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08-27-2013 09:50 PM

VFD minimum running frequency

What is minimum frequency that a variable frequency drive (VFD) can run?
08-27-2013 09:53 PM
Top #2
08-27-2013 09:53 PM
AC drives with DTC motor control can operate motors at zero speed with 100% torque. For example marine winch applications using often open loop DTC motor control to avoid the speed feedback encoder device.
08-27-2013 09:54 PM
Top #3
08-27-2013 09:54 PM
Most modern VFDs can operate at very low frequency outputs. A reasonable expectation for open loop (no motor speed feedback) operation would be .5Hz. That is true for variable torque and constant torque loads. With motor speed feedback, typically an encoder, you can get 100% torque at zero speed.

All of these modes can easily be simulated with a VFD and motor on a work bench. How they will actually operate your manufacturing plant may require some extra planning. As was pointed out before, the application is key.
08-27-2013 09:55 PM
Top #4
08-27-2013 09:55 PM
As has been mentioned by myself and my learned colleagues, it is possible to have a motor/vfd combination which is capable of high torque at zero frequency, but without information related to load/ application an informed answer is not possible. The other aspect to consider is hoist/elevator applications which require additional safety considerations related to suspended loads and emergency braking.
08-27-2013 09:56 PM
Top #5
08-27-2013 09:56 PM
Industrial cranes (hoist motions) have safety and performance as most important features,here a motor encoder bring additional safety for control and supervisions.Marine winches have reliability and performance as most important features and here open loop DTC motor control a possible solution.Yes the application itself is a important thing, which decide open loop or closed loop motor control.
08-27-2013 09:57 PM
Top #6
08-27-2013 09:57 PM
I am agree with Mikael. I , my self, tested this experience. I started an induction motor in the speed near the zero. Additionally, i worked with starter frequency converter for starting of gas turbine 200MVA. Usually it starts from the speed 100 r.p.m. But i started it from zero.

I think it is not important to start from. It is important what time it can work in minimum frequency. Smooth and pricise.
08-27-2013 09:57 PM
Top #7
08-27-2013 09:57 PM
There are 2 issues the VFD and the motor. The VFD will have 1 of 3 types of controls methods: V/Hz, sensorless vector or flux control. V/Hz will control to ~5Hz, sensorless vector ~3 Hz and flux down to 0 Hz. A standard motor cools itself based on speed of operation, in terms of frequency ~30Hz. However, you can get a motor with an external cooling system that can literally allow full torque at 0 Speed!
08-27-2013 09:58 PM
Top #8
08-27-2013 09:58 PM
Take in consideration, that at low output frequency the IGBTs are more stressed. I'm not sure, but I think the range is between Zero and One Hertz. So keep in mind, that one size bigger Drive max be needed.
08-27-2013 10:01 PM
Top #9
08-27-2013 10:01 PM
 I think the first two responses from Danial Mahoney ("What is your goal?") and Dennis Braun ("The limitation is not in the inverter but in the application, . . .") are still the most relevant.

I did not see mention of what is connected to the VFD (VFDs are not always used to power motors) so let's assume it is a motor.

In simplest terms, the VFD starts at zero hertz (Hz) and can run there indefinitely. I can make such a bold statement because at that frequency, and with the motor stationary, there is only magnetizing current being taken through the semiconductors. The VFD normal cooling system can disperse the heat that would be generated in this situation.

The motor in this case is likely one of two types; Induction or Permanent Magnet (PM). We also need to be clear when discussing 'frequency' and 'speed' as they pertain to motors. While they are related, they are not necessarily the same.

In the case of the Induction motor, at zero frequency or even up to 0.2 or 0.3 Hz there is little or no induction of the rotor bars taking place so only magnetizing current and low level active (torque producing) current is being taken from the VFD, even at zero speed.

A typical high efficiency induction motor needs approximately 0.75 to 1.0 Hz of rotor bar frequency to develop full torque. That is to say, with a locked rotor condition (zero speed) the motor shaft would be developing rated torque when the rated (slip) frequency and rated stator magnetic field strength is circulating around the rotor. This relationship of frequency and speed is consistent throughout the rated speed range of the induction motor.

In the PM motor, the rotor flux is established by means of the permanent magnets rather than induction. Therefore, with the proper the stator flux the motor shaft would be developing rated torque almost immediately as the stator frequency beings to rise. This also means that the PM motor, with proper stator magnetic field strength, would tend to hold its rotor position relative to the stator field poles, even at zero Hz.

If position accuracy is the objective, an encoder feedback and a field oriented control algorithm are likely to give the best results.

Whether it is an induction motor or PM motor, the important factor is the proper management of the stator field flux and orientation so that the needs of the rotor are fulfilled by the VFD. As Tom Michalski mentioned, attention must also be given to the cooling needs of the motor at low speeds.

In either case, the properly sized VFD is not likely to have an issue with low frequency operation.

I hope this information is helpful, more...
08-27-2013 10:02 PM
Top #10
08-27-2013 10:02 PM
I also recommend to have a perspective view of the assembly VFD Motor and in witch application you use it.Some examples:* recommended min 25-30Hz for pumps, sometimes ventilation* min rpm for cooling a motor, if equipped with internal ventilation- min Hz for full load @ zero speed, if no encoder
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