Induction motor for frequent start stop operation

[agreatstar]

I am making a robotic football goalkeeper. My application requires my 15kg human dummy(6ft in height, MI=20kgm^2) to turn by a specified angle (which can be max 90deg) in 0.5 sec. From acceleration and deceleration characterisitics, I have calculated my accelerating torque to be 450Nm, and decelerating torque to be 900Nm assuming that I will accelerate the dummy for 0.3 sec, and decelerate for 0.2s. One of my vendors is suggesting a 3HP induction motor for the application and it does satisfy the requirements. I am not planning to use a servo because of budgetary constraints. My question is will induction motor be able to turn 90deg in 0.5s at such high load? Isnt the starting time for induction motor high because of its own inertia? Also is it recommended to use induction motor for such a frequent start-stop operation(start and stop in 0.5s)?

- - - Updated - - -

Yes, I want the motor to stop after turning a specific angle. The angle of turn will be max 90 deg. It can be lower than that sometimes. Once the motor turns by specified angle(say 90), it has to stay there for next 3-4 secs and reset to initial position by rotating as much angle in reverse direction (-90deg).

 

[srizbf]

IndMotor cannot be started and stopped like a stepper or servo , i.e, turn to 90 degree and stop and then start.

 

[agreatstar]

IndMotor cannot be started and stopped like a stepper or servo , i.e, turn to 90 degree and stop and then start.

I am planning to use a VFD and an encoder along with the motor

 

[KlausST]

Hi,

Srizbf is correct in this.

A gearbox could help.

But with a gearbox the revolution of the motor is multiplied with respect to your dummy.
This multiplies (squared?) the inertia of the motor.

For the inertia of the dummy it is optimal if the rotation center is at the mass center. And most of the mass should be near the rotation center (to avoid additional inertia according Steiner). Maybe you can improve your dummy on this.
The mass of the dummy itself is not that important, but it is more important the distance of the mass to rotation center.

Klaus

 

[agreatstar]

Somehow due to strength considerations, the mass of the dummy cant be concentrated near the rotation center. It is at 1m (ie at center point of humanoid dummy which is 2m in height). I am using a worm gearbox, but I am not confident about the motor characteristics. I can tolerate 0.5 to 1 degree error in precision, But I am not sure whether or not motor will achieve the desired speed to turn 90deg in 0.5 sec. Also will this kind of system be durable(at least for a year), assuming motor will be operating in this start-stop manner throughout the day.

 

[FvM]

I have seen induction motors controlled by vector oriented VFD working as high dynamical servo drive. In so far it's surely possible.

You need to check if the motor has sufficient torque without a gearbox.

 

[KlausST]

Hi,

I don´t think the motor has a problem with the start stop.
I even think you can overload the motor a lot for the short times - for sure there must be time to cool down.

I´m sure there are position regualtors combined with inverters for 3 phase asynchron motors.

I´m more concerned about the lifetime of the gearbox.

Klaus

 

[FvM]

You can assume, that the available torque of a VFD driven motor is roughly independent of the motor speed and calculate the rated torque from the motor's rated power and speed. Short time torque will be limited by the VFD peak current and also by possible thermal motor overload. Factor 2 sounds generally feasible, higher peak torque only with an oversized VFD.

A 3 HP motor can't supply 900 Nm without a gearbox.