Wound rotor motor
Wound rotor motor
(OP)
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.
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.





RE: Wound rotor motor
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Wound rotor motor
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.
RE: Wound rotor motor
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
RE: Wound rotor motor
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
RE: Wound rotor motor
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
RE: Wound rotor motor
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?
RE: Wound rotor motor
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
RE: Wound rotor motor
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Wound rotor motor
A question on insulation values. Are rewind shops now using insulation suitable for VFD drives or is that an extra cost option?
Thanks
respectfully
RE: Wound rotor motor
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Wound rotor motor
RE: Wound rotor motor
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
RE: Wound rotor motor
RE: Wound rotor motor
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
RE: Wound rotor motor
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
RE: Wound rotor motor
RE: Wound rotor motor
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.
RE: Wound rotor motor
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.
RE: Wound rotor motor
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?
RE: Wound rotor motor
VFD's do create an increase of leakage currents due to the fact that capacitors are used internally, the VFD generally incorporates high switching frequencies and the recommended practice (in certain quarters) of shielded motor cable all come together to create higher leakage currents than you would generally want, especially on an installation that requires the hull to be the ground. It is only a problem if you do not take it into consideration prior to purchase/installation. There are many thousands of VFD/VSD's installed onto ships so it is not a problem as long as your supplier knows. We had a problem on a ship in the Med where somebody replaced an old damaged drive with a new one but installed it with a standard RFI filter. This filter has high leakage currents through the Y connection internally and kept tripping the GFD breaker. The leakage was in the 100's of mA so had the breaker not tripped then I imagine the sacrificial anode (correction on my earlier 'cathode') would erode quicker, although this is not my expertise I'm afraid.
So, don't worry about it, just make sure you talk to a reputable supplier who knows your VFD is going on a boat/ship/vessel (I was got my knuckles wrapped by calling a boat a ship or a ship a boat so now I call them everything, just to keep the peace...)