Computations and circle diagrams:Starting of Slip-ring Motors

Starting of Slip-ring Motors

These motors are practically always started with full line voltage applied across the stator terminals. The value of starting current is adjusted by introducing a variable resistance in the rotor circuit. The controlling resistance is in the form of a rheostat, connected in star (Fig. 35.22), the resistance being gradually cut-out of the rotor circuit, as the motor gathers speed. It has been already shown that by increasing the rotor resistance, not only is the rotor (and hence stator) current reduced at starting, but at the same time, the starting torque is also increased due to improvement in power factor.

The controlling rheostat is either of stud or contactor type and Slip-ring electric motor may be hand-operated or automatic. The starter unit usually includes a line switching contactor for the stator along with no- voltage (or low- voltage) and over-current protective devices. There is some form of interlocking to ensure proper sequential operation of the line contactor and the starter. This interlocking prevents the closing of stator contactor unless the starter is ‘all in’.

Rheostat As said earlier, the introduction of additional external resistance in the rotor circuit enables a slip-ring motor to develop a high starting torque with reasonably moderate starting current. Hence, such motors can be started under load. This additional resistance is for starting purpose only. It is gradually cut out as the motor comes up to speed.

The rings are, later on, short-circuited and brushes lifted from them when motor runs under normal conditions.

Starter Steps

Let it be assumed, as usually it is in the case of starters, that (i) the motor starts against a constant torque and (ii) that the rotor current fluctuates between fixed maximum and minimum values of I2max and I2min respectively.

In Fig. 35.23 is shown one phase of the 3-phase rheostat AB having n steps and the rotor circuit. Let R1, R2 ….etc. be the total resistances of the rotor circuit on the first, second step…etc. respectively. The resistances R1, R2 …, etc. consist of rotor resistance per phase r2 and the external resistances r1, r2 …. etc. Let the corresponding values of slips be s1, s2 …etc. at stud No.1, 2…etc. At the commencement of each step, the current is I2max and at the instant of leaving it, the current is I2min . Let E2 be the standstill e.m.f. induced in each phase of the rotor. When the handle touches first stud, the current rises to a maximum value I2max, so that