The rating of a motor should exceed that of its driven machine by a suitable margin. The selection of this margin is often made by the manufacturer of the driven machine, unless advised otherwise. The actual choice depends on various factors e.g.
Approximate rating of the motor or machine (kW) | Margin of the motor rating above the machine rating (%) |
Up to 15 | 125 |
16.0 to 55 | 115 |
`Above 55 | 110 |
• The absolute rating of either the motor or the driven machine i.e. small or large machines.
• The function of the driven machine e.g. pump, compressor, fan, crane, conveyor.
• Expected operating level e.g. often near to maximum performance, short-term overloading permitted.
• Shape of the operating characteristic of the machine e.g. pressure (head) versus liquid flow rate in a pump.
• Change in energy conversion efficiency of the machine over its working range.
• Machine is driven at nearly constant speed.
• Machine is driven by a variable speed motor.
• Harmonic currents will be present in the motor.
• The nearest standard kW rating available of the motor.
• Ambient temperature.
Some rule-of-thumb methods are often stated in the purchasing specifications of the motor–machine unit, see for example Table 1.11, which applies to low voltage three-phase induction motors. Where the driven machine is a centrifugal type i.e. pump or compressor, the shaft power may be taken as that which occurs at the ‘end of curve’ operating point. This rule-of-thumb point is defined as being 125% of the power required at the maximum operating efficiency point on the designed curve of pressure (head) versus fluid flow rate, at the rated shaft speed. These rule-of-thumb methods can be used to check the declared performance and ratings from a machine manufacturer.