Load types and characteristics
Type of load is relevant for VFD hardware dimensioning and selection of most suitable VFD topology [1]. Sometimes the load type determines what kind of rectifier (front end) is required. Also, load type may affect some options and features that the VFD shall have. Therefore, it might be worth to look a bit closer at the load types and their characteristics.
The load, i.e. the driven equipment, can be classified based on several criteria. The categories of interest are among others:
– load torque characteristic
– dynamic behavior
– moment of inertia
– need for active braking
– elasticity of shaft string
– torsional and lateral behavior and sensitivity to vibration
Load profiles
Load profile describes the load torque as function of speed, often called “torque-speed characteristic” or “torque-speed curve”.
Torque-Speed characteristic
The torque-speed characteristic describes the load torque as function of rotational speed. The load profiles are typically classified in following groups:
– quadratic torque (square torque, variable torque)
– linear torque
– constant torque
– inverse torque
– combination of above torque characteristics
– special torque characteristic (user defined)
Kindly note that above classification is a simplification. Real torque characteristic might not exactly fit the category. However, for many considerations such rough classification is sufficient. For instance, rules like need for encoder would consider the load torque profile, minimum continuous speed and starting torque as inputs. Applications requiring high starting torque and operating at lower speed for longer period of time likely need an encoder. It does not really matter whether the load is a perfect purely constant torque or whether it has high starting torque that is either dropping or further increasing at higher speeds.
Quite often load torque envelopes are a combination of the load torque categories. For example, centrifugal compressors have a square torque profile. However, this holds only for the start up to minimum speed. Afterwards, the outlet pressure and flow is controlled by the process. The envelope might be a blend of quadratic torque from standstill up to minimum speed, constant torque in the continuous operating range and inverse torque in field weakening range (narrow range for compressors, but fairly wide for some other applications).
All these figures 1a, 1b and 2 show the load torque versus speed. The required motor torque shall be larger to provide additional torque for acceleration. VFD motor start is explained in [2].
Figure 3 shows a VFD driven motor start up from standstill up to nominal speed. The load has a quadratic torque profile. The motor torque has almost constant offset during the start-up process, i.e. there is almost constant acceleration. This is common behavior when using a speed reference ramp. Note that the figure is in speed domain and not in time domain. It shows the motor and load torque during start-up, but from this figure you can’t see how long time the start-up takes.
Dynamic behavior
The paragraph about torque-speed characteristics considered a steady state operation or start-up and braking with limited dynamics. Now we look at the dynamic aspects.
Many applications do not really require any dynamics. The rate of change of speed and load torque is so slow that it can be considered as quasi steady state from VFD point of view. Pumps, fans or compressors are typical applications with low dynamics and hardly any overload.
On the other side of the spectrum there are high dynamic applications, such as e.g. certain test stands. Some applications do not change speed very quickly, but the load torque varies quite significantly. Examples are extruders, crushers, some conveyors etc. Rolling mill drives are the champions of dynamics.
Motor control methods such as direct torque control (DTC) offer highest dynamic response and high static as well dynamic accuracy.
Moment of inertia
Moment of inertia (often just called ‘inertia’ and denoted as ‘J’) is relevant for start-up and braking considerations. The larger the inertia the slower the rate of change of the speed (both acceleration and braking!). Therefore, applications with large moment of inertia require longer start-up time than those with smaller inertia when applying the same acceleration torque.
The moment of inertia also defines the amount of energy stored in the rotating mass. The kinetic energy is a product of inertia and square of angular speed.
E_kinetic = ½ ⋅ J ⋅ ω²
Figures below show a start-up with small and large inertia. The VFD and motor have the same rating in both cases.
The inertia does not only impact the acceleration rate, but also the braking capability. The larger the inertia the slower the deceleration rate, i.e. longer braking time. In order to brake down the load to standstill the kinetic energy corresponding to the initial speed needs to be:
(a) regenerated into the grid
(b) dissipated as heat/losses
That brings us directly to our next point – need for active braking.
Note that the moment of inertia can be expressed in different units and mistake can easily be done. Be careful whether the value is corresponding to wr² or GD². Another topic is also the conversion between SI units and imperial units.
Need for active braking
Some loads require active braking while others can simply coast down. What are the decision factors?
Many loads can simply coast down. It means that the drive stops providing any torque (motor torque ramped down to zero) and the driven equipment is decelerating due to load torque and system losses. For example, pumps practically never require active braking. Their inertia is rather low and the load torque when immersed in the liquid is rather high → pump stops very fast (typically in less than 2 seconds). Compressors are also normally stopped by a cost down. Compared to pumps the coast down takes much longer time, but it is usually no issue. Anyway, compressor services are not meant to stop very frequently. Other application without active braking requirement is e.g. extruder.
Other applications require some sort of active braking. The reasons are:
(i) practical aspects
(ii) energy efficiency
(iii) safety concerns
(iv) process requirements
(v) application principle
(i) Practical aspects
One reason can be e.g. unacceptably long coast down. Applications such as certain test stands would be very limited when they would have to wait a long time (range of minutes) for the driven equipment to stop. Test stands with compressors or blowers are typical examples. Active braking is used to cut the braking time and increase the productivity.
Active braking can be realized as loss braking for small braking capability, resistor braking with braking chopper or regenerative braking. Selection depends on the frequency of braking, required braking power, braking time etc.
(ii) Energy efficiency [3]
Energy efficiency can be a motivation for regenerative braking. Especially when the braking periods are longer and braking is more frequent, the regenerative braking becomes technically and commercially attractive. As explained, the available kinetic energy tells how much is available for regeneration. Note that the system (typically grid) needs to be capable to absorb the energy. The larger the inertia the more energy is stored in the rotating mass and the more potential for regenerative braking when supported by the grid connection. Principle of regenerative braking is depicted in figure 6 in a simple way. The figure rather fits for a traction vehicle or a servo drive, but as illustration it is good enough.
(iii) Safety concerns
For some applications it is not acceptable to let them coast down due to safety aspects. There might be a time limit within which the load shall be brought to standstill. When the braking is safety critical, there are usually redundant braking systems (e.g. independent electrical and mechanical braking).
(iv) Process requirements
Certain processes require the speed to be maintained constant or almost constant while the load is dynamically changing. In such case active braking is required in order to keep the balance (example: rolling mills). Regenerative braking is often used for this purpose. For extreme dynamics a combination of regenerative braking and resistor braking might make sense.
(v) Application principles
In some applications the active braking (often regenerative) is an essential part of the functionality. These are typically loads that have the capability to drive the machine in generator mode. Examples are downhill conveyors, mine hoists, cranes etc.
Elasticity
Although the shaft string is normally a steel structure, it is not infinitely stiff. Components such as e.g. couplings need to tolerate certain misalignment and cannot be designed completely rigid. In fact, couplings are typically the sections with lowest stiffness and exhibit the torsional twist.
Torsional and lateral behavior
Torsional vibration is a general concern [4]. However, certain applications, particularly the turbomachinery, are especially sensitive due to very low inherent damping and corresponding large amplification. Torsional oscillation affects the lifetime of the mechanical components. Excessive torsional stress leads to fatigue or even mechanical failures. For more information refer to our series on torsional vibration.
For sensitive applications a “torsional stability check” is recommended to avoid adverse interaction between VFD control system and most critical torsional mode shape.
Lateral vibration needs to be taken care of as well. The lateral analysis is rather complex as it considers not only the rotor system, but also the bearings, foundations etc.
Summary on load types and characteristics
The character of the load impacts the selection of VFD topology (e.g. type of front end converter), hardware dimensioning of VFD (e.g. impact of high torque at low output frequency) as well as some options (e.g. braking chopper).
Since VFDs are used in almost any kind of industrial motor drive applications one would find practically all types of load torque profiles. The most common categories are quadratic torque (square torque, variable torque) and constant torque.
References
[1] How to choose a medium voltage VFD: Application requirements, https://mb-drive-services.com/choosing-mv-vfd-app-req/
[2] Motor start with VFD, https://mb-drive-services.com/vfd-motor-start/
[3] VFD energy efficiency series, https://mb-drive-services.com/category/energyefficiency/
[4] VFDs and torsional behavior, https://mb-drive-services.com/category/torsional/
[5] Medium voltage AC drives – Overview, https://new.abb.com/drives/medium-voltage-ac-drives
