A brake in the action

Feb. 1, 2000
Brakes and motors form a close-knit team that creates rotary motion as well as stop-and-hold action

Brakes and motors are often joined closely together to function as a compact integral unit. But the similarity in terms that describe the components of such a unit, namely motor brakes and brake motors, can be confusing. Here in a nutshell are some simple definitions to clear up the differences:

A motor brake is a type of brake designed to work with a motor to stop and hold the motor and its driven load. The brake can be one of several basic types.

A brake motor is a combined brake and motor package, usually bolted together through mating C-face flanges. It too produces stop and hold action, but it also produces motion. The motor is usually an ac induction or dc type, although servo and step motors can also be used. This article discusses mainly ac and dc motors. (For information on servomotor brakes, see PTD, “Choosing servomotor brakes,” 8/97, p. 51).

Motor brakes

Motor brakes are similar to other industrial brakes in many respects. They come with the same design features and they operate in essentially the same way. But they differ in the way they are mounted.

Operation. Motor brakes generally use friction between mating surfaces to stop or hold a load. The same brake can be used for both of these functions, but if stopping time is important, the brake must have sufficient torque to stop within the allotted time.

Actuation is either through electrical, mechanical, or pneumatic methods. Most motor brakes are mechanically actuated and electrically released. Called power-off brakes, they provide fail-safe operation by engaging when electrical power is removed for any reason, thereby holding the driven machine or load. When power is restored, the brakes release.

They generate friction and braking torque in one of two ways — spring set or permanent magnet. Both methods use an electrical coil that, when voltage is applied, moves the friction faces apart to disengage the brake. Pneumatic actuation is described later.

With spring-set brakes, you have little control over the deceleration rate — they simply stop at a rate dictated by their mechanical characteristics (mainly spring force and coefficient of friction). Some spring-set brakes have an adjustable nut for changing the spring force, and thus the deceleration rate. Other types require using a unit with a different spring or a different friction material. Either approach lets you reduce the deceleration rate. But it also increases the stopping time and reduces the braking force.

With permanent magnet brakes, a controller applies a step function or gradually applies the brake to control the rate at which it decelerates, thereby producing a soft stop.

Spring-set types are less expensive than permanent magnet types. They are well suited for static holding applications, such as on inclined conveyors, and low-cycle dynamic operation. Permanent magnet types are generally better suited for applications that require high cycling rates, high torque capacity or accurate positioning.

Other brakes, called power-on brakes, are electrically actuated and mechanically (spring) released. They provide soft starting or soft stopping when used with an electrical controller. Power-on brakes are used mostly with clutches to slow down and stop at a precise position, which is often needed with bar code readers, shrink-wrap equipment or filling machines.

As an option to spring or permanent- magnet actuation, brakes can be actuated pneumatically. With this method, a power-off brake is spring engaged until air pressure in a cylinder disengages it.

Most electrical and pneumatic brakes for ac or dc motors are in the 1/2 to 10 hp range, although some go up to 150 hp. Higher power motors typically use pneumatically actuated brakes.

Frequent cycling may cause heat buildup in some motor brakes. In such cases, motor cooling fans help to dissipate this heat. Pneumatic types generally don’t need fans because they use finned frictional discs for this purpose.

Controls. In simple applications, a spring-set brake is usually controlled in conjunction with the motor. A controller sends voltage to the motor starter, which turns on the motor. Simultaneously, the motor supplies voltage to the brake, so that when the motor starts, the brake releases.

For more complex operations, permanent magnet brakes come with a separate power supply and controller. By adjusting the controller, a user changes the amount of voltage to the brake, thereby controlling the rate of deceleration and providing a soft stop.

Electrical brakes operate on either ac or dc voltage, with ac being more common. Ac types offer fast response time for set and release (as low as 5 millisec). They are used in many types of applications, such as hoists and conveyors, as well as in machine tools, and equipment for food processing, printing, packaging, and textile making.

Dc versions, in combination with controllers, provide special control functions. For example, one set of wires may control the motor, while another set independently controls the brake. This allows the brake to delay in shutting off after the motor starts, or to start braking before the motor stops so it doesn’t coast. Dc types are used mostly for cranes as well as for unit handling (package) conveyors in distribution centers (Walmart, UPS, Post Office, baggage handling).

Brake motors

Single and 3-phase ac motors are generally available in ½ to 30 hp capacities for use in brake motors. Other versions include customized ac and dc motors up to 500 hp. Specific motor types include totally-enclosed, fan-cooled (TEFC) with and without C-face; open drip proof with and without C-face; and explosionproof with C-face or foot mount. Fan cooled motors comprise up to 80% of the applications.

Brakes and motors are combined into a brake motor package mainly for user convenience. Because the two components are directly coupled, the package is smaller and has fewer parts (no shaft coupling), which reduces the chance for error. The shorter length is an advantage particularly in conveyor and hoist applications.

The user doesn’t need to separately mount the components, align the brake (important for brake function) or install separate wiring to operate the brake. The package is less expensive because the brake has no shaft support bearings and there are no shaft couplings.

Packaging also simplifies warranty considerations. One vendor — the motor manufacturer — supplies all the components and takes responsibility for the entire package.

On the downside, a packaged unit can’t be used with a soft starter or inverter unless you disconnect the brake so it can be separately energized.

Putting it together

In most brake motors, the motor brake attaches to a flange on the motor (without a shaft coupling). In the U.S., the brake mounts on either the auxiliary (fan) end or the drive (output) end of the motor, most often with a NEMA C-face flange connection.

In one common arrangement, a double C-faced brake mounts on the drive end of the motor and has a Cface flange for mounting a geared speed reducer, belt pulley or chain sprocket.

In another arrangement, the brake and cooling fan both mount on the auxiliary end of a double-shaft, fancooled motor. This requires a longer shaft to accommodate both fan and brake. A geared speed reducer mounts on the drive end.

The shaft on the auxiliary end carries only a light load — a fan, brake, or encoder — so it is usually one size smaller than the drive end shaft.

In Europe, a motor brake typically mounts on a motor with a DIN flange, and the brake is often enclosed in the same housing with the motor.

Brake mounting configurations also depend on the motor type and size. Ac induction motors and dc motors typically require C-face mounting whereas servo and step motors use DIN flanges. Also the shaft diameter and length may differ. In the U.S., mounting arrangements differ between brake motors up to 10 hp and above 10 hp. Through 10 hp, the brake and motor are integrally connected (bolted together via C-face connection).

For large machinery applications, over 10 hp, the brake and motor are often foot mounted to a common base and their shafts are connected via flexible couplings. The brake has an output shaft for mounting a speed reducer that connects to the driven machine. This type of arrangement is more expensive because the brake requires an additional shaft and bearings to support the speed reducer. It also requires alignment of the brake and it consumes more space.

Most servomotor applications use a spring-set, power-off type brake that has a static torque 50% higher than that required to hold the load. If light holding is the only requirement, a small brake (with less holding torque than the motor torque) can be mounted inside the motor on its auxiliary end. Applied to a tool changer on a spindle, for example, such a brake has enough torque to keep the chuck in place while a tool is changed. A small brake of this type can operate very fast, a common requirement in servo applications.

Applications that require controlled deceleration to a stop may need a servomotor with a controller to handle the deceleration. Other options include a vector drive with position controller, and a dc SCR control with a position controller.

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