# Braking 3ph Induction Motor

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#### Marvin

I would like to know if 3 ph capacitors connected in parallel to a induction motor (3/4HP 208V,460V)can brake the motor to a stop after the power is turned off to the motor?

The capacitors will induce a DC current into the motor. I have been told that this method is used in Europe. We do NOT want a safety braking system. Once the motor has been stopped, the motor must be able to be rotated by hand.
Has anyone used this method and what are the calculations for selecting the size of Capacitors.

#### PhilCorso

The use of a capacitor, permanently in parallel with the motor terminals will produce a braking torque. However, I am unaware of a tried and true
sizing calculation. Trial and error will be necessary.

The capacitor can be sized using the technique for power-factor correction. However, the capacitor's braking efficacy is dependent on
load inertia and friction. To size the capacitor, its capacitive kVAR [2xPixfxCx(kV)^2] should not exceed the motor's inductive kVAR [kW/pf]
input at the motor's minimum operating load. Excessive capacitance will produce overvoltages that will endanger motor winding insulation.

By the way, a more effective, controllable, and testable method would be to introduce a small DC current across one phase of the motor, after the
motor's supply voltage has been disconnected.

Regards,
Phil Corso, PE
(Boca Raton, FL)

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#### Dan Perry

To brake, you need to remove energy from the system. I have sucessfully dynamically braked 3 phase motors (as well as single phase) by installing a second braking contactor which places a resistive load between all 3 phases of the motor. For 1 HP 3-phase motors I've used 5 Watt, 100 ohm resistors (480V motor). Note, the control of the contactors is critical. Acitvating both at the same time will explode the resistors. The motors will still manually turn with the brake applied.

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#### sandeep kapoor

To brake the 3 phase induction motor by injecting the dc current in to it. it is possible by the power correcting capacitors. first of all as per the power factor you should calculate the capacitor required to make the power factor unity for this motor and then put a contactor between the motor- capacitor combination and the main incoming line power as soon as you switch off the power by dropping the contactor the energy stored in the capacitor will released through the motor winding. the maximum size of capacitors can only be the power factor correction value. oversizing will cause trouble to the system.