Forward-reverse induction motor switching

Hi.

Is it safe to switch between forward-reverse with induction motor every 4 seconds? Will I not burn or damage it?

You will almost certainly burn out an induction motor by reversing it every 4 seconds. Such applications are why clutch/brake systems and servo motors exist.

There are tables of "Allowable Starts Per Hour" for various NEMA motor designs. Just google "NEMA AC motor allowable starts chart" to get an idea of how various NEMA motor designs of various sizes can be expected to perform in terms of starts per hour.

Thanks a lot for being helpful.

It depends on the size of the motor, the load at start and during run, whether you are 'plugging' (going straight from one direction to the other), the cooling you can provide to the motor, the ambient temperature and the quality of motor you buy in the first place.

This is information from ABB:
http://www04.abb.com/global/seitp/seitp202.nsf/0/12c3580f179a9d58c125761f0057ca5c/$file/motor+guide+gb+02_2005.pdf

Pages 54 and 55 gives details of their maximum starts per hour and how to de-rate depending on the load.

Perfect application for a VFD

Assuming, of course things are properly sized, you can limit the current (thus heat) in the motor during the ramp up/down. You likely will need a DB resistor or regen front-end top cycle at this rate, but it's an application we've done numerous times.

What is the application?

Even with a VFD, changing direction every 4 seconds is a challenge. That means accelerating in 2 seconds and decelerating in 2 seconds with zero run time at full speed. So let's say you want 2 seconds at full speed, that's a 1second accel/decel time. Yes, a VFD can limit the current, but does so by limiting speed, so your accel time will increase. So you take the motor mass plus the load mass and the accel time to calculate the required torque to make that happen, and because of the constraint on accel time you run into a loop wherein the torque required means the motor mass increases, which increases the torque requirement, which increases the motor mass etc. eventually you will overcome it, but in my experience, the end user is not happy with the motor size you end up with!

As previously mentioned, this is where servos shine, as long as the overall power falls into their range. They have a much better torque to mass ratio.

PS: another aspect of reversing every 4 seconds is going to be the contactors to accomplish this. Most are rated for at best, 1 million operations and with plug reversing, even less. Assuming just 8 hours per day, 20 days per month, you are looking at the contactors lasting 7 months or less. So now you would need to seriously over size the contactors as well. That's actually another good argument for doing this electronically, but I am still in the servo camp if it fits.

We have a depositer motor that reverses every 4-5 seconds on a large sludge dryer. It's on a VFD and only ramps up to 10 Hertz and -10 Hertz. I put it in my calendar to replace all the relays once a year. They won't make it much longer. No major problems so far though.

Really depends on the application.

Saturn Europa1-What relays? I hope you don't have relays on the input of the VFD and are turning them on and off every 4 seconds.

Highland Controls said:

Saturn Europa1-What relays? I hope you don't have relays on the input of the VFD and are turning them on and off every 4 seconds.

Click to expand...

Not a 100% sure I know what you mean.

But yes, we switch a forward relay and a reverse relay every 4-5 seconds based on an encoder reading/PLC command.

Switching L|1 and L|2 on a Altivar 71

Highland Controls said:

Saturn Europa1-What relays? I hope you don't have relays on the input of the VFD and are turning them on and off every 4 seconds.

Click to expand...

Saturn_Europa1 said:

Not a 100% sure I know what you mean.

But yes, we switch a forward relay and a reverse relay every 4-5 seconds based on an encoder reading/PLC command.

Switching L|1 and L|2 on a Altivar 71

Click to expand...

HC, That means he is using relays for the LOGIC commands to the VFD inputs, not for switching the power.

S_E1, I stand corrected; 0-10Hz is something a VFD can do in a second, so you are right, it depends on the application.

HOWEVER, if the OP is CURRENTLY planning on or using a reversing contactor, then he is NOT going to accelerate to only 10Hz, he is going to accelerate to full speed and decelerate to zero. THAT will be difficult for a VFD every 4 seconds.

Without knowing the specifics of the application, it's impossible to make a definitive judgement, however, as jraef pointed out, the contactors to do it at full voltage will not live long. The other issue of full voltage starting is that *at best*, a 'normal' induction motor may be rated as high as 10 starts per hour, due to the incredibly poor efficiency and massive power required for them to accelerate. The heat must be dissipated.

We've done many indexing and reversing applications in the 5hp and down area with great success. You just have to look at it and decide if it fits. If it doesn't, then a servo is the right answer... just add a decimal place to the cost

But, for example, we've had several applications with low inertial loads, such as motorized brushes, which reverse more often than every 4 seconds.

Gene Bond said:

Perfect application for a VFD

Assuming, of course things are properly sized, you can limit the current (thus heat) in the motor during the ramp up/down. You likely will need a DB resistor or regen front-end top cycle at this rate, but it's an application we've done numerous times.

What is the application?

Click to expand...

This application is for chemical bath in plating machines. They use a motor, coupled it with gears and cam, to move the bath up & down at 60 Hz. Unfortunately, the gear got broken and got to be replaced. However, replacement unit will take 2-3 months as they need to order overseas.

They are asking if we can find "temporary ways" to run the motor, restricting its full rotation so it avoids the broken part of the gear. Hence, the reversal option was being looked at.

The cam's full rotation takes 10 seconds at 60Hz. Hence, I am thinking to reverse the motor every 3-4 seconds. By the way, the motor has an inverter, and the bath is quiet heavy.

If you can't do it yourself, better get someone to run some load calculations. The normal method requires less energy because once the motor and cam have reached full speed, the motor load is just frictional losses and the energy required to raise the bath.
In your alternate operation you will also have to accelerate the cam and the components in the drivetrain. Depending on how large the drive components are compared to the bath you're probably talking about significantly higher load. It's quite likely the motor isn't big enough to operate the machine that way.

Assuming the motor was rated correctly for the original setup

Then you will almost guarantee you will need a larger motor say 1.5 times larger
as reversing the motor at 4 sec's intervals will create more heat for the
motor to deal with this would then require a larger Inverter say 2 times larger
as it also will create more heat to deal with

My thoughts
I feel that pulling off a satisfactory result will be a challenging mission

And you will probably need forced air cooling for the motor.
How far do you have to move how much weight?
Perhaps there is another way to do this?
Is the gear the final drive before the cam?
In a cement kiln clinker cooler we often use a large sprocket with an offset pin driving an arm to oscillate the cooler grates.
On the clinker cooler the whole thing is actually a big flywheel as well and the stored energy helps it go "over the top".
One thing to make certain is that there is no binding at the end of travel.
The torque at the top due to the very low slope of the "incline" at the end of travel means that there is a treemendous force generated an d this could easily explain the breaking of the gear.
In the case of the accursed clinker cooler grate drive it will snap 8" I beams if binding.
You will also see the drive go into current limit at the extreme if it is bad enough.
Been there done that. No TEE shirt however.