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thingstodo · 09-04-2010, 11:18 PM

Some points about Vector control and VFDs in general.

I don't exactly understand vector control, but I use it. The parameters are measured by the VFD when you set it up during commissioning. It sends some DC pulses through the motor and measures a few items. Every VFD vendor is pretty specific about what they call the numbers and how they calculate them. Basically they get a resistance and an inductance for the windings. As the rotor starts to spin, the current taken by the motor changes. Another couple of inductances are measured.

The result is that the VFD puts out a 'magnetizing current' at the right voltage and phase to build a 'good' magnetic flux across the gap between the windings and the rotor. This generates current in the rotor. The magnetic flux 'rotates' as the voltage and current are changed and 'drags' the rotor with it. The current in the rotor cutting through magnetic flux is what generates torque. If the rotor was made out of magnets, then the rotor speed would be identical to the VFD output frequency. I think those are called BDLC motors. Induction motors turn slower than the VFD frequency.

A Volts per Herz VFD puts out a pretty much linear voltage as you vary the frequency, up to the point when the voltage is at the maximum that the VFD has. Then the voltage stays the same and the frequency keeps going up.

A vector VFD puts out what the motor needs to turn at the specified frequency. It knows the magnetizing current. It increases the voltage with frequency to keep the magnetizing current the same as the speed changes. The current increases non-linearly with load on the motor. Pardon my personification of electronics.

I have 400 HP motors driving belt conveyors that are 1500 feet long started with tonnes (literally) of product sitting on the belt conveyor. And the VFDs we use don't put out thousands of amps. Our VFD vendors (Mitsubishi, Siemens, Toshiba, Allen Bradley) all use their own variants of IGBTs and they all seem to agree that a single IGBT should not have to deal with more than 100 - 150 amps at 600V. All of our VFDs 150 HP and below use 1 set (6) IGBTs, 200HP VFDs use two sets of IGBTs, the vendors disagree about the 300's, some use 2, some 3, the 400 and 500 HP VFDs have more variation in how many IGBT sets are used.

Driviing more current into a motor gives more torque for the same speed. If a operations person screws up and overloads a belt, we can put 200% current through the motor for a couple of minutes to get it cleared. But we keep an eye on that motor for an hour or so to make sure the temperature is not rising.

There is a limit. I have not tried 300% on a larger motor, but there is definitely not the same gain going from 200% to 300% that there is going from 100% to 200% current on a 25 HP motor.

The IGBT's, at least the ones I deal with, put out a lot of heat. Running a VFD at rated power will give you 1.5% to 3% heat. So a 10 kw VFD is also a 300W heater. Air flow is important.

09-04-2010, 11:18 PM	#295 (permalink)
thingstodo Master EcoModder Join Date: Sep 2010 Location: Saskatoon, canada Posts: 1,488 Ford Prefect - '18 Ford F150 XLT XTR Tess - '22 Tesla Y LR Thanks: 749 Thanked 565 Times in 447 Posts	Some points about Vector control and VFDs in general. I don't exactly understand vector control, but I use it. The parameters are measured by the VFD when you set it up during commissioning. It sends some DC pulses through the motor and measures a few items. Every VFD vendor is pretty specific about what they call the numbers and how they calculate them. Basically they get a resistance and an inductance for the windings. As the rotor starts to spin, the current taken by the motor changes. Another couple of inductances are measured. The result is that the VFD puts out a 'magnetizing current' at the right voltage and phase to build a 'good' magnetic flux across the gap between the windings and the rotor. This generates current in the rotor. The magnetic flux 'rotates' as the voltage and current are changed and 'drags' the rotor with it. The current in the rotor cutting through magnetic flux is what generates torque. If the rotor was made out of magnets, then the rotor speed would be identical to the VFD output frequency. I think those are called BDLC motors. Induction motors turn slower than the VFD frequency. A Volts per Herz VFD puts out a pretty much linear voltage as you vary the frequency, up to the point when the voltage is at the maximum that the VFD has. Then the voltage stays the same and the frequency keeps going up. A vector VFD puts out what the motor needs to turn at the specified frequency. It knows the magnetizing current. It increases the voltage with frequency to keep the magnetizing current the same as the speed changes. The current increases non-linearly with load on the motor. Pardon my personification of electronics. I have 400 HP motors driving belt conveyors that are 1500 feet long started with tonnes (literally) of product sitting on the belt conveyor. And the VFDs we use don't put out thousands of amps. Our VFD vendors (Mitsubishi, Siemens, Toshiba, Allen Bradley) all use their own variants of IGBTs and they all seem to agree that a single IGBT should not have to deal with more than 100 - 150 amps at 600V. All of our VFDs 150 HP and below use 1 set (6) IGBTs, 200HP VFDs use two sets of IGBTs, the vendors disagree about the 300's, some use 2, some 3, the 400 and 500 HP VFDs have more variation in how many IGBT sets are used. Driviing more current into a motor gives more torque for the same speed. If a operations person screws up and overloads a belt, we can put 200% current through the motor for a couple of minutes to get it cleared. But we keep an eye on that motor for an hour or so to make sure the temperature is not rising. There is a limit. I have not tried 300% on a larger motor, but there is definitely not the same gain going from 200% to 300% that there is going from 100% to 200% current on a 25 HP motor. The IGBT's, at least the ones I deal with, put out a lot of heat. Running a VFD at rated power will give you 1.5% to 3% heat. So a 10 kw VFD is also a 300W heater. Air flow is important.