Introduction
This application note provides guidance on selecting the appropriate reactor for DC motor drives. It outlines step-by-step methods to ensure optimal reactor selection, enhancing drive protection and power quality.

Reactor Selection for DC Motor Controllers
MTE three-phase AC reactors can be used on the input of DC motor controllers to protect drives and improve power quality. A 3% impedance MTE reactor is recommended to reduce supply voltage notching caused by three-phase DC motor drives.

Quick Selection Guidelines
For lower horsepower applications, select reactors directly from the MTE reactor selection table. For higher horsepower DC drives, which may require more line current than AC drives of the same horsepower, choose the reactor for the next larger horsepower rating. Specific recommendations include:

• 600 Vac, > 75 HP DC
• 480 Vac, > 100 HP DC
• 208-240 Vac, > 20 HP DC

For example, a 200 HP DC drive with 480V 3-phase power should use the 3% impedance selection for 480V, 250 HP.

Mathematical Selection Method
To calculate the required reactor, gather the full-load DC current, three-phase supply voltage, and supply frequency. The current demand from the AC supply is approximately 0.85 times the full load amps of the DC motor. The inductance for 3% impedance is calculated using the formula:
L = (Z * V) / (I * 2 * π * f * √3)
Where:

• L = inductance in Henries
• Z = percent impedance desired (0.03)
• V = supply voltage (line to line)
• I = fundamental load current from AC line (Amperes)
• f = supply frequency (Hz)

Choose a reactor with a fundamental current rating equal to or greater than the calculated demand and an inductance close to the calculated ideal value.

Example Calculation
For a DC motor drive with 480V 60 Hz three-phase power, a 500Vdc, 125 HP motor with a full load current of 205A:

• Full load current demand: 0.85 * 205 = 175 Amperes
• Required inductance: L = (.03 * 480) / (175 * 2 * 3.1416 * 60 * 1.732) = 0.126 mH

The nearest MTE reactor with a fundamental current rating not less than 175A is the 200A rating, with an inductance of 0.110mH, model RL-20002.