Abstract:
Variable speed constant flux AC drives are an accepted choice for many industrial processes on account of their predictable response under steady state and transient conditions. However, exact constant flux control is difficult to achieve at reasonable cost, because of the inaccessibility of the air gap or the rotor in cage induction motors (CIMs), and the commonly used "nominally" constant flux approach causes significant problems at low frequencies; especially with the boost voltage which is difficult to determine and temperature dependent. In this thesis a new flux estimator is proposed which, when applied to a simple scalar drive, removes the boosting problems without requiring modifications to either a star or delta connected motor. The technique uses measurements of the third harmonic (zero sequence) harmonics within the machine, generated from saturation of the motor iron, as an accurate (though nonlinear) steady state and transient indicator of the air gap flux. Under ideal conditions this method is applicable over a 100:1 frequency operating range although, for a delta connected machine, the measured third harmonic current circulating in the stator windings is affected by the changing machine impedance at low frequency; consequently, some compensation is required for this effect. In a star connected machine, third harmonic voltages at the neutral point (ideally) remain unaffected by any machine impedance. An experimental constant flux controller is developed for a delta connected CIM. The performance of this controller is evaluated using computer simulations and experimental measurements. It is shown that this control strategy enables true four quadrant capability and has a load step response which betters other similar (established) constant flux controllers which do not employ any speed feedback. A detailed experimental analysis of the flux and third harmonic components in a saturated machine is undertaken which reveals the existence of additional sources of third harmonic not related to main flux saturation. These additional sources are found to be dependent on the level of fundamental current and are attributed to saturation of the leakage branches in the motor. In some cases these load dependent sources can be present with sufficient magnitude to limit the accuracy of the proposed flux control. When the CIM operation is restricted to near (or above) rated flux conditions, main flux saturation always predominates. Therefore, under this strategy, the additional third harmonic components (if present) have little effect on the accuracy of the flux control, on account of the high nonlinear gain of the third harmonic/flux relationship. On the other hand, harmonically controlled inverters for variable flux operation (or using field weakening) can have significant errors. Accordingly, accurate flux control and/or parameter estimates based on this technique (under such operation) may be impossible, as both the magnitude and the phase of this measured signal can be adversely affected in some highly saturated machines. This thesis concludes that a scalar constant flux drive can be developed using these third harmonic components and applied to either star or delta connected CIMs. Such a drive may be operated over a wide frequency range and has a load step response which betters that of other similar flux controlled drives, without speed feedback. The controller requires tuning - ideally dependent on both the frequency and load - but the settings are not critical and it may be applied to a wide range of conventional induction motors relatively easily.