calculate supply current from InstaSpin FOC signals

Martin Bayer

Other Parts Discussed in Thread: TMS320F28069M

Hello,

I am using TI InstaSpin FOC software on a TMS320F28069M, integrated in a high voltage board with PMSM motor.

Actually I am faced to the task, to calculate the current, which my board sinks from the supply.

Because there is no current sensor in the supply line, the idea is to do the calculation from given signals of TI InstaSpin FOC software.

Following the thread

https://e2e.ti.com/support/microcontrollers/c2000/f/902/p/470634/1693712#1693712

there is given the approach over calculating motor-power and divide this by the supply voltage:

motor power + inverter losses = buss power.

P_motor = (Vd * Id + Vq * Iq) * 1.5

P_buss = P_Motor * 1.04 (assuming 4% inverter losses)

I_buss(avg) = P_buss / V_buss

After I implemented this calculations in a cyclic task and did some tests I found out, that it works in principal, but that there is a systematical mismatch dependant from supply voltage.

I.e. on the test rig, when the motor is operating in a constant load situation (speed, torque= constant) and the supply voltage is increased from min. to max. supply level, the calculated power (P_motor) is decreasing.

Monitoring the input signals for the P_motor calculation, I found out, that the signals Iq and Id stay constant during the test, but the signals Vq and Vd decrease with increasing supply voltage. In effect the calculated motor power decrease, instead staying constant.

From my understanding, the Vq and Vd signals should not vary, when Iq and Id are constant (at constant motor speed and torque).

In fact, the power which is delivered from the supply does not vary, but the calculated motor power varies with changing supply voltage.

Below some results from the measurement.

First 3 columns show the values from the power supply.

Next 4 columns show the InstaSpin FOC signal values.

The last 2 columns show the calculated power and supply current (losses ignored).


calculated Power Supply [W]	measured Power Supply [V]	measured Power Supply [A]	InstSpin I_q [A]	InstSpin I_d [A]	InstaSpin V_q [V]	InstSpin V_d [V]	calculated Motor Power [W]	calculated Input current [A]	ratio motor power calc./ input
3150	250	12,6	18	4,8	173,16	93,24	5346,648	21,39	0,58915418
3164	280	11,3	18	4,8	155,4	79,92	4771,224	17,04	0,6631422
3180	300	10,6	18	4,8	142,08	75,48	4379,616	14,60	0,72609105
3185	325	9,8	18	4,8	128,76	71,04	3988,008	12,27	0,79864434
3185	350	9,1	18	4,8	119,88	62,16	3684,312	10,53	0,86447619
3225	375	8,6	18	4,8	115,44	62,16	3564,432	9,51	0,90477249
3200	400	8	18	4,8	106,56	57,72	3292,704	8,23	0,97184563
3239	410	7,9	18	4,8	102,12	53,28	3140,856	7,66	1,03124753
3234	420	7,7	18	4,8	97,68	48,84	2989,008	7,12	1,08196432

What is the fundamental mistake in this scenario ?

Friendly regards

Martin

over 9 years ago

0 Yanming Luo over 9 years ago

TI__Guru** 117110 points

Calculate the input and output power of motor as below

Pin_motor = (Vrms*Irms)*sqrt(3) = (Va_rms*Ia_rms + Vb_rms*Ib_rms + Vc_rms*Ic_rms)/sqrt(3), need to calculate the RMS of phase voltage and current first.
Pout_motor = Torque(N.m)*Speed(rpm)/9550

And then calculate supply current as below
Iin_dcbus = Pin_motor/Vin_dcbus/k, k is the inverter efficiency which is about 0.94~0.98.

0 Martin Bayer over 9 years ago in reply to Yanming Luo

Prodigy 140 points

Hello Yanming,

thanks for your proposal, but the description is a very generally way to calculate the motor power from AC signals.

I would use this, if I had 'traditional' measurement equipment in the motorlines (like currentclamps and voltage meters) .

And I think, I would need to consider although the phase between current and voltage, when I am using AC signals.

I am looking for a solution, using the TI InstaSpin FOC signals, which are already transformed into 'steady' signals, like Iq, Id and so on.

Friendly regards

Martin

0 Yanming Luo over 9 years ago in reply to Martin Bayer

TI__Guru** 117110 points

Three phase Voltage and Current have been sensed in InstaSPIN, so you could using these signal to calculate Vrms and I rms. As you said, this is a generally way, but it is more precise, and is often used in industry inverter and automotive.
Another way, you may try to use motor output power to estimate the input current
Pout_motor(w) = Torque(N.m)*Speed(rpm)/9.549, Torque can be get from InstaSPIN-FOC.
Iin_dcbus = Pout_motor/Vin_dcbus/k1/k2, k1 is the inverter efficiency which is about 0.94~0.98, k2 is the motor output efficiency which is about 0.6~1.0(k2 is lower with light load, and is higher with heavy load).

0 Martin Bayer over 9 years ago in reply to Yanming Luo

Prodigy 140 points

Hello Yanming,

from my understanding, one big advantage of TI InstaSpin FOC software is, that currents and voltages are transformed from AC scope to DC scope. So you can control the machine like a DC Motor.

I walked through the software and found no spots, where Vrms or Irms are calculated or used.
It seems to me to go a step back, if I would recalculate AC signals from DC scope.
Another disadvantage of power calculation with AC signals is, that the phase information between voltage and current is needed.

Or am I completely wrong, when interpreting your Vrms and Irms as AC signals ? (supposing rms does mean 'root mean square')

Starting supply current calculation from motor output power (speed,torque) will not be accurate enough, due to operating point dependant efficiency of the motor.

So maybe you can provide a solution closer to the issue of my first post ?

Friendly regards

Martin

0 Yanming Luo over 9 years ago in reply to Martin Bayer

TI__Guru** 117110 points

Please refer to below solution using InstaSPIN-FOC as your first post, the current is not very accurate, but may is close to input current.

// calculate inverter output power in kW
gMotorVars.Power_coef = _IQmpy(_IQ(1.0*1.5/0.96), gMotorVars.VdcBus_kV); // _IQ(1.0*1.5/0.96) for MW16

gMotorVars.Power_kW = _IQmpy((_IQabs(_IQmpy(gMotorVars.Vd, gMotorVars.Id_A)) + _IQabs(_IQmpy(gMotorVars.Vq, gMotorVars.Iq_A))), gMotorVars.Power_coef);

gMotorVars.Iin_A = _IQdiv(gMotorVars.Power_kW, gMotorVars.VdcBus_kV);

0 Martin Bayer over 9 years ago in reply to Yanming Luo

Prodigy 140 points

Hello Yanming,

the proposal seems to me unplausible, because regarding the physical types
- the variable 'gMotorVars.Power_coef ' is from type 'voltage'
- the variable 'gMotorVars.Power_kW ' is from type 'power*voltage'
- in result the variable 'gMotorVars.I_in_A ' is from type 'power'

Friendly regards

Martin

0 Yanming Luo over 9 years ago in reply to Martin Bayer

TI__Guru** 117110 points

In InstaSPIN-FOC lab projects, gMotorVars.Vd and gMotorVars.Vq as below is a scale voltage from _IQ(-0.5)~_IQ(0.5) for MW15 or _IQ(-1.0)~_IQ(1.0) for pre-MW15, not a real voltage value, so need to multiply the dc bus voltage to get the real value. Use the variable 'gMotorVars.Power_coef ' to convert Vd&Vq from the scale value to kV.

// read Vd and Vq vectors per units
gMotorVars.Vd = CTRL_getVd_out_pu(ctrlHandle);
gMotorVars.Vq = CTRL_getVq_out_pu(ctrlHandle);

// read Id and Iq vectors in amps
gMotorVars.Id_A = _IQmpy(CTRL_getId_in_pu(ctrlHandle), _IQ(USER_IQ_FULL_SCALE_CURRENT_A));
gMotorVars.Iq_A = _IQmpy(CTRL_getIq_in_pu(ctrlHandle), _IQ(USER_IQ_FULL_SCALE_CURRENT_A));

0 Martin Bayer over 9 years ago in reply to Yanming Luo

Prodigy 140 points

Hello Yanming,

thank you for your support.

With latest information, the issue is clear.

I implemented the current calculation as suggested and everything works fine now.

Friendly regards

Martin Bayer

0 Yanming Luo over 9 years ago in reply to Martin Bayer

TI__Guru** 117110 points

Hi Martin,

Great. Thanks for your information.

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calculate supply current from InstaSpin FOC signals