Is the torque constant of a DC motor the same regardless of voltage?

[bbq_build]

I see. So, what is the purpose of measuring the resistance by stalling the motor shaft while measuring the voltage across the motor and the current in series with the motor? I am a bit confused now.

In my previous post, I mentioned that based on the torque vs current curve from the company, 0 kgcm at 1.2A and 135 kgcm at 17.5A. After unit conversion, 0 Nm at 1.2A and 13.24Nm at 17.5A. Since Torque = Kt * I, Kt = 13.24Nm/17.5 = 0.76. Since Ke = Kt and they are not affected by the input voltage, then I can use Kt = Ke = 0.76 in my Simulink model. Is this the right thing to do? From the data given by the company, their Kt =Ke= 3.55 which is quite different from my calculated Kt=Ke=0.76. Another confusion.

 

[David Lewis]

So, what is the purpose of measuring the resistance by stalling the motor shaft while measuring the voltage across the motor and the current in series with the motor?

You can get an approximate value for resistance without a tachometer or having to know what Ke is.

I can use Kt = Ke = 0.76 in my Simulink model. Is this the right thing to do?

Yes. The precision of the calculated armature resistance value will depend on how accurate your Ke is, of course.
You can also use T = (I - I_o)Kt, where I_o = 1.2A

 

[bbq_build]

Thanks. So, in my model, will it be reasonably good if I set Kt=Ke=0.76 and Resistance to be 0.1 Ohms?

 

[bbq_build]

None of the above. Normally you measure Ke first by back driving the motor,
and using a tachometer and voltmeter.
After you know Ke, calculating back EMF is easy.

I have ordered a tachometer. It will arrive in about two days. Also, ordered a LCR meter to measure the inductance.

From what we have gone through, it looks like the results are also affected by the order of measurements. Shouldn't the results be the same regardless of method and order of measurements? Could anybody please let me know the proper order of measuring the following parameters: resistance (done if 0.1 Ohms is acceptable), Kt=Ke, inductance, back EMF, moment of inertia of the motor.

So, the parameters and curves I got from the company are useless if I plan to drive the motor at a different voltage? Thanks.

 

[David Lewis]

V = IR, but 7V does not equal 19A*0.123Ω, nor 1.2A*0.123Ω. Anybody knows what is going on?

V = (I*R) + (back EMF)

Note that V = IR only applies to your locked rotor scenario.
When the motor is turning, on the other hand, the motor generates a back voltage.

 

[David Lewis]

The torque constant Kt is torque/current. Is this Kt the same regardless of the supply voltage?

Yes, within reason. The steel armature core can only conduct so much flux. When it nears saturation, the B/H curve flattens out.
Secondly, this is the grand total torque produced by the motor. Net torque at the shaft available to do useful work is less,
and that will make effective Kt less.

 

[bbq_build]

None of the above. Normally you measure Ke first by back driving the motor,
and using a tachometer and voltmeter.
After you know Ke, calculating back EMF is easy.

Thanks. My tachometer has arrived. Could you please let me know the proper way to measure Ke by back driving the motor and using a tachometer and voltmeter?

 

[David Lewis]

Will it be reasonably good if I set Kt=Ke=0.76 and resistance to be 0.1Ω?

Not sure. Let me do some cross checking of the data you've presented.

How about the torque vs. current line? The more the torque, the more the current. Am I right that when the voltage increases, this line also shifts upward?

When voltage increases, the current vs. total torque curve does not change. However, the current vs. net usable torque graph does shift upward, and Kt decreases as applied voltage increases.

The solid blue line represents current as a function of total torque. Point A shows torque absorbed by bearing friction, commutator friction, and windage. Moving a copy of Point A horizontally to Point B gives a plot of current as a function of net usable torque.

At Point C, the motor is stalled, so mechanical losses are zero. Net torque and total torque are the same. The dashed red line from B to C is not quite straight because windage is roughly proportional to the square of speed.

 

[bbq_build]

Thanks David for the graphs.

I connected a small motor hub to the gearhead of the motor. Then, I measured the voltage, current and speed of turning (in RPM) using a tachometer. I found that the measured voltage is a bit less than the supplied voltage. This is probably due to the Back EMF. I am not sure if the added weight/inertia due to the hub could be neglected in the calculations. Here is the data:

Supplied voltage: 11.1V
Measured voltage across the motor: 10.2V
Measured current in serial with the motor: 1.2A
Speed (with the gearhead and hub): 78RPM (8.17 rad/s).

What can we do with the measured data? Any advice appreciated. Thanks.

 

[David Lewis]

I connected a small motor hub to the gearhead of the motor. Then, I measured the voltage, current and speed of turning (in RPM) using a tachometer. I found that the measured voltage is a bit less than the supplied voltage.

I am not clear on what you mean by a small motor hub.

I have derived a formula for current draw based on the torque absorbed by the load.
The equation of a straight line follows the form: y = mx + b (where m is the slope, and b is the y-intercept).
Referring to the graph in post #58:

I = [T(Is - Io)/Ts] + Io

I = motor current
T = load torque
Is = stall current
Io = no-load current
Ts = stall torque

Note that some of these variables depend on motor voltage (refer to the second graph in post #58).

 

[AFZAL AMANULLAH]

What is the relation of Kt and Ke in BLDC motor?
How to find it?
Also how to find R, L of motor?
What is the setup suitable for in finding Kt .
I want to find the transfer function of BLDC motor for simulation.
How to find the value of B and J ?
Photo is attached herewith.