Electrical – FOC speed limit motor

brushless-dc-motormotor controller

I have here an FOC controller with encoder and a 2200kv, 40A, 324W motor. I can run the motor just fine and I limited the current to 4A and the motor spins up to around 1400rmp. When I now increase the current limit, I would expect the motor to turn faster, since it allows higher voltage across the phases, but this seems not to work at all and the motor turns maybe around 1450rmp at 10A, where also the PWM on timings are changed only a small amount. I run the motor not in the speed control mode, just in the current control mode, which will spin the motor to the maximum possible rmp.

What could cause the problem here? Is it possible, that the FOC with encoder somehow goes into saturation?

Edit: Here is a plot of (Iq,Id,Vq,Vd) for this motor, with a reference quadrature current Iq=-3A, where I first let the motor run full speed, then I slowly put a load on it with my hand until stall. Is it possible, that if |Vq|<|Vd|, the motor cannot accelerate anymore? Since this is the case here, when the motor holds the speed.

Edit 2 I made now some new measurements, where also the speed is included and the phase currents. Iq_ref is set to 2A and I hold the motor and release it with my hands. I really don't know what really does limit my speed here. Id grows, when the motor reaches the current limit Iq_lim of 2A. The plotter is not the fastest, so that the phase currents could only be truly depicted at very low speeds.

S2

Best Answer

You have two current controlers in your FOC Iq and Id. If you drive a PMSM motor, then Id has to be 0, and Iq is the one which controlls the torque.

It is obvious if you limit the Iq to 4A, the motor will accelerate until it will deliver a setpoint torque which is related to the 4A setpoint.

Now if there is no load presen and the motor spins at high revs, the controller can't inject more current because the back EMF is now the same as supply voltage. This BEMF is related to the motor constant.

EDIT:

d-axis stand for excitation, menawhile q- axis for torque. It is similar to brush DC motor with separate excitation and with permanent magnet. Iq is the armature current, Id is the field current. Since you already have a permanent magnet, then Id shall be zero.

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Vq and Vd are also similar to brush DC, except they are also cross linked with Iq and Id respectively.

$$u_d = R_s i_d+L_s\dfrac{di_d}{dt}-\omega L_s i_q $$ $$u_q = R_s i_q+L_s \dfrac{di_q}{dt} +\omega (L_si_d+\lambda_{pm})$$

If the id=0 and it's kept constant, neglecting the resistance , then the vd has to be: $$ -\omega L_s i_q $$ That's the feedforward component that user JonRB refered at. Meanwhile the vq can be simplified to the similar formula as brush DC motor armature voltage:

$$u_q = R_s i_q+L_s \dfrac{di_q}{dt} +\omega k_e$$