DC Motor Starters

[ControlConcepts]

Anyone know of a maufacturer of DC motor combination starters? If not how can you put overload protection off a regular contactor and make it into a DC motor starter. I know current is current but if you use a thermal overload relay as overload protection how much cooler does DC current run than AC current in order to select heater size?

 

[ScottyUK]

Heating effect of a DC current is exactly the same as for an AC current of equal RMS value. If your overload relay is a three-phase type, you should wire all three heaters in series.

Larger overload relays are often CT based. CT's don't work on DC. DC thermal relays are expensive - Allen Bradley make a magnetic type which uses a dashpot to give an inverse-time characteristic which we have used successfully in the past, but it was an expensive item. PB Golds also make relays suitable for DC protection, and these make the A-B part look cheap!

Regular (does this mean a typical AC3 type?) contactors will either have to be drastically de-rated or have poles in series to handle DC. Better to use a purpose-made DC contactor if space and cost allows it.

 

[RadarRay]

It strikes me that you might want to start your motor from an existing DC bus. If this is true you will require a step starter. You cannot apply a large DC voltage across a motor when the motor is at zero speed without damaging the motor and or supply. In effect you will have very low armature ohms with a large DC voltage applied to it, giving you very high current.

 

[jbartos]

Suggestion: Visit

http://www.abb.com

for dc motor drives

 

[jbartos]

Suggestion: Contact the motor combination starter manufacturer for DC application of that starter including the rating of thermal overload or electronic overload relays.
Regular catalogs, e.g.
1. General Electric Co. GEP-1260J GE Electrical Distribution & Control
indicates on page 1-75
CR324 Block Overload Relays for Panel Mounting
3-pole, 600VAC/250VDC 135A Maximum Ampere Continuous
This clearly implies that there are different ratings for AC and DC. Pages 1-80 - 1-84 do not explicitly address DC Heater Selection.
2. Square D Industrial Control Catalog 169, December 1992, does not explicitly address DC heater selection for DC Thermal Overload Relays
3. Allen-Bradley Industrial Controls Publication A112 September 1997 does not explicitly address DC heater selection for DC Thermal Overload Relays. However, it provides five examples of AC motor overload heaters selection on pages 1-203 to 1-205

 

[philnacc]

Need starter relay square D class 8805 EQ-1733-G4 or equivlent for AC motor (long allis 2364890) 115/230 60 Hz.

 

[abcd3286]

WAIT ONE MINUTE

How large a motor.
THis is a BIG factor

Small Dc / low voltage 12 to 24 VDC (car truck starter)can get away with across the line.

Medium size medium volt (120) probably want to get into resistive starter bank

Large high volt 250 and more definitely gets complicated.

THEN you may want to do speed control on top of that. A good DC speed control may take care of all issues stopping starting, overcurrent etc etc

Dan Bentler
Seattle

 

[ScottyUK]

Dan,

It's an AC motor.




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