Kuormitettavuus ja momenttikäyrä

Taajuusmuuttaja

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Tasavirtamoottorin momentti

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Synkronimoottorin momentti

The maximal torque, as a function of the rotational frequency, has a simple form as seen in the figure below. With constant supply frequency there is torque only at corresponding rotational speed, if the maximal torque is not exceeded. If exceeded the motor drops from pace but starts again to work properly when the load has been reduced sufficiently.

If you want your synchronous motor to develop the same torque with different speeds and constant stator current, you must keep the ratio V_S/ω constant, for the motor obeys the following equations:

                      Ψ_S = V_S/jω,

                      T = i_R x Ψ_S.

Here Ψ_S is the rotating flux linkage or winding flux, V_S the stator supply voltage, ω the angular velocity of the rotor, T the machine torque and i_R the rotor current, which is considered to be constant. - So, if you want more speed, ω, you must add voltage, also, if you want the torque to remain in the old value.

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When changing the frequency of the power supplied to an AC-motor, the ratio of the applied voltage to the applied frequency (V/Hz) is generally maintained at a constant value until the maximum of supply voltage is reached. Operation at a constant voltage (maximum voltage), above the so called base speed, provides reduced torque capability (reduced V/Hz) and constant power capability. The frequency or speed in which constant voltage operation begins is called the base frequency or speed, see Figure below.

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