What does 'rated voltage' we see written on the name plate of synchronous machines mean? Is it the generated voltage or terminal voltage? By generated voltage I mean the EMF induced in the machine and by terminal voltage I mean the voltage obtained after considering the synchronous impedance.

Why am I confused?

Consider the synchronous machine as generator. If I consider rated voltage to be generated, voltage produced when rotor rotates at rated speed then what about the field current? Since induced voltage also depends upon field current, we may obtain different values for generated voltage if we vary field current. So for which value of field current is rated voltage printed on machine if generated voltage is actually the rated voltage in first place?

Coming to considering the rated voltage as terminal voltage... I can hardly get my head around with this case. How can we fix the terminal voltage? It will vary as we vary the load. For example, a DC battery rated as 9 volts means that the generated voltage in the battery is 9 volts and as you vary the load across the battery, the terminal voltage will subsequently also vary.

The same thing is when I consider the synchronous machine as a motor.

What does the rated voltage of a synchronous machine mean?

  • \$\begingroup\$ Add a photograph of the nameplate please. \$\endgroup\$
    – Andy aka
    Feb 23, 2023 at 17:34
  • 4
    \$\begingroup\$ 'Rated' is always quoted under certain conditions, however those conditions are often assumed, or implied. If it's a motor, then it means terminal voltage, and the motor can be expected to run normally, with a load within rated limits, at rated RPM, and field current if it has a separately excited field. If it's a generator, then that means the terminal voltage, given a rated load and RPM. Try not to overthink. \$\endgroup\$
    – Neil_UK
    Feb 23, 2023 at 17:42

1 Answer 1


Provide a synchronous machine with electrical power and it creates mechanical power. Provide a synchronous machine with mechanical power and it creates electrical power.

From Synchronous Motor Nameplate Details Explanation

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\$P_{m(echanical)} = 21,000hp = 15.67MW\$

\$ \begin{align} P_{e(lectrical)} =&\ \sqrt {3}\ V_L\ I_L\ cos\ \theta + P_{DC excitation} \\ =&\ \sqrt {3}\times 6,600V\times 1,404A\times 1 + 125V \times 5.2A \\ =&\ 16.05MW + 650W = 16.05MW \end{align} \$

\$ \eta = \frac {P_m}{P_e} \times 100\% = \frac {15.67MW}{16.05MW} \times 100\% = 97.6\%\$

As a synchronous motor, provide excitation of 125V at 5.2A and line voltage of 6,600V at 60Hz and the motor will continuously provide 21,000hp at 1,200 rpm drawing 1,404A at an efficiency of 97.6%.

A motor or generator of this size would have significant protection to make it only operate as a motor or generator. As in a generator can absorb power from the network and try to run as a motor, called motoring, which is bad so circuit protection would take it offline.

But in principle, the synchronous motor can run as a generator.

Provide excitation of 125V at 5.2A and spin synchronous machine at 1,200rpm with a mechanical power of 16.05MW and the synchronous machine will generate 6,600V, 60Hz providing 15.67MW at 1,404A.

This reflects the info on the nameplate, but if you do not provide these values, then the actual output will vary.

A motor will deliver enough torque to drive actual load. Vary the excitation and the motor may not synchronize, but run as an induction motor.


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