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If the speed of a 3-phase, 400 V, 50 Hz synchronous motor is trebled, the efficiency of the machine will _________.
become zero
reduce to one-third
remain constant
become 3 times
remain constant
The efficiency of a synchronous motor is independent of its speed because the machine is designed for a fixed synchronous speed dictated by the supply frequency and number of poles. Changing the speed (typically by changing the frequency) shifts the machine's operating point, but the overall efficiency remains relatively constant across its operational design range.
The efficiency of a synchronous motor is independent of its speed because the machine is designed for a fixed synchronous speed dictated by the supply frequency and number of poles. Changing the speed (typically by changing the frequency) shifts the machine's operating point, but the overall efficiency remains relatively constant across its operational design range.
NsтАЛ=P120fтАЛ тАФ Synchronous speed formula
╬╖=PoutтАЛ+PlossesтАЛPoutтАЛтАЛ├Ч100% тАФ Efficiency calculation
The speed of a synchronous motor is governed by the formula NsтАЛ=P120fтАЛ. Efficiency is defined as ╬╖=PoutтАЛ+LossesPoutтАЛтАЛ. While changing frequency affects magnetic flux (if voltage is not adjusted proportionally), for a fixed load, the efficiency remains governed by mechanical and electrical loss components that do not scale linearly or proportionally with simple speed changes in a way that significantly alters machine efficiency.
Synchronous motors are constant speed machines
Efficiency depends on output power and internal losses (copper, iron, windage, and friction)
Operating speed changes require frequency variation (V/f control)
Constant speed operation
High efficiency at rated load
Requires DC excitation
Not self-starting without auxiliary methods
Power factor correction
Constant speed industrial drives
Option B is incorrect because efficiency is a ratio of power, not a linear function of speed.
Constant speed machines maintain their design-specific efficiency characteristics unless the load profile changes significantly.
C is correct тАФ The efficiency of a synchronous motor remains constant regardless of variations in speed as it is an inherent property of the machine design.
Always remember that efficiency is a ratio of power outputs and losses; never correlate it directly with speed unless specified by a load-dependent torque profile.
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