The Basics of Motor Control Centers (MCCs)

Motor Control Centre (MCC)

This guide explains the role of motor control centers (MCCs) in a power distribution system and it explains the need for circuit protection. You will learn how to identify various components of a MCC and the difference between the various classifications and types of motor control center wiring.

Basic Motor Control

Wherever motors are used, they must be controlled. In Basics of Control Components you learned how various control products are used to control the operation of motors. The most basic type of AC motor control, for example, involves turning the motor on and off.

This is often accomplished using a motor starter made up of a contactor and an overload relay.

The contactor’s contacts are closed to start the motor and opened to stop the motor. This is accomplished electromechanically using start and stop pushbuttons or other pilot devices wired to control the contactor.

Figure 1 – Power distribution and Motor Control Centre connections

The overload relay protects the motor by disconnecting power to the motor when an overload condition exists. Although the overload relay provides protection from overloads, it does not provide short-circuit protection for the wiring supplying power to the motor.

For this reason, a circuit breaker or fuses are also used.

Typically one motor starter controls one motor. When only a few geographically dispersed AC motors are used, the circuit protection and control components may be located in a panel near the motor.

Figure 2 – Basic motor control schematics

Motor Control Centers (MCC)

Numerous commercial and industrial applications necessitate multiple electric motors, and it is frequently advantageous to manage some or all of these motors from a centralized location. The device built for this purpose is the motor control center (MCC).

Additional devices related to the motor, such pushbuttons and indication lights, may also be incorporated.

Figure 3 – Typical motor control panel

Power Supplies used by Motor Control Centers

The primary source of electrical power for motor control centers is an AC generator situated at a power generation facility. AC generators function based on the principle of electromagnetic induction. This indicates that as conductors traverse a magnetic field, a voltage is induced within the conductors. A fundamental generator comprises a magnetic field, an armature, slip rings, brushes, and a resistive load.

An armature consists of many conductive wires (conductors) arranged in loops that rotate within a magnetic field. Only one loop is depicted for clarity.

Figure 4 – When conductors are moved through a magnetic field, a voltage is induced into the conductors

Tracking the rotation of the AC generator over a full 360° round reveals that voltage rises throughout the first quarter, peaking at a maximum positive value at 90°. The voltage will diminish throughout the second quarter of the revolution, until reaching zero at 180°. In the third quarter of a revolution, voltage would rise in the opposite direction until it attained a maximum negative value at 270°.

In the final quarter of a revolution, the voltage would diminish until it hit zero at 360°. This constitutes a full cycle or a complete alternation between positive and negative.

Figure 5 – Voltage cycles

The armature of the simple two-pole AC generator depicted above revolves at 3600 revolutions per minute (RPM), resulting in the generation of 60 cycles of voltage every second, equivalent to 60 hertz (Hz).

Should the generator had four poles, it would produce the requisite 60 Hz at a rotational speed of 1800 RPM.

Three-Phase Voltage

Three-phase power is utilized in the majority of extensive commercial and industrial motor applications. In a three-phase system, the generator generates three voltages. Each voltage phase oscillates at the same frequency (60 Hz in the U.S. and 50 Hz in several other nations).

The phases are displaced by 120° relative to one another.

Figure 6 – Three-phase power

Motor control centers receive electricity via intricate distribution networks comprising power distribution wiring and associated devices. Transformers operating with three-phase electricity necessitate three interlinked coils in both the primary and secondary windings. These transformers can be configured in either a wye or delta arrangement.

The transformer type and real voltage are contingent upon the specifications and capacity of the power company, as well as the customer’s requirements. The subsequent figure depicts examples of the secondary windings of wye and delta transformers.

Note that these are merely examples, and alternative transformer secondary voltages may exist.

Figure 7 – Examples of the secondary windings of wye and delta transformers

Motor Rotation

Three-phase voltage is utilized in extensive commercial and industrial establishments to operate AC motors. An AC motor consists of a stationary component, known as the stator, and a rotating component, referred to as the rotor.

Three-phase alternating current is supplied to the stator via the power connections.

Figure 8 – Three-phase motor

The rotational direction of a three-phase AC motor rotor is determined by the phase sequence of the supplied power. In this example, L1 (A) is linked to motor lead T1, L2 (B) is linked to motor lead T2, and L3 (C) is linked to motor lead T3.

When electricity is supplied via the “F” connections, the motor rotates in a clockwise (forward) direction.