Why Do VFD's
Need Brakes?

In many motor applications, the load on a motor is continually changing. Using a hoist as an example, the motor is powered to lift a load to a certain height. Once this is achieved, that motor must allow the hoist to return to it’s starting position by lowering the load. When this happens, gravitational force helps push the hoist back down, and instead of consuming power to lift the hoist, gravitational energy drives the motor and generates power. This power flows back to the VFD, causing an overvoltage situation. To remedy this, something must be done with the excess power. This is where braking technologies come in.

Resistive Braking

Typical Braking Solutions
In most cases, excess power is dissipated through an external bank of resistors. When the VFD detects an overvoltage on the DC bus, it closes the brake transistor circuit, or “chopper” to shunt power towards the resistors. The resistors dissipate the power until the DC bus voltage drops sufficiently and the VFD can reopen the transistor circuit to function normally. This allows the VFD to properly drive the motor under load when needed, and protect itself from harm when power is flowing from the motor.

The “Downside” to Resistive Brakes
These external resistors, or “toasters” can be a hazard. Braking resistors dissipate electrical energy through heat, and can reach temperatures as high as 375OC. This is an obvious safety hazard in many industrial environments. In environments such as sawmills, these resistors require frequent cleaning, and can represent a significant fire risk if this maintenance is neglected. While toasters are usually protected by high limit thermal switches minimizing the fire risk, this can cause equipment to shut down unexpectedly leading to expensive downtime. Braking resistors dissipating energy as heat also waste the opportunity to re-use that energy elsewhere on location. This is costly and environmentally unfriendly.

The Solution

Regenerative Braking
Instead of using a braking resistor, it is possible to use a regenerative drive. When the DC bus voltage on the system is above a certain threshold the power section can commutate current right back to the line. When the DC bus voltage falls back below the threshold, the power section functions normally to drive the motor with no interruption to normal functioning.

The Benefits of Regenerative Braking
With a regenerative braking solution, the need for dangerous and wasteful braking resistors is gone. There is no need for tedious brake maintenance, and the excess power can be reused in the same facility. The switching ability enables two-way flow of energy through the VFD, meaning no energy is wasted. This lowers electrical costs, and reduces the environmental footprint.

The Siemens PM250 Power Module
A great entry to regenerative braking is the Siemens G120 VFD with a PM250 power module. This module is equipped with regenerative braking and also has a very desirable “power factor” or “PF” of 0.94, saving the owner additional cash. This PF is compared to a typical drive without regenerative braking’s PF of 0.81. This demonstrates that not only does this drive consume less power overall, this drive draws much less “apparent power” and therefore cleaner power from the line supply than a comparable drive under the same conditions. All these benefits from these regenerative drives means a lower power bill for the owner. The cost of this drive remains quite low when compared to competitors regenerative braking solutions. This provides a mechanism for industrial consumers to obtain all the benefits of a regenerative drive without the excessive cost of the competitors regenerative braking systems.

The Savings

What is the Power Factor? How is it Measured?
Power factor is a way to measure how efficiently your equipment is converting electrical energy into mechanical energy. Technically, it is the “apparent power consumption” divided by “real power consumption”. Apparent power is the square root of the sum of the squares of real and reactive powers. Real power is the actual amount of power drawn. When a system is performing efficiently, the power factor will be higher. A power factor above 0.90 is considered quite efficient for a VFD. Most power suppliers will add a surcharge for a poor power factor to the user’s bill. In BC, a power factor of 0.81 will earn you a 9% surcharge.

How Much Power Can I Save With Regenerative Braking?
Say you had a bucket hoist, using two 75HP VFD’s, drawing 55KW/hr each. If each cycle without regenerative braking consumed 10.7kW, using regenerative braking could lower that to a mere 5.8kW. That is a 54% energy savings! Without regenerative braking, all that power is being discarded as waste heat.

Are the savings significant?
Based on the same VFD’s above, let’s say you ran them for 8 hours a day, from Monday to Friday, for one month. At a standard BC Hydro rate of $0.1247/kWh, you would save $605.36 per month. In addition to the improved power consumption, BC Hydro would eliminate the 9% surcharge for the poor power factor. Improving your power factor from the 0.81 of a typical VFD to 0.94 using the regenerative braking of a PM250, would save approximately $118.44 a month. In a year, you could save $8,685.60 on just two drives. That is no small amount of savings!

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