Adaptive design of UPS for motor loads

In some applications where it is necessary to ensure that the load is not disconnected, customers may find that the UPS frequently has DC BUS high protection or negative power protection. Some customers may think that this is a quality problem of the UPS. In fact, in most cases, this is caused by the motor load behind it. In industrial applications, motors are a major form of load. When the key links in industrial applications must have a sufficiently high power protection level, the coordination between UPS and motor loads is a factor that must be considered.

Usually, the original intention of UPS design is to protect key IT equipment, and the circuit structure is mainly designed based on the characteristics of IT equipment. For example, current IT equipment mainly uses switching power supplies, and EU regulations stipulate that equipment above 75W must have power factor correction. Therefore, UPS mainly faces loads with power factor correction, which usually has the characteristic of a constant power load with a power factor close to 1. In terms of high-power electrical equipment, there are still some old equipment in use. These equipment are usually based on 6-pulse rectification or 12-pulse rectification technology, and the characteristic is a constant power nonlinear rectification load.

Whether it is a switching power supply with PFC or a pulse rectifier power supply, the real part of its power can only be positive, and the energy will not be fed back to the mains. Therefore, in the design of UPS, more attention is paid to the reliability under constant power load, the harmonic control ability under nonlinear rectifier load, and the voltage steady-state accuracy and dynamic recovery speed, without special requirements for energy feedback ability. Especially after UPS has a lot of intelligent design, the energy fed back from the load to the DC bus of UPS is often treated as a fault state. Therefore, when there is a motor load, the energy generated by the motor regeneration can easily trigger the protection conditions of UPS.

On the other hand, the commonly used structure of UPS circuit architecture is rectification + battery boost + inverter. A large part of UPS rectification and battery boost parts use Boost or deformed circuits. Energy can only flow from the mains and battery to the DC bus, but not in the opposite direction. In this way, even if the software allows energy feedback, when energy feedback occurs, since the energy is stored in the DC bus, the DC bus will increase, which will eventually cause the UPS to trip protection.

The characteristics of motor loads are completely different from the common switching power supplies of IT equipment. They have many working states such as starting/braking, and they vary greatly with the load behind the motor. They are completely different from IT switching power supplies that only have loading/unloading. Therefore, the specific solution also needs to consider the load behind the motor and handle it separately.

The motor has a high transient impact when starting. If there are no additional auxiliary measures, the UPS power supply needs to be able to supply very large power instantly. UPS designed for IT equipment is generally only designed based on 2 times the power in a short period of time, and some UPS are only 1.5 times. For loads with higher power, the software current limiting algorithm or hardware current limiting circuit will take effect, thus affecting the motor start. Fortunately, UPS is generally designed with Line Support function, which can be solved by bypass power supply when the load power is large. However, in battery mode, the power cannot be shared through the bypass, and there is a possibility of abnormal motor starting process. For this reason, in occasions where the ability to supply current instantly is very critical, it is necessary to choose a UPS with higher power.

The motor has energy regeneration during braking. The energy fed back at this time is not only the energy stored in the motor itself, but may also include the inertia and potential energy stored in the load connected behind the motor. Take the elevator as an example. When the elevator goes up, the motor needs to provide energy. When the elevator goes down, if the weight of the elevator exceeds the resistance during the descent, it will become a power generation device, driving the motor to generate electricity. In this way, the regenerated electricity may be fed back to the UPS.

In addition, there is another factor that needs to be considered in applications with motors, which is the variable frequency speed control device. Different variable frequency speed control devices have different effects on the UPS system. The most common frequency converter practices are as follows:

At the input end is a six-pulse rectifier and an additional DC or AC side filter, and a braking resistor is connected to the DC bus. When the motor generates energy feedback, the DC bus of the inverter will be charged high. When the DC bus reaches the preset voltage point, the feedback energy is consumed by turning on the braking resistor control. This approach is currently the most common method in the industry. Its advantages are simplicity and reliability, and for UPS, the inverter is a standard nonlinear rectifier load, which is very close to IT loads. Of course, its disadvantage is that the energy fed back by the motor is converted into heat and consumed, and is not reused.

In order to save energy, some high-end inverters adopt a back-to-back structure, while ordinary inverters can also return the energy fed back by the motor to the input end by adding an energy feedback module. See the figure below.

For this type of inverter, the electric energy regenerated by the motor will still be fed back into the UPS, causing the UPS to face similar problems as directly connecting the motor.

There is also a special type of inverter that uses a matrix converter structure, as shown in the figure below. Since there is no energy storage element, all energy is directly transferred between the input and output ends, which is no different from directly connecting the motor to the UPS.

Of course, if the UPS DC bus has a large enough capacity, and the DC bus charging caused by the motor feedback energy is within an acceptable range, then the UPS can still be used with confidence because the average energy provided to the motor is still positive. However, UPS usually does not have such a large DC bus capacitor, so other ways must be considered to solve the motor load energy feedback problem. For simplicity, only the solution for a single UPS is discussed here. The negative power protection problem in the parallel system involves many other parallel system module design issues, not just the processing of motor regeneration energy.

Some UPS use a fully bidirectional structure in circuit architecture, as shown in the figure below, where PFC, inverter and even battery DC/DC can ensure bidirectional energy flow.

In principle, as long as the software removes the power restrictions at the PFC level and the inverter, this UPS can work in bidirectional mode: when the motor generates energy feedback, in AC mode, the energy is fed back to the AC through PFC; in battery mode, the energy is fed back to the battery through bidirectional DC/DC. However, there are some factors that must be considered, such as:

Does the grid allow energy feedback?

What specifications does energy feedback to the grid need to meet?

What charging power does the battery allow?

The first question is whether the power grid allows regenerative energy feedback. The requirements of power grids in different places may be different. For some loads with particularly high power, the power grid may not want energy feedback for stability reasons. If the input end uses a normal diesel generator, then energy cannot be fed back.

Under the premise of allowing energy to be fed back to the grid, the safety issues faced at this time must be considered, which is what solar power generation systems have already faced. When energy is fed back to the grid, and the grid is powered off at this time, the so-called island effect problem will occur. If the grid has abnormal conditions such as short-term low voltage during the feedback process, the energy feedback of the UPS should also work normally for a period of time. For this reason, technologies suitable for renewable energy generation, such as island detection and low voltage ride-through, need to be equipped on the UPS.

In battery mode, the current allowed by common lead-acid batteries during charging and discharging is different, and the maximum current during charging is much smaller. This means that if the load feedback energy is large, the charging current will also be large. Therefore, in battery mode, it is necessary to use enough battery packs to share the charging current in order to be compatible with motor loads. On the other hand, the charging power of general UPS is equipped according to the capacity of common battery packs. If the charging power is to be increased, this part of the circuit also needs to be specially designed.

For UPS with other circuit architectures, such as the common structure below, the battery boost and PFC work unidirectionally, which means that the motor regenerative energy cannot be fed back to the AC power or battery, and another solution must be found.

In AC mode, the simplest solution is to use bypass. As long as the energy fed back by the load is found to be too large, the UPS is switched to bypass mode to absorb the regenerated energy of the motor through the bypass. However, this method can only be used when the bypass is actually AC power and under normal circumstances, so its application has some limitations. If the UPS is required to work with motor loads regardless of whether it is in AC or battery mode or using a generator as input, other methods must be used.

Another simple way that is not limited by the AC power and battery modes is to add a braking resistor to consume excess energy like the inverter. This design is very mature in the inverter and can be easily transplanted to the UPS. Since the traditional UPS does not have an IGBT specifically used for braking, the braking resistor and the braking IGBT need to be designed separately as a module and used as an optional accessory as needed.

Energy feedback module is also a mature technology in inverter, and of course it can be used here. However, the principle of energy feedback module is to convert the energy fed back by the motor into AC and return it to the mains. Therefore, in battery mode or when the input is a generator, the energy feedback module cannot be used.

In the design of UPS chargers, a common practice is to draw power from the DC bus and charge the battery after reducing the voltage through the circuit. In this way, a workaround is provided for the processing of motor energy feedback: whether in AC mode or battery mode, the excess energy is transferred to the battery for storage through the charger. When the battery is charged to a certain level, it switches to battery mode and releases the energy to a relatively low level. In this way, by slightly reducing the battery backup time, the motor load problem can be solved. This process is shown in the figure below.

The figure shows the standard flow of energy. In mains mode, energy is generated from the mains, through PFC, DC BUS, INV to generate AC voltage output and provide it to the load, while the charger draws power from the DC BUS to charge the battery. In battery mode, battery energy is provided to the load through DC/DC, DC BUS and INV.

When the motor generates energy feedback, the energy flow direction will change. In the AC mode, if the BUS voltage is high due to the feedback energy, the AC power supply needs to be stopped and the charger transfers the energy to the battery.

When the energy feedback ends, you need to check whether the battery is fully charged. If it is fully charged, you need to release part of the power in battery mode to leave room for the next motor energy feedback.

In battery mode, it is relatively simple. As long as the BUS surges due to the inverter power recovery, the battery DC/DC is turned off and the charger is turned on until the motor energy feedback is completed, and then the battery DC/DC is switched back to work. The advantage of this solution is that the energy fed back by the motor will only return to the battery and then be released at the appropriate time later, instead of returning to the mains, thus preventing problems caused by similar solar grid-connected power generation methods.

Obviously, this process is very consistent with the principle of hybrid electric vehicles. Similarly, intelligent battery energy management is also critical here. If the charging threshold is set too high, the battery may be damaged; if the discharge threshold is set too low, it may affect the backup time during power outages. Similarly, the capacity of the charger and the maximum charging current allowed by the battery are also important factors to be considered during design.

in conclusion:

In UPS applications, when the load is a motor that generates regenerative energy, the general UPS system is more likely to generate inverter negative power protection or DC bus high voltage protection due to the problem of motor braking energy being fed back to the UPS DC bus. In order to be compatible with this type of load, the UPS system needs to add additional functional modules to achieve reliable operation.

The most reliable and simple method is to equip the UPS with an optional braking module, which contains a resistor and a switch tube. When the motor brakes, the energy fed back from the motor can be consumed by the braking module.

In order to further improve energy efficiency, the battery capacity and charger power can be adjusted appropriately, and the battery energy storage method can be selected to recover the energy regenerated by braking. Through intelligent battery energy management, the battery pack can always have room for the next energy feedback, which can make the UPS work reliably under motor load conditions and save more energy.