How to start 1hp DC Motor

[AndreyG]

What is proper way to start 1hp 24V DC Motor? Permanent magnet type.
Can I just use contactor with sufficiently high current rating?
(like, 100A, this is what my boss wants)
Or do I need to rig resistive ladder and use multiple contactors?
(I see lot's of literature on that, mostly old)
Or do I need to use switching controller with Triacs?

My goal is pretty minimal: turn the Motor on and off. I am looking for compact economic solution that does not damage the motor.

If I would use Contactor, should I switch both lines (like AC motor) or can I switch positive line only and leave Ground line permanently connected (it probably will work either way but what is recommended practice?)

 

[waross]

What is the source of the 24 Volts DC? At rest a DC motor has no back EMF and the initial current may be very high. In such a case the internal resistance of the source matters.
What is this motor driving?

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

 

[AndreyG]

The motor is powered up from large 24v battery.
It drives a pump.
The application is marine steering gear.

 

[SceneryDriver]

Start it across the line; 1hp is a small motor. Switch one or both leads to the motor; it really doesn't matter. I'd switch both leads, but that's just my preference. Just make sure to use a proper DC rated contactor. Must contactors are only rated for AC, and will not last long switching DC, even at low voltages. See this thread:

http://www.eng-tips.com/viewthread.cfm?qid=349050

Keeping things simple will help Cletus and Bubba out when it comes time for troubleshooting. Unless you need speed control or reversing, a simple (properly rated) contactor should suffice.


SceneryDriver

 

[ScottyUK]

Depends how big a current surge the commutator can handle, and how much the battery sags under starting current. DOL starts on DC machines are usually avoided in larger sizes because a battery large enough not to have a big voltage sag will likely deliver enough current to damage the commutator. 1HP at 24V is about 40A, and a DOL start might well be nearer to 400A. That's enough to damage the commutator on most machines. I'd consider either a semiconductor controller - they aren't all that expensive - or a resistor timestarter as you've seen in the old texts. The latter can be made bomb-proof reliable.

 

[OperaHouse]

I fix a friends construction lifts when he gets into trouble. He buys 24V 100A contactors and says they only last about three months. I've taken some apart and the plastic guides seem to wear and then the contacts hang up. He doesn't try to be cheap, just gets what the parts jobber has. If it was my lift I would be putting in a FET switch. Steering doesn't sound like something to have fail.

 

[crshears]

More questions:

Will the motor start once and be kept in continuous operation the entire time the vessel is underway, or will it be starting and stopping cyclically? Huge difference on heating effects and dissipation!

If continuous run using a closed-centre hydraulic system, it might make sense to use a DC-to-DC convertor that not only soft-starts the pump/motor assembly but uses a strain sensor transducer to control the hydraulic pressure of the oil by modulating the speed of the motor as required, with a hydraulic bypass valve used to unload the pump completely at times of zero demand...

Marine steering gear: pump is almost certainly positive displacement so as to provide the high pressure needed for the appication. So...will the pump be unloaded during starts or will the motor be starting under load? Or is the plan to have an open-spool hydraulic system? [The latter would almost certainly start unloaded by its very nature.]

The choices made will determine the best design...

CR

 

[AndreyG]

Thank you everybody for comments and suggestions.
This really makes good thinking reference for me.

To answer the questions:
the motor start once and be kept in continuous operation the entire time the vessel is underway.
The motor will start with minimal load - when the system is not engaged and hydraulic pump runs idle - dumps all oil back into the tank.

 

[waross]

Go to a heavy truck parts supplier or a diesel engine parts supplier. Look for a starter solenoid for a 24 Volt starter for a diesel engine. (Why diesel? 24 Volt starters are not common for gasoline engines.)

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

 

[AndreyG]

>> Go to a heavy truck parts supplier or a diesel engine parts supplier. Look for a starter solenoid ...
Thank you, good advice! And I did ... an learned that starter solenoids are often intermittent duty and i need continuous one. Nothing is easy but this is right direction.

 

[mikekilroy]

too clugy to use the truck intermittent for the hi inrush start then smaller dc rated relay in parallel for running current (drop out start solenoid)?

 

[iop995]

24V starter have minim 200-300A capability (depending of motor stater power / car engine type) and intermittent refer to start-stop car profile not that such starter can't support continous a maxim 50A motor current. I think is much cheaper than DC contactor / converter and remain money to buy 2 pcs, one for back-up...

 

[crshears]

AndreyG wrote:

'...learned that starter solenoids are often intermittent duty...'

Since this would be an off-design use, a question to answer is 'Why is the starter solenoid rated intermittent duty only?' Is it because in its intended service its contacts are only rated to carry their rated current for a short period of time? Or is it that the solenoid itself is only intended to be energized for a short period of time and would overheat and burn out if continuously energized?

In the first case it might be possible to use a starter solenoid anyway.

In the second, the clugy [?] method of starting the pump with starting solenoid 'A', parallelling the SS with a continuous-duty DC contactor 'B', then dropping out the SS 'A' would seem a quick and dirty and fairly inexpensive way to go.

CR

 

[iop995]

Good point crshears. I checked a starter datasheet and it have 6A hold-in current / 24V, so about 144W in heat.