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How to control speed of ac motor?

Precise regulation of AC motor speed regardless of load is critical for automated machinery and industrial processes that require repeatable results.

Precisely regulating AC motor rotational velocity is crucial for automated machinery and industrial processes requiring repeatable outcomes. Several control techniques enable optimization of speed profiles.

Variable Frequency Drives

VFDs alter the frequency of the three-phase AC power supplied to the motor stator windings, allowing adjustment from 0-150% of the motor's synchronous speed. By incrementally increasing or decreasing the AC frequency in small amounts such as 0.5Hz, 0.1Hz or even 0.01Hz using microprocessors paired with IGBT semiconductor switches, the magnetic field alternations affecting the rotor can be slowed or sped up with high resolution. Some advanced VFDs provide closed-loop control where a tachometer, encoder or other position sensor provides real-time speed feedback enabling the drive to dynamically correct any deviations with better than 0.5% accuracy through modulating voltage and frequency.

AC Motor & Load Configuration

Proper pairing of AC motors with matched loading ensures stability at all speeds. By following guidelines to select a motor sized between 75-100% of the maximum expected load torque requirements, large fluctuations are mitigated. Correctly grounding motor housings, lead wires and other electrical components according to hazardous location classifications serves to reduce electromagnetic interference from electrical noise that could disrupt sensitive speed regulation circuits.

Field Weakening Controls

On high performance VFDs capable of operating motors at elevated synchronous speeds exceeding base ratings, field weakening control schemes are employed. These complex modulation algorithms dynamically cancel generated magnetic field strengths as motor frequency increases, which allows the magnetic pull on the rotor to stay balanced as rpm rises into the thousands.

Sensorless & Encoder Feedback

Some advanced AC motor control schemes have the ability to estimate rotor position without sensors by monitoring characteristics of back EMF voltage pulses generated in the motor windings due to the spinning permanent magnets. While computationally intensive, sensorless techniques offer reduced system costs. Encoder feedback provides the highest level of precision where a rotary shaft-mounted position sensor reports real-time shaft angular position values to the VFD multiple times per revolution enabling closed-loop regulation to within 0.1% of the target speed.


The ability to accurately regulate AC motor rotational velocities regardless of load disturbances is essential for automated systems. Modern control methods provide flexibility to optimize speed profiles.

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