Induction Motor in the Dynamometer Application

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Thread Starter

ElecEngEnas

Dear Sirs,

I have an application of dynamometer where a diesel engine is being under test. so to provide a simulation for variable loading on the diesel engine, an induction motor (controlled by vfd) is being mechanically coupled to the induction motor. The diesel engine will rotate the induction motor. so the induction motor will be in the (regeneration) mode where it's going to convert the mechanical energy of the diesel engine into electricity that will be dissipated in a braking resistor that is connected to the inverter driving the motor.

As the experiment requires variable loading of the diesel engine, we are going to control the torque of the induction motor during the regeneration or the dynamic braking mode to vary the loading torque on the diesel engine.

My questions are:

1- is it possible to control the braking torque with the speed being fixed at a constant value? does this mean I'm going to perform both speed and torque control in parallel?

2-when the diesel engine drives the induction motor at a speed which is less than the synchronous speed, will the induction motor converts to the dynamic braking mode? or for this mode to be activated the diesel engine must drive the motor at speeds greater than the synchronous speed?
 
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Robert Scott

> 1- is it possible to control the braking torque with the speed being
> fixed at a constant value? does this mean I'm going to perform both speed and
> torque control in parallel?

The torque in an induction motor is based on the difference between the electrical frequency and the mechanical frequency. If you want to apply braking torque you must drive the motor with an AC voltage whose frequency is slightly lower than the mechanical rotation frequency. This gets hard to do at very low RPMs because the induction motor is not very efficient at very low drive frequencies. Even at higher frequencies this becomes a challenge if you are trying to apply a fixed torque over a changing RPM. It means you would have to constantly monitor the mechanical RPM and adjust the electrical drive frequency accordingly.

You can't control two independent variables (speed and torque) without two independent control inputs. In this case you could use the electrical drive frequency and the engine throttle control as the two inputs. But if using the engine throttle control is not allowed for this purpose, you can only control one thing - speed or torque, but not both.

> 2-when the diesel engine drives the induction motor at a speed which is less
> than the synchronous speed, will the induction motor converts to the dynamic
> braking mode? or for this mode to be activated the diesel engine must drive
> the motor at speeds greater than the synchronous speed?

The second is correct. If the motor is turning at less than the synchronous speed, the motor will be applying torque to try to make the motor go faster. If the motor is turning at greater than the synchronous speed, the motor will be applying a braking torque.

Robert Scott
Real-Time Specialties
Hopkins, MN
 
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Dear Robert, I really appreciate your prompt response. I have a couple of questions.

1- Is it more convenient to obtain a braking torque when the slip is of higher or lower value? I mean depending on your reply, I need to slightly differ the mechanical speed from the electrical one. does that mean the negative slip is to be of low value rather than high value to obtain a braking torque? or the higher the negative slip is, the higher the braking torque?

2- The diesel engine is controllable through its throttle. so you mean I will control the speed using the throttle of the diesel engine and the braking torque through the induction motor driven by the inverter?

Thanks
 
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