I have an induction motor which is fed by a three phase inverter. I am trying to use vector control with d-axis aligned with the rotor flux vector. I have a load which increases its torque as the speed increases. I use a fixed value for direct component of current (Id) reference.

I start to test at low speed and everything goes fine, voltage amplitude increases with frequency and current is almost constant. But when I try greater speeds, at some point, the voltage amplitude reduces notoriously (instead of keep increasing), and quadrature component of current (Iq) also increases notoriously.

I also observed that Iq has a very low value before the problem occurs. Almost all current is contributed by Id. Is this low value of Iq a normal thing? or should it be increasing with speed? Is there some kind of relationship betbeen Id and Iq, that I should follow? I suspect this is related to saturation. How can I avoid this?

UPDATE: I tested with a larger value of field generating current (which I call Id) and the problem reduces, but still happens at higher speeds. So I believe may be is not saturation, but could it be like the motor looses its field?

is it normal that my controller tries keeps Iq at low values, even at high torque?

Thanks for your help.


Id and Iq are perpendicular to each other. We can say that Id is the excitation current, while Iq is torque current in the same fashion as brush DC motor with separate excitation, where Id is the field and Iq is the armature current.

There is a certain relationship between voltage and current, V/f characteristics. The voltage increases with speed and at certain point it can't go further (supply voltage). Beyond this speed it is possible to increase the speed by field weakening method - reducing the Id current, which reduces the flux.

The output torque is the product of both currents Id and Iq. within the nominal speed operation the Id is constant, while the Iq is has a mirrored value of the load torque, when accelerating is positive, when deccelerating is negative - generator mode, when the motor rotates unloaded, steady speed it should have a minimal value - bearing friction torque.

Without more details is difficult to find where the problem in your case is. The Id should be constant, the Iq small if you spin it with low acceleration. Did you made this vector control by yourself? What is the hardware used : eval board, FOC library,...? Did you placed current transformers correctly?

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  • \$\begingroup\$ Thanks for your answer. I made the hardware and the controller, it runs in a microcontroller. The mechanical load, increases its torque with speed.I keep the field producing current (Id) constant. I know that in theory Iq should increase, as you say, with greater torque, but I notice that, in my controller, Iq does not increase with higher torque, what could be causing this? \$\endgroup\$ – berto Dec 13 '17 at 23:42
  • \$\begingroup\$ You could be doing wrong calc. like swapped Id and Iq current controllers, or rotor flux estimator, or.....lots of possibilities. If you did the program and hardware you're the only one to solve the mistery. \$\endgroup\$ – Marko Buršič Dec 13 '17 at 23:51

Think of it: Iq is 90° rotated from Id. It represents reactive power. Reactive power is connected to slip in an induction motor and as you are using a VFD, the slip is held constant over a wide speed range, and so is Iq.

Above a certain speed your load requires more power than the motor can deliver. This is determined by the load's torque over speed characteristic. You still increase the frequency of the VFD, but you cannot speed up the drive. The only thing you increase that way is the slip. That's why your reactive power increases.

The VFD reacts by decreasing the voltage amplitude, as high reactive power could also mean oversaturation for a given motor.

  • \$\begingroup\$ Thanks for your answer. I made a test with higher field generating current (which I call Id), and I noticed that it works better, altough the problem still happens at high load, so I think I have a good (not to high) value for Id. Do you mean that high slip can cause saturation? even if I have a good value for Id? \$\endgroup\$ – berto Dec 13 '17 at 23:47
  • \$\begingroup\$ Slip causes reactive power, not saturation. But higher slip indirectly also increases active power through the motor torque/speed characteristic. A VFD will usually try to maintain a constant slip to keep reactive power (which it had to deliver without generating any torque with it) low. Now that I read your comments on the other answer, I think you messed up Id and Iq in your control program. \$\endgroup\$ – Janka Dec 14 '17 at 1:11
  • \$\begingroup\$ do you mean a swap between Id and Iq? perhaps it is just a convention issue, the book I'm following has Id for field current, and Iq for torque. I know it is strange because in electric power it is the opposite, d is real power and q is reactive. \$\endgroup\$ – berto Dec 15 '17 at 0:00

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