a) The rated voltage of a three phase induction motor is expressed on its name plate as 110V/220V. Can this motor be directly used in our country supply network? Justify your answer. Say and draw how the stator windings of this motor will be coupled in an environment of supply voltage 110V/220V. b) One-way switches are cheaper than push buttons; explain why push buttons are preferred to be used in motor circuits instead of one way switches. c) Draw the power diagram of a star delta starter two directions of control.

Part (b)

Step 1: Identify motor ratings and supply voltage.
Motor rated at $110\,V~\Delta / 220\,V~Y$. Assume country three-phase supply is $220\,\text{V}$ line-line.

Step 2: Check direct connection (typically $\Delta$).
Rated $\Delta$ line voltage $= 110\,\text{V}$.
Supply voltage $= 220\,V > 110\,\text{V}$.

Step 3: Consequence of direct $\Delta$ connection.
Phase voltage $= 220\,\text{V}$ (vs rated $110\,\text{V}$).
Overvoltage causes excessive flux, high magnetizing current, core saturation, overheating, insulation failure.

No, the motor cannot be directly used on $220\,\text{V}$ supply.

Part (c)

Step 1: Use star ($Y$) connection for $220\,\text{V}$ supply.
Rated $Y$ line voltage $= 220\,\text{V}$, matches supply.

Step 2: Calculate phase voltage in $Y$.
$V_ph = \frac{220}{\sqrt{3}} \approx 127\,\text{V}$
Rated $\Delta$ phase voltage $= 110\,\text{V}$. Difference $\approx 15\%$, acceptable (no overload if current-rated).

Step 3: Winding connection.
Connect ends of three windings to form neutral point $N$.
Line $R$ to winding 1 start, $S$ to winding 2 start, $T$ to winding 3 start.
Winding 1 finish, 2 finish, 3 finish joined at $N$.

Justification: Matches rated $Y$ voltage, safe operation.

Part (d)

Step 1: Compare switch types.
One-way switches: Simple on/off (SPST), cheap for lighting.

Step 2: Motor control requirements (star-delta starter).

• Timed star-to-delta transition.
• Start/stop push buttons.
• Contactor interlocking (prevent simultaneous star+delta).
• Overload relays, fuses.
• Two-direction control needs reverse contactors.

Step 3: Why one-way switches fail.
Cannot sequence operations, no interlock, no remote control, unsafe arc on inductive load, no protection.
Adding auxiliaries increases cost/complexity beyond standard contactor setup.

No, low cost will not be achieved for motors.

Part (e): Power diagram of star-delta starter (two directions)

Description (schematic):
Supply: $L_1, L_2, L_3$ ($220\,V, 50\,\text{Hz}$).

• KM0: Main line contactor (3-pole).
• KM1: Star contactor (connects motor $U,V,W$).
• KM2: Forward delta contactors ($U\!-\!V_2$, $V\!-\!W_2$, $W\!-\!U_2$, where 1/2 distinguish terminals).
• KM4: Reverse delta contactors ($U\!-\!W_2$, $V\!-\!U_2$, $W\!-\!V_2$).
  Motor terminals: $U_1,V_1,W_1$.

Start forward: Close KM0 + KM1 (star), timer opens KM1, closes KM2 (delta forward).
Reverse: KM0 + KM1, then KM1 open, KM4 close (delta reverse).
Overload relay in each phase.

Part (f): Power diagram of star starter (two directions)

Description (schematic):
Supply: $L_1, L_2, L_3$.

• KM0: Main contactor (3-pole).
• KM1: Star contactor.
• KM2: Forward run contactor (direct delta).
• KM3: Reverse run contactor.

Start: KM0 + KM1 (star), then KM1 open + KM2 (forward) or KM3 (reverse).
Simpler than star-delta (no delta shorting during run).

No for direct use; use Y connection; No for low cost; diagrams as described