Machine Design

Choosing the Correct Motor Brake

Ed Lazorchak
Vice President
Electroid Co.
Springfield, N.J.

Automation is putting ever-increasing demands on brake motors as equipment moves faster, with more stops and starts, and the need for tighter positional accuracies grows. But how does an engineer choose between three-phase ac brakes, dc brakes, and single-phase ac solenoid/ linkage brakes? When the different types of brakes are examined for their advantages and drawbacks, choosing the correct brake becomes a matter of matching application to capability.

Dc brakes use springs to exert an axial force against the armature plate. It squeezes the friction disk between the armature plate and mounting plate. Braking torque is generated by the hub, which is attached to the motor shaft. Energizing the coil attracts the armature plate to the magnet, releasing the friction disk to rotate with the hub and motor shaft. Dc brakes also feature a manual release handle and adjustable torque.

Three-phase ac brakes work similar to dc brakes except for the magnet. The armature and field coil assembly are made from wound-strip laminated stock to minimize magnetic losses. Six coils, two per phases, are inserted in the coil assembly. The electromagnetic effect is created by the threephase current flow, with each phase having two coils energized 180° apart. A constant magnetic force acts on the armature. Torque can be adjusted from 100 to 33% of full rating by altering the number of springs.

Single-phase ac solenoid/linkage brakes have a linear solenoid that pulls a mechanical linkage to release two pressure springs. This pulls the pressure plate up, allowing the friction disk to rotate freely with the brake hub. There is no way to adjust the torque, but it is fitted with a resetting manual release.

For applications requiring many stops and starts and exact positioning, three-phase ac brakes are the better choice over dc or solenoid brakes. Three-phase brakes are released electromagnetically. Torque comes from springs, giving them superior response times. For power-on (brake off), response time is 4 msec for NEMA sizes 56, 140, 180 and 210; for power-off (brake on), it’s 5 msec. Power-off times are calculated with the brake wired separately using a common motor contactor with dual interlocks to isolate the motor’s reverse EMF from the brake. This means that in power-on mode, the brakes release before the motor can attain sufficient rpm to create heat or brake pad wear. The power-off time of 5 msec is the obvious reason for the brake’s exact positioning. Most three-phase brakes operate reliably for more than 10,000 cycles/hr, a figure which decreases as brake size increases.

Response times for dc brakes in the same NEMA sizes can be from 6 to 100 times greater than those for three-phase ac brakes. And as the dc brakes get larger, so does the response time. This is due to the time needed to magnetically saturate the magnet assembly at power-on and drain the residual magnetism at power-off. The response time can be slightly reduced by switching the brake on the dc side of the rectifier rather than the ac side. The slow response time also produced more wear and heat. Maximum operations per hour exceeds 1,500 cycles, and this drops as size increases. Dc brakes cannot provide the switching times or positioning accuracy of three-phase brakes.

Solenoid/linkage brakes have even longer response times than dc brakes due to the 0.5-in. air gap between the plunger and solenoid body. Response times deteriorate even more as mechanical components wear. Ac-solenoid brakes cannot provide the switching times, positional accuracy, or life of ac or dc brakes.

The choice of which motor brake to use must take into consideration the surroundings and noise. Theaters, hospitals, and offices, for example, have different noise requirements than warehouses and factories. Both ac and dc brakes have only one moving part (the armature) which moves 0.01 in. going from the friction plate to the magnet assembly. An audible click accompanies the movement. The brakes are then almost silent. Soft rubber plugs in the armature plate press against the hub to prevent the armature plate from rattling. Because there’s only one moving part, the brakes can be mounted in any attitude, or, in the case of vehicles or amusement rides, changing attitude.

Solenoid brakes have an ac actuating solenoid with the usual single-phase hum. They also must be mounted horizontally. Many require hush-kits to prevent the friction plate from rattling if used in any other attitude.

Using the Guidlines
Suppose engineers have to decide on a drive for a highspeed roll-up door in a large meat-packing building. The doors are 12-ft high, 12-ft wide and operated by electrooptical sensors which allow meat to be moved rapidly in and out of the refrigerated interior by forklifts. The doors are driven up at a motor speed of 1,750 rpm in 3 sec, and are powered down at 875 rpm in 6 sec. Upon reaching the up or down position, the brake must immediately stop the drive for accurate positioning. Positional errors and response times were calculated. Engineers concluded that a dc brake would have an error 163% greater than that of a three-phase ac brake and a wear rate 43% higher. Solenoid/ linkage brakes were not considered since they are best suited to applications that are “on” in the morning and “off” at night.

© 2010 Penton Media, Inc.

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