OT: VSD's and DC power

zankorel

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Join Date
Dec 2008
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Hunter Valley
Posts
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This may be a bit offtopic, but I have a situation where only DC power is available, and i want to run an AC motor off a VSD (for a Crane Hoist). I am worried about the performance of the VSD when using an Inverter due to poor power waverforms.

Has anyone had any experience in doing this? Any recommendations on what to do/not to do?

I have done a little bit of research, and found that Seimens drives allow you to bypass the rectifier stage and power it directly from DC, which sounds pretty good for the application.
 
DC powered VSD inputs are rather common but the DC voltage has to be at the right level. plcnoob's suggestion of researching "common DC bus" applications is right on.
 
DC Bus

common DC bus is quite a common thing these days where we can power the inverter directly from another DC source(540 V DC)

but you need a DC source for that which in most of the cases a rectifier unit which convert AC to DC ,but as your problem is of AC supply so think of some other source like a battery ....
 
This may be a bit offtopic, but I have a situation where only DC power is available, and i want to run an AC motor off a VSD (for a Crane Hoist). I am worried about the performance of the VSD when using an Inverter due to poor power waverforms.

Has anyone had any experience in doing this? Any recommendations on what to do/not to do?

I have done a little bit of research, and found that Seimens drives allow you to bypass the rectifier stage and power it directly from DC, which sounds pretty good for the application.

What size (HP or Kw) is crane motor? What is voltage of motor?

Want to drive vehicle using AC 3 phase motor. Vehicle is 56 Chev pickup and motor is Leeson 50 HP.

Have researched using "low DC" voltage (< 96 VDC) equipment to drive AC motors - stuff is out there most typically forklifts. Big stopper there is most forklift makers will not give software to change paramaters in VFD to allow modification to adapted need.

Gave up on this option and researched using off shelf industrial VFD to drive vehicles.

Basically tie DC source to DC bus in VFD. Main drawback is "high voltage DC". DC bus voltage will be 1.4 times that of motor ie for 230 volt motor DC bus voltage will be about 325.
Second drawback is how to bypass the AC input and recitifiers and connect to DC bus.

Common bus is the term to search for.

Two options I have found are
Hitachi SJ 300 - this can be modified for driving from DC source. You will have to retain the capacitors (per manual) thus will have to build a precharge protection circuit in DC line to protect the capacitors.

Allen Bradley Power flex 700 has option for direct DC supply. Advantage to me is it is all in one package ie hook up DC, hook up motor and turn on.

Both of these units are vector drive which depending on the demands of your hoist will be a good feature to have. I believe either will allow you to do vector sensorless (no encoder) or with an encoder to sense motor shaft speed - if you can do this I believe it to be the better option.

If you are driving from a battery you may not need braking resistor - dynamic braking will charge the battery which will act as a braking resistor. You will still need some method to charge the battery you can only count on dynamic braking to supply 15 to maybe 20% recharge.

If you are driving VFD from a DC source ie generator or rectifier you will need the brake resistor.

Allen Bradley unit is about 6,000 and Hitachi is about 4,400 US $. I will probably go with Hitachi mostly because of better local support and lower cost even though from technical standpoint I prefer AB.

Another issue with DC at 325 or more is that it is difficult to find equipment disconnect switches, fuses etc with proper ampere AND voltage ratings. Major fuse makers Little Fuse etc make the fuses but you may need to call them. I found Little fuse most helpful and knowledgable and they have a watts line.

Dan Bentler
 
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This may be a bit offtopic, but I have a situation where only DC power is available, and i want to run an AC motor off a VSD (for a Crane Hoist). I am worried about the performance of the VSD when using an Inverter due to poor power waverforms.

Has anyone had any experience in doing this? Any recommendations on what to do/not to do?

I have done a little bit of research, and found that Seimens drives allow you to bypass the rectifier stage and power it directly from DC, which sounds pretty good for the application.

SSD 890's offer a comon bus dc version
 
Most drives over 20hp provide DC bus connections on the power terminal strip. Not all have the ability to turn off the phase loss or low voltage faulting that can occur when the AC input is not present.
 
Thanks for the replies all, I have been taking a look at the suggestions and have a much better understanding of where to go.

fyi the hoist is probably going to be two 3phase 415V 21kW motors and two smaller motors for the cross travel. The crane runs off a 240V DC collector rail system, so i would need to investigate DC step up to ~600V.
 
Chris
Assuming your whole site is not DC and you have 3 phase.

It may be easier and cheaper to convert the whole crane to 3 phase. By the time you change out the motors add VFDs change the cab or pendant controls add a way to get higher voltage DC, the cost of higher voltage DC equipment, the cost of changeout of DC wiring to 3 phase may be less.

Granted the cost of converting the bus on the craneway say if 500' wont be cheap but overall conversion to 3 phase may be the way to go.
Dan Bentler
 
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The Crane travel is about 800m long , which gets pretty expensive to install new collector rails for :) The hoist crab assembly has to be replaced anyway also.
Addition of 3 phase AC is actually my preferred solution, but there are other political forces at play. Schedule is also a large component of the decision.
If only I didnt have to get approval for the work I want to do!
 
You can also look at Bosch-Rexroth for direct DC connect drives. They will operate on a wide range of DC supply, which may help.

The 240VDC - 600VDC step-up will probably get pretty expensive in the power level you are looking at. Can you go with lower voltage motors and modify the gearing knowing you won't get rated speed out of them?

Keith
 
Thanks for the replies all, I have been taking a look at the suggestions and have a much better understanding of where to go.

fyi the hoist is probably going to be two 3phase 415V 21kW motors and two smaller motors for the cross travel. The crane runs off a 240V DC collector rail system, so i would need to investigate DC step up to ~600V.

Chris

I have my reasons for changing out DC to AC with VFDs. I know them and understand them and of course since they are mine they are justifiable and good.

What I am curious about is
what are your reasons for wanting to do it?

Dan Bentler
 
Just a thought here. Why not convert the two-rail slidebar system to 240VAC single phase. Then use oversized 230V three phase drives to power 230V three phase motors. Other than the hoist, the bridge and cross travel motors are likely low hp. You might even find drives at or under 5hp designed for 240V single phase input power.
 
Chris

I have my reasons for changing out DC to AC with VFDs. I know them and understand them and of course since they are mine they are justifiable and good.

What I am curious about is
what are your reasons for wanting to do it?

Dan Bentler

I cant go into too much detail, but there are enormous gains to be made in this process if i can provide a hoist system that can do a low torque, low speed setting, in addition to normal operations.

For this i need VFD driven AC motors. As a result, my main problem is obtaining/expanding suitable AC power on the crane. The cranes are also 50 years old which makes things complicated also.
 
Just a thought here. Why not convert the two-rail slidebar system to 240VAC single phase. Then use oversized 230V three phase drives to power 230V three phase motors. Other than the hoist, the bridge and cross travel motors are likely low hp. You might even find drives at or under 5hp designed for 240V single phase input power.

It is a good idea, and one I had briefly considered. The problem is that that would need me to completely redo the entire crane electrics, which is way outside the allowable scope. The other problem is that there is more than one crane on these rails, and only one can be out of service at any one time, due to the criticality of the process.

It is a very interesting project, with a lot of dimensions. Thanks to everyone for the discussion thus far.
 

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