Powering the leveling jacks

[Jim Blackwood]

In our last episode:
https://www.busconversionmagazine.com/forum/index.php?topic=34365.0
our hero installed the Jost landing gear in a tripod configuration in the '96 MCI 102DL3 and used surplus 3 phase gearmotors to power them. This worked just fine if somewhat slow but leaves the job of supplying power to the 3 phase motors, which is the subject of this episode. There are several concerns that have to be addressed in order to make this part of the build a success. The project thread is here:
https://www.busconversionmagazine.com/forum/index.php?topic=34360.30

Because of the slow speed, an automatic shutoff of the jacks would be the best. This is because it would be impractical to sit on the control switches for the time required to fully raise or lower the jacks. This issue has two modes: retract for travel, and extend to bring the coach to level, both of which have different triggers. Also an extension limit is required to prevent damage from over-extension.

Available power must be converted to a suitable voltage and amperage of 3 phase power to drive the motors. It is necessary to decide where that power is going to come from, whether house batteries, hook-ups, or genset. The required power conditioners will depend on the source of the power. And of course the power conditioners must be conveniently located.

A means is required for reversing the motors. More than one option exists.

The driver interface must have manual overrides and an automatic leveling function. It must be capable of using the jacks for servicing requirements such as for tire changes or access to the bottom of the bus, suspension, etc.

That's the short list, I may have left some things out. So, it just happens that  3 phase motors are easily reversed by switching 2 of the three power leads. Any 2 will do just fine. So that is the most basic reversing scheme and of course I'm not going to use that. But it remains a very viable option. Instead, several of the needed functions can be combined in a single power conditioner known as a VFD or Variable Frequency Drive. This is a box that is normally used as a speed control for 3 phase motors and originally they were powered with 3 phase and put out a pulsed power waveform that was varied to get the required speed control. But then someone bright decided that since the unit essentially converts the input power to DC and then modifies it to drive the motor, it could be powered with split phase 220vac or in some cases 110vac single phase power. Usually these last are low power applications of under 1.5Kw but that just happens to be enough for this application. Well the question that came to my mind was, "How is it possible to get 220vac 3 phase power out when you have 110vac in and no power transformers? Seems like a reasonable question and I struggled with it for awhile. But the key is in the voltage of each of the legs of the 3 phase and 220 split phase power. OK for you purists I'm not worrying about being exact on the voltages and will consider for these purposes 110vac and 120vac to be the same thing. Anyway, the important bit is that from any leg of 3 phase to ground is 110vac, which is the same as from either side of split phase 220 to ground. So with 110vac only as an input you have enough voltage to put 110vac on each leg of the 3 phase. The limitation is the current, and it's probably easier to think in terms of power. A standard outlet will power an 1875watt space heater so that is the practical limiting factor. I think that's usually on a 15 amp outlet so it might be possible to double that but finding a 110vac VFD over 1Kw gets expensive fast. 750Kw is about 1 HP though and in this case I have 1hp motors on 2 jacks and 1/2hp on the other.

750W VFDs are available on ebay for about $50-60 each. The "plus" features however are equally important. First, the motor can be reversed at the touch of a button or in this case the closing of an external switch. So there is built in directional control. Next, these units all come with some form of current limiting. This is very useful to prevent damage to the jack at the travel limits. Essential in fact because the gearmotor will easily drive the jack with enough power to shear the internal roll pin, without any indication that it is under strain other than the sudden rise in motor current by a factor of about 10x. So there's an effective means of limiting travel. Then for leveling purposes the motor can be turned on and off using external inputs. Pretty simple so far, though it might get a little more complicated later.

So the VFD handles many of the concerns and is pretty compact therefore easy to locate and mount, but it needs 110vac power. OTOH, considering when it's most desired to use the jacks, 110vac may not always be the most convenient. True, if hook-ups are not available the genset is, but is it really neighborly to pull in and immediately fire up the genny? Maybe not. The alternative is an inverter to power the VFD, and I have some surplus 28vdc 900w inverters so it appears that solution should work well, one inverter and one VFD per jack.

So that's the initial plan, the next step being to source and buy the VFD units. DC power will come from an existing 28v breaker panel that was originally used to power the surplus inverters which were located in the parcel shelves.

Jim