4.3.5. Choosing the type of chopper
Select the correct mode chopper is an important consideration, since this affects the stability of the system and the dissipation (thermal effects). Table 4.7. shows a relative comparison of different modes of chopper for chopper frequency, motor current through the motor inductance and fixed.
Chopper mode
| Current ripple
| Dispel motor
| Dispel bridge
| Minimum current
|
| Inhibition | high | high | high | very low |
| A phase | low | low | very low | low |
| Two phases | high | low | low | ipp / 2 |
Table 4.7. Comparison of different types of chopper.
4.3.5.1. Ripple Current
The current change is directly related to the voltage applied to the winding, which is determined by the equation:
The ripple current is determined primarily by the switching frequency and second, the voltage across the winding. When the winding is powered, the voltage is the power supply less voltage drop at the terminals of the power transistors of the driver (see paragraph 4.3.4.). Moreover, the tension in the winding during the period of recirculation depends on the chopper mode selected.
When you select the mode chopper by inhibiting or two steps, the voltage across the winding during the recircualción is the supply voltage minus the voltage drop across the diodes or transistors (on a straight drop or voltage bipolar R * I if you use a DMOS). In this case the slope of the current decays rapidly, so the ripple it is bigger.
When using the chopper stage, the voltage across the winding during the recirculation is the saturation voltage of the circuit breaker (Vsat for bipolar I * RDS ON for a DMOS) but the drop in the diode. In this case the current decays very slowly, so that the slope and the ripple is very small compared to the previous case.
4.3.5.2. Losses in the motor
The motor losses include losses (I 2 * R) of the winding resistance and losses resulting from eddy current. The last group, parasitic losses, generally increases with increasing the current ripple and the frequency. Switching techniques have high ripple current causes higher losses in the motor, directly affecting its temperature, these are the chopper and the chopper by inhibition of two phases. Generally for small losses in the engine chopper is used per phase.
4.3.5.3. Dissipation in the bridge
Conduction losses are derived from the saturation voltage of the power components, and depending on the current flowing through them, and the time of driving. The intensity and the voltage drop across the transistor are fixed parameters, which depend on the chopper, but the drive time if that is the type of chopper used.
The chopper by inhibiting the recirculation current drops quickly. If the period is constant, the driving time is high, so the power losses in the driver with this system are high.
While the chopper by phase, the current decays slowly, so most of the time period is the recirculation of the flow, and to a lesser extent driving. Therefore, the power losses in this system are lower than those produced by inhibition in the chopper. This can be seen by comparing the waveforms of current through the coil in both cases, located in Figures 4.37. and 4.39.
