VFD Behavior When Motor Stops

let me add one more thing in this question that what happen when i give DC supply to rotor or stator of Squirrel Cage induction motor while stopping?

Most probably due to inductive feedback on the electrical power lines to the motor. Can be caused by the motor remaining under full load while stopping.

If you give DC supply to the rotor (but is it not a cage rotor? then how would you give DC Supply to the rotor? Your DC Power Supply fuses would fail on short circuit) assuming it is a wound rotor with slip rings, and with the rotor in rotation, it would generate a high voltage in the stator (Synchronous generator principles).

Altenately, if you give DC supply to the stator, with the shorted rotor in rotation, it is nothing but DC Injection Braking and the rotor would come to an abrupt stop.

Dynamic orr Rheostic breaking is much better than the school injection braking. In DC injection braking the generated power gets dissipated in the rotor of the motor. There is chances of rotor winding insulation failure if DC is applied for longer period.

If motor is operated through VFD, the dynamic braking must be preferred.

Does the motor have something attached to it that has large rotational inertia?

Good question...

Consult your VFD manual, it should clearly indicate that the minimum deceleration time that can be set depends on the motor load and braking method used. Additionally consult the drives stored fault history as it too will tell you exactly why the drive stopped under a fault condition and the actions necessary to fix the problem.

It should all be in the manual, and what isn't should be available on the VFD company's website (such as brake resistor sizing and application notes for stopping high inertia loads).

It seems your motor would stop during about 10 secs without supply power by the mechanical load. If you set the deceleration time to 10 or more secs that results a smooth deceleration with a small but continuous power consumption. If you set the deceleration time to less than 10 secs you have to break the motor with the VFD, the motor will be in generator mode (negative slip!) and VFD should be able to handle recuperation, a reverse energy flow.

Good one. First check whether DBR is available. Second question you put is applying DC voltage to induction motor. Yes, by applying DC instead AC to phase you can break the motor. Such breaks circuits is used in motorised valve actuator. http://www.claytonengineering.com/Training/myweb6/Module16/Output/BrakingMotor.html

I think the motor needs to be stopped in a more smoother way with appropriate deceleration time.

Depends on the type of application,like in some pump motors and compressors,smooth starters(soft starters) are used to gradually increase the voltage untill rated voltage and also command is initiated to gradually reduce the voltage untill the machine stops normally.

Patrick Whowha

For a 75kW, 380volts supply and 1400rpm motor, driving a belt conveyor , what should be the DC voltage for DC Injection braking and how long should thee DC supply remain ON? How will you calculate these two parameters?

let me know one more thing, what happen when i reverse motor supply polarity for few seconds while stopping?

DC current control via PWM is not so easy when the motor has low resistance, it is better to programme the current level if possible, not voltage. If the motor is about 135A I would expect to try about 100ADC but some inverters are not good enough to control DC at this capacity, so it depends on the make and how good it is. I have experience of VFD DC control and it has improved over time but much easier on smaller (higher resistance) windings. If the VFD has a problem, you should get overcurrent trip, try reducing DC current level.

Since its a conveyor, why do you need to stop it so quickly?

The DC may work OK and worth trying, I guess a chopper and resistors will be needed otherwise as most VFD have the braking transistor as an (expensive) option at this power.

If you give the VFD a reverse signal, it ramps down just the same so no difference to deceleration time / braking option aspect at all, I'm afraid.

When you stop a VFD is a RAMP down condition with time set shorter that the inertia generation by the rotor you will have a voltage rise in the DC bus to about 1.4 of the incomig rated voltage. The drive is programmed to trip out on OV (overvoltage) to protect the IGBT's. At tha point the connected load becomes free coasting for a fraction of a second and cog.

Solution: 1) Add a DB resistoir across the DC bus to absorb the excess energy.

2) reprogram the VFD to coast to stop rather that rammp to stop. It will stop by the friction of the motor and the load friction. If this uncopntrolled condition is noit desired go back to solution 1.

Joe

YOU CANNOT JUST CONNECT RESISTORS TO THE DC BUS!!!

Almost all VFDs are capable of having Dynamic Braking enabled in them. DB is where the VFD removes the excess kinetic energy in the spinning load and converts it to electrical energy, then gets rid of it outside of the motor. A VFD is inherently capable of absorbing a SMALL AMOUNT of this excess energy, but not a lot. Once the DC bus rises above the danger level, the VFD will trip off to protect itself. Small (i.e. <10HP) VFDs usually have a built-in "chopper transistor" that acts as the switch to begin dumping excess energy into the resistors.Above that, you usually need an external device to make that happen, sold as an accessory to the VFD because it is expensive, and if not needed it's a waste of money. This accessory is usually called a "Braking Module", "Chopper Module" or something like that. You also need the resistors, and each VFD manufacturer will provide guidance on the resistor sizing based on the current capacity of their components. In other words, read your manual!

But DB will not FULLY stop a load, because the braking energy is dynamically reducing with load speed, which is of course diminishing too. So to finish the job, VFDs will ALSO have built-in DC Injection Braking. There is typically a speed threshold, i.e. 10Hz or 5Hz, below which the DCIB will auto-engage (if you have enabled braking). But DCIB as it exists in VFDs is limited in it's scope for stopping a load, and DCIB is hard on the motor, because ALL of that kinetic energy now gets absorbed in the motor, especially the rotor. I don't suggest using it other than as it was intended by the VFD manufacturer.

By the way, DO NOT add an external DC injection brake to this, you will damage the VFD if you don't know what you are doing and get everything EXACTLY right..

This is the case of regeneration. The motor is acting as a generator when it is stopped. The kinetic energy of the rotor must be dissipated in the resistor to avoid the DC BUS OVER VOLTAGE. When the voltage of DC Bus exceeds above its set point the drive trips with over load.

The Dynamic Braking resistor must be used to dissipate the generated energy.

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