Induction motor Across the line - reduced torque

[retirede]

The braking is provided by a mechanical stop in the system. The motor drives the system against this stop.

The OP wants to reduce torque and current when the motor drives against this stop.

Jon

Nothing described will reduce the torque and current more than simply de-energizing the motor. Other than a VFD.

 

[winnie]

Nothing described will reduce the torque and current more than simply de-energizing the motor. Other than a VFD.

Agreed, but where's the fun in that?

Hopefully @W@ttson will come back with more information about the constraints pushing the look at old school torque reduction techniques.

Jon

 

[Joethemechanic]

Seems like I work on lots of equipment that has an electromechanical brake on the motor. I can't give you a time it takes to apply, but it's pretty damn fast. Fast enough that hoists don't drop and fast enough to stop equipment dead when someone hits an E-Stop

 

[W@ttson]

I'm thinking a "wound rotor induction motor" you control the speed and torque by varying the rotor resistance with external resistance. We used them on overhead cranes for years

View attachment 2565286

yes, old systems use WRMs and use a drum controller to change the resistance in the rotor. With that the torque can be reduced. However, I don't want to go down the WRM path. I want to stick with squirrel cage.

 

[W@ttson]

Agreed, but where's the fun in that?

Hopefully @W@ttson will come back with more information about the constraints pushing the look at old school torque reduction techniques.

Jon

I can stop the motor no problem, but what I need is to hold the load down because there is potential to "bounce back" if not pressed down and held there with the motor until the brakes hit. This is for a backup system. That is why it needs to be super simple. The main system is with a VFD. In the main system, the VFD goes down to that press down mode, senses that it is in that region, pops into reduced torque mode and holds the load down ever so gently but with authority until the brakes set.

Before drives, people would do it with the WRMs, but they are a pain to specify and use. Which is why I want to do the same with Squirrel cage.

 

[zbang]

until the brakes hit

How long is that really? Most of the mechanical brakes I've seen apply almost instantly (remove current, large springs immediately apply the brake; think "overhead traction elevator").

Repeating ActionDave- this sounds like trying to solve a mechanical problem electrically, and avoiding things that would make it easier.

 

[Joethemechanic]

Running out of Ideas here. Reduced voltage with something like a starting compensator maybe? How big of a motor? How much do you want to reduce the torque? I take it you want a steady torque at zero RPM?

 

[gar]

230515-0138 EDT

An electro magnetic brake or clutch can operate within about 100 milliseconds.
.
.

 

[W@ttson]

How long is that really? Most of the mechanical brakes I've seen apply almost instantly (remove current, large springs immediately apply the brake; think "overhead traction elevator").

Repeating ActionDave- this sounds like trying to solve a mechanical problem electrically, and avoiding things that would make it easier.

between 1 - 5 seconds based on the hydraulic setting. If you have a massive machine going full speed, and you hit E stop, you don't want the brakes to actuate immediately. It would blow up the machinery. Therefore, hydraulic needle values are adjusted in the brake to slow the actuation of the brake.

 

[W@ttson]

Running out of Ideas here. Reduced voltage with something like a starting compensator maybe? How big of a motor? How much do you want to reduce the torque? I take it you want a steady torque at zero RPM?

Motor would be about 50HP. I need about 150% of FLT at 0 RPM. Then in constant velocity it will run at about 50%FLT. I would like the torque to be reduced to anywhere between 10% and 50%.

 

[W@ttson]

230515-0138 EDT

An electro magnetic brake or clutch can operate within about 100 milliseconds.
.
.

we are using a hydrualic brake that has a large shoe. It has a a delay on braking so that it doesn't brake the machinery on E stop when it is going full speed.

 

[W@ttson]

Ok it is sounding like there is no easy way to accomplish this where all of my criteria will work out. Either drive (which is not what I want since this is a backup and should have no electronics) or old school drum controller with a WRM.

Thank you

 

[Besoeker3]

I'm thinking a "wound rotor induction motor" you control the speed and torque by varying the rotor resistance with external resistance. We used them on overhead cranes for years

View attachment 2565286

We designed wound rotors for that application (static Kramers) for that application. Quite a lot more expensive than cage motors.

 

[gar]

230515-0819 EDT

W@ttson:

You could play with an electromagnetic, DC excitation, clutch or brake, and adjust current to the coil to adjust braking torque. You would probably require momentary over current to the clutch to bring the plates together, and then adust current to determine the amount of slip force.

My work with this type of device in the past was to quicken release time of the clutch. This we did by mechanically controlling the clutch air gap. But with an air gap and adjustment of the magnetic force one can control the slip torque.

..

 

[winnie]

@W@ttson I believe you are on the right track looking at reduced voltage to meet your various requirements. (A wye/delta system changes to motor configuration to reduce volts/turn, so can be considered a form of reduced voltage.)

You wish to reduce motor torque and current using an ordinary squirrel cage motor, without using power electronics.

You agree that using a VFD to change drive frequency is the best way to control this motor, and you _are_ using a VFD, but want a non power electronic approach as a backup. You also agree that absent a VFD, various old school techniques such as wound rotor machines would be an option, but now we are looking at much more custom/expensive hardware given present day manufacturing.

So given these design constraints (no VFD, 'modern' COTS motor), voltage reduction is the only tool you have available to reduce torque.

@Jraef mentions in post 18 that there is a voltage and current spike during the wye/delta transition. I don't know how much of this transient is caused by the phase angle change and how much by the effective voltage change. This spike is presumably a short transient which won't cause appreciable heating, and also won't damage power electronics that isn't present in the system.

Using a motor starting autotransformer https://americas.hammondpowersoluti...ol-automation/motor-starting-autotransformers and suitable tap selection contactors you can reduce the voltage applied to the motor, without any significant phase change. This would let you drop torque in a non-electronic fashion. Standard tapping is for 50%, 65%, and 80% voltage, giving you a selection of torque values to consider (25%, 42% or 64%)

I personally feel that at the 50 Hp size, spending money on two VFDs is probably the best 'bang for the buck' in terms of redundancy. But maybe you need this system to function after an EMP or something. So I'll needle you a bit but respect the design constraint you are setting

-Jon

 

[Joethemechanic]

We designed wound rotors for that application (static Kramers) for that application. Quite a lot more expensive than cage motors.

I can't remember ever having to buy one. Fixed quite a few, bearings, brushes, slip rings, etc.. Had a few of the armatures rewound. Even had some that were 2 phase rewound for 3 phase. I think the conversion was a little pricy, but not anything crazy

 

[Besoeker3]

I can't remember ever having to buy one. Fixed quite a few, bearings, brushes, slip rings, etc.. Had a few of the armatures rewound. Even had some that were 2 phase rewound for 3 phase. I think the conversion was a little pricy, but not anything crazy

Well we didn't buy those - we manufactured the variable drives. Typically those were upwards of 2,000 kW and 11kV.

 

[synchro]

... Soft starters are not good for my application as I need high starting torque due to the high inertia load.

I can stop the motor no problem, but what I need is to hold the load down because there is potential to "bounce back" if not pressed down and held there with the motor until the brakes hit. This is for a backup system. That is why it needs to be super simple. The main system is with a VFD. In the main system, the VFD goes down to that press down mode, senses that it is in that region, pops into reduced torque mode and holds the load down ever so gently but with authority until the brakes set.

Does the main system with a VFD apply braking (i.e., negative torque) to slow down the high inertia load before going into a reduced torque mode and hold the load in position? This would reduce or prevent any rebound when the load hits the stop. Has any analysis or evaluation been done which shows that switching to a single reduced torque value will be sufficient to avoid "bounce back" before mechanical braking is applied?

A soft starter could reduce the torque when approaching the mechanical stop as Jraef mentioned, and some can also apply DC braking to slow the load down if it's not applied repetitively because it will heat up the rotor. A DC injection braking module could also be used with other reduced voltage approaches, such as an autotransformer. But I agree with Jon that having a redundant VFD would have the least risk in terms of meeting requirements, and it would also allow normal usage to continue while a replacement VFD is acquired.

 

[zbang]

Either drive (which is not what I want since this is a backup and should have no electronics)

I don't understand why the backup systems isn't being made identical to the main system. From the discussions there won't be much cost reduction and it'll be a different system that the operators need to learn and use periodically so they stay proficient. To me, that last one is a powerful force for identical drives/controls.

(And why "no electronics"?)

 

[Joethemechanic]

You know if you're going to use a squirrel cage motor we used to use ones for high inertia loads like flywheel type presses and shears with across the line starting. If I remember right they were "Class H" with a higher rotor resistance. They might be a thing of the past????? But if you started with one of those it might be better suited to your application