Stopping Methods
6
-
15
The concept of HSB
If we want to stop very fast without using a braking resistor, the only way is to reduce the amount of current
regeneration. This is done by reducing the motor efficiency. Most of the energy will be dissipated as heat
within the motor and just a little bit of energy is regenerated to the Inverter.
The way to reduce the motor efficiency is to force a high slip in the motor and keep it working in the non-lin-
ear zone of its Torque/slip curve.
Fig 6.17 Torque/ slip curve
The normal behaviour is to work in the linear area with a small slip (s<<1).
High Slip Braking Procedure
•
1. When HSB is applied, suddenly you output frequency will decrease to half of the actual value. That
means that you are working at the point of Torque/slip curve near S=0.5. This is the low efficiency area
and the mechanical energy is dissipated mainly as thermal losse in the motor. As the electrical regeneration
is low the DC voltage does not increase.
The voltage applied to the motor is the corresponding voltage according the V/f curve. The voltage might
be higher if the regenerated current is bigger than the value programmed in N3-02.
As the mechanical speed is approaching the output frequency, the slip is decreasing and the motor is going
back to the linear zone where it increases efficiency.
•
2. When the motor is in the linear area of the Torque/slip curve (normal behaviour) the efficiency is high
and the regenaration to the inverter is big. This causes the increase of the DC bus. If the DC voltage
reaches the Overvoltage (OV) level the inverter reduces suddenly the frequency, according the value pro-
grammed in N3-01 and returning to the High slip/ low efficiency zone of the Torque/slip curve and the
process of step 1 is repeated.
Generator Mode
Braking Mode
Input Power
Motor
Mode
Core Loss
Net power
returned to
source=0
