TORQUE-SPEED RELATIONSHIP?

[BoboyeOkeya]

I have a question, I am presently working with a bldc motor. And I also did a simulation on matlab.

So I observed this:

1. when I applied a certain load torque (let’s say 0.4Nm), then I give my controller a set point speed (400RPM) the controller is not able to stabilize the system at the set point instead it stabilizes at a wrong value : but when I change the load torque to a certain value e.g 0.3Nm, the controller is able to stabilize the speed at set point. Do you know why this is so? Does it relate to the torque-speed curve I’m unaware of?

2. From the physical experimental setup I made, I am not fixing any load torque to the bldc motor. Also, my present power converter board can only handle up to 16A current, and I even limited the current controller limit to 14A, so that it incases of oscillations or overshoots, the machine won’t draw more than 16A at least, but it still does. The bldc machine itself is rated at 24V, 3200RPM, and 1.7Nm,500W. I really don’t know if they are rated values or nominal values or if the term rated and nominal values mean the same thing. But those are the values written on the motor.

Now, based on those values I calculated that the maximum current the motor windings can handle is about 500W/24V= 20.833A. But my power converter board can only handle up 16A, which is less than the 20.833A

So the main issue with the experimental tests I did was that:

a)  At no load, the speed controller is able to stabilize the rotor speed to a certain speed value limit.

b.) But at torque values greater than 0, the controller no longer is able to stabilize the speed. Instead it stabilizes at lower values.

Could this also be due to issues with the torque-speed relationship?

I would appreciate an answer.

Many thanks.

[Muhammed Güler]

Engine speed and torque are inversely proportional. You cannot increase both at the same time.

[BoboyeOkeya]

Okay that makes sense looking at the relationship between power, torque and speed.
But does that mean that if I apply a certain load torque on the motor’s shaft. Then the maximum speed I can get is fixed at that operating point determined by the load torque?
I hope you understand my question. If not let me rephrase.
Let’s say I have the torque-speed curve of my motor. Now if I apply a certain constant load torque. Then the load torque intersects the torque speed curve at a certain point which should give the corresponding speed. So does that mean that given that applied load torque, I won’t be able to exceed that corresponding speed?

[Cristiana SCARAMEL]

Hi @BoboyeOkeya​ 

I confirm that your assumptions are correct.

In the end the concept is that each motor has a relationship between voltage, current and specific torque.

Regardless of the range that is able to manage the inverter, you will never exceed the physical limit given by the characteristics of the motor.

Please click on the 'Select as Best' label to close the topic and help other user to find the answer to a similar question.

[BoboyeOkeya · Accepted solution]

Hi @Cristiana SCARAMEL​  yes Now i get it. It is important to also state that for the purposes of motor control, the operating point actually shifts across different torque speed characteristic curves. This is true especially if the terminal voltage between two phases is modulated -- and that was what I had in my project. The speed controller modulates the output voltage between two phases of my inverter to get my BLDC motor to set-point speed.

So basically, the torque-speed curve changes as the voltage changes from 0 to DC-link voltage. So, the DC link voltage sets the maximum torque speed curve i.e the maximum achievable speed (no-load speed) and torque(stall torque). Therefore, it explains why regardless of the amount of current I supplied to my system, I just could not get it to a speed higher than the no-load speed set by the DC-link voltage.

Also, there is a more complex but simple relationship between the voltage, current, torque and speed like you said. When driven by a speed controller, especially if the speed controller has an output current saturation limit, sometimes it would not be able to achieve the setpoint speed even though it is less than the no-load speed determined by the DC-link voltage. So increasing the output current saturation limit can make the motor achieve the setpoint speed.

I hope this will be useful for someone in the future so they don't have to pass through the confusion I had to go through.