Braking Systems: Three-phase Asynchronous Motors with Short-Circuit Rotor by Electrobrake

GLOSSARY

Apparatus An organized set of parts that fulfills a function

determined within an electrical circuit.

Control apparatus Manually operated devices that, included in the
auxiliary circuits allow control of the devices
maneuvers arranged in the main circuit.

Starter Combination of all the devices required for the
starting and stopping an electric motor in relation to
appropriate overload protection.

Assigned Current Current for which the devices are designed
maneuver.

Device A device that enables the operation of an appliance

interlocking maneuvers depend on the position or effect of one or more
components of an installation.

Start-up time Time that extends from the connection until the motor
reaches operating speed.

Autotransformer of It is used to implement a soft motor start with

start squirrel cage. The autotransformer with a ratio
The appropriate amount reduces the voltage at the motor terminals during the
start.

Contact State in which two conductive parts intended for this
function, they join with a certain force and allow passage
of an electric current.

Auxiliary contact Contact located in an auxiliary circuit. According to its function
The operating mode can be Normally Closed (NC),
Normally Open (NA), Inverter (I) or pass-through.

Timer relay Electronic time-delay switching device or
electromechanical that, once a time had passed
Adjust, close and/or open your contacts.

Service voltage Voltage verified “in situ” between the conductors
that power an electrical appliance or installation.

BRAKING SYSTEMS

INTRODUCTION

If an electric motor is disconnected from the power line, due to inertia, it takes some time to stop and may even accelerate or start rotating in the opposite direction after stopping, due to the weight of the load, as in the cases of overhead cranes, freight elevators, lifts, etc.

When, due to the needs of the system or machine coupled to a motor, it is desired that it stop immediately after disconnecting the motor from the power line, or that its speed can be reduced, some type or system of braking is used to make this possible.

motor control course

BRAKING OF THREE-PHASE ASYNCHRONOUS MOTORS WITH SHORT-CIRCUIT ROTOR

In many applications of three-phase asynchronous motors with short-circuited rotors, a safe system is needed to allow the motor to be stopped at a specific moment; that is, a braking torque is required. This occurs, for example, in machine tools where the precision of the work or the safety of personnel demands a rapid shutdown of the driven machine, and also during elevator operation.

The most commonly used procedures for braking three-phase asynchronous motors are as follows:

ƒ – Electrobraking.
ƒ – Braking against the current.
ƒ – DC injection braking.

ELECTRIC BRAKE

For braking three-phase asynchronous motors, three types of electro-braking are used, which are based on a system of a movable plate attached to the motor shaft, and brake shoes or bands that act on it:

ƒ – Electromagnetic brake.
ƒ - Electro-hydraulic brake.
ƒ - Brake built into the engine.

In the first two cases of the braking systems indicated, the plate is fixed to the motor shaft and the shoes are actuated either by an electromagnet or an electro-hydraulic drive; whereas in the case of a brake incorporated into the motor, it is the plate that slides and presses against the braking band when the motor is disconnected from the power supply line.

ELECTROMAGNETIC BRAKE

This type of brake consists of an electromagnet, which can be single-phase or three-phase, connected in such a way that when the motor is running, the electromagnet is energized and keeps the brake shoes open, allowing the motor shaft to rotate freely. When the motor is disconnected from the power supply, the electromagnet also de-energizes, and the brake shoes, by means of opposing springs, press against the disc, which is fixed to the motor shaft, stopping the motor instantly.

This type of braking is very fast and effective; it can be used for small and medium power machines, since in large power machines, the size and consumption of the electromagnet makes it unfeasible.

ELECTRO-HYDRAULIC BRAKE

This braking system differs from the previous one in that the shoes are actuated by a complex element consisting of an asynchronous motor, an impeller pump, and a hydraulic cylinder.

When the main engine is energized, the brake system motor is also energized, which rotates the pump impeller. This impeller sends oil to the cylinder, which raises its piston and rod. As the cylinder rod rises, it moves a lever that activates the brake shoes, thus
a way that releases the brake plate so the main motor can rotate freely.
When power to the main motor is cut, the brake motor is also disconnected. The piston and rod of the cylinder descend, assisted by an opposing spring, causing the brake shoes to press against the brake plate and instantly stopping the main motor's rotor.

When the piston in the cylinder moves down, the oil returns to the reservoir, either directly or through a return valve.

The most significant advantages of this type of braking compared to electromagnet braking are its smooth and quiet operation, as well as its safety.

service and low consumption, so it can be used in all types of engines, mainly in medium and high power engines.

BRAKE INCORPORATED INTO THE ENGINE

In this special type of motor, the short-circuit rotor is slightly truncated conical and offset from the stator core by a spring. At the opposite end of the shaft from the coupling point, a slightly truncated conical plate is attached. When displaced by the rotor spring, this plate presses against a braking band located inside the rotor housing.

When the motor is not connected to the power line, it is always braked, due to the spring force that presses the plate against the brake band; on the contrary, when the motor is energized, the magnetic fields of the stator and the rotor force the latter to center with the former, resulting in a small displacement of the rotor that will rotate freely, as a consequence of the release of the brake.

This type of motor, although more expensive than those of normal construction, is usually used in machine tools, elevators and any other small power machine that requires rapid braking.