Confused about motor nameplate voltage?
Have you ever observed the difference between power system voltage and motor nameplate voltage in North American region? One can get easily confused seeing that the nameplate voltage of the machine does not fit the supply voltage.
For AC industrial motors it is a common practice in North America that the motor nameplate voltage is slightly less than the nominal system voltage. The difference is approx. 4% as can be seen in Table 1.
Table 1: Motor nameplate voltage vs nominal power system voltage
The difference in above voltage ratings may be explained due to certain voltage drop in the utlity grid when the motor is fully loaded or due to other consumers.
When the voltage at motor terminals varies from the nameplate rating the performance of the motor may be affected. Higher terminal voltage means higher motor torque. higher starting current and typically decreased power factor. Vice versa, lower terminal voltage means lower machine torque and marginally increased power factor.
However, certain variation is natural. The NEMA standards typically provide ±10% tolerance from the nameplate rating. In this range the motor shall be able to operate without significant impact on performance. Besides grid voltage variation there is also a certain variation of grid frequency.
Please note that described considerations apply for direct on-line (DOL) motors only. When the motor is fed from a VFD the situation looks quite different. There is much more freedom for optimization!
First of all, the motor voltage doesnot have to fix the grid voltage unless the drive shall be equipped with a bypass. That is an advantage. Especially in the lower power range it is not necessary to provide a high voltage motor. For example, in power range 2-3 MW it is not economical to use 10 kV or 11 kV motor just to match the grid voltage. Instead, a more cost effective 6 kV design can be used (beneficial for motor as well as VFD).
Another thing is the influence of grid voltage variation on motor performance. The VFD can partly or fully compensate the reduced grid supply voltage. VFD can “shield” the machine from grid disturbances through its buffering function and advanced control. If the starting time is important then a VFD fed motor will almost always surpass his direct on-line counterpart. Moreover, the field weakening point can be specifically optimized and the machine flux can be shaped as required.
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References
[1] Motor start with VFD, https://mb-drive-services.com/vfd-motor-start/
