OT: frequent motor reversal questions

Hi,

I have a 1hp 3 phase 220 volt motor on a fixture with a very light load that is required to reverse very frequently. It is required to run for 1.5 seconds forward, stop as fast as possible, reverse for 1.5 seconds, wait 1 second and then repeat.

The motor I am using has a dc brake and I engage it with the motor windings through a rectifier. Right now it is applied for 1.5 seconds and then again for only 250ms while I attempt to reverse the motor as fast as possible. I control it with a set of reversing contactors across the line.

The motor is getting quite hot and trips the overload after half an hour.

Whats the best way to deal with a situation like this? VFD? VFD with a braking resistor? Bigger motor?

Thanks

Laws of physics are against you here. It takes just as much energy to start a motor as it does to stop it. Normally, we don't see the stopping energy because it is spread out as friction, heat, wind, etc. and distributed across an amount of time that we often do not care about. In your case however, the time frame is important. So the energy to stop it is more visible.

You basically have two choices. Cool it in such a way as to remove the heat as fast or faster than it is created, or over size the motor so that it can absorb that heat and dissipate it better. Most likely you will need a combination of both.

Motors have a finite limit of "Starts-per-hour", based on the number of starts, the cooling time between them, and the mass that needs to be started and stopped. There are standardized formulae in a design specification for motor manufactures called NEMA-MG1 which shows you how to determine your Stars-per-hour capacity. Using that, you can keep increasing the motor size and redoing the calculations until you find the size of motor necessary for it to survive this abuse. The easy way out is to contact your motor supplier, tell them EXACTLY what you want to accomplish, and let them tell you what to use.

Where I used to work we made some tapping machine that required motors to do quick reversing. We used to call it "plug reversals". Not sure if that is industry standard or just what our boss called it.

Anyway, in these applications we always used Demag motors with a built in conical brake and rarely had issues.

I like what jraef said on this subject but, if you don't have room for a larger motor, you have three methods for reducing heat in the motor with the same start/stop cycle.

First, change to a TENV (Totally Enclosed Non Vent) motor. This motor cools thru its skin without the benefit of any fan. And they are almost always Insulation Class H which is good for 175 degrees C---very hot.

Second, install a VFD to drive the motor and install a snubber brake system. This transfers the braking energy from the motor to a resistor bank. The DC injection braking presently in use leaves all the braking energy in the motor as heat.

Third, if you use a sensorless vector drive and do an optimal job programming it, you will also reduce the inrush current on start and the resulting heat. The VFD will probably have to have 150% shortterm current capacity and maybe more----like 200%.

Since this probably a very small load compared to the capacity of his distribution system, would there be any benefit to a line-regenerative drive that transfers excess energy to the 3 phase incoming line, versus an ordinary braking resistor that burns off the excess energy from the DC bus ?

Why not just fit a servo motor and drive

Another consideration I had when doing tappers with Demag brake motors - since the motor was not running all the time, but frequently starting up, add a 120VAC cooling fan to the motor that was running any time the motor was and for 3 minutes after the last motor run to keep it cool, controlled by it's own contactor & timing circuit.

I'd leave the motor running and clutch it through a gearbox that can reverse.

rdrast said:

I'd leave the motor running and clutch it through a gearbox that can reverse.

Click to expand...

+1
Let's face it...This is not an electric motor only application...Even the most advanced technology cannot defeat physics but slightly delay the inevitable end...
Clutch/Brake- FWD/REV- solenoid directional shift gearbox; turn the motor on and forget about it -> Clutch Off Input -> Brake Output -> Release Brake -> Shift Output -> Clutch On Input -> REPEAT.

I vote with Tim

Low inertia servo motor + servodrive.So forget all the heat and stuff.

I am wondering this application not being so demanding as for servodrive if a smart drive live PowerFlex 755 might do the job.

It is the inrush current causing the heating. I don't know squat about servos, so that might well be the solution. For a conventional motor you have options:
- soft start if you don't need the rapid acceleration of across the line
- over sized motor for more heat and current capability
- external fan cooled combined with RTDs to monitor actual temperature and over sized heaters in the overloads (the fan won't cool them)

And "plugging" is a common term for this kind of service.

Is there room for a reversing transmission, like you would find in a washing machine?

I installed an application like that two years ago I used a ACS800 and a servo drive.

Maybe there is not a lot of profit in this machine so I would go a step at a time.
Try removing the injection braking system and adding an external brake (Warner?). That will transfer the braking heat from the motor to the brake. If you still require cooling just add a fan.
Posts above indicate that run tapping machines run satisfactorily with a Demag brake motor.

If its linear i would use a pneumatic cylinder.
if it is rotation is it possible to use mechanical like a wiper. same as a cam solution.
Or indeed use a servo solution, with a separate fan to cool the motor.