Single-Phase Asynchronous Machine

Description

This model uses two single-phase asynchronous motors respectively in Capacitor-Start and Capacitor-Start-Run modes, in order to compare their performance characteristics, such as torque, torque pulsation, efficiency and power factor. The two motors are rated 1/4 HP, 110 V, 60 Hz, 1800 rpm.and they are fed by a 110V single phase power supply. They have identical stator windings (main and auxiliary) and rotor squirrel cages.

Motor 1 motor operates in capacitor-start mode. Its auxiliary winding, in series with a 255 uF starting capacitor, is disconnected when its speed reaches 75% of nominal speed. The starting capacitor is used to provide a high starting torque.

Motor 2 operates in capacitor-start-run mode. This operation mode uses two capacitors: The run and start capacitors. During the starting period, the auxiliary winding is also connected in series with a 255 uF capacitor but, after the disconnection speed has been reached, the auxiliary winding stays connected in series with a 21.1 uF run capacitor. This capacitor value is optimized to mitigate torque pulsations. The motor operates efficiently with a high power factor.

The two motors are first started at no load, at t=0. Then at t=2 sec, once the motors have reached their steady-state regime, a 1 N.m torque (nominal torque) is suddenly applied on the shaft.

Simulation

Start the simulation. The Scope block displays the following signals for the capacitor-start motor (yellow traces) and capacitor-run motor (magenta traces) : total current (main +auxiliary winding), main winding current, auxiliary winding current, capacitor voltage, rotor speed and electromagnetic torque. The mechanical power, power factor and efficiency of motor 1 and motor 2 are computed inside the Signal Processing subsystem and displayed on 3 Display blocks.

During the starting period, as long as the disconnect switch remains closed (from t=0 to t=0.48 s), all waveforms are identical. After opening of the switch, differences are observed as explained below.

1. Capacitor-Start:

Observe the 120 Hz torque pulsations which produce 120 Hz mechanical vibrations of the rotor and decrease the motor efficiency. Peak to peak torque ripple is about 3 N, or 300% of rated load when the motor is operating at no load. Observe that the starting capacitor remains charged at its peak voltage when the auxiliary winding is switched off.

2. Capacitor-Start-Run:

Observe that the torque pulsations are substantially reduced. The run capacitor value has been optimized to minimize torque pulsations at full load. Torque pulsations magnitude is 2 N.m peak to peak (200 % of rated torque) at no load, while it is only 0.04 N.m peak to peak (4 % of rated torque) at full load. Power factor and efficiency at full load (respectively 90 % and 75%) are higher than with the capacitor-start motor (respectively 61 % and 74 %).