Electric Motors and Control Systems

Internal components of an AC rotating machine.
Internal components of an AC rotating machine.

An electric motor is the workhorse that converts electrical energy into mechanical energy using the principles of electromagnetism. Magnetic fields created by electrical charges are the driving force behind motors which generate the torque required to perform useful work.


  • Rotor – The rotor is the moving part It turns the shaft that delivers the mechanical power.
  • Stator (and Stator Core) – The stator is the stationary part of the motor’s electromagnetic circuit and usually consists of either windings or permanent magnets.
  • The Bearings – The rotor is supported by bearings, which allow it to turn on its axis.
  • Windings – Windings are wires that are laid in coils, usually wrapped around a laminated soft iron magnetic core so as to form magnetic poles when energized with current.
  • Air Gap – The air gap is the distance between the rotor and stator. The motor’s air gap has important effects, and is generally as small as possible, as a large gap has a strong negative effect on performance.
  • Commutator – The commutator is a mechanism to switch the input of most DC motors and certain AC motors.

Important Ratings


The most basic and common rating for an electric motor is its horsepower rating. Shaft horsepower is a measure of the motor’s mechanical output rating. Expressed as its ability to deliver the torque required for the load at rated speed. For an electric motor, one horsepower is equivalent to 746 watts of electrical power and is the standard rating in the United States. In Europe, the kilowatt rating of a motor has become standard.

Synchronous Speed

The speed at which the rotating field inside the motor operates depends on the input power frequency and the number of electrical magnetic poles inside. It’s nearly impossible for a motor to reach synchronous speed because even an unloaded motor still has some form of friction to overcome.

Rated Full Load Speed

The rated full load speed is the actual RPM value listed on the motor nameplate.


The difference between the synchronous speed of the electric motor magnetic field, and the shaft rotating speed is slip – measured in RPM or frequency. Slip is typically expressed as the ratio between the shaft rotation speed and the synchronous magnetic field speed. Slip increases with load – providing a greater torque.

Full Load Amps

The full-load current (FLA) is the maximum expected amperes drawn by the motor when operating at maximum torque and horsepower. The nameplate FLA is a very important rating that is used to select the correct wire size, motor starter, and overload protection devices necessary to serve and protect the motor.


Efficiency is defined as output power divided by input power expressed as a percentage five different types of motor losses:

  1. Core Losses: The energy required to magnetize the core and eddy current losses in the stator core.
  2. Stator Losses: I2R heating of the stator due to current flow in the stator windings.
  3. Rotor Losses: I2 heating of rotor bars as induced current flows
  4. Friction and Windage Losses: Bearing and air friction on the rotor shaft and cooling fan.
  5. Stray Load Losses: Leakage reactance fluxes induced by the load current.

Motor Control Centers

Different types of motor control center.
Some MCC’s control large medium-voltage motors, while others may serve small HVAC systems and other low voltage loads.

In many commercial and industrial applications, quite a few electric motors are required, and it is often desirable to control some or all of the motors from a central location. Motor Control Centers are basically switchboards fitted with motor starter “buckets” rather than traditional circuit breakers.

The fact that an MCC principally contains combination motor control units is what differentiates a motor control center from other power distribution equipment. A combination starter is a single enclosure containing the motor starter, fuses or circuit breaker, and a device for disconnecting power. Typically one motor starter controls one motor.

Motor Control Center "Buckets"
Motor Control Center “Buckets”

Full-Voltage Starter

Full-voltage starters are sometimes referred to as across-the line starters because they start an induction motor by applying the full line voltage to the motor’s stator windings when the contacts of the motor starter’s contactor close.

Soft Starter

Solid-state, reduced-voltage starters, often called soft starters, limit motor starting current and torque by ramping up the voltage applied to the motor during the selectable starting time. Soft starters accomplish this by gradually increasing the portion of the power supply cycle applied to the motor windings, a process sometimes referred to as phase control.


Overload relays are rated by a trip class, which defines the Trip Classes length of time it will take for the relay to trip in an overload condition.

Variable Frequency Drive (VFD)

The variable frequency (or speed) drive controls the induction motor speed, changing the flow volume of air through a space. They are often used in HVAC systems to control compressor speed, pumps, fans, etc.

VFD Components

  • Rectifier, Diodes in Parallel (Provide Forward/Reverse Bias)
  • DC Bus and Filter (Capacitors or Inductors to filter and smooth ripple)
  • Inverter – IGBT (Insulated Gate Bipolar Transistor) Electronic switches control flow direction and frequency of output, metal–oxide–semiconductor field-effect transistor (MOSFET)

Pulse Width Modulation

Controls the output voltage by controlling how long the switches are closed for. Changes the amount of flow occurring per square wave segment. The more segments will result in a cleaner wave. Frequency is changed by controlling the timing of the switches.

Pulse Width Modulation Example
Pulse Width Modulation works by adjusting the length or “width” of the pulses within a cycle (V) to create an average that mimics a sine-wave (B).

Programmable Logic Controller

The programmable logic controller (PLC) is used in simple and complex automation, and operates on simple outputs based on inputs and fixed rules (IF, AND, THEN statements). Typical components of a PLC include: Input Module (analog/digital), CPU (microprocessor, memory, com, other integrated circuits), Output Module (Lights, valves, motor starters, etc.), Interface (screen, com, etc.), and Power Supply.


One of the most commonly used types of industrial control system, SCADA (supervisory control and data acquisition) can be used to manage almost any type of industrial process. SCADA systems include hardware and software components. The hardware gathers and feeds data into field controller systems, which forward the data to other systems that process and present it to a human-machine interface (HMI)

SCADA is used to assist in automating and managing industrial processes that have become too complex or cumbersome for human monitoring and control. SCADA is particularly useful for processes that can be monitored and controlled remotely, especially in cases where it is possible to reduce waste and improve efficiency.