What is the right strategy to retrofit fix-speed motor into VFD operation?
As addressed in our previous post, a retrofit of a direct on-line (DOL) machine into VFD duty may be a very attractive choice [1]. This post focuses a bit on the practical aspects. What do I need to consider when converting a fix-speed motor into variable speed operation using a VFD? How to make sure that the VFD and motor will be compatible?
Motor friendliness
Motor friendliness is what we seek in general when retrofitting an existing fix-speed motor. Such motor had not been designed for inverter duty and care shall be taken to respect the design limits. After all, it is about drive-motor compatibility [2]. Of course, motor friendliness is a broad term and can be split into several subcategories.
i. Compatibility with insulation system
The inverter duty operation shall not adversely impact the machine insulation. This refers mainly to the inverter voltage waveform (peak voltages phase to ground, dv/dt rate of change, voltage steps etc). Factors such as age of the machine and original insulation design play an important role. Note that motor insulation levels are not much standardized globally and the selection of insulation system depends on the manufacturer or even the specific factory.
Unless the VFD has an output sine filter the impact of the inverter duty operation should be considered and carefully checked. The more original documentation on the motor design is available the better.
ii. Thermal design and effect of harmonics
Thermal aspects mainly refer to additional heating due to current harmonic distortion. VFDs with a sinusoidal output waveform do not cause any issue. Other VFD topologies introduce certain amount of harmonics. Their thermal impact ranges from negligible (multilevel topologies with lot of voltage levels and/or high switching frequency) to moderate to significant. Harmonics do not depend on VFD hardware only. They can be minimized by smart modulation and control strategy. Finally, the amount of harmonics and their thermal impact partly depends on motor design itself.
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Strategy to retrofit fix-speed motor
To fulfill the requirements of above two areas multiple strategies are possible. The motor friendliness can be addressed directly in the VFD hardware (e.g. output sine filter) or on a drive system level. A direct solution may not always be available. For instance, a VFD with an output sine filter is not available in the power or voltage range that you would require. Sometimes, you may be in a situation where you need to prioritize one aspect over the other as they cannot be entirely fulfilled at the same time. One means may smooth the current waveform but increase voltage peaks or vice versa. If in doubts, talk to a motor/VFD expert or contact us.
iii. Machine cooling
Besides thermal effect of harmonics the cooling concept shall be proven in general. Externally cooled machines typically do not have any problems with cooling capability at variable speed. However, self-cooled machines with a cooling fan on the shaft need more attention. At lower speed the cooling is naturally less efficient. It may be okay, especially for relatively narrow speed range and variable load torque profile. On the other hand wide speed range and/or constant torque profile are most likely not suitable for self-ventilated machines. It also depends on the duty cycle and the fact whether the operation at low speed and high torque is just temporary (thermal time constant helps) or continuously.
iv. Bearing considerations
Bearings may need to be modified or replaced if they are not suitable for the operation with VFD. There are few topics to be checked [3], such as proper lubrication (especially when continuously operating at low speed) or need for insulation (to prevent bearing currents and electric erosion of bearing races). A general recommendation is to have at least one bearing insulated.
Estimation of motor actual condition
Sometimes it can be useful knowing the remaining lifetime of the machine. This can be estimated by a service specialist. However, the best way is to perform a series of dedicated tests that reveal the overall health status of the machine and estimate the remaining service life. For example, ABB offers such capability – it is called LEAP [4] (Life Expectancy Analysis Program).
In LEAP program there is a high focus on the condition of the stator insulation system. LEAP can accurately predict the remaining lifetime of the insulation system (excluding inverter duty operation). Especially for larger and more expensive machines LEAP is a great choice with real added value.
Motors in hazardous area
Motors operating in hazardous area naturally require a bit more attention. For example, overheating due to harmonics may not just cause accelerated aging of insulation but could be a root cause of ignition or explosion. Therefore, the user shall be careful and ensure a compatibility of VFD and motor.
– Recertification (such as ATEX, IECEx etc) may be required
Benefit from historical data
As already mentioned in [1], the history of a fix-speed motor can be very valuable source of information for the retrofit project. While a greenfield project may have several unknowns and uncertainties, in the retrofit case the user may know quite well how the system will be operated. Utilizing this knowledge contributes to best operational experience – both technicaly and commercially.
Summary
Retrofitting a fix-speed motor into variable-speed operation may bring many benefits. At the same time attention shall be paid to ensure VFD – motor compatibility. There are obviously multiple criteria to focus on. It may not be possible to fulfill all of them at the same time. Therefore, compromise must be made. By combining user’s knowledge of the process and historical data from previous motor operation together with VFD manufacturer’s know-how on motor friendliness and compatibility there is almost no risk to fail.
Do you have a retrofit succcess story to share with us? Please let us know!
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References
[1] Retrofits of DOL motors: Low-hanging fruit? https://mb-drive-services.com/retrofits-of-dol-motors-low-hanging-fruit/
[2] How to choose a medium voltage VFD: Motor compatibility, https://mb-drive-services.com/category/choose-mv-vfd/
[3] How to diagnose bearing issues, https://mb-drive-services.com/how-to-diagnose-bearing-issues/
[4] ABB Ability™ LEAP, https://new.abb.com/motors-generators/service/advanced-services/abb-ability-leap-for-hv-motors-and-generators
