Wound rotor motor 3

[NEWFIEBOY]

Hi Guys
I have a 1960s 15 hp 230 volt 3 ph wound rotor motor. It is presently connected to a motor starter and resistor bank. The motor is reversable and can be operated at either of three speeds selected by remote control switches. This motor is the power unit which operates a capstan on the deck of a tug boat. The problem is that the motor starter is original, it is not reliable, parts to keep repairing it are hard to find and expensive. I would like to replace the motor starter with a new AC drive. My question is has anybody had any experiences with using an AC drive along with an older wound rotor motor? Is it feasible to short circuit the rotor windings to make this motor resemble a squirl cage motor? I would appreciate any advice.

 

[jraef]

Yes it can be done, and being only 230V you should not have any problems with the motor windings. Just make sure the VFD is sized for the motor stator Full Load Amps, not HP because that's really all that counts, and sometime WR motors have different FLA ratings than typical squirrel cage motors.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[aolalde]

Hi NEWFIEBOY.
I agree with Jraef. Short circuit the rotor winding and apply a Variable Frequency Drive. Since your motor is 46 years old you will not loose to much trying. The worse case scenario is winding failure and or bearing failure. Chances are the motor will survive due to your application for reduced operation time.
If it will fail look for a qualified repair shop that could analize the failure and apply a winding or bearing protection according with the failure analisys.
Nice result is a universal posibility of speed and acceleration setups.

 

[waross]

Hello NEWFIEBOY
I agree with jraef and aolalde.
One other suggestion. If the motor is working well you can replace the individual contactors in the starter with any similar rated contactors that will fit in the physical space.
A Variable Frequency Drive is a good solution.
The contactors may cost as much as the drive. I would guess you have at least four of them.
The advantage is that the same people that fix it now will be able to continue fixing it. I don't know if this is a consideration or not.
respectfully

 

[jraef]

Assuming that he even has contactors. A lot of those old WR controllers were tap switches, motorized or manual.

This is the only picture I could find of one, a relatively large fire pump controller, but it's fairly typical of a manual tap switch controller. On a little 15HP motor it may have been a small drum switch for the resistors.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

Hello jraef
You're probably right. I thought that the remote control may have implied contactors. The drum switch can still be replaced with contactors.
I have seen quite a few crane installations with wound rotor motors. Most of them predated the 60's and had contactors.
One plant had a couple of old cranes that predated WWII. They did have drum controllers.
There's still the resistors to consider. That may even be where the major parts problem is.
Those pictures must be getting hard to find. How about a picture of the old 300 hp compensator I fought with years ago?
respectfully

 

[NEWFIEBOY]

Hi guys

The motor starter does have contactors. The Original timers were changed to Omron timers, the romote control switches are now Allen Bradley. These modifications, as well as others have been carried out on as needed basis. One of the problems which we have now is that the contactors will sometimes stick closed ( not a fault of the timers). This will keep the motor running for up to a minuite after the operator has selected the motor stop position on the selector switch. We have also had problems with the resistor bank connections. I think it may be time to eliminate the whole control system and install an AC drive. My concern is that since the motor is not inverter rated and that I am planning to short circuit the rotor windings am I going to cause more problems by doing this?

 

[jraef]

Probably not at 230V. There are in general 2 categories of issues to deal with in applying VFDs to old motors: Heat and Voltage Spikes.

Heating can be a problem on retrofits where the motor is going to be run at low speeds for extended periods. Because your hoist is most likely geared, that probably is not the case here, so I won't bother with the details of what's involved.

The issue of voltage spike problems is more prevalent at 380V and above, because older motor winding insulation was typically only rated for 1000V, and the spikes associated with VFD use can reach 2 - 3x the DC bus voltage. So on a 460V drive where your DC bus is 650+V, the spikes could easily be close to 2000V, far higher than the insulation rating. At 230VAC, the DC bus voltage will only be around 310V so when the spikes get to be 3x the DC bus, that's still only 930V. Cable lead length plays a part in this as well, but typically hoist controls are mounted near the motor so leads are relatively short.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[jraef]

By the way, don't forget to check out your brake situation. If you have a holding brake coil that was energized by the stator power, you will need to run a separate 230V power source to it and design a brake control system to use it with the VFD.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

Hello jraef
A question on insulation values. Are rewind shops now using insulation suitable for VFD drives or is that an extra cost option?
Thanks
respectfully

 

[jraef]

Good ones will at least offer it to their clients. Most of the magnet wire suppliers have a product to offer now, whereas just 5 years ago there was really only one supplier, which made it difficult for smaller shops. As with everything, increased volume is driving the cost down, so at some point there will probably no longer be a premium for 2000V rated wire. From what I hear we are not at that point yet.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[DickDV]

Just a thought here, fellas, but wouldn't it be wise to consider leaving a little resistance in the rotor circuit even when running on a VFD. I've seen wound rotor motors that become too tight with all of the resistance taken out. This is a hoist application so minimum slip and maximum efficiency is not a big consideration.

 

[jraef]

I used to subscribe to that Dick, but I have been swayed by good arguments in this forum and have since done it in the field, although admittedly not on a winch. It certainly holds true for using soft starters on WRIMs, but modern sensorless vector drives are now so good at torque control it theoretically doesn't matter.

I probably should have qualified my original endorsement to include that caveat and another.
1) A V/Hz scalar drive or an el-cheapo SVC drive may not offer the necessary control. Don't skimp because the difference in the drive will be the speed and power of the microprocessor, which affects performance.
2) The slip rings should be shorted on the rotor itself, which may necessitate taking it to a motor shop. Shorting the connections after the brushes can lead to problems and instability as the brushes wear.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[NEWFIEBOY]

Thanks for all the replies. Jraef do you have an opinion on which manufacturers drive would be more suitable for this application and why?

 

[jraef]

My caution was to warn against choosing the cheapest one, because in a very competitive VFD market, you pretty much get what you pay for. This site is supposed to be free of commercial endorsements, but what matters more anyway is the local support you are going to get. From your handle I would assume you are in Newfoundland, so you will likely have a very wide variety of manufacturers available to you. My advice would be to go to some vendors that you trust, describe exactly what you want to do, tell them you are going to expect them to stand behind their claims if they say they can handle it, and hold their feet to the fire if they were mistaken, i.e. make them taske it back. If you don't know of any good vendors, ask around at other local industrials for who they recommend. I would avoid buying them from websites for your application, you will get almost no meaningful support.

A reasonable measure of the VFD capability is if it has an option for "Closed Loop" Vector control or not. You don't necessarily need Closed Loop, but that can be a tool to weed out the ones that are less likely to perform in this specific application. Many of the less capable drives only offer "Sensorless / Open Loop Vector" control, with no option for the more complex and capable "Closed Loop" or "Flux Vector" control. If they can't or won't offer it, it's usually because they don't have the expertise in house to design it, or the microprocessor doesn't have the kahunas to do the job. Either way it probably reflects on what you can expect from the Sensorless Vector version they offer.

If the drive needs to be on-deck, NEMA 4X packaging will be a necessity as well, which narrows the field considerably so that would be the first place to start. If you can though, locate the VFD below deck in a dry environment.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

Hello jraef and NEWFIEBOY;
The VFD will probably fit in the existing control cabinet after you throw out the contactors and you have the power leads and motor leads there.
respectfully

 

[jraef]

Good point.

 

[Simmers]

Desirable motor rewind shops have a specification for VFD rewinds. We have used this spec successfully, but I would suggest that you have the motor rewind shop tell you what they stand behind (warranty).
MOTORS
Existing Motors will be rebuilt as follows (assumes windings are adequate - advise otherwise before proceeding):
1. Test, disassemble and clean motors.
2. Clean and bake windings.
3. Remove all shorts and grounds.
4. Reinsulate with class H insulation.
5. Remove slip rings and holders.
6. Permanently short rotor windings.
7. Install new bearings (front and back).
8. Dynamically balance the rotor.
9. Reassembled, paint and test.
10. Test reports will be submitted for each motor.

At 15 HP, normally not very expensive.

There are drive manufacturers who specialize in material handling applications: Drivecon, Electromotive, Power Electronics. They are all well versed in the most difficult of applications and environments. If addition, they can even "unitize" your remote control receiver into their control enclosure and test it at their facility (or supply new remote control).

Local support is available throughout North America.

 

[ozmosis]

Just another tip on the usage of a VFD on a boat/ship. Depending on the configuration and overall design of the VFD, you will get leakage currents. As your 'ground' is your hull, then you have to limit the level of leakage currents otherwise you will get problems with hull corrosion (I'm no expert on boats/ships but I do know that ships with sacrificial cathodes have them to absord leakage currents but most boat designers would prefer them not to for high cost reasons) and also the protection you may have on-board for leakage currents could be tripping when the VFD runs. There are certain criteria outlined by Lloyds, ABS etc that restrict the use of devices that will increase leakage currents. Certain companies have compliance with these regulatory bodies (ABB, Siemens, Danfoss and Vacon[Cutler-Hammer] that I know of).
I don't want to be alarmist and it might be nothing depending on your vessel but certainly do not take a VFD with an RFI (Radio Frequency Interference)filter as this will increase the leakage currents. I would check with your chosen supplier before fitting.

 

[NEWFIEBOY]

Hello sed2developer

There is a ground detection system attached to each buss bar at the main switchboard. The ship's electrical system is 3 phase delta with no grounded conductors. The ground detection is accomplished by putting 3 pilot lights in series and a normally closed push button between a common connection point at the pilot lights and the ships hull ( ground). There is one of these detection system attached to the buss bars of differect voltage including the DC supply. On the hull's exterior there are a number of zinc anodes which act as sacrifical anodes to combat the electrolitic wasting due to leakage currents from the electrical system to ground. Do you think we would accelerate the errosion of the zinc anodes or interfere with the ground detection system by installing a VFD?