Electromagnetic brakes

Electromagnetic brakes (also called electro-mechanical brakes or EM brakes) slow or stop motion using electromagnetic force to apply mechanical resistance (friction).

These brakes use a single plate friction surface to engage the input and output members of the brake. This style of brake is used in applications ranging from copy machines to conveyor drives. They are the most common type of electromechanical brakes. Other applications for these brakes could include packaging machinery, printing machinery, food processing machinery and factory automation.

Engagement: Electromechanical brakes operate via an electric actuation, but transmit torque mechanically. When voltage/current is applied, the coil is energized creating a magnetic field. This turns the coil into an electromagnet that develops magnetic lines of flux. The magnetic flux attracts the armature to the face of the brake. The armature and hub are normally mounted on the shaft (customer supplied) that is rotating. Since the brake coil is mounted solidly, the brake armature, hub and shaft come to a stop in a short amount of time.

The fields of electromagnetic brakes can be made to operate at almost any DC voltage and the torque produced by the brake will be the same as long as the correct operating voltage and current is used with the correct brake. If you had a 90 volt brake a 48 volt brake and a 24 volt brake all being powered with their respective voltages and current, all would produce the same amount of torque. However if you took a 90 volt brake and applied 48 volts to it you would get about half of the correct torque output out of that brake. This is because voltage/current is almost linear to torque.

A constant current power supply is very important if you want accurate and maximum torque from a brake. If a non regulated power supply is used the magnetic flux will degrade as the resistance of the coil goes up. Basically the hotter the coil gets the lower your torque will be.

Disengagement: When current/voltage is removed from the brake, the armature is free to turn with the shaft. In most designs, springs hold the armature away from the brake surface when power is released, creating a small air gap.

Cycling: Cycling is achieved by turning the voltage/current to the coil on and off.

There are two engagement times to consider in initial electromagnetic brake action. The first is the time it takes for a coil to develop a magnetic field strong enough to pull in and attract an armature. Within this scenario there are two factors affecting this. The first one is the amount of turns in a coil which will determine how quickly a magnetic field is generated. The second one is the air gap which is the space between the armature and the face of the brake. This is because the magnetic lines of flux diminish quickly in air. The further away the attractive piece is from the coil the longer it will take for that piece to actually develop enough magnetic force to be attracted and pull in to overcome the air gap. For very high cycle applications floating armatures can be used that rest against the brake face. In this case the air gap is zero but more importantly the response time is very consistent since there is no air gap to overcome. Air gap is an important consideration especially with a fixed armature design because as the unit wears over many cycles of engagement the armature and the brake face will wear creating a larger air gap which will change the engagement time of the brake. In high cycle applications where registration is important even the difference of 10 to 15 milliseconds can make a difference in registration of the driven material. Even In a normal cycle application this is important because a machine that was at one time good, can eventually see a “drift” in its’ registration.

Second factor in figuring out response time of a brake is actually much more important than the magnet wire or the air gap. It involves calculating the amount of inertia that the brake needs to decelerate. Many customers refer to this as time to stop. In reality this is what the end customer is most concerned with. Once it is known how much inertia is required for the brake to start or stop then the appropriate size of brake can be chosen. Remember to make sure that the torque chosen for the brake should be after the brake has been burnished.