Induction motor Across the line - reduced torque

[W@ttson]

Hello,

I have an application where I need to have a motor drive a load into a parked position and keep it there until brakes can set and hold it in that position. The motor controller is just a regular full voltage reversing starter.

With a drive I can have the drive output at a reduced torque. However, for an across the line controlled motor how can I press the load into place without having the motor go into locked rotor current and trip out. We are talking maybe 3-5 seconds of pressing/holding till the brakes set.

I am thinking about all sorts of delta wye starters or introducing some large power resistors into the lines of the motor in series.

 

[gar]

230511-2346 EDT

W@ttson:

An ordinary induction motor is not well suited for what you want. A DC shunt wound motor would be very good for your application.

.

 

[Jraef]

…

I am thinking about all sorts of delta wye starters or introducing some large power resistors into the lines of the motor in series.

Then you are no longer talking about “Across the Line”…

The point is, you will have to do SOME form of torque reduction, either electrically or mechanically. Personally, I would do it with a good Solid State Soft Starter, but I’m biased that way. Some Soft Starters have a “Smart Motor Braking” option that will ramp down into a DC Injection Brake to hold it in place until a mechanical brake sets.

 

[W@ttson]

230511-2346 EDT

W@ttson:

An ordinary induction motor is not well suited for what you want. A DC shunt wound motor would be very good for your application.

.

Don’t want to go DC

 

[W@ttson]

Then you are no longer talking about “Across the Line”…

The point is, you will have to do SOME form of torque reduction, either electrically or mechanically. Personally, I would do it with a good Solid State Soft Starter, but I’m biased that way. Some Soft Starters have a “Smart Motor Braking” option that will ramp down into a DC Injection Brake to hold it in place until a mechanical brake sets.

Sorry, what I wanted to mean by “across the line” is non electronic. Soft starters are not good for my application as I need high starting torque due to the high inertia load.

I am hoping that with the wye- delta starter as long as I can manipulate those electromechanical starters I can get the reduced torque at that press down point, by reducing the voltage. The torque should be proportional to voltage squared.

 

[winnie]

Changing the connection from delta to wye will have the effect of reducing torque and current, as you suspect.

Keep in mind that the torque drops as the square of the voltage but current only drops linearly with voltage. So you will see a large fraction of locked rotor current. You will get 33% LR torque, 58% LR current, 33% LR heating.

As far as adding external elements to drop voltage, you might look at starting autotransformers.

Jon

 

[ActionDave]

Sorry, what I wanted to mean by “across the line” is non electronic. Soft starters are not good for my application as I need high starting torque due to the high inertia load.

I am hoping that with the wye- delta starter as long as I can manipulate those electromechanical starters I can get the reduced torque at that press down point, by reducing the voltage. The torque should be proportional to voltage squared.

Stop looking for an electric motor to solve a mechanical problem.

 

[ptonsparky]

Changing the connection from delta to wye will have the effect of reducing torque and current, as you suspect.

Keep in mind that the torque drops as the square of the voltage but current only drops linearly with voltage. So you will see a large fraction of locked rotor current. You will get 33% LR torque, 58% LR current, 33% LR heating.

As far as adding external elements to drop voltage, you might look at starting autotransformers.

Jon

Is an oversized VFD a better fit? If one can be found.

 

[winnie]

A VFD wouldn't even need to be oversized. You get torque at zero speed with normal operating current.

If the load requires more than full torque to accelerate, then the VFD might need to be sized for the acceleration requirement.

OP didn't specify just how big this system is.

Jon

 

[retirede]

Sorry, what I wanted to mean by “across the line” is non electronic. Soft starters are not good for my application as I need high starting torque due to the high inertia load.

I am hoping that with the wye- delta starter as long as I can manipulate those electromechanical starters I can get the reduced torque at that press down point, by reducing the voltage. The torque should be proportional to voltage squared.

The torque available from the motor will drop if you transition back to wye, but that doesn’t get you braking. It’s still a motor that will try to run at rated speed.

To get braking, you need to reduce the power frequency so that the motor tries to run slower.

 

[gar]

230513-1643 EDT

W@ttson:

Look at the speed vs torque curve for an induction motor. You will find that characteristic curve for normal motors to have very little variation until a break down point. The shape of this curve can be changed by changing the rotor resistance.

If you connect the rotor coils to connect thru slip rings, then externally you can adjust rotor resistance and get a means to adjust rotor slip. With a large enough rotor resistance you can get a curve that does not fold back.

You can also use on-off switching on a induction motor to have a degree of speed control. A speed sensor and feedback is needed to do this.

Fix frequency power sources are not a good source for induction motor speed control. DC motors are much simpler to control.

.

 

[Joethemechanic]

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

 

[winnie]

If we are opening the window to different motors, I saw one with a conical air gap.

A spring held the rotor against a brake. When power was applied, the attractive force between rotor and stator pulled the rotor sideways away from the brake.

Release power and the brake applied immediately.

As I understand what the OP is doing, the motor is driving the system against against a stop. Once the system literally crashes, the motor needs to be held for a few moments until the brake is applied.

Question for the OP: is there any gearing involved? Could you simply use a non back drive gearbox?

Jon

 

[Joethemechanic]

Probably cheaper and easier to just use hydraulics. We built some wastewater clarifier drives that needed to supply full torque at or near zero RPM and we just used hydraulics. Not exactly energy efficient, but they solved a lot of operational problems.

 

[ptonsparky]

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

Those were interesting to work with, but it has been 40 years since.

 

[Joethemechanic]

Those were interesting to work with, but it has been 40 years since.

We had a 20 ton bridge crane in a place I worked at in the early 80's. It was a Cleveland Crane built around 1900. It actually used carbon piles for rotor resistance.

Wild crane to operate, two trollies with hoists, operator's cab moved with one of the trollies. Had magnets on it, it was in the plate bay of a steel co

I used to come across a lot of DC bridge cranes too

 

[ptonsparky]

We had a 20 ton bridge crane in a place I worked at in the early 80's. It was a Cleveland Crane built around 1900. It actually used carbon piles for rotor resistance.

Wild crane to operate, two trollies with hoists, operator's cab moved with one of the trollies. Had magnets on it, it was in the plate bay of a steel co

I used to come across a lot of DC bridge cranes too

We installed power and wired up a 40 ton one at Rockwell Valve. They built railtrack etc so they could load valves onto train cars. All the other tracked overheads were wound rotor as well. Whole plant was shut down and remodeled into a Cabela's a few years later.

 

[Jraef]

Sorry, what I wanted to mean by “across the line” is non electronic. Soft starters are not good for my application as I need high starting torque due to the high inertia load.

I am hoping that with the wye- delta starter as long as I can manipulate those electromechanical starters I can get the reduced torque at that press down point, by reducing the voltage. The torque should be proportional to voltage squared.

The problem with Wye/Delta is the transition, because there is a phase shift when that happens and there is a resulting voltage and current spike, neither of which is going to serve you well in this scenario.

If you want to stick to electromechanical, I would suggest a Part Winding motor arrangement. You will have the option of ATL starting, then drop out part of the windings when you want to reduce torque on stopping.

But as one last pitch toward RVSS, there are several products on the market that are designed to be capable of ATL starting if you want that, then use the decel function to accomplish your torque reduction.

 

[retirede]

The problem with Wye/Delta is the transition, because there is a phase shift when that happens and there is a resulting voltage and current spike, neither of which is going to serve you well in this scenario.

If you want to stick to electromechanical, I would suggest a Part Winding motor arrangement. You will have the option of ATL starting, then drop out part of the windings when you want to reduce torque on stopping.

But as one last pitch toward RVSS, there are several products on the market that are designed to be capable of ATL starting if you want that, then use the decel function to accomplish your torque reduction.

Help me out….how would a part-winding motor provide braking?

 

[winnie]

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