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*Instructions are at the bottom*
This circuit represents a way to start up a wound-rotor motor. First some information about that type of motor.
Normally, with a squirrel-cage motor type, the rotor consists of conductive bars which are shorted on both ends. Due to the created magnetic field by applying power to the stators coils, the rotor will spin.
In the case of a wound-rotor type, the rotor also consists of coils. The stator has the same construction, as a squirrel-cage motor stator.
The stator has 3 coils, split in 2. This is called a "pole pair". If one coil of the pole pair has a North magnetic field, the other one has a South magnetic field and visa versa. Both coils of one pole pair are shifted 180°. The coils have 2 connectable connections (6 for 3 phases). On U1, V1 and W1 the power is applied. On U2, V2 and W2 is chosen what kind of circuit is applied on the coils (Y or ∆). In the example below, it's a Y circuit.
øU1 øV1 øW1 < Motor terminals
B B B
B B B < Stator coils pole 1
B B B
B B B < Stator coils pole 2
øU2 øV2 øW2 < Motor terminals
|______|______| < Connection bars
connected in Y circuit
The rotor of a wound-rotor motor also consists of coils (windings, this is why it's called a "wound-rotor" motor). These are shorted on 1 side (Y connection) internally, the other side runs to "slip rings". Each phase has an own slip ring. Those slip rings are mounted on the shaft of the rotor and are spinning with it. There are brushes mounted on the inside of the stator. Those brushes will touch the slip rings, one brush for each slip ring. There are connections on these brushes (1 per brush). These can be connected to resistances for reducing current during start up. Those can be adjustable resistances, or some fixed-value resistances is series, with bypasses between them.
The connections on the brushes are usually K, L and M.
_____________
| | | < Internal Y connection
B B B
B B B < Rotor coils pole 1
B B B
B B B < Rotor coils pole 2
∪ ∪ ∪ < Slip rings
# # # < Brushes
øK øL øM < Brush terminals
| | | < to resistances
When power is applied on the stator coils, current is induced on the rotor coils. Because the rotor coils are connected in series with resistances, the induced current is limited to a certain value. This means that the stator coils can't induce more current, than the rotor coils allow to be induced. When decreasing resistance by bypassing resistances, the induced current is increased. The resistance is decreased untill both sides of the rotor coils are short circuited. This means that the rotor will act the same way as a squirrel-cage rotor.
_____________
| | | < Internal Y connection
B B B
B B B < Rotor coils pole 1
B B B
B B B < Rotor coils pole 2
∪ ∪ ∪ < Slip rings
# # # < Brushes
øK øL øM < Brush terminals
|______|______| < short circuited with last
set of contactors
How to use:
When starting up, close the far left switch. This closes the mains contactor. The motor will start up with all resistances in series. Look at the current of the stator and rotor coils, as well at the rotor voltage.
Close the far right switch. This will bypass the last set of resistances. Look at the increase of currents and decrease of rotor voltage.
When the second switch is closed, another set of resistances will be bypassed, and the currents will increase further and the rotor voltage is decreased more.
When the third and last switch is closed, all the resistances will be bypassed, and the current will reach full nominal value. The rotor voltage will be very low.
When shutting down, open the far left switch. Then open the other switches to make the circuit ready for next start up.
Note: the "grounds" on both sides of the rotor coils, is done because the circuit wouldn't work without it. In a real circuit, the rotor wouldn't be grounded.
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