Why are switches derated when used with DC motors?

• A. They run cooler at startup
• B. DC motors draw several times their rated current during start
• C. They require less current at start
• D. They are unaffected by temperature

Answer: (B)

When a DC motor starts, there is no back-EMF yet, so the winding behaves like a simple resistor and the current is limited only by the winding resistance. That means the starting current can be several times higher than the running current. That surge lasts only briefly, but it causes extra heating and greater electrical stress on the switch contacts. To avoid overheating or contact wear, switches are chosen with a current rating higher than the motor’s running current to handle this inrush. So the need to derate comes from the startup current spike produced when the motor has zero back-EMF.