As shown in the illustrated 4-speed, 3-phase motor controller, contactor "M2" is electrically interlocked with what other contactors? See illustration EL-0170.
• Electrical interlock vs. mechanical interlock in multi-speed motor controllers • How normally closed auxiliary contacts of other contactors are wired in series with a coil to prevent simultaneous energizing • Reading the ladder diagram for the M2 coil rung and identifying which contact labels are in series with that coil
• On the control schematic at the bottom, trace the circuit that energizes the M2 coil starting from the Low Speed pushbutton. Which contact symbols must all be closed for M2 to pick up? • Look for all NC auxiliary contacts that will open if another speed contactor energizes. Which contactor labels appear on those NC contacts in the M2 rung? • Compare what contactor names are used for interlocks in the other speed rungs (M1, M3, M4, M5, M6). Which ones are intended to be mutually exclusive with M2?
• Be sure you are looking only at the control (ladder) diagram, not the power diagram above. • Confirm which contacts are normally closed (NC) in the M2 rung; only those provide an electrical interlock. • Verify that every contactor you choose appears as an NC auxiliary contact in series with the M2 coil, not just somewhere else in the circuit.
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