The standard power source for an ac motor is a three-phase sinusoidal current waveform. Add the amplitudes at any point and you’ll see that the resultant vector sum has a constant magnitude.
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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.

Motor basics

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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