Question E4E05:
What is used to suppress radio frequency interference from a line-driven AC motor?  

Choices:  
A: A high-pass filter in series with the motor’s power leads  
B: A brute-force AC-line filter in series with the motor’s power leads  
C: A bypass capacitor in series with the motor’s field winding  
D: A bypass choke in parallel with the motor’s field winding    

Correct Answer is  B: A brute-force AC-line filter in series with the motor’s power leads  

To suppress RFI from a line-driven AC motor, you use a brute-force AC-line filter in series with the motor’s power leads.

Why B is correct:
- A “brute-force” AC-line filter is a mains-rated low-pass EMI filter (typically series inductors/chokes plus shunt capacitors: X caps line-to-line and Y caps line-to-ground).
- It attenuates both differential-mode and common-mode RF noise generated by motors (brush/commutator noise, TRIAC controllers, VFD edges) before it escapes onto the AC line.
- Placing it in series with the power leads ensures RF currents see high series impedance and are shunted by the capacitors.

Why the others are incorrect:
- A: High-pass filter in series would pass RF and block low frequency. That’s the opposite of what you want; it would also interfere with 50/60 Hz power delivery to the motor.
- C: A bypass capacitor in series with the field winding is not a “bypass.” Series capacitors impede 50/60 Hz ($X_C$ can be significant), altering motor operation and not providing an RF shunt path.
- D: A bypass choke in parallel with the field winding is ineffective as a bypass for RF. An inductor’s reactance increases with frequency, so at RF it looks like an open circuit, not a shunt. Also, putting an inductor across the mains or winding can draw excessive current or resonate undesirably.

Useful rules of thumb:
- Low-pass EMI filters use series inductance and shunt capacitance because:
  - Inductive reactance: $X_L = 2\pi f L$ increases with frequency, so series L blocks RF.
  - Capacitive reactance: $X_C = \dfrac{1}{2\pi f C}$ decreases with frequency, so shunt C diverts RF to the return/ground.
- For line filters, use safety-rated capacitors (X across line, Y line-to-chassis) and a common-mode choke sized for the motor’s current.