Induction motor Across the line - reduced torque

[winnie]

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

This is a very good point.

As the rotor stalls, the frequency 'seen' by the rotor goes up. There are rotor bar designs where the effective resistance of the rotor changes as slip increases because of 'deep bar' effects. Essentially these motors change rotor resistance without having a wound rotor or external resistance control. Selecting the correct motor design might be sufficient to reduce locked rotor current.

But this might be a problem for the OP during starting when they want maximum torque at start.

-Jon

 

[Joethemechanic]

Think they were a Class-H for the higher temperature rated insulation. They ran kinda hot because of the higher resistance of the rotor and they had more slip.

One time I replaced one on a big shear with a standard T Frame motor. It was what we had and I couldn't get the right one anytime soon. Tried to start it and blew a primary fuse. I guess that was 40 years ago. I think it was 100 HP or more. That shear had a massive flywheel on it

 

[Jraef]

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

"Class H" would be the insulation temperature rating. I think maybe you were thinking of "NEMA Design D" which would be the torque / speed curve design of the motor, with D being for highest inertial load applications. But Design D motors also provide the highest Locked Rotor Torque, so for this application, that is the exact opposite of what the OP is after.

 

[Jraef]

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.

I get it, because I have done similar applications, albeit with VFDs and / or soft starters. The issue is that if you simply cut power to the motor and the brake sets, there is a chance that the load "bounces" (for lack of a better word) off of the end stops before the brakes can set and lock, so you want a SMALLER amount of torque to continue to be applied by the motor to keep it from moving off of the end stop until the mechanical brake sets and holds.

This is where I have used either the "brake and hold" functions in some soft starters and/or VFDs. Is it POSSIBLE using an electro-mechanical starting method where you want Across-the-Line starting? I think possible, especially with the PW option I mentioned. It might also work with a Primary Reactor or Primary Resistor starting method, but I don't think a Wye-Delta or Autotransformer would be appropriate.

 

[Joethemechanic]

"Class H" would be the insulation temperature rating. I think maybe you were thinking of "NEMA Design D" which would be the torque / speed curve design of the motor, with D being for highest inertial load applications. But Design D motors also provide the highest Locked Rotor Torque, so for this application, that is the exact opposite of what the OP is after.

Yeah I think you are right they were "D"

I'm wondering what the speed/torque curve would be like with one of them and a compensator for voltage reduction.

Depending on duty cycle, I kind of see heat becoming a problem

 

[winnie]

It might also work with a Primary Reactor or Primary Resistor starting method, but I don't think a Wye-Delta or Autotransformer would be appropriate.

I'm curious: what do you see as the problem of using an autotransformer to reduce terminal voltage and thus torque?

Thanks
Jon

 

[gar]

230515-1921 EDT

W$attson:

Apparently your application has more inertia in the load than in the drive motor. Thus, when you remove power from the motor the motor will slowdown faster than the load. If this is the case and there is slop in the drive from motor to load, then the motor will move from a driver to a gap with no motor inertia load. When the motor slows enough ( a small amount ) there will be no torque between the two rotating mechanisms. After this short time the motor will become an inertia load on the previously driven load.

If your brake is on the motor and it does not come on until the motor is providing a drag torque on the previously driven load, then other than some possible mechanical oscillation this is no different than if there was no backlash
in the motor coupling.

If your brake is on the load, then a shift in the torque from brake to motor will change direction. Is this a problem? I don't know.

If you maintain driven power from the motor while applying excitation to the brake, and at the time the brake actually engages, then removing motor drive, is this a problem?

.

 

[Joethemechanic]

I'm curious: what do you see as the problem of using an autotransformer to reduce terminal voltage and thus torque?

Thanks
Jon

I'm thinking about the "hold" part of it's operation. It's applying torque but there is no movement. Force without movement is not work. You have electrical energy flowing into the system but no work being done. Energy can't just disappear, so there has to be heat being created. How much heat, and how much can be safely dissipated ,, I don't know

 

[Joethemechanic]

I'm curious: what do you see as the problem of using an autotransformer to reduce terminal voltage and thus torque?

Thanks
Jon

OOps. Sorry, I didn't have my glasses on and I thought you were replying to me

 

[winnie]

I'm thinking about the "hold" part of it's operation. It's applying torque but there is no movement. Force without movement is not work. You have electrical energy flowing into the system but no work being done. Energy can't just disappear, so there has to be heat being created. How much heat, and how much can be safely dissipated ,, I don't know

This is exactly the issue that the OP is dealing with. The motor is forced to a stop, and will be pulling its locked rotor current. This is on the order of 6x normal full load current. Lots of power going into the motor, no mechanical power coming out, lots of heating.

The best way to deal with this is to use a VFD. The VFD drops frequency and voltage, to minimize the power being delivered to the motor when it is producing torque at zero speed. By carefully selecting frequency, full load torque can be maintained at zero speed with essentially the same heating as seen during normal full load operation.

If you reduce the voltage going to the motor while it is holding at zero speed, you reduce the power and the heating. But in this case torque also goes down.

-Jon

 

[W@ttson]

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.

..

this kind of reminds me of an Eddy current drive I saw once:

How Dynamatic® Technology Works | Adjustable Speed Drive System

Our Electromagnetic Eddy Current engineering is simply better by design. Call 800-548-2169 to find out more.

www.dynamatic.com

 

[W@ttson]

@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

Thank you very much for the detailed layout of this. All what you say is accurate. I will look more into this motor starting autotransformer scheme.

You mention the EMP, and that is more or less what we are after. Not so much a bomb, but rather often times we get lightning events, and they sometimes cause havoc on some our electronics. By the time we can get the system operational, we can have tons of fines for not being able to operate. This way, having a really simple "dumb" control scheme, that is more or less impervious to such transients/surges is desired.

 

[W@ttson]

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.

Yes, the VFD applies overhauling braking through a dynamic braking resistor if needed.

It also slows it down to 10% of full speed when it gets close to that location.

 

[W@ttson]

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"?)

See post 52.

 

[W@ttson]

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

Yes, the back up squirrel cage has super HHH insulation and is a NEMA D.

 

[W@ttson]

This is a very good point.

As the rotor stalls, the frequency 'seen' by the rotor goes up. There are rotor bar designs where the effective resistance of the rotor changes as slip increases because of 'deep bar' effects. Essentially these motors change rotor resistance without having a wound rotor or external resistance control. Selecting the correct motor design might be sufficient to reduce locked rotor current.

But this might be a problem for the OP during starting when they want maximum torque at start.

-Jon

We choose NEMA D motors so that we can get that BDT at starting.

 

[W@ttson]

"Class H" would be the insulation temperature rating. I think maybe you were thinking of "NEMA Design D" which would be the torque / speed curve design of the motor, with D being for highest inertial load applications. But Design D motors also provide the highest Locked Rotor Torque, so for this application, that is the exact opposite of what the OP is after.

Its a catch 22, I need high starting torque because of the high inertia, but at the Parking point, I need to reduce my locked rotor current.

 

[W@ttson]

I get it, because I have done similar applications, albeit with VFDs and / or soft starters. The issue is that if you simply cut power to the motor and the brake sets, there is a chance that the load "bounces" (for lack of a better word) off of the end stops before the brakes can set and lock, so you want a SMALLER amount of torque to continue to be applied by the motor to keep it from moving off of the end stop until the mechanical brake sets and holds.

This is where I have used either the "brake and hold" functions in some soft starters and/or VFDs. Is it POSSIBLE using an electro-mechanical starting method where you want Across-the-Line starting? I think possible, especially with the PW option I mentioned. It might also work with a Primary Reactor or Primary Resistor starting method, but I don't think a Wye-Delta or Autotransformer would be appropriate.

Yes, you describe my issue exactly. I will need to look more into the Part winding and primary resistor methods you mention here. Thank you

 

[W@ttson]

230515-1921 EDT

W$attson:

Apparently your application has more inertia in the load than in the drive motor. Thus, when you remove power from the motor the motor will slowdown faster than the load. If this is the case and there is slop in the drive from motor to load, then the motor will move from a driver to a gap with no motor inertia load. When the motor slows enough ( a small amount ) there will be no torque between the two rotating mechanisms. After this short time the motor will become an inertia load on the previously driven load.

If your brake is on the motor and it does not come on until the motor is providing a drag torque on the previously driven load, then other than some possible mechanical oscillation this is no different than if there was no backlash
in the motor coupling.

If your brake is on the load, then a shift in the torque from brake to motor will change direction. Is this a problem? I don't know.

If you maintain driven power from the motor while applying excitation to the brake, and at the time the brake actually engages, then removing motor drive, is this a problem?

.

Way more inertia in the load.

There is some slop in the gears. This backlash in the gearing we want to eat up by pressing down into the stop. The brake sets when power is removed. Its Spring set, power release.

 

[W@ttson]

This is exactly the issue that the OP is dealing with. The motor is forced to a stop, and will be pulling its locked rotor current. This is on the order of 6x normal full load current. Lots of power going into the motor, no mechanical power coming out, lots of heating.

The best way to deal with this is to use a VFD. The VFD drops frequency and voltage, to minimize the power being delivered to the motor when it is producing torque at zero speed. By carefully selecting frequency, full load torque can be maintained at zero speed with essentially the same heating as seen during normal full load operation.

If you reduce the voltage going to the motor while it is holding at zero speed, you reduce the power and the heating. But in this case torque also goes down.

-Jon

Yes, exactly. I typically have a timer circuit in this end stop position so that power is removed even if brakes don't set after say 5 seconds, just in case. But with this simple system, drawing 6x FLA, even 5 seconds I think is too long.