0
\$\begingroup\$

I'm working on a project that requires four (4) electrical linear actuators to work together. They must be driven at the same speed, simultaneously. Let's assume that I have four identical actuators with the following characteristics:

-> 12V DC -> Max draw of 2.5A

(Something like these... http://bit.ly/1QXNqve)

These guys are all rated for 200 pounds or more, which is about 4 times the lifting power I need, and so I figure I can drive them all from a single power source, with the following characteristics:

-> 12V -> Max 6A

(Something like this... http://amzn.to/22m8D7j)

Now, what I'm wondering is whether I should drive them in parallel or in series? My understanding is that running them in series will draw, at most, the max 2.5A, but at a decreased voltage. It makes sense that they would each get about 3V (12V / 4 motors). Would the decrease in voltage only manifest as a decrease in speed, or also a decrease in lifting strength?

On the flip-side, running them in parallel will provide them all with the 12V they are designed for, but will limit their current to a combined 6A. It makes sense, then, that they would each get about 1.5A, assuming they are equally sharing the load (6A / 4 motors). Would the decrease in current only cause a decrease in lifting strength, or would it also cause a decrease in speed?

I assume that a combination of the two would land somewhere in the middle? (i.e. two motors in series, parallel to another two motors in series)

Please let me know if the logic isn't right. Makes sense to me, but my experience is that motors aren't exactly equivalent to resistors or other constant-draw components, particularly when one motor might have a slightly higher or lower draw/load than the others.

The application requires four actuators, but only a combined total lifting capacity of about 250#. Speed is not a huge concern, they are only required to travel about 15 inches in roughly a minute or less. The quicker, the better, but it isn't a huge deal.

Finally, is one method preferable over the other for maintaining nearly identical driving speeds? These actuators will be responsible for moving a platform, that must remain as level as possible while it is being raised and lowered. If the platform has a concentration of weight on one side/corner, will the opposing motors always drive faster? If I run them in parallel, it makes sense to me that the constant voltage should drive them identically, regardless of load.

I am trying to make this as simple as possible, and so want to avoid additional electronics for monitoring and changing speeds with microcontrollers, etc. Just a simple up/down switch, and hopefully everything else falls into place. The single power source is desirable for space/cost savings, as well as any slight variances in output that two power supplies might have. Math, sources, experience are all welcome!

\$\endgroup\$
  • \$\begingroup\$ If you connect the actuators in parallel, you will need a 10 Amp (or more) power supply, as the motors will all attempt to draw the advertised 2.5 Amp. If your power supply can't provide the required current, its voltage will drop, and it may be damaged. \$\endgroup\$ – Peter Bennett Mar 17 '16 at 15:46
  • \$\begingroup\$ Are you planning on a 4-point lift? What happens if one of the motors runs significantly different speed to the others or if one stalls? \$\endgroup\$ – Transistor Mar 17 '16 at 16:30
  • \$\begingroup\$ @transistor That's why I'm trying to figure this all out before I go ahead and invest a bunch of $$ on something that won't work out. It is a 4-point lift. I'm looking into possible ways to do it with a single motor, maybe chain-driven worm screws instead of actuators in each corner. \$\endgroup\$ – Birrel Mar 17 '16 at 19:16
  • 2
    \$\begingroup\$ I'd recommend that. Variations in motor resistance, etc., along with variations in load will result in slight variations in speed. I'd be afraid that the whole thing will bind up and you may not have an easy release mechanism. Chain or toothed belt with one big motor may be a better option. \$\endgroup\$ – Transistor Mar 17 '16 at 19:20
  • \$\begingroup\$ Non-hydraulic automotive hoists use cables and pulleys with 1 motor. To prevent drop downs they usually have a toothed pawl that needs releasing before lowering the car. \$\endgroup\$ – John Canon Nov 23 '18 at 2:23
2
\$\begingroup\$

If your actuators are DC brush motors, the question of when to wire in series and parallel will depend upon how you need things to behave. Motors wired in parallel will tend to run at the same no-load speed, which will fall off somewhat with applied torque; motors wired in series will produce the same torque, which will fall off for all motors in the series based upon the total of all the speeds of all the motors.

To be a little more precise, a DC brush motor may be pretty accurately modeled as an ideal motor in series with a resistor and an inductor. At any moment in time, the rotational speed of the motor will be directly proportional to the voltage across the "ideal" part of the motor, and the torque on the motor will be proportional to the current. These are both bidirectional relationships, so changing the speed will change the voltage (on the ideal part of the motor) and vice versa; likewise changing the current will change the torque and vice versa. While this model is not absolutely precise, the difference between most practical DC brush motors motors and an ideal model is in fact very nearly equivalent to a series resistor and inductor.

If your actuators are physically connected such that they will run at the same speed, wiring them in series will balance the torque on them. If they are not connected that way physically but instead should run near the same speed, wiring them in parallel will cause them to do so, though if they subjected to differing amounts of torque that may cause the rotational speeds to differ somewhat.

If you think in terms of the model described above (and if you're running things with unmodulated DC you may be able to ignore the inductance part) it should be pretty clear what is happening and how you need to wire things.

\$\endgroup\$
1
\$\begingroup\$

I would wire the actuators in parallel.

The chances are pretty good that the load on the actuators is NOT equal.

The actuator with the greater load will attempt to consume more current than the others.

If they are wired in series, the voltage across the actuator with greater load drops. This will cause it to run slower.

If the actuators are wired in parallel, the voltage across all the actuators remains the same. The actuator with the greater load will still run slower than the others, but to a much less extent.

\$\endgroup\$
0
\$\begingroup\$

EDIT:

Your actuators are strong enough to lift 250lbs when work in parallel, even a torque will decrease with current. Connecting motors in series would probably make differences in rotational speed (they are not ideal probably).

\$\endgroup\$
  • 1
    \$\begingroup\$ It would be a pretty poor design of a DC motor to actually interrupt power to the armature as it rotates. In fact, if that were the case, then there is a good possibility that the motor could never start. Typical brush riggings have the brushes in contact with multiple commutator bars on the armature. \$\endgroup\$ – R Drast Mar 17 '16 at 16:34
  • \$\begingroup\$ @Haris778: why don't you actually either measure a brushed DC motor with an ohmmeter while rotating the shaft OR apply power to a small brushed DC motor and manually stall the motor to just barely turning. When you are satisfied that the motor doesn't interrupt the DC current, come back and fix up your answer. I'll gladly remove my down-vote when you have corrected the inaccuracy of your answer. \$\endgroup\$ – Dwayne Reid Mar 18 '16 at 2:31
  • \$\begingroup\$ I apologize for answering unadvisedly and I am sorry. Answer is edited, but almost everything is said with other answers. \$\endgroup\$ – Haris778 Mar 18 '16 at 8:07
  • \$\begingroup\$ Downvote removed. Many thanks for editing your answer! \$\endgroup\$ – Dwayne Reid Mar 22 '16 at 0:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.