Motor Control Panels

The workings of large companies can often be complex. This is often more so in the case of their power distribution systems, since there are a large number of variants that power distribution systems need to take into account. There is a variety of ways that companies can try and distribute power. They can be distributed through:

  • Transformers
  • Switchgears
  • Switchboards
  • Panel boards

Power has a number of different applications for large companies. It can either be used for heating, cooling, and running machineries.

Most power distribution units that have been mentioned above tend to be used in combination with different types of loads. On the other hand, motor control centers exercise control over the distribution of powers to electric motors.

Motor controls are the devices or mechanisms which govern the performance of the motor.

A motor control could include an automatic or manual method to start and stop the motor, select the rotation direction (forward or reverse), limit or regulate the torque, regulate the speed, and/or protect the motor from an accidental overload or fault. High performance motor controls are essential in most industrial applications.

Motor Control Applications

All electric motors have some type of control. Motor controls have different features contingent on the function it will perform.

In an uncomplicated case, such as a switch used to hook up the motor to its power source, it is usually a simple on/off flip switch, button, or knob. Simple switches in most cases rely on human intervention; however, a sensor connected to a contact or relay could be utilized as an automatic start/stop for the motor.

Other switches might have several choices or settings to select specific motor connections and controls. These selections could involve voltage regulation, multiple speed, or reverse rotation control on start up. Overloads and high current insurance might be skipped in smaller motor controls, relying instead on circuit breakers, other current protectors, or human intervention. However, some small motors might use a built-in device to control current.

On the other hand, a large motor might have sensors or relays that control temperature or protect against current overload; yet, these sensors, relays, and other controls could be external as well. Those interrupters might include circuit breakers or fuses. Automatic motor controls could include a limit switch or another device to insulate motors from damage.

In some connected motors, a highly complex motor control could be utilized for more accurate control of torque and speed. Close loop controls, as well as other systems and devices are more accurate for controlling torque and speed for those connected motors.

Classifications of Motor Controls

Electric motor controls are classified into groups depending on the kind of motor the controls drive, these include Servo, permanent magnet, separately excited, alternating current, or series. Motor controls are operated manually, automatically, or remotely.

The motor controls are connected to power sources, such as power supplies or battery packs and control the motor’s circuitry through input signals that are either analog or digital.

Design Standards for Motor Controls

The following organizations establish the regulations and standards applied during design of motor controls:

  • National Electrical Manufacturers Association (NEMA)
  • Underwriters Laboratories (UL)
  • National Fire Protection Association (NFPA)
  • International Electro Technical Commission (IEC)

NEMA standards are most widely applied to large motor controls, whereas UL is very common in smaller controls, particularly in small appliances in the US and Canada. IEC standards are more widely used throughout Europe and Asia.

Servo Motor Controls

  • Exact close loop control positions
  • Rapid acceleration rates
  • Servo motors are commonly made up of:
    • DC motors with brushes
    • DC motors without brushes
    • AC motors

A Servo motor control closes control loops using position feedback, which is usually implemented with a Hall effect sensor, a resolver, or an encoder that directly measures the position of the rotor.

Methods used to measure position feedback include:

Stepper Motor Controls

Stepper motors are synchronous, high pole count, brushless poly-phase motor is normally, yet not entirely done with open-loop controllers (when the rotor’s position is supposed to adhere to a rotating controlled field). Exact positioning is cheaper and easier with steppers than using controls that are closed loops.

Contemporary stepper motors use much higher voltage than the rated voltage on the nameplate, while limiting the current with chopping; therefore, the motor controls are much more complex. Normal setups have the positioning control, called an indexer, send direction and step pulses to the segregated high volt drive circuit that is responsible to limit current after computation.

To control the motor’s shaft movements, power is supplied to an electromagnet that attracts the teeth of the gear. Aligning the teeth of the gear with the electromagnet, it produces a slight offset with the next magnet. When that next magnet is supplied power, the power to the first is shut off and a slight rotation occurs to create the alignment in the next. Repeating this process constitutes a “step” and a specific number of steps equals a full rotation. The motor is turned to an exact angle using this method.

Motor Starters

Small motors could be started simply by plugging it in or by using a circuit breaker or switch, these are known as manual switches. Large motors, however require specialized motor contactors or switch units, often referred to as automatic or remote control using magnetic contacts. A very large motor commonly using medium voltage might utilize a circuit breaker as its switch element.

The direct on line (DOL), the most straightforward motor starters, instantly connect straight to motor terminals in its power supply when energized. Whereas, the star-delta, soft starters, or reduced-voltage starters use voltage regulators to connect the motor to its power supply. These regulators increase the supplied voltage in steps or gradually.

The DOL starter may contain protective devices or devices that monitor conditions. A large-sized DOL uses a relay or electromechanical contactor to switch motor circuits.

Asynchronous motors draw high startup current until reaching full speed, normally exceeding six or seven times its full load current. A variable speed drive or reduced-voltage starter is used to reduce current inrush and minimize dips in voltage to its power supply.

Reverse starters are connected for motor rotation in forward or reverse. These starters use two circuits, one for forward and one for reverse operations. Electric and mechanized interlocks prohibit concurrent closure. In a three-phase motor, wires that connect any of the two phases can be swapped to achieve this objective. In single-phased AC, as well as DC motors, additional devices are required.

Utilizing series inductance or autotransformer starters, which were first patented circa 1908, permits reduced voltage at motor terminals, thus reducing inrush currents and starting torque. When the motor reaches the predetermined speed, an elapsed time, or when triggered by the current sensor, full-voltage is applied to the terminals. Since all this takes place in a matter of seconds, the device is smaller comparative to the continuously rated mechanism.

Summary of Motor Control Functions

Whether the application or function of the motor control is simple or complex, manual or automatic, used to regulate torque or speed, or as a preventive measure against motor damage, motor controls are indispensable in commercial industry applications that use electric motors to power their operations, both AC & DC. At WATTCO, we feature a variety of Motor Control products and panels, if you have questions regarding any of our products, their function or applicability to your project, give us a call, 1-800-4-WATTCO or send us a request for information without obligation.