Electronic braking can extend machine life and improve safety. Here’s what you need to know about it.
Three-phase induction motors drive huge sectors of industry, but getting them to stop is important, too. Braking is required for a myriad of reasons including tool changing, conveyor unloading, and press clearing. It’s also a part of controlled stopping (as opposed to coasting), which helps to increase occupational safety while reducing wear on power transmission belts, sprockets, and gears.
Besides mechanical brakes, today’s options include electronic brakes. When used together, the two most common types (discussed here) are especially effective. Regenerative braking provides slowing; injection braking finishes the job. Though not meant for holding or safety braking, dc electronic braking provides reliable load deceleration and stopping, and conserves energy as well.
First things first — before we delve into electronic braking we need to understand how a threephase ac motor moves and rotates with its load. Three equally spaced voltage phases (120° apart) vary sinusoidally for a sum resultant vector of constant magnitude. As these signals change amplitude and sign, their associated windings modulate magnetically and change both in amplitude and polarity. Consequently, the windings take turns in repelling the fixed magnets of the rotor, pushing it along like standing children spinning a merry-go-round. In this way, two-pole three-phase motors are not unlike motors with four, six, eight, or ten poles.
In effect, the combined magnetic field rotates inside the stationary stator, and induces current in the rotating rotor that spins the attached load. In this way, electrical energy is converted into usable mechanical energy in the form of motor shaft torque and angular velocity.
Motor windings, already present to drive the motor, are energized by a dc power source to generate a stationary magnetic field. This stationary field exerts a static force on the rotor that brings it to a stop. Following is a discussion of two prevalent dc brake types that work well together to form a complete braking system.
Regenerative braking rundown
Electronic regenerative brakes work mainly by slowing the systems to which they’re applied. They take dynamic energy from a spinning rotor and load, convert it into electrical energy, and feed it back to the brake power line. Alternatively, in the same system the regenerated electricity may be dissipated as heat in a resistor or rheostatic brake.
The problem with regenerative braking is that as the load decelerates, energy recovery obviously decreases along with it, and braking force diminishes until backup brakes — such as injection brakes — are required for full stop. Regenerative brake challenges also include heat dissipation limits and transistor size; both restrict braking torque (and deceleration). If used, rheostatic brake resistors must have sufficient resistance to limit braking current as well as the specified wattage to accommodate braking cycles.
Though drives aren’t usually selected based on brake requirements, brake duty cycle and braking magnitude are important considerations when braking is frequent. If braking is especially heavy, brakes are better protected when continuously rated for about 150% of the peak braking level because this reduces thermal stress fatigue caused by cycling.
Drive manufacturers usually offer regenerative brakes with low-watt rheostatic brakes, or none at all. However, when load braking is important, a drive with these braking features is essential. Most new scalar or vector pulse width modulation (PWM) drives offer excellent braking features; with PWMs, the biggest challenge is selecting a braking resistor with proper watts rating. Regenerative braking is often standard on adjustable speed drives. However, line regenerative braking requires a drive with a transistorized front end, and its cost is justified only for rapid-cycle processes like centrifuges or dynamometers.
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Dc injection braking
Dc injection brakes, unlike regeneration brakes, have their own power source. Supply ac power is changed into pulsating dc current, which flows in one of the motor’s windings used for braking. Because in dc injection braking energy dissipates in the motor itself, a deceleration mechanism — such as regenerative brakes — is required to lessen motor wear. Otherwise, the braking current required is too high, risking saturation of stator windings and overheating (and no gain in braking torque). Typically used with threephase motors, injection brakes are either added to existent motor control circuits, or integrated into new motor control applications.
Current as well as the subsequent injection braking force are functions of the applied dc voltage and stator winding properties; this is key when connecting dc brakes to multiple motors, or motors with six or nine leads for multiple windings (as in two-speed or wye-delta motors) because their properties (such as effective resistance) differ. High currents affect line voltage, so power systems need good voltage regulation during braking. Also, injection brakes are usually sized for fully loaded motor current and voltage.
For safety reasons — dc injection brakes generate a lot of heat — power circuits are usually incorporated into the motor’s thermal and overload circuitry. Thus, when the motor is critically overheated, brakes won’t turn on.
When dc injection brake power is supplied through motor circuitry, the brakes either require their own fuses or oversized motor circuit fuses. With such an integrated setup, however, brakes must be prevented from energizing during motor startup or operation. Otherwise the resulting short circuit would wreak havoc with the brake, motor branch circuit, and other apparatus. Also, dc injection brakes should not be connected to manually controlled motor branch circuits, as they’re more designed for use with electromechanical, three-phase contactor motor branch circuits.
Input contactors should be electrically connected to motor-run contactor coils, which in turn should be controlled by brake logic circuitry. Control circuitry monitors motor voltage to determine safely applicable dc brake voltage levels; the circuitry also checks for zero crossings — firstly to prevent brakes from leaking dc into the energized motor, and secondly to release braking when the motor stops. (Some motors actually use dc injection brakes at stops for holding, but because they’re activated by line power, this technique is not usually recommended.)
The control circuit modulates voltage and controls contactor and electronic board power. (Though these features are helpful, current isn’t fed back — braking is instead controlled by means of RMS magnitude modulation of chopped, rectified dc waveforms.) Most dc injection brakes also have safety time delays. This, along with voltage monitoring, tends to compromise the speed of response to stop signals; in fact, braking may be delayed for a couple of seconds. Another challenge: loss of voltage may occur with unbalanced voltage supplies, causing temporary singlephasing of the three-phase line. These transient conditions may affect braking circuit logic, and are a good reason to properly wire brakes into the motor contactor circuitry.
Wiring the brake as an integrated part of the motor control circuit will prevent malfunction if the brake is set to automatically engage with the loss of motor lead voltage. If not, temporary power outages, even for just a few seconds, could result in motor flux collapse and voltage loss on motor leads. The brake may then detect this condition as an intentional stop signal, and dc injection braking may initiate. The potential mix of dc and ac voltage at power restoration would be catastrophic.
One alternative to integrating dc injection brakes into motor circuitry actually costs less than investing in an adjustable speed drive. Reducedvoltage, solid-state, soft-starters often come with dc injection brakes as an option; silicon controlled rectifiers (SCRs) power both devices so the brakes a natural added feature.
Ac is best
Electronic braking generally works better with ac motors than with dc types. For one thing, ac motor brakes are not limited by speed. Only motor specs, current, and voltage limit the braking of a three-phase ac motor. They’re also capable of developing a braking torque equal to their starting torque.
In contrast, dc brush-type motor brakes are limited to about 120% of their full-speed torque. It has to do with commutation current limits; commutators and brushes, when running at high speeds, are subject to flashover, a phenomenon that defeats commutation and wears out brushes. Electronic braking is also more difficult with permanent magnet motors, limited by magnet hysteresis and possible degaussing, or loss of magnetic strength.
There are, of course, many ways to bring a spinning shaft to a halt. Still common is mechanical braking. Essentially, friction brakes are coupled to a motor shaft, disengaging when voltage is applied to the motor windings. Removing voltage has the opposite effect; it applies the brakes.
One drawback with this approach is that brake motors tend to be large; many retrofit or upgrade applications will not accommodate them. Also, mechanical brakes have no torque adjustment, and will not work if the motor is powered by an adjustable speed drive or reduced voltage starter.