Braking capability of VFD fed motors
Not every VFD is capable of active braking. And not every application requires active braking either. In today’s post we are going to review the braking capability of most common medium voltage VFDs.
Let’s start with the need for braking. In fact, many applications do not require any braking capability at all. When the motor drive reduces its driving torque below the load torque, the load will decelerate and eventually stop (coast down). On the other hand there are systems where active braking is essential part of working principle or it is necessary due to safety concerns.
Below we list few examples of applications that usually do not require active braking as well as those that rely on one of the active braking options.
Applications typically not requiring braking capability
– Pumps
– Fans and blowers
– Compressors
Applications that usually require active braking
– Mine hoists
– Downhill conveyors
– Rolling mills
– Test stands
Braking capability
The VFD, depending on its topology and configuration, offers one of the following braking options:
– No braking capability
– Loss braking
– Braking chopper
– Regenerative braking
No braking capability mostly applies to VFDs with a diode rectifier (DFE) and with absence of any kind of braking chopper. They are perfectly suitable for applications that can coast down and do not require specific deceleration ramp. The time to stop depends on the initial speed before the torque was cut, on the counter torque during coast down process and on the inertia of the system.
Next option is a loss braking. This is a little trick how to get at least small braking capability without any extra hardware. The idea is based on temporary increase of losses in the system (inverter, motor). As you probably guess such braking power is rather low. However, it is sufficient in cases where you just want to reduce the coast-down time a bit and do not expect anything powerful.
Braking chopper is the well-known option for resistive braking. The chopper consists of self-commutated semiconductors and is connected to the dc link. A modulation controls the dc voltage in a defined hysteresis band. Regarding the braking resistor there are two obvious options: a) inside the VFD or b) external. The choice depends on the braking energy per cycle and frequency of braking.
Regenerative braking is the most sophisticated braking method. To make it happen a bi-directional rectifier, so called active front end (AFE) unit is needed. Instead of converting the electric power into heat it is supplied back to the grid (i.e. regenerated). Therefore, regenerative braking may help to conserve energy. However, it is not just black and white. For very seldom and short braking events the braking chopper option will actually be more efficient.
Braking and energy efficiency
When to select resistor braking via braking chopper and when apply regenerative braking? One important factor is the frequency of braking. Another one is the duration. For frequent and long periods of braking the regenerative option might be the best from energy efficiency point of view assuming that the grid can absorb the power. On the other hand, for infrequent and short braking events a braking chopper may generally be a better solution.
Summary
Whether active braking is required or not depends mainly on the drive application and process control. There are several technical solutions how to achieve desired braking capability. You just need to select the one that suits you the best.
- How to evaluate braking options from energy efficiency point of view?
- How does loss braking method actually work?
- What variants of braking chopper do exist?
- How to calculate braking time for specific application?
- What are the pros and cons of listed braking options? How to select the best option for your specific case?
Do you want to know more? Sign up for premium content.
