How are VFD and motor frame sizes defined?
When talking about the size of variable frequency drives (VFD) and motors, the word frame is often used. What is the definition of VFD and motor frame size? That is shortly explained in this post.
Why using frames?
The main reason is standardization. Manufacturers produce the equipment in discrete steps when it comes to size or capacity. Development is faster and cheaper. The standardization reduces production cost by using less types of components and leveraging higher volumes. It allows to have common spare parts. The discrete frames can be type tested so that expensive testing can be avoided. Finally, standardization increases quality and reliability as the manufacturer can build up on the experience from the installed base.
A. VFD frame sizes
The standardization of VFD frame sizes is not governed by standards. Thus, it is more the decision of the manufacturer. The frames are usually based on certain steps in the inverter size or the size of a power cell.
The capacity of a VFD is mostly defined as apparent output power or output current. Certain overloadability may be available on top of the continuous power rating (e.g. 110% for 60 s every 600 s). The frame sizes are often defined by the rating of power semiconductor or rating of a power cell.
- Voltage source inverters with central DC link
- Frames are mostly defined by the rating of power semiconductor. Larger VFD frame uses power semiconductor with higher current rating or it utilizes parallel connection of inverter modules.
- Voltage source inverters with power cells
- The frame sizes depend on the rating of the power cell and total number of cells
Note that some VFDs scale the power with current, i.e. using semiconductors with higher current rating, paralleling the semiconductors or (more often) paralleling the inverter modules. Other VFDs scale the power with voltage, i.e. adding more semiconductors or more power cells in series. For more information refer to our post:
What changes between the VFD frame sizes?
Differences between two VFD frames may consist of:
- Different model of semiconductor (e.g. with higher rated current)
- Different power cell hardware (not only semiconductor but maybe also thicker busbar, larger dc capacitor, another snubber circuit etc)
- Identical power cells but in higher quantity (more power cells in series)
- Paralleling of power modules or inverter modules
Topology based examples
Case A: Load-commutated inverter (LCI)
- 2-3 types of thyristors with different current rating, say 1’200 A, 1’800 A and 2’500 A
- Number of thyristors in series defines the output voltage
- VFD output power is a result of thyristor model and number of thyristors per phase
Case B: 3-level neutral point clamped (3-L NPC) voltage source inverter (VSI)
- Several different power modules, rated e.g. 4 MVA, 7 MVA, 12 MVA
- Higher power achieved by paralleling of entire inverter modules, e.g. 2 x 7 MVA, 2 x 12 MVA
- Depending on required output power we can combine e.g.:
- 4 MVA: using 1 x 4 MVA single inverter
- 12 MVA: using 2 x 7 MVA in parallel
- 16 MVA: using 3 x 7 MVA in parallel
- 22 MVA: using 2 x 12 MVA in parallel
Case C: Cascaded H-bridge (CHB) voltage source inverter (VSI)
- Cell based topology
- Several cell types with different rated current (usually many smaller steps, e.g. 70 A, 88 A, 105 A, 132 A, 175 A, 220 A, …, 525 A, 612 A, 700 A)
- Number of cells per phase (series connected) depending on output voltage
- Typical voltage classes 3.3 kV, 4.16 kV, 6.0-6.6 kV, 10-11 kV, 13.8 kV
- Output power capability is a result of voltage class and cell current rating, i.e. number of cells in series and current rating of the IGBT inside the cell
Output current and power ratings of possible CHB drives are listed in Table 1. The numbers shall be considered for illustration purpose (though realistic). Each combination of output current and power could be considered as one specific frame size of this VFD. Alternatively, one could define frame sizes of the power cells just based on their output current and the VFD power hardware is identified by specific power cell and the quantity of cells.
Case D: Modular multilevel converter (MMC, M2C)
- Cell based topology
- Only 2-3 types of power cells (streamlined hardware design for high repetition and optimized cost), e.g. 700 A and 1100 A output
- Output power depends on output voltage class and type of cell
B. Motor frame sizes
For motors, it is not the mechanical power that defines the frames. Instead, so called “shaft height” is defined. It is the distance from the center line of the motor shaft to the mounting surface (typically bottom of the motor feet). Such standardization shall ensure mechanical compatibility between different manufacturers and shaft alignment of motor with the driven load.
The shaft heights are defined in IEC 60072-1. Standardization of motor frame sizes creates the advantage that there are only a limited number of different flange dimensions and shaft diameters.
- S = Short
- M = Medium
- L = Long
IEC frame sizes
IEC frames: 355, 400, 450, 500, 560, 630, 710, 800, 900, 1000, 1120, 1250, 1400, 1600 etc
Shaft height and motor power
The shaft height does not determine motor power directly. The tendency is clear: the bigger the shaft height the larger the mechanical output of the motor.
Motor output power depends on additional factors such as rotor length, rated speed, allowable temperature rise, number of pole pairs, rated voltage, motor type (induction versus synchronous), cooling type, area classification, ambient conditions, type of inverter (if applicable) etc.
There is a formula expressing the output (mechanical) power of an electric motor:
What can be observed:
- Shaft height ~ D/2 thus having major impact on motor output power
- D²⋅L is proportional to rotor volume which is related to motor torque
- Then, D²⋅L⋅n is proportional to power
- Machine constant is the most “tricky” part in the equation as it defines sort of power density and combines factors such as motor type, cooling type, allowable temperature rise etc. Thus, it can differ from one motor to the other significantly.
As explained, there is no simple correlation between motor frame size and its output power. Anyway, very indicative ranges are shown below:
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Summary
VFD frames:
- Frame is directly linked with certain output power or output current
- Frames are not standardized in international standards
- VFD manufacturers are free to define the frame sizes themselves
- The frame sizes typically reflect the semiconductor/power cell capability and their quantity (in series or parallel)
Motor frames:
- Frame is defined by the heigh of the shaft centerline
- The frame size does not directly tell the output power
- Frame sizes are unified in international standards such as IEC or NEMA
- Output power of the motor depends on far more factors than just the shaft height
- There is no simple way to translate the frame size (shaft height) into mechanical output power of the motor
Take away
VFD frame sizes are discrete steps of VFD rating in terms of output power or output current at given voltage, typically linked with hardware steps of the specific vFD model. The VFD frames are globally standardized. Instead, each manufacturer defines his own frame sizes based on the hardware design.
Motor frames are associated with discrete steps of the shaft height. The shaft heights are defined in international standards such as IEC or NEMA.
References
[1] Power scaling of VFD, MB Drive Services, November 2020, available online, https://mb-drive-services.com/power-scaling-of-vfd/
[2] Power, speed and torque, MB Drive Services, October 2020, available online, https://mb-drive-services.com/power-speed-and-torque/
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