Influence of Magnetic Saturation of Induction Motor with Massive Ferromagnetic Shields on the Analysis of Electric Drive Startup Modes

Abstract

Asynchronous electric drives have become widely spread for different industrial applications, many of them are unregulated. Their main disadvantage is small starting torque at high starting currents.

In unregulated asynchronous electric drives to reduce the value of the starting current with an increase in the torque value, is possible with the use of asynchronous motors with massive face ferromagnetic screens of the elements of the rotary circuit outside the working air gap. With the appropriate choice of screen geometry in such asynchronous motors, it is possible to form a mechanical characteristic of the “excavator type”. This allows obtaining the starting torque of the motor, with the value close to the maximum, while simultaneously limiting the value of starting current. Limiting the value of starting currents with increasing the magnitude of the torque of an asynchronous motor with massive face ferromagnetic screens of elements of the rotary circuit outside of the working air gap improves the dynamic properties of the electric drive and reduces energy loss in the starting modes.

However, the presence of ferromagnetic screens leads to additional losses in the steady-state operating modes due to an increase of the active resistance of the elongated rods of the rotor winding and the electromagnetic effects of the screens themselves. The use of such motors is expedient, in electric drives, which operate with frequent startups, when the reduction of energy losses in the starting mode exceeds additional losses at operation on the steady speed.

The parameters of the rotor circuit of such motor depend on slip and rotor current, and for their calculation, iterative procedure is used. In startup modes, the currents of the asynchronous motor significantly exceed the nominal values, which lead to a change in the value of magnetization of the magnetic circuit. The procedure for simultaneous determination of variables of two different contours that affect each other is rather complicated.

The paper deals with a comparative analysis of the loss value, taking into account the saturation of the magnetic circuit of the machine and without it. The processes considered in the direct start of the electric drive and with the saturation of the current value due to the use of soft-starter devices. The results of the research are compared with the similar operation modes of the electric drive with serial asynchronous motor with squirrel-cage rotor of the same power.

The results of the studies have shown that under such conditions the influence of magnitude saturation on the parameters of starting modes decreases, therefore, the error in calculating energy indices when neglecting this parameter is 7...8 %, which is acceptable for a considerable part of engineering calculations.