The Use of Rectifiers in Brake Motors
By: Roberto Vasquez
A rectifier is an electrical brake control device used to take inbound AC voltage and convert it to DC voltage needed to energize a DC brake coil. AC type brake coils are available in the industry but are less common for us.
There are several versions of rectifiers in the industry. In this article we will review the ones used in our brake motors, figure 1. We use three types: half-wave, bridge and fast excitation. Special attention is needed in regards to specifying the appropriate rectifier based on the available voltage supply, brake coil voltage rating and net effect in regards to braking behavior.
Before we start to talk about where, how and why a rectifier works, we should discuss the functionality of a brake on a motor.
The way that a motor's brake works is the following: In an off state, meaning no power to the motor or the brake, the springs inside the brake housing push a disc lined with a friction material up against the motor to create a rotational stopping force, figure 2. The brake "releases" by receiving voltage across the various windings inside the housing, creating a magnetic field. This magnetic field creates an attraction force, which pulls on the disc and opposes the force from the springs, figure 3. With enough voltage/magnetic force, the disc will pull all the way back and release its hold on the motor shaft allowing it to spin freely.
The rectifier acts as a filter to allow only DC voltage to reach the DC brake coil, figure 4. Pay special attention not to supply the DC brake coil with AC voltage. This would result in the brake coil heating up from the vibrations brought on from the AC supply and result in overheating and eventual burning through of the wire insulation. The rectifiers use a set of diodes wired in such a way that the oscillating input is reduced to a single polarity.
For scenarios that call for voltage supply of 460V, a half wave rectifier and a 195V DC coil are used. For times that the voltage supply is 230V, either a half-wave rectifier can be used with a 102V DC coil or a bridge rectifier can be used but with a 195V DC coil. Bridge rectifiers offer smoother DC supply than a half wave rectifier. There also exists a 24V DC brake coil, but typically that connection is done directly from the controls, as a specific feed thus no rectifier is necessary.
The half-wave rectifier works in that it only allows the positive portion of the AC sin wave and smoothes out the voltage in between waves. This allows a single polarity voltage to reach the brake, figure 5. The bridge rectifier works in that it not only allows the positive aspect of the sin wave but also converts the negative aspect into a positive wave with some smoothing out in between. As a general note, a half-wave rectifier allows 46% of the input voltage to pass through. This is approximately 50%, (only half the wave/voltage goes through) but due to some inefficiency, 46% is closer to actual. In a bridge rectifier, the two halves of the sin wave equate to about 90% throughput of the input voltage.
A fast-acting rectifier works to help the brake become energized faster and release the motor quicker, figure 6. This has the benefit of the motor seeing less of a momentary resistance when it turns on. This momentary resistance is associated with the quick but gradual time it takes the brake springs to retract far enough as to no longer apply the stopping force. The fast acting rectifier allows for a sudden jolt of voltage (~90% of inbound feed) to pass through but then after a few milliseconds switches to allow approx 46% of the input voltage. This works in essence as a hybrid of the half-wave and the bridge rectifier with a time-release aspect.
Often times the function of the brake is intertwined with the motor to create a smooth engage/release transition. When the motor is off, the brake is engaged. When the motor is powered to operate, the brake should release its holding effect allowing the motor to turn freely. For this same reason, the brake is often wired together with the motor which allows the two to function simultaneously.
Another consideration to the way that the brake/rectifier can be wired is the time it takes for the brake to engage and hold onto the motor. The standard connection is via AC switching which allows the braking to occur at a standard speed. This connection has the brake switch for on/off ahead of the rectifier, usually through means of the contact breaker. This AC switching allows for the magnetic field to dissipate/weaken at a certain rate. The springs in the brake always have a pushing effect, so once the magnetic field that was holding back the brake disc dwindles, the springs will push the disc with the friction lining plate against the brake and apply the holding force.
In applications where you need the brake to be applied faster in order to keep the motor shaft from moving, DC switching is an option, figure 7. DC switching allows the magnetic field to dissipate even faster than AC switching. For DC switching, a switch is not only necessary ahead of the rectifier but also between the rectifier and the brake coil. It is necessary to know that DC switching results in high spikes of voltage at the switching contacts. This negative effect can be mitigated through the use of protective wiring in the form of a Varistor wired in parallel to the brake coil or by use of a recovery diode also in parallel to the brake coil.
To make use of a DC brake with an AC power supply, the means to the end is through use of a rectifier. Knowing what voltage you have available is usually the telling sign of what type of rectifier you will use, but other times knowing what is readily available will be the decision making criteria. As a general rule, have an idea of what you have to work with and see which pieces will help complete the picture. Have an idea of what the main task is at hand so that you can determine you need a half-wave or bridge or fast-acting rectifier. Also, consider whether you need to apply braking in a shorter period of time, hence DC switching or if a standard time is satisfactory. Consult the manufacture to determine these times.
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