24VDC or 48VDC

[tetwin11]

I'm an ME here looking for some input from EE types.
We have a system (mobile robotics application intended to be used in an industrial environment) that was designed around a 24VDC/160Ah battery system. For several reasons, it would be possibly beneficial to reconfigure our battery system to a 48VDC/80Ah battery system. However... there must be some kind of drawbacks to that, right? I vaguely recall something about SELV at 24V being a good thing for some kind of regulatory approval, but I don't know much. Any thoughts would help!

 

[waross]

It depends.
What are the I²R losses in your system?
What is the cost and availability of both battery configurations?
A rule of thumb for power systems is that when the HP exceeds the system voltage, consider a higher voltage.
That said, after consideration, some motors are used with HP exceeding the system voltage.
Increasing the system voltage will generally reduce the losses but after calculating the actual losses of a system it may not be worth while to change.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[tetwin11]

Thanks for the reply.
I really don't know what the I2R losses are in the system.
We could configure the battery either way with pretty minimal difference in price and availability.
We're consuming 2-3 HP at 24V, so by that rule of thumb, we're good. I'm not sure if that would apply here very well though for this battery powered system. If we're using 24HP at 24V (the tipping point for that rule of thumb) our battery would run out in about 15 minutes when we want it to last some 8 hours.
One of the big drivers to considering a 48V system is driving our motors. The motors are rated for 48V, but since we're driving at 24V we're only getting 1/2 the power out of them. I was under the impression some "Extra Low Voltage" factor was key in deciding to go with 24V over 48V, but I have no idea about those regulations.

 

[waross]

I'm apologize for misleading you.
That rule of thumb does not apply to small DC systems.
It was meant to illustrate that generally as current increases, higher supply voltages are selected by the designer.
2 HP at 24 Volts will draw about 70 Amps.
A simple calculation shows a run time of about 2 hr 20 min.
However as the current increases the available AHr decreases due to internal losses in the battery.
I would guesstimate a run time of about 1.5 hr at that discharge rate.
With the information supplied, you may need more voltage and more Amp hours.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Here is an online battery calculator.
Link
The calculator uses Peukert’s Law to determine the reduced battery capacity at higher discharge rates.
For the 24 Volt solution and a 2 HP load you need 69 Amps.
That looks like about 552 AHr to run 8 hours.
But in the real world, with a discharge rate of 69 Amps you will need 1456 AHr with flooded lead acid cells.
With gell cells you can get down to 1264 AHr.
At 48 Volts the current drops to 35 Amps.
With a discharge rate of 35 Amps you will need 728 AHr with flooded lead acid cells.
You will often see the statement:
"20 Amp hours means that you can draw 1 Amp for 20 Hours or 20 amps for 1 hour."
That statement, while true is very misleading.
A 20 Amp hour rated battery will deliver 1 Amp for 20 Hours, but it WILL NOT deliver 20 Amps for 1 hour.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

If you're in a part of the world where the IEC definitions are accepted, ELV is up to 50V AC rms, or 120V ripple-free DC. So that's unlikely to be the driving reason behind selecting 24V as the battery source.

 

[itsmoked]

I would also imagine that running a 48V motor at 24V is considerably less efficient than running it at 48V. You will save some weight by being able to use smaller wire.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I hope that those are PM motors, not shunt motors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

With PM motors, as long as you don't hit the voltage limit as you increase the voltage you increase the full load speed with the same full load torque.
Often the voltage is limited by the commutator.
The commutator will generally flash over at much lower voltage than the winding insulation will fail.
Mechanical damage when starting may be an issue unless there is some type of current limit when starting.
Locked rotor current is limited by the brush-gear voltage drop and the rotor resistance.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

The motors are rated for 48V, but since we're driving at 24V we're only getting 1/2 the power out of them.

Hmmm...not necessarily...

Depending on the type of motor and any field control arrangements, you may be getting something considerably different. I would have thought you could get into hitting component current and/or thermal limits...

Also, again depending on field control arrangements, what may presently have a safe no-load speed could possibly double to a dangerously high level, posing hazards to personnel and/or driven machinery.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]