Daisy chaining motors, bulding PRC have questions

I have been unsuccessful at finding a used 7.5 to 10HP 3phase motor locally... well reasonably priced. I did find a 7.5 used for $280, think that's a bit high based on ~$10/hp.

I did find 2 3hp motors today, and reasonably priced. My question, will the combined 6hp be enough to run my 1942 Monarch 10EE, and if so, how difficult is it to set up the system with two motors compared to one motor?

I'm inclined right now to wait until I can find a 7.5 motor.

Any thoughts or suggestions?

Ryan

theres a distinct differance building units of above and below 5 hp size
below is easier to build do to limitations of potential relays

Look down the posting about 10 or 12 and you will find a thread started by "BadDog" about building a dual idler RPC which is what you are asking about. He is pursuing the idea. He plans to use a three phase breaker panel which is a nice way to connect the multiple RPCs into the circuit. Your RPCs must be connected in parallel which is exactly what a breaker panel does. There has been some discussion about problems when using two idlers of the same size. I do not remember the details but do remember it is best to have them different sizes. In the "Dual Idler 3 hp + 10 hp RpC" thread you will find a control diagram showing how to wire the controls so you can have a start circuit in only one but will be able to run either once the RPC is up an running.

Ryan,

You posted:

'I have been unsuccessful at finding a used 7.5 to 10HP 3phase motor locally... well reasonably priced. I did find a 7.5 used for $280, think that's a bit high based on ~$10/hp.'

Have you tried that big auction house in the sky? One of the more cost prohibitive things with respect to motors, especially 5 HP and up is shipping. If you do a modified search for motors within 200 miles you might can find an affordable 7.5 HP or 10 HP. A couple of years back I scored 2 Lincolns, one a 7.5 and the other a 15.

The 7 1/2 is a 1750 rpm, 208/240/480 "Y" and was built in October of 1993. The cost was $39.95 + shipping which was at that time less than $50 (air, no less). The motor was a factory spare and was still in it's Lincoln box mounted on a mini-pallet. The wiring diagram/manual/parts sheet was still inside the junction box. Wires were factory strip.

The 15 is a 1750 rpm, 208/240/480 "Y" and was built in July 1992. The cost was $90. The motor would've been over $100 to ship the 80 miles from Chatanooga to Huntsvegas. After winning it I drove and picked it up ....... about 4 hours due to bad directions. The motor was also a factory spare, mounted on a 1/2" steel plate (no wonder the shipping quote was so high!!!), it has an aluminum shrouded Lovejoy connector on it. It too had the factory paperwork inside the junction box and had the factory strip. Neither motor had been run in 15 years. Worst case scenario would be new bearings. I shot a litlle Mobil Polyrex in each and ran them for a while ...... no excess heat. Kewl.

Here are the motors mounted prior to becoming a Dual-idler RPC:




Toolnut wrote:

'There has been some discussion about problems when using two idlers of the same size. I do not remember the details but do remember it is best to have them different sizes.'

I remember reading something along that line but can't see why it would be problematic. I have also seen more than a few RPC's where people insist on starting one motor first, usually the smaller HP motor. I can understand not wishing to have a second starting circuit but that is NOT how I constructed mine. To extract maximum versatility I essentially constructed two RPC's and then connected them. In that way I can start and run the system as a 7.5 HP RPC, a 15 HP RPC, or with the combined 22.5 HP. Each motor has a seperate single phase contactor to energize it's circuit, a seperate Steveco 90-66 relay to moniter/engage/disengage their respective start caps. Each motor also has it's own balanced (within 5%) run caps. To add icing on the cake, I designed a starting circuit which, to the best of my knowledge, is unique. With my RPC, since either motor can be started seperately, both motors can be started seperately at the same time. Additionally, I have a 3 pole contactor to engage either or both idlers to the system output. Realizing this, I chose 240 V. coils for the output contactors. I also kept the regenerative shunt on the "black" side of each coil. Since the output system is "hot" if either side (idler) is running and engaged, when I push the start button for the non-running motor it's own start circuit (single phase supply, potential relay, start caps, run caps) engages. At the same time, the 3 pole output contactor closes and the generated leg is at high voltage. When starting either motor individually, start times are quick ....... under 1 second. If a second motor is brought on line the motor looks like it is starting on utility 3 phase ........ under 1/2 second. Either motor will help the other one start. I have been meaning to do an mpeg of the start up but just recently got a digital camera with mpeg capabilities. I'll try to do it by weeks end. I am currently acruing the components to make my converer in it's final form. Right now the contacters are IEC stuff. I am planning on going with NEMA for the main contacters .... 2 pole single phase, 3 pole motor, and heaters for idler protection. I think I will still use IEC (Squre D) 3 pole contactors for the start caps. More later. here is my initial Dual RPC. The output wiring was not in place when photoed:



Jeff

Great photos Jeff!!

Thanks Bruce.


wippin' boy wrote:

'theres a distinct differance building units of above and below 5 hp size below is easier to build do to limitations of potential relays.'

That is only true if you use the relay to supply the current to the start capacitors. IIRC, the Grainger (GE) 6X500 series of potential relays is limited to about 35 A. A Steveco 90-66 relay is rated at 50A. But, if you modify the Steveco relay, as Bruce showed me when we met at a mutual friends place, the relay can operate a load rated contactor to engage and disengage the capacitors. You can then start any size motor you wish. The coil draw on even a size 5 Allen Bradley contactor (starter) is less than 500 VA. Before Bruce showed me a Steveco 90-66 I had been repeatedly frustrated trying to get a Grainger 6X550 to operate a contactor. Finally I blew the fool out of a electrolytic start and was about to throw in the towel and try a push and hold momentary switch to manually hold the start caps in the circuit ......... doable but why? When Bruce showed me the Steveco and how easy it was to modify it (takes about the same amount of time as heating up and cooling your soldering iron) to isolate the coil from the contacts I knew the problem was solved.

The starting circuit I use for either side of my dual RPC is essentially the same as Bruce supplies in his plans, namely using the PR to operate a seperate contactor for the start caps. My addition to the circuit merely allows for either motor to assist starting a second motor, or third, or fourth, ad nauseum. If you are contemplating building your own RPC you could do alot worse than obtaining a copy of Bruce's plans gaged to the size of your RPC. As the basis for a self-starting, voltage monitered, capacitor balanced RPC I know of none better. Then if you have the knowledge of circuits, motors, & such you can add any bells and whistles you desire.

Here is a pic of the unmodified Steveco relay (left) and the modified relay (right).



I know that Peter has since rethought the topologies and diagrammed two circuits allowing for the use of the Steveco relays in an unmodified form to start both light RPC's (> 5 HP) and larger ones. I have not tried these circuits and can't speak towards them.

jeff