DC Drives

Most DC drives do not have capacitors to generate reactive current, they draw current from the AC lines only. When the motor is moving slowly with high torque, the drive has to produce full current even though little work is being done. This current has to come directly from the line side. So, even though there is no power factor on the DC side (just high current and low voltage), a demand for reactive power is created on the l ...

I do not understand how Power Factor can be affected with typical DC drives, as power factor only exist in AC system when the current wave is either leading or lagging the voltage wave. For DC drives, since phase angles do not exist, why would power factor be affected? This is also my understanding on why power factor is not affected on the line side of variable frequency drive (VFD), as AC voltage is first converted into the DC link volt ...

For a given torque rating, the AC motor will almost always be physically LARGER than the equivalent torque rating in a DC design. This is particularly true for slower (< 450 rpm) shaft speeds. The reason for this is two-fold. First, the AC machine must run at a lower magnetic saturation to enable equivalent performance in terms of speed control (generally the windings are higher inductance than the DC version, since more turns and less amps). Second, the AC machine is often relying on an i ...

Conversion of DC drive to AC drive is complex. Most important are the changes, impact or improvement in your power system (motor sizing, power factor, power quality, harmonics, efficiency, operation/maintenance, controls, etc.).

Many of the considerations for comparing AC and DC drive systems can be covered if you have a procedure for making an economic evaluation between the two technologies. Areas of comparison include: system purchase price, spare parts costs, motor cooling fac ...

My choice is not only a shunt-wound motor, but a straight-shunt-wound motor. With the advent of SCR-based DC drive controllers, the need for specific motors has diminished. Especially in the case of a compound-wound, or a stabilized-straight-shunt-wound motor, the addition of a series field was an attempt to keep the motor under stable control during extended-speed operation, where the shunt field is weakened to attain speeds beyond base speed. They're also somewhat useful is containing armat ...

The rectifier switches (SCRs or IGBTs) are likely the only components that will be similar between AC drives and DC drives. There will be many differences some of which are the design of the energy storage and filtering components (inductors, capacitors), the control of the DC section and the inverter control and switching in the AC drive, the design range of the DC bus voltage (for the DC drive it needs to go to zero; for the AC drive it does not).

An AC drive does contain an int ...