Comparison of Various Drive Systems

Machine HP Function Torque Speed Speed control range Regeneration or braking 

Relative cost Moderate Moderate High Moderate 

EMI and line noise Good Poor Poor Poor 

The power factor is also an important factor to consider. The power factor of the DC drive reduces slightly with load, but it drops drastically with speed. This becomes more severe as the horsepower increases. The power factor of the variable frequency AC drive is higher than the power factor of the DC drive and it is much less affected by speed. In fact, for small to medium horsepower variable frequency AC drives, the power factor is essentially independent of load and speed. The effect of the power factor on overall energy cost cannot be simply calculated by multiplying the line-to-shaft efficiency with the power factor. 

The cost of electric power generally depends both on the actual power (KW) and the apparent power (KVA). The power factor is the ratio of actual power to apparent power. Most utilities incorporate a certain cost penalty if the power factor is consider- 

---