This causes the magnetic field of the stator to be of a fixed, as opposed to the rotating magnetic field of the 3-phase AC supply, as the speed of rotation of the air-gap field is directly proportional to the stator frequency, it should be clear that since DC is effectively zero frequency (as it is linear wave form), the air gap field will be stationary, and it is known that the rotor of a AC motor will always try to rotate at the same speed as the air gap field, if this is zero then the rotor will in turn try to become stationary. when the poles of the stator are alined (N-S and S-N) the rotation of the rotor will stop, the only thing that will over come this stopping power is the inertia of the rotor and its load.
The overall effect is that the motor will stop quickly and smoothly, the rotor will be held in place and resistant to any external force for the period that the DC voltage is applied. The strength of the braking force is controlled by the magnitude of DC voltage applied, the higher it is the stronger the braking force will be. The braking characteristic for a AC cage motor is shown below
Tinamics. (2009) DC injection braking. http://www.tinamics.com/s0103/index.php?tpid=0062&pgid=0010&menusub=12. Accessed 3/1/10.
The above graphic shows that the reverse troque falls to zero as speed reduces to stand still. It can be also be said that the speed torque braking characteristic is a morrior image of the motoring speed torque curve.
If the DC voltage is kept on the stator the rotor will be held in place, but if DC injection is continued over a prolonged preiod of time it can cause damage to the motor. Although the DC injection technique can be used for any load or torque level it is prefred that a smaller load is breaked as the larger the load the more detremental the braking system is on the motor.
DC injection is a standard feature of AC drives today. If the DC injection was not applied the motor would slow when the feed is removed but would take a long period of time to stop and may not be acceptable under emergency conditions or for use with potentially dangerous machinery. The machanical maintainace is not increased as the braking is provided without a need for touching parts.
A DC injection break in a 3 phase motor system would be installed as per the depiction in the graphic below.
The applications of the DC injection break are many; one example of this is in use with synchronous wind turbine generators. When wind turbines are under great stress from excessively quick winds, the propellers will attempt to rotate at a rate which will damage the turbine structure, eventually the turbine could rip its self apart. In order to slow the rotation of the propeller blades a DC injection break can be implemented as shown in the following diagram
Law, G. (n.d.) FdSc_-_Lect_Sync_Generator_Wiring. http://moodle.cityplym.ac.uk. Accessed 1/1/10
The above diagram shows, from right to left, the propeller and synchronous generator setup is shown. The AC energy from the generator is converted to a DC form via the diode rectifier. The high frequency interference is dampened by the inductor giving a cleaner DC supply. The DC power is stored within the capacitor which in turn feeds the inverter allowing for control of the AC output in terms of voltage and frequency. The central SCR Blocks the DC voltage from being fed back into the generator, if this device was to be switched on a DC injection would be fed into the stator of the generator slowing its rotation, the level of voltage, controllable with PWM, determines the strength of the braking. Using this process the propeller rotational speed will be slowed or even stopped and held in position in high wind conditions.
DC injection braking has the following disadvantages in terms of energy efficiency
• Losses through heat when used for a prolonged period of time or if it is used with an existing supply present.
• Requires, in some cases, a separate DC supply. This produced DC energy is all lost through the braking process and heat.
• Losses in rectification process from the transition from AC to DC, if the AC supply is used to create the DC voltage.
• Mechanical energy is lost, by heat, by stopping the machine at a greater rate.