Standard motors and ratings

Standard motor features

There is no IEC publication covering standard ratings associ­ated with frame sizes, but British Standard BS 5000 : Part 10 does give ratings against frame size and shaft number gener­ally from 56 up to 315 sizes depending upon the type of motor. Although this standard was first published during 1978, and has been amended more recently, it is still current and forms the ba­sis for standard ratings for motors within this range.

The motors covered by the Standard are described as “general purpose induction motors” and meet various parts of British Standard BS 4999 (this generally therefore meets the IEC pub­lications on which BS 4999 is based where appropriate). The motors are suitable for connecting to 3-phase, 415 V, 50 Hz supplies but by agreement may be wound for any voltage not exceeding 660 V. Class E, Class B or Class F insulation may be used with the ambient conditions not exceeding 40°C or 1000 m altitude. BS 5000: Part 10 should be consulted for full details.

Standard motor ratings

The standard ratings are specified for single-speed motors with synchronous speeds of 3000,1500,1000 or 750 r/min. Inmost cases the shaft sizes are the same for all speeds, except for 3000 r/min on some of the larger standard frame sizes.

Table 13.6 Standard outputs and shaft numbers for totally enclosed fan-ventilated (TEFC) cage motors

In the case of airstream rated motors with pad or mountings classified as IC418, the ratings are as given in Table 13.7 with the average air velocity at least the value given by Table 13.8 when measured 50mm radially from mounting pads.

The standard ratings for enclosed ventilated cage motors are given in Table 13.9. These motors have a cooling system clas­sified as IC01 and a degree of protection classified as IP22.

It should be noted that the air velocities specified in Table 13.8 are in many cases extremely low for low hub-to-tip ratio axial flow fans which have a high flowrate. In consequence the air velocities flowing over the motor will be considerable greater than those given in the Table. The power produced can there­fore be appreciably greater, without exceeding safe tempera­ture rises in the windings or the motor surfaces. Fan motors may therefore take advantage of this situation provided that the nose motor bearing can accommodate both the increased torque requirement and also the radial and thrust loads im­posed by the fan impeller.

This has lead the major fan manufacturers, some of whom manufacture their own electric motors, to develop machines specifically appropriate to the application. Such solutions are especially the case in the smaller frame sizes where quantity requirements make such motors economically viable.

Posted in Fans Ventilation A Practical Guide