Section 15.7: Starting of Induction Motors cont.
Key assumption: |Is| » |Ir|, which is reasonable since the (rotating) transformer’s |jXm||Rc| >> |Rr/s + jXr|
Then, from Examples 15.4 and 15.5: TST / TFL » sFL(|Is,ST|/|Is,FL|)2
If the motor is delta-started, |Is,ST|/|Is,FL| = 9 so that TST / TFL » sFL(9)2 = 0.04´81 = 3.24
However, if it’s wye-started, |Is,FL| is attenuated by Ö3 because |Vt| is attenuated by Ö3 (and |Is,FL| » |Vt| / |Z|, where: Z =(Rs +Rr/sFL) + j(Xs + Xr) is fixed)
Ergo, TST / TFL » sFL(|Is,ST|/|Is,FL|/Ö3)2 = 0.04´(9/Ö3)2 = 1.08
An induction motor employs a wye-delta starter, reduced-voltage starting scheme, which connects the motor phases in wye on starting and in delta when the motor is running. The full-load slip is 4%, and the motor draws 9 times the full-load current if started directly (in delta) from the mains. Determine the ratio of starting torque to full-load torque.