How to Estimate Part-Load of an Electric Motor

Electric Motor

An electric motor is a device that converts electrical energy into mechanical energy for use in driving mechanical power. It consists of an electromagnet and a rotating armature. The armature contains wire windings on a soft, iron, ferromagnetic core that generate magnetic poles when energized with current. These poles interact with the permanent magnet of the field magnet to drive the rotor and deliver the mechanical output.

An important feature of an electric motor is that it produces maximum torque from the get-go. As the rotor rotates within its internal magnetic field, it induces back-EMF in the stator that opposes the supply voltage and acts like a natural braking force. This is why the armature in an electric motor must be carefully designed to avoid excessive currents at high speeds.

As the rotor turns, it produces a proportional increase in shaft torque that is transmitted to the load by the magnetic fields of the rotor and a shunt or series resistor connected across the armature terminals. The amount of current required to produce a given amount of torque varies between different motor/generator types. Typically, this is limited by core saturation or safe operating temperature rise and the number of pole pairs.

How to Estimate Part-Load of an Electric Motor

When a motor is under load, the armature current tends to decrease from its rated full-load value down to about 50% of this value. The resulting non-linear amperage curve is the basis for using root mean square (RMS) current measurements to estimate motor part-load. The motor nameplate’s full-load current rating applies only at rated motor voltage; to accurately determine part-load, the supply voltage must be corrected for reactive power.

An additional factor to consider when estimating part-load is that the armature’s induced back EMF increases as the load decreases. The net effect is that a large portion of the applied voltage’s power is lost as waste heat. Capacitors are often used to correct this low power factor; they can be thought of as “electrical reservoirs” that capture and reflect back-EMF to return it to the motor at an optimum level.

When a motor is plugged, the rotor connections are reversed and a braking resistor is connected across the armature. Plugging gives a greater braking torque than rheostatic braking, but it also consumes power as electrical energy is converted to heat. It is generally used for applications where sudden application of reverse torque would damage the machinery (such as lowering a load in an elevator). The motor can be unplugged at any time. This method is not suitable for large motors, as the sudden application of reverse torque could cause it to overspeed and break. This is why many large electric motors are equipped with a power limiter or brake.

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