PlantFlow 3D › Guides › What is a Motor Control Centre (MCC)? Feeders, buckets and wiring
What is a Motor Control Centre (MCC)? Feeders, buckets and wiring
What an MCC is, how motor feeders are arranged, what each feeder contains, how the control transformer and E-stop work, and how an MCC is fed from a transformer.
A board full of motor starters
A Motor Control Centre is an assembly of motor starters fed from one common busbar. Each motor has its own feeder (often called a bucket or drawer) with a breaker, a contactor, an overload relay and its own control circuit. Plants use MCCs so that pumps, fans and conveyors can be started, stopped and isolated from one place.
What is inside a feeder
A typical DOL feeder contains: a motor-protection breaker or MCCB for short-circuit protection and isolation; a contactor for switching; a thermal or electronic overload relay set to the motor full-load current; START and STOP buttons or a PLC output; and a running lamp. Larger motors use star-delta, soft starter or VFD feeders instead.
Every feeder must work independently: pressing STOP on feeder 1 must not stop feeder 2. Only a common emergency stop, or the loss of the incoming supply, stops all of them.
Control supply from a control transformer
MCCs usually make their 230 V or 110 V control voltage with a control transformer connected between two phases of the busbar. The secondary is separate from the supply, so a fault in the control wiring stays in a small, fused circuit. If the primary loses a phase, the control voltage disappears and all contactors drop out, which is the safe direction to fail.
Fed from a transformer
An MCC in a plant is often fed from its own distribution transformer, for example 11 kV to 400 V Dyn11. The primary current is much smaller than the secondary current: I1 ≈ I2 × V2 / V1. For 40 A on the 400 V side, the 11 kV side carries about 1.5 A.
Try it yourself
Load “MCC with transformer (2 feeders)”. Start both feeders, then press Stop 1 and see that feeder 2 keeps running. Press the E-stop to stop both.
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.