Need Quick Braking of Induction Motor 2

[RajeevCell]

Hi all,

We have replaced a DC motor with an induction motor, in a cigarette packaging line. All mechanical and electrical work have been done successfully, and the product line is working fine with the new motor and drive. Except one thing. When stopped, the motor is not braking as quickly as the DC motor did. We need the motor to stop very quickly (stand still within 0.3 seconds of pressing STOP).

We are using the following machines

Motor
400V, delta
50 Hz
22.5 A
11 kW
cos phi = 0.83
1450 rpm


Inverter
Unidrive SP2403
400 V
25 A
11 kW

The relevant params of the drive are
Ramps
acceleration rate 8.0 second/100Hz
deceleration rate 0.5 second/100Hz

Braking resistor value
19 ohm, 33.1 kW (instantaneous)

We do have braking resistor. We have tested with 43 ohm and 19 ohm resistors. The 19 ohm one, indeed, works better.

The motor is taking about two seconds to stop completely, after pushing STOP button. We need to bring that down to 0.5 seconds, which was the case for the previous DC motor. How can we do that?

Does anyone has experience with this drive?

I appreciate all types of help, suggestions, link to resources, general advices, etc. Thanks in advance.

YR

 

[boaterbill]

I am not familiar with that drive, but since you are already using Breaking resistors, see if you can also use DC injection braking from the inverter. There should be a paramter setting for this.

Good luck

 

[RajeevCell]

Thanks much boaterbill. Very quick response! The drive has provision for DC injection. But we have other packaging machines in the factory that have the same rating IM and Unidrive inverters. Those are not using DC injection. Still, the braking is quite fast for them. How do they do it?

Respectfully.
YR

 

[ozmosis]

Is this fast stop a requirement of an emergency stop?(related to any safety regulations)?
If so, you may need to investigate further as to the legalities of using basic regenerative braking for this function.
Without knowing the details on the load (inertia, gear-train etc) it's difficult to determine if your 0.5secs is achievable. Looking at your ratio of acceleration to deceleration, this gives some indication as to what is possible. Have a look at the current drawn on accel and this will give an (approx) indication as to how much energy can be dissipated into you brake resistors.
You may run into limitations on the internal brake chopper as this will have a maximum current rating dependant on the resistor package you fit. Don't keep lowering the resistance and increasing the wattage otherwise you'll simply pop the brake transistor.
Ask CT, they are usually pretty good on matters such as this.

 

[RajeevCell]

Thanks Ozmosis.

The quick stop is not for emergency. It is needed for reducing wastage during unexpected stops.

The load does not have much inertia. Motor is belt coupled with the mechanical system. It is running at about 1280 rpm, usually. But may go above 1500.

I am not reducing the brake resistance any further, the drive manufacturer's min value is 19 ohm.

Respectfully
YR

 

[sreid]

Do you have the current limit set to maximum (often 150% of continously rated current?).

 

[Marke]

One of the issues to check is what the resistors are selected for.
You can select resistors for 10% or 40% or higher duty. If you have 10% resistors fitted, they may not be able to pull out the energy fast enough and so the deceleration rate willbe restricted.

There are two issues on the sizing of the resistors, the power rating determines the total energy that can be pulled out of the load, the the resistance determines the rate that the energy is pulled out. You may need to reduce the value of the resistors, even if the power rating is adequate.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[davidbeach]

And we should care about a cigarette packaging line why?

 

[itsmoked]

Now, now. (same thought I had) At least it's not a Solent Green line.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[RajeevCell]

Thanks sreid. The drive max current is set to 30 A. I'll check if there is any other current setting for the braking resistor section. A different IGBT inside the drive operates the braking resistor.

Marke, thanks much. The resistor doesn't have percentage duty cycle mentioned on it. Only a serial number is there. It is heaksink mounted and I can measure the resistance from outside. Need to ask the machine integrator about it.

Dear davidbeach and itsmoked, I sincerely appreciate your concerns about a risky industry. I myself am a non-smoker.

But I was looking for suggestions on a technical issue, from willing contributors only. Your comments will discourage others to share useful knowledge that could be a help to many members.

Respectfully
YR

 

[LionelHutz]

If you have the same VFDs running on similar machines then have you looked at the VFD vs motor size and the settings in those VFDs?

Does the VFD drop the frequency to 0hz in 0.5 seconds but the motor not stop or does the VFD take 2 seconds to ramp down even though it's set to 0.5 seconds?

A typical motor can produce over 2 times it's rated torque for acceleration and deceleration when connected to a VFD. Basically, A VFD can run a motor at it's breakdown torque. However, to do this it may require the current to be around 3 times it's rated current. A typical VFD sized to match the motor can not supply this much current. An oversized VFD may help the problem.

If the VFD says the min resistor size is 19 ohms then you're at the limit there. Unless you've maybe got a wirewound resistor that adds impedance or the VFD has settings to protect the resistor. Otherwise, the only thing indicating a change is necessary is if the resistor gets too hot on stopping meaning you need a higher wattage resistor.

 

[RajeevCell]

If you have the same VFDs running on similar machines then have you looked at the VFD vs motor size and the settings in those VFDs?

Thanks. I have investigated the parameters in other Unidrive inverters on the plant. They are a different model from the one I'm working on now, but the parameter list and menu structures are similar. I tested with the copied values. Did not work.

Does the VFD drop the frequency to 0hz in 0.5 seconds but the motor not stop or does the VFD take 2 seconds to ramp down even though it's set to 0.5 seconds?

I have not measured the output frequency. But the drive shows an rpm reading on the front panel display, and that reading decays to zero in about two seconds. Actual motor rotation also stops in the same time.

Other drives in the plant are not using DC injection.

I tried to measure current through the resistor during motor stop. Could not see a change in the clamp on meter display.

I looked at the bus voltage also. It was 575 volts normally, after STOP it momentarily rises to 720 volts. Then drops to 575.

Best regards
YR

 

[LionelHutz]

If you are stopping from 60hz then that 0.5sec/100hz setting should get you 0.3 seconds. So, something is causing the VFD to extend the deceleration time. It will either be a current limit or a form of over-voltage protection. I don't know enough about that VFD so look in the manual for these 2 things for possible ideas of what is happening.

Typically when you set too short a deceleration time with a VFD you either get an over-current fault or an over-voltage fault. So, this VFD has some type of protection that is extending the deceleration time to avoid the trip.

 

[DickDV]

If the drive spec's a minimum brake resistor of 19 ohms and you are there, you can't go any lower on that. You could set the decel time down to zero but the actual time is already longer than the .5sec setting.

I strongly suspect that there is a parameter in the drive that provides overvoltage fault avoidance. This is used to extend the decel time when the DC bus reaches a high voltage point. THIS SHOULD BE TURNED OFF WHENEVER A BRAKE CHOPPER AND RESISTOR IS INSTALLED. Check it out. I'll bet it is still on which would account for the extended decel times.

 

[Marke]

The minimum resistance for that model is 19 ohms, so provided that the resistor has not failed open circuit, there is probably not much that you can do.
I will investigate more at this end.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Tumppi]

The setting of Unidrive SP you need to change is parameter 2.04 which is ramp mode selection. At default it is Std = standard ramp and it will longer deceleration time if dc-link voltage is rising up to trip level.

Set pr.2.04 to Fast = fast ramp and you will decelarate according set ramp rates and current limits.

If this does not help and motor still rotates after drive goes stop or zero speed, then you will need an encoder for the motor and set unidrive sp as closed loop vector. Then you can have full torque at zero speed too to stop the motor.

Regs,

Tumppi

 

[Marke]

As it appears that the deceleration is better with the 19 ohm resistor than with the 3 ohm resistor, that suggests that everything is working correctly and you have too much kinetic energy to pull out of the load in 0.5 seconds. The energy that you pull out of the load goes into the DC bus. If the rate of decel is too high, the voltage will go too high and you will have an over voltage trip and possible damage.
It would appear that things are operating at their limits. The only alternative is to change the decel to a DC injection braking. This transfers the kinetic energy to the rotor of the motor and may be a quicker stop. - I am not sure if you can DC inject on the SP.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[RajeevCell]

Dear Tumppi,

The param 2.04 is linked with param 0.15 (both are the same thing). I set that to FAST already. Thanks anyway.

 

[RajeevCell]

Problem solved! I set the max current to 35 Amps. That did it.

Indeed the discusson here led me to the solution. A star for sreid.

Thanks to everyone. The tips are invalueable. I've printed this page and filed the hardcopy in my "Motor Conversion" folder. I'll be replacing a dozen more DC motors in the same plant. This was the largest though.

Respectfully
YR

 

[ozmosis]

Good to see it`s solved and thanks for letting us know.