Types of Motor Enclosures:
In order to protect machines against the ingress of dirt and dust or larger foreign bodies like spanners, vermin etc. into it, it is desirable that some suitable enclosure be provided. But the enclosure provided for this protection will interfere with the free entry of cool air into the machine with the result that an enclosed machine does not give the same output as does an open type machine of the same dimensions for a given temperature rise. The purpose of providing enclosure is two fold-firstly providing protection to persons against contact with live or moving parts inside the enclosure and protection to machines against the ingress of solid foreign bodies, and secondly providing protection to machines against harmful ingress of water. Various types of motor enclosures giving various degrees of protection and cooling facilities for use in different working conditions can be built. ISI 4691 provides an ingress protection code which consists of the letter IP followed by two numbers-the first characteristic numeral designates the extent of protection of persons against contact and of the machine against ingress of solid foreign bodies, while the second characteristic numeral indicates the extent of protection to the machine against harmful ingress of water.
Further addition of letter S or M indicates that the degree of protection against ingress of water is applicable only while the machine is stationary or rotating respectively. Absence of both the letters means that protection is provided under both the conditions. For example enclosure marked IP 22 will provide protection against ingress bodies of diameter more than 12 mm and against ingress of water falling up to 15° from vertical under both the conditions. Similarly enclosures marked IP 44 and IP 56 will provide protection against ingress of bodies of diameter more than 1 mm and against water splashed under both the conditions and protection against ingress of bodies in powder form and water projected by a nozzle under both the conditions respectively. For further details refer to ISI code 4691.
The various types of motor enclosures, as specified by relevant ISI are as follows:
- Open Type
- Protected Type
- Screen Protected Type
- Drip Proof Type
- Splash Proof Type
- Totally Enclosed Type
- Totally Enclosed, Fan Cooled Type
- Pipe Ventilated Type
- Flame Proof Type
1. Open Type
The bearings are supported on simple pedestals or end brackets. This type of enclosure does not protect the machines against dust, dirt and larger foreign bodies but facilitates in cooling. The machines of this type are usually large dc generators and dc motors, which are installed in a neat and clean separate room such as in case of power stations, substations, collieries, rolling mills. Because all the moving parts are exposed, hand rail and other suitable rail guards are provided to prevent unauthorized persons coming into contact specially with moving parts.
2. Protected Type
Protected type machines are such in which access to live and rotating parts is so limited as to prevent accidental contact with such parts or ingress of foreign bodies while ventilation is not materially obstructed. Protected type motors are used mostly for industrial purposes.
3. Screen Protected Type
In this types of motor enclosures, machines openings provided for ventilation are covered with wire screen, expanded metal or other suitable perforated covers having apertures not exceeding 3.25 cm2 in area but not less than 0.6 cm2 in area. Though such an enclosure does not protect the motor against dirt and dust but larger bodies and big insects cannot enter into the machine. Such motors are very widely employed for most of industrial applications where dry clean conditions prevail and gas or chemical fumes are not present. Fig. 1.112 (a) illustrates a screen-protected motor in which no attempt has been made to exclude dust or fluff from the windings. It is suitable for most forms of electric drive and permits ample ventilation.
4. Drip Proof Type
In such machines openings provided for ventilation are so protected by overhanging cowls that liquid drops or dust particles falling on the machine vertically cannot enter the machine. Such motors are used in damp situations such as for pumping sets. Fig. 1.112 (b) illustrates a drip-proof motor, similar to the screen-protected type, but having protection against dripping moisture.
5. Splash Proof Type
In such machines the ventilating openings are so constructed that liquid drops or dust particles falling on the machine or coming towards it in a straight line at any angle not greater than 100° from the vertical cannot enter the machine. Such motor can be used in rains.
6. Totally Enclosed Type
Such machines have solid frames and end shields but no opening for ventilation. They get cooled by surface radiation only. In such machines no dirt or foreign matter can enter the motor and clog the air passage. These machines are used in situations where dust, water, oil, chemicals, gases or other injurious substances are likely to enter the machine but natural cooling is sufficient such as for driving coal handling plant, saw mills etc. In these machines due to cooling difficulties the output obtainable is only about 50 to 70% of that obtainable from a protected type of same dimensions and for same temperature rise. Totally enclosed motors are seldom furnished in sizes above 1.5 or 2 kW because higher rating motors require frames of much larger sizes than fan-cooled motors of corresponding rating.
7. Totally Enclosed, Fan Cooled Type
In order to assist in heat dissipation, most of the manufacturers have developed totally enclosed fan cooled motors in which an outer casing is built around the motor and air is blown by means of a fan mounted on the shaft between the casing and external surface of the motor. To increase cooling rate, motor is provided with external fans. Such motors are used in saw mills, flour mills, cement works etc. These motors are not water proof and should not be used in places likely to be submerged. Fig. 1.112 (c) illustrates a totally enclosed motor in which the main means of cooling is by radiation and internal air circulation.
8. Pipe Ventilated Type
These are totally enclosed type machines in which to overcome the heat dissipation difficulties, a duct or pipe is provided through which clean air is led to the motor from outside the dust laden building or area. The cost of the piping may he overweighed by the saving in cost of motor, as motor of smaller size can be used with same output.
Pipe ventilated motor is shown in Fig. 1.112 (d). Fresh clean air is brought to the motor by piping and drawn in through a big inlet, as illustrated in (i), and air is then expelled through louvres or taken away by pipe. as illustrated in (ii).
9. Flame Proof Type
The flame proof type machine is provided enclosure designed and constructed to withstand an explosion of the gas or vapour which may occur within it and to prevent the ignition of the gas or vapour surrounding the machine. The ventilating openings in the machine are made long enough for the flame to be contained within the frame. This types of motor enclosures are used in collieries, petroleum stations or chemical plants.