GE Industrial Solutions AF-650 GP General Purpose Drive - Manual (Page 28)

9 Alternative Braking Methods
9.1.1 DC Injection Braking
If the three-phase winding of the stator is fed with direct current, a stationary magnetic field Φ will be set up in the stator bore causing a voltage to be induced
in the bars of the cage rotor as long as the rotor is in motion. Since the electrical resistance of the rotor cage is very low, even small induced voltages can create
a high rotor current. This current will produce a strong braking effect on the bars and hence on the rotor. As the speed falls, the frequency of the induced voltage
falls and with it the inductive impedance. The ohmic resistance of the rotor gradually becomes dominant and so increases the braking effect as the speed comes
down. The braking torque generated falls away steeply just before standstill and finally ceases when there is no further movement. Direct current injection braking
is therefore not suitable for actually holding a load at rest.
Drive-Series:
An over-modulated DC current added to the AC current works as an eddy current brake (par. B-02 DC Braking Time ≠ 0 s).
9.1.2 AC-braking
When the motor acts as a brake the DC-link voltage will increase because energy is fed back to the DC-link. The principle in AC-brake is to increase the magnetisation
during the braking and thereby increase the thermal losses of the motor.
Drive-Series:
The brake energy is distributed in the motor by changing the loss conditions in the motor. The AC brake function cannot be used in applications with high cycling
frequency since this will overheat the motor (par. B-10 Brake Function = [2]). Using factory settings it is possible to brake with about 50 % of rated torque below
2/3 of rated speed and with about 25 % at rated speed. The function is not working at low speed (below 1/3 of nominal motor speed).
9.1.3 Mechanical Holding Brake
A mechanical holding brake mounted directly on the motor shaft normally performs static braking. In some applications the static holding torque is working as
static holding of the motor shaft (usually synchronous permanent motors). A holding brake is either controlled by a PLC or directly by a digital output from the
frequency converter (relay or solid state).
NB!
When the holding brake is included in a safety chain:
A frequency converter cannot provide a safe control of a mechanical brake. A redundancy circuitry for the brake control must be included in the total installation.
AF-650 GP Brake Resistor Design Guide
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