Motor Controllers

Section 8

Motor Controllers

8.1. Definition and devices
8.2. Motor starters
8.3. Motor Control Centers (MCC)

8.1. Definition and devices

A motor controller is a device or group of devices that serves to govern in the performance of an electric motor. A motor controller might include a manual or automatic means for starting and stopping the motor, selecting forward or reverserotation, selecting and regulating the speed, regulating or limiting the torque, and protecting against overloads and overcurrents and short-circuits.
The motor controller will have differing features and complexity depending on the type of the motor and the task that it will be performing.
An electric motor controller can be classified by the type of motor it is to drive such as permanent magnet,servo, series, separately excited, and alternating current. At this course it will only be discussed the case of 3-phases alternating current motors.
The protection against overcurrents and short-circuits can be performed by fuses, associated to switch-disconnectors or instantaneous trip circuit breakers.
The protection against overloads is normally assured by thermal overload relay. This relay canbe of the following types:

Bi-metallic strip
A thermal overload will accommodate the brief high starting current of a motor while accurately protecting it from a running current overload. The heater coil and the action of the bi-metallic strip introduce a time delay that affords the motor time to start and settle into normal running current without the thermal overload tripping. Thermaloverloads can be manually or automatically resettable depending on their application and have an adjuster that allows them to be accurately set to the motor run current.
This type of protection is normally used on LV motors.
Electronic digital overload relays
This type of protection is used for large LV motors and HV motors, and contains a microprocessor. These devices may model the heating of themotor windings by monitoring the motor current and they can also include metering and communication functions.
Large LV motors and HV motors shall also be protected against overvoltages.
Other motor protection devices that can be used are:
* Low Voltage Protection
* Phase Failure Protection
* Phase Reversal Protection
* Ground Fault Protection
* Bearing Temperature Monitorsand Protection
* Winding Temperature Monitors and Protection Devices
* Current Differential Relays (Phase Unbalance)
* Vibration Monitors and Protection
* Blocked rotor
Start and stopping of motors is done normally through contactors, known as “motor starters”. The type of starters to be used depends on the power of the motor, their characteristics and their function.
Motorcontrollers can be manually, remotely or automatically operated. They may include only the means for starting and stopping the motor or they may include other functions.
Safety practices require that an emergency stop button should be installed close to motors.

8.2. Motor starters

Small motors, usually with rated powers up to 10 kW, or those where low starting torque is not acceptable (the caseof motors of elevators, as an example), use normally “direct on line” (DOL) starter, which diagram is shown on the image below.

Image 8.1 – DOL starter
In the case of an asynchronous motor, at starting moment the motor will draw a high starting current until it has run up to full speed. This starting current is typically 6-7 times greater than the full load current.
When motors are not inthe above referred conditions, to reduce the inrush current, larger motors will have reduced-voltage starters or variable speed drives in order to minimize voltage dips to the power supply.
The common methods used at 3-phase squirrel-cage motor “reduced-voltage starters” are:

Star-delta starter

Image 8.2 – Star-delta starter
Autotransformer starters

Image 8.3 – Autotransformer starter...