DC motor starters combine a controller (most often a contactor) with overload protection, allowing a motor or motor-controlled equipment to be turned on and off. There are many different types of products. Multi-speed DC motor starters are designed to be operated at a constant frequency and voltage. Reduced voltage starters (RVS) also change speeds, but use motor windings that can be reconnected to form different numbers of poles. Reducing or reversing DC motor starters are designed for applications which must avoid overload conditions, or which need to avoid unnecessary wear and tear on equipment. There are five main types of reduced voltage starters: primary resistor, autotransformer, part winding, wye delta, and solid state. Full voltage, non-reversing DC motor starters have an overload relay and two contactors. These contactors are mechanically and electrically interlocked. Manual DC motor starters are also commonly available. They connect the incoming power directly to the motor, and are well-suited for squirrel-cage motors.
DC motor starters use three types of overload protection: eutectic, bimetallic, and solid-state. Eutectic overload relays consist of a eutectic (melting) alloy and a heater coil. When an overload occurs, the coil heats the tube. When the tube melts, a tripping device initiates a switching action. Bimetallic overload relays are made of two metal strips that are joined together permanently. Because metals expand and contract at different rates, heating the bimetallic strips causes it to bend. In turn, a spring pulls the contacts apart and breaks the circuit. Solid-state relays for DC motor starters do not generate heat to trip a circuit. Instead, they measure current or a change in resistance. Solid-state overload protection may also provide advanced functions such as ambient temperature sensing, programmable trip-time, and current-change detection. DC motor starters with low voltage protection, phase failure (loss) trip, and phase reversal (unbalance) trip are also available.
DC motor starters follow rating standards from the National Electrical Manufacturers Association (NEMA) and the International Electrotechnical Commission (IEC). There are 11 NEMA size ratings: 00, 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. IEC sizes are rated at the thermal current rating or the operational current. Choices include A, B, C, D, E, F, G, H, J, K, L, M, and N. DC motor starters also carry starter ratings for continuous current, thermal current, motor voltage and DC horsepower. Number of poles, control voltage, current-line acceleration, time-limit acceleration and operating temperature are also important parameters to consider when selecting DC motor starters.