Torque Control for an Electric Kart

[darylviorel22]

Hello, I want to use Field Oriented Control Strategy for PMSM of the electric kart i am working on.
I need to have a torque reference, but on most diagrams there are only speed references.
May you provide a good diagram with torque reference?
Thanks,
Andrei

 

[rjenkinsgb]

The simplest approach I can think of is to sense the motor current, compare that to the torque command, and use the difference as the speed command to the motor drive.

It's a bit backwards from the conventional servo drive approach, but should allow you to use a simple off-the-shelf BLDC motor driver board with minimal extra electronics.

 

[shortbus=]

Unless that is your only choice of a motor, or this is a"model" type/size kart, in my opinion your not going to like the end result. A PMSM of large enough size to move a kart with a person on board will also need a "starting motor" to get it to move.

Quote, " Above a certain size, synchronous motors are not self-starting motors. " From - https://en.wikipedia.org/wiki/Synchronous_motor

 

[Diver300]

Unless that is your only choice of a motor, or this is a"model" type/size kart, in my opinion your not going to like the end result. A PMSM of large enough size to move a kart with a person on board will also need a "starting motor" to get it to move.

Quote, " Above a certain size, synchronous motors are not self-starting motors. " From - https://en.wikipedia.org/wiki/Synchronous_motor

I think that only applies to full-frequency starting. If the frequency is near zero when the motor has to start, I think that a PMSM will be fine.

 

[shortbus=]

I think that only applies to full-frequency starting. If the frequency is near zero when the motor has to start, I think that a PMSM will be fine.

Then why don't the use them instead of the more common BLDC motors? Not trying to start a fight, but just to understand the thinking of this.

 

[Diver300]

Then why don't the use them instead of the more common BLDC motors? Not trying to start a fight, but just to understand the thinking of this.

A BLDC motor has some sort of position feedback so needs to have a more complicated interface between the motor and the drive circuit, but may be a simpler driver circuit. A three-phase motor will work with just three wires, but the drive circuit may be more complicated.

The different parameters lead to different engineering choices as to which is used.

 

[shortbus=]

I appreciate your point of view, but don't think it's correct. If it was there would be far more of the PMSM's in common use. But like I said earlier, not wanting a fight.

 

[rjenkinsgb]

A normal stepper motor is a common type of permanent magnet synchronous motor, although named differently.

 

[shortbus=]

A normal stepper motor is a common type of permanent magnet synchronous motor, although named differently.

I understand that but they aren't commonly used to drive a kart either. There has to be a reason that large high power PMSM isn't used more often. Other than needing a "pony motor" to get them close to speed, like in an elevator. The drives are similar in all instances, so I doubt that is the reason.

 

[rjenkinsgb]

They only need to "get to speed" if they are to run on a fixed-frequency supply.

If the drive starts from zero speed and the motor torque is not excessive, the motor rotor will simply follow the stator field as that rotates.

eg. All common RC drones use PMSMs for the lift motors.

There is no reason at all for them to be any less powerful than a BLDC motor with rotor feedback - the difference is the driven device must not be critical on motor start angle, as a feedback-less motor may kick slightly in the wrong direction as power is first applied.

And there is no feedback for the motor starting to slip, so the acceleration etc. must be carefully controlled.

I agree that a kart does not seem a very practical application, especially as it should be simple in such a DIY project to add sensors to the motor shaft for angular feedback & then run it as a BLDC motor.