Starting a three-phase a-c motor gets most of the attention. Starting current is high, causing a temporary voltage sag on the power system. Starting a high-inertia or a high-torque load can overheat the motor. Internal forces on the winding can get dangerously high during the start. We hear a lot about special “soft start” controllers that can mitigate some of these concerns.
Stopping, too, can cause problems. In most applications, we can just turn the motor off and let it coast to rest. But that won’t work for many machine tools, for saws in a lumber mill, for cranes, or for a palletizer at a cement plant. Several problems arise. One is that the process machinery involved can’t wait for a shut-off motor to coast down before beginning the next operating cycle. In tapping threaded holes, for example, keeping production going requires quickly stopping, then reversing, the tool in the bottom of the hole, to back it out and go on to the next hole.
Another concern is the unforeseen emergency. If a large band saw binds or strikes an obstacle, the drive needs to be physically stopped quickly. In such applications as the crane lift or palletizer, the motor-driven equipment must be firmly held when stopped so that the load won’t drop, or the palletizer move on past its proper position. For all those reasons, many motors need brakes for stopping, holding, or both. Two common alternatives are available. The first is the friction brake — either a disk or a shoe type, just as in automobiles — that is either mounted directly on the motor, or (in large sizes) mounted on the floor with a motor shaft extension coupled to the brake wheel. For safety reasons, these brakes are normally spring-set and electrically released. The brake release coil is wired to the motor’s power circuit, so that when the motor is shut off, when a wire breaks, or any other electrical failure occurs, the brake automatically sets and holds against the maximum torque the driven machinery can exert. (Very large brakes may use air or hydraulic power, controlled in a similar way.) Whenever the motor is running, regardless of its load, some power is being used to keep the brake released — which has a slight effect on overall drive efficiency.
The second braking method is “d-c injection.” A separate braking power supply, usually including a rectifier, connects a d-c voltage to two of the motor’s three leads whenever the motor is turned off. Packaged “electronic brakes” are available for most standard motors. This form of braking has two drawbacks. First, even a large d-c current develops only a fairly low braking torque. So this doesn’t usually provide a quick stop. A second problem is that once the motor has stopped, no holding torque is developed. Any machine that must be held in position at zero rpm still needs a friction brake.
A third method, “plug stopping,” involves reversing the phases of the a-c power source while the motor is running. That causes full torque to be developed in the reverse direction, which drags down the rpm in a hurry. Unfortunately, unless the circuit is fully opened at just the right instant, once the motor comes to a standstill it will immediately start accelerating in the reverse direction. And this method, like d-c braking, offers no holding torque.
Efficiency is not a concern with these stopping methods because braking power is used only during the deceleration process.
Richard L. Nailen, P. E