Large DC and AC motor drives are often supplied with terminals for mounting braking resistors. What are these resistors and how do they slow down a machine? What hazards and precautions must be considered?
Any machine in motion has kinetic energy. This energy is the result of some stored potential energy being "dumped" into the motor or actuator, resulting in the conversion of energy into completed work. When a machine is in motion, there are two factors that help determine how much energy must be dissipated to bring it to a stop: mass and velocity.
High kinetic energy can come from a massive system or high speed, or both. When the system has a large mass (large flywheel, heavy drum), we call it a high inertia system, which means that the motion will continue unless it is acted upon by a significant external force (Newton's second law of motion).
When both kinetic energy and inertia are high, like many industrial equipment, the process of slowing down or stopping a system can be a daunting task.
Figure 1Internal view of the braking resistor. **Courtesy of EAK Resistors.
To freeze frame, you only have a few options. In a technical sense, "stopping" means converting the kinetic energy of the system into other by-products that will result in a gradual decrease until the kinetic energy reaches zero.
If the device continues to spin, there are a few things that can stop it:
Friction between the bearing and the contact point (this can take a long time to slow down if the bearing is good).
If the geometry of a moving object interacts with the air around it, the air resistance may be unexpectedly significant.
If the device is driven by an electrical or fluid system, the machine may find itself trying to drive the fluid or current in reverse. This acts as a form of frictional resistance.
In all three cases, we have to ask a key question about kinetic energy. You can't simply remove energy from a rotating object, so that energy actually goes **?
In all three cases, heat is generated at the energy conversion point. If the air around the bearing or object is the source of friction, that is the heat of the day. The greater the friction (e.g. bearing wear), the more heat is generated. Because of this, ** maintenance checks for problem hotspots in the machinery to indicate excessive wear and friction.
In the case of reverse drive of fluid and current, this also generates a lot of heat. Hydraulic fluids are very viscous and can stop moving faster and dissipate heat faster, so they tend not to be much of a problem. However, in the case of electricity, driving the current back into the drive circuit can have an immediate and catastrophic effect on the control system.
Figure 2Large floor-mounted braking resistor. **Courtesy of EAK Resistors.
Resistors are one of the most common passive electronic components. The simple function of the device is to convert electrical energy (current) into heat. This can accomplish a variety of tasks, such as dividing voltage in series circuits, limiting current in solid-state circuits, or simply generating heat in baseboard heaters.
In the case of heavy load rotation, it is crucial that the kinetic energy dissipates safely as soon as possible, as a long deceleration can mean a downtime of minutes or even hours. Resistors are the obvious choice for this task, as long as the resistor is chosen correctly for the task.
The amount of energy to be lost is enormous. No small prototype board resistor is suitable for the job. These resistors are rated between 1 and 8 watts and are only 5-10 watts at maximum.
To get an idea of what kind of power rating this resistor might need to perform braking tasks, imagine a resistor with a diameter of 3A 3-foot (1 meter) steel flywheel with a mass of 330 pounds (150 kilograms) that spins at 250 rpm. We want to slow it down to a stop in 3 seconds.
For rotational motion, the kinetic energy equation is as follows:
energy = frac times moment of inertia times angular velocity 2$$
Moment of inertia of the cylinder:
moment of inertia = frac times mass times radius 2$$
moment of inertia = 1875~kg \cdot m^2$$
Angular velocity of this wheel:
angular velocity = frac$$
angular speed approx 26 frac$$
So, substituting the first energy formula, this system has an energy of 6337 joules.
This unit, joules, is not a particularly common unit, but when the energy is divided by the time (the three-second stop time target), we will see the maximum power dissipation in watts.
power = frac=2112 watts$$
Therefore, when checking the power brake resistor spec sheet, we should expect to see a power rating of hundreds to kilowatts.
You can't simply install a braking resistor on the motor leads, as this will transfer the power from the drive unit through the resistor. Instead, the drive unit is equipped with a brake resistor terminal specifically for this purpose.
Figure 3Some braking resistors don't need to be that big – smaller masses, speeds, and dump rates will vary. This one is ceramic.
Many VFD and DC motor drive units have clearly marked resistive terminals.
When a controlled stop is required, the drive unit automatically switches the internal contacts from the drive source to the output resistor. Suddenly, instead of receiving power from the mains, the motor power is rerouted through resistors. The motor is now a generator, and the generator drives the resistor to dissipate energy faster.
The power dissipated by the resistor is not constant. When the machine is traveling at full speed and a stop is applied, the kinetic energy is at its maximum, so the power dissipated by the resistor will also be maximum. As the load slows down, the energy also dissipates.
In our example, it would be wrong to say that the resistor consistently consumes 2112 watts throughout the deceleration cycle, but it will be at that value for a while, so the resistor must be the right size.
Ultimately, when the load is close to stopping, the energy dissipation will be very small. This total energy expenditure is calculated by calculus (the sum over time when the situation changes).
In a physical sense, this appears to the viewer to be a huge change in velocity, as well as a large amount of heat output at the beginning of the resistor, but gradually decreasing over time.
In modern drives, the amount of current discharged into the braking resistor is carefully adjusted to control the power dissipation and thus the braking effect, so dynamic control is more useful in a variety of situations.
EAK brake resistors are suitable for large loads. The drive unit can take advantage of these devices to achieve faster deceleration cycles. In some cases, the energy may not be converted into heat, but it may be converted into battery chagrin circuitry. In these cases, we call this "regenerative braking" and are used in electric vehicles, train locomotives, etc.