there is no field winding, and hence no copper loss in the field. Hence PMDC motors have a higher efficiency than the wound field motor. Thus they are highly suitable for applications demanding low drain on the power source such as battery driven applications. ( All the portable equipment such as tape recorders, or Automobiles use PMDC Motors ). The other major factor is that the space needed for the Field winding in a wound field motor is much larger.The PMDC motor has a lower diameter and can be made axially longer for a higher outputs. 1.6 MAJOR APPLICATIONS PMDC MOTORS are widely used in : CONVEYORS, TREADMILLS, WINDING MACHINES, PRINTING MACHINES, RAILWAY TRACK SETTING, GOLF CARTS, VALVE CONTROLS, DOMESTIC APPLIANCES, AUTOMOTIVE, REMOTE CONTROLLED VEHICLES, ROBOTS, MACHINE TOOL DRIVES, SOLAR WATER PUMPS, SPACE APPLICATIONS, INVALID CHAIRS, FORK IIETS FANS PUMPS, GARAGE DOOR OPENERS, CURTAIN CONTROLLERS, HOSPITAL BED CONTROLS Etc 2. SINGLE PHASE INDUCTION MOTOR. These motors are run from a Single Phase supply which is the main source of supply in an Household environment. Hence these are very popular and come in a variety of ratings. An average household uses many of these motors without even realising their existence, For example household equipment like Ceiling and Table Fans, Kitchen Mixers and Grinders, Bathroom, or Kitchen Extractors,Refrigerators, Air Conditioneers etc use these type of motors. 2.1 TYPES OF SINGLE PHASE MOTORS: These are avialable in: INDUCTION MOTOR TYPE : CAPACITOR START INDUCTION RUN, CAPACITOR RUN, SPLIT PHASE START, CAPACITOR START AND RUN. These are very popular the most popular being the Capacitor Start for its high starting torque. SHADED POLE TYPE : Much more economical than the Induction motor in the smaller range ( less than 50 W output ), these are extensively used in the low cost appliances. These have lower efficiency and low starting torque. COMMUTATOR MOTOR TYPE : These have high starting torque and controllable speed and hence used in Kitchen appliances. 'ROHINI' brand motors are available only in INDUCTION MOTOR type. The Capacitor Start Motor ( Capacitor start Capacitor Run Motor is a variant ) has a Starting torque of 250 % or higher and hence these are the most commonly used motors in higher output range (above 150w ). Usually these motors are available off the shelf from stockists. Very large output motors are also made in this type. Split Phase Motors have a moderate Starting torque ( 150 - 200 % ). These are used on specific appliances which need moderate starting torques. Capacitor Start and Run Motors are also very popular as these have no starting switch to disconnect the auxiliary winding. The efficeincy is higher and the power factor can be almost unity. They are economical to manufacture in the sub-fractional range ( 200w or lower ). These are extensively used in Fans, Washing Machines, Pumps etc. 2.3 SPECIAL MOTORS : Motors having special torque characteristics such as Torque Motor, Multi speed motors for Fan applications, Motors fitted with Thermal Overload Protection, Dual Voltage Motors, Special Mechanical construction etc are available |