Motors are manufactured in standard frame sizes, corresponding to the rated output. The size of the frame naturally increases with the rated output. The frame size standards stipulate various dimensions like the shaft center height, axial distance between shaft end and the nearest pair of mounting holes, axial distance between the sets of mounting holes and other mounting dimensions. This ensures interchangeability between motors of the same frame size manufactured by different vendors. In a given frame size, a number of designs are available to suit various applications, for example, types of mounting like foot-mounting or flange-mounting.
The type of enclosure required, depends upon the conditions under which the motor has to work. It is therefore selected so as to protect the internal parts against the ingress of dust and water. At the same time the enclosure is expected to protect the surrounding areas from the internal, live, and moving parts of the motor.
The type of insulation and the type of cooling required is selected, depending on the temperature rise and the operating temperature limits.
The position of the terminal box, which is either on top or on the side, is often required to be specified while ordering a motor.
The frame itself is available in a number of designs to suit the requirements of site conditions, duty, etc. Some of the standard designs are as follows:
1. Totally enclosed, non-ventilated type
Such motors are limited to sizes up to 2 or 3 kW. The cooling is by surface radiation as there are no openings for ventilation. The frame is of solid construction.
2. Splash-proof type
The frames of such motors, incorporate ventilated openings, so constructed that the liquid drops and dust particles that fall vertically or greater than 10º from the vertical angle, cannot enter the motor directly. They also cannot enter the motor by striking or running along the outer surface. This is the extension of drip-proof motors and is called the hose-proof type.
3. Totally enclosed fan-cooled
Totally enclosed fan-cooled motors, incorporate a fan, mounted on the motor shaft. The fan draws air and forces it between the inner fully enclosed frame and an outer shell. An internal fan carries the heat that is generated internally to the enclosed frame, which is cooled by the air drawn by the fan on the shaft. The enclosing frame protects the motor against corrosive and abrasive effects of dust, moisture, etc.
Protected-type motors contain feature-perforated covers for the openings in the end shields. Thus the internal and live parts of the motor are mechanically protected using wire mesh or metal covers without affecting the flow of air.
5. Drip-proof type
The frames in such motors afford protection against liquid drops or dust particles falling on the machine at angles greater than 15° from the vertical. Water drops and particles cannot enter the motor directly or indirectly by either striking or running along the surface. The ventilated openings are protected by use of a hood.
6. Pipe or duct ventilation type
Sometimes the air surrounding the motor is such that, if it is passed through the motor winding, it can damage it.
In such cases, clean air can be brought from outside, and by means of a pipe or a duct, it can be used for motor ventilation. For force induction of air, a blower is installed at either the entry side or the exit side of the duct.