Wound rotor motor with external brake option

[pitrdvorsky]

I was on a visit to a customer.
He wants to add frequency inverters to a bridge crane that uses ring induction motors (WRIM) for smooth ramp start-up.
I read on other forums that it works and there is no problem if the rotor short-circuits.
What surprised me is that there is an option on the electric drive that I cannot identify exactly. The photo shows the motor and on the back side there is an induction brake under the perforated sheet metal. I have not come across this tip yet.

In the photo where there is a piece of iron in the shape of a gear wheel, there is a disk with metal rectangles that remind me of embedded magnets. The wiring diagram states that the motor has an induction brake with a 24V power supply. When inspecting, I did not test whether there are really magnets in the disk. Does anyone have experience with a similar brake?
There are two motors and they are used to move the crane bridge. At the other end of the motor there is an electrohydraulic cylinder that controls the parking brake.
The motors are controlled via DOL (direct on line) via contactors. The stopping sequence can be in the following order: 1. electrodynamic braking using an external brake. 2. activation of the parking brake.

This does not make sense to me, because electrodynamic braking uses braking with DC current, which is fed into the stator circuit after the AC power is disconnected.

 

[edison123]

The brake operates mechanically by springs in case of power loss/stop to hold the hoist in position. The DC supply releases the brake (by pulling in the springs) when the hoist is moving up/down.

After seeing the excellent performance of my new 60 ton EOT crane which came with VFD's for hoist/LT/CT, I did a VFD retrofit for my old 10 ton crane which had a DOL hoist cage motor and couldn't be happier. Negligible inrush current, excellent control of the hoist with much better starting torque.

Go with a VFD with a brake resistor for quick start and stop and inching. My second 10 ton hoist is being retrofitted with a VFD right now.

 

[waross]

This does not make sense to me, because electrodynamic braking uses braking with DC current,

Google "eddy current brakes".
That does not look like an eddy current brake but similar.
It looks like there are cores and windings just behind the cogs.
That probably develops a braking action when DC is supplied to the coils.

 

[edison123]

This is not an eddy current brake. It plain old fail-safe spring activated mechanical brake found in hoists.

 

[waross]

That does not look like an eddy current brake but similar.

The plain old spring applied, thruster released drum brake is on the other end of the motor.
The cogged piece appears to be solidly bolted to the fixed end piece.
The cogs extend to very close to the segments.
Speculation, this may be a device to prevent overspeeding when a load is lowered.
There may be items behind the segmented part that make the purpose of this obvious but not much can be deduced from the pictures.
With the drum brake on the other end of the motor, this may not be a parking brake.

 

[jraef]

This does not make sense to me, because electrodynamic braking uses braking with DC current, which is fed into the stator circuit after the AC power is disconnected.

There are multiple kinds of braking technology to go with AC motors. Dynamic Braking (DB) is one type, DB pulls the kinetic energy of the rotating load into a DC bus and then burns it off into resistors. It is very commonly used for motors with VFDs because it is dirt cheap to add it to the VFD and then all you need are resistors.

Another option is called “DC Injection Braking” (DCIB) where DC is put onto one set of windings in the motor to establish a non-rotating magnetic field and thus counter-rotating torque to bring the load to a stop.

Next is what Bill mentioned, Eddy Current Braking, where DC is applied to a series of coils near a disk attached to the rotor, and the eddy currents that happen in that disk also cause retarding of the rotation.

You can also do Plug-Reverse Braking, where you throw the motor in reverse while still moving forward (not recommended, but people do it all the time).

The BEST technology in terms of wear and tear on the motor or other parts is taking the same concepts in Dynamic Braking and making it Line Regenerative Braking, which means instead of taking the kinetic energy in the rotor and burning it off into resistors, it’s converted back into electrical energy using an inverter to feed that energy back into the line source (assuming there is a place for it to go).

But none of these purely electrical braking methods can HOLD a load, because in general their braking force weakens as the speed reduces, AND if power fails, so do they! So you then also still need to have good old electro-mechanical brakes. This older technology is almost always used on hoists because it will designed to employ on the ABSENCE of power in order to assure that a load is not dropped if power fails. In many cases a hoist will have one of the electrical braking means AND an electro-mechanical brake that is used primarily as a “holding brake” once the load is stopped electrically, but SIZED to be able to take over completely if power fails.

I agree with Bill that the unique looking toothed wheel arrangement appears to be part of an eddy current braking assembly to help retard the downward force on lowering a load. But like Muthu said, I’m sure it also will have an electro-mechanical braking mechanism built into it. If it is an eddy current braking system, it likely has a separate DC power source for it which means you can continue using it with the VFD on the motor. But electro-mechanical holding brakes often have DC coils that will be energized by the stator power circuit, using an internal rectifier, but if there is an eddy current brake, it may be tied to the DC source for that. So to use a VFD you will have to find that circuitry and figure out how it is powered so that you can either separate it from the stator or leave it alone.

No matter what though, you will have to figure out how to interface whatever braking technology you have to the VFD. Most decent brands of VFD will have I/O that can be programmed to take care of this issue, but you have to make sure before buying it.

 

[jraef]

OK, I can’t believe I left this out but rather than edit, I’ll just do it as a new post. When you are applying a VFD to a hoist, you have to understand that not all VFD‘s are capable of being used for this. You need one that is capable of close loop flux vector control, and has a feature called “torque proving”. Without it, the VFD may drop the load slightly when you release the break to change position. If the load starts to move before the VFD can establish torque, it can take more torque to stop a moving load than the VFD is capable of generating. So if you’ve never done this before, I highly recommend doing it alongside somebody with experience on applying a VFD to a hoist. Everyone needs to start somewhere but the risks involved make it problematic for a novice.

 

[waross]

He wants to add frequency inverters to a bridge crane that uses ring induction motors (WRIM) for smooth ramp start-up.

I would exhaust the possibility of adding resistance to the first resistor step, adding acceleration steps (add a contactor and resistors), and reviewing the acceleration timing.
There may be an issue with the existing equipment.
WRIMs generally work well on cranes and do provide smooth ramp-up.
It will be very much cheaper to tune up and/or repair the existing equipment than to add a VFD.

 

[pitrdvorsky]

The customer wants to go for a VFD solution at all costs.
I received the motor wiring diagrams, the diagram indicates an induction brake or an eddy current brake. So, an eddy brake is applied to slow down the load and then an electromechanical brake is applied using a time relay.
In such an application, there is a problem when a frequency inverter without an encoder is used. We had problems that the load collapsed due to gravity after opening the brake on the crane. The motor and the inverter were not from the same manufacturer. The converter did not know what the parameters of the motor were and therefore it was controlled in U/f control. With VFC, the inverter crashed. The customer did the commissioning himself and was not knowledgeable enough to know what this problem entailed. Later, autotuning was done and the converter controlled the motor in vector control, which solved the problems with load collapse. The torque that the motor was spinning during operation was measured. This value was set in the inverter with a larger margin for motor excitation before the brake opens. In older inverters, we solved this by setting the minimum speed to e.g. 15 rpm before the brake opens. The inverter had a problem regulating the minimum zero speed without an encoder, since the feedback consists of current measurement and at low speed the currents are low.

WRIM motors are not built to operate with VFD. The winding insulation is designed for sinusoidal voltage. With VFD, the winding insulation of the old motor is more stressed. There is a solution using a sinusoidal filter. But here there is a problem with the voltage drop on the filter of about 10%. This would mean a reduction in torque on the motor. As far as I know, that is why the sinusoidal filter is prohibited for lifting applications. We justified this to the customer, but even with this risk he wants to replace the contactors instead of VFDs.

In this whole case, I am worried whether the VFD will measure the correct parameters of the WRIM motor with a short-circuited rotor and will be able to control it in vector control in open loop. With U/f regulation there are problems when lifting heavy loads.

I have no experience with a WRIM motor operated on a VFD.

 

[waross]

WRIMs provided good, dependable crane control for decades.
If the customer is willing to expend the money for the possible slight advantage of a VFD, they should also spend the money on a suitable motor rather than trying to use an unsuitable motor.
The issue that you describe are a good reason why a VFD is a bad idea on a WRIM.
A possible alternative:
Years ago, I saw a custom application for fan speed control of WRIMs.
Rather than resistors in the rotor circuit, an inverter was used in the rotor circuit.
The inverter controlled the rotor current and returned the rotor energy back to the line rather than wasting it as resistor heat.
While the technique worked for fans, it may not have the speed range or load range needed for a hoist.

 

[FacEngrPE]

I have been involved in replacement of wound rotor hoist systems with VFD / systems. The conversion works, HOWEVER you need to be careful with these points with hoists, other motions are less critical. My rules of thumb. Your millage may differ.

1. select a hoist duty 4 quadrant VFD, size for test weight torque + margin for acceleration (about 150% of proof test weight torque) then choose next larger size VFD. Then discuss with mfgr and verify that the drive will provide good service with the recommended motor.
2. Select a good mechanical brake (you need to hold the load when stopped), control the brake from the drive brake contacts.
3. understand the motor duty cycle (get the motor manufacturers engineering department involved is very helpful). Blower for hoist motor cooling is a must. Encoder feedback is necessary if any precision is needed. Matching the motor and VFD is desirable to ensure correct shaft torque and duty at all speeds of interest.
4. Make sure the regen brake resistor has ample duty cycle.
5. Verify at construction with a proof test.

All of the jobs I was involved in were end of hoist system life replacements so I did not need to provide economics justification. I can't see passing the hurdle rate unless replacement of major components is required regardless.

 

[waross]

I have been involved in replacement of wound rotor hoist systems with VFD / systems.

Are the resistors still used or is the rotor shorted?
Thanks.