Need some input on a piece of motor control kit

Thread Starter

JonathanL

Joined Jun 12, 2020
16
Hi guys

I have attached a photo of the inside of a very very old motor control panel which i have in a plant where i work. I have no electrical drawing for this panel but in short it is just running a large motor which is running some heavy mixing blades. My question is ,that is piece of kit on the top a very old resistor bank? If so what exactly is it doing. I have never really seen something like this before but to me it looks like it is a large heat sink but i could be wrong. If you look at the photo you can see some cables running from it to the A2 or 0volts on the contactors which are running the motors etc, but again i am not 100% so any input or previous experience would be appreciated.

Ideally i am looking at upgrading this panel to more modern switchgear but i do not want to start on anything until i figure out what this piece of kit is.

Thanks in advance guys.
 

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Thread Starter

JonathanL

Joined Jun 12, 2020
16
Welcome to AAC.

Looks like it might be a series of resistors with heat dissipating fins attached.

I was thinking so too. I just dont really see them anymore in modern panels, but again I can not be sure. I just did a further search on grid resistors and some visually similar images popped up but not much info on what they function for
 

mcardoso

Joined May 19, 2020
192
AC motor drives often have an option to connect a external shunt resistor to bleed off rotational energy and stop the motor faster than coasting it to a stop.

I'm unfamiliar with the exact device you have posted a picture of, but it could be used for the same purpose.
 

Thread Starter

JonathanL

Joined Jun 12, 2020
16
AC motor drives often have an option to connect a external shunt resistor to bleed off rotational energy and stop the motor faster than coasting it to a stop.

I'm unfamiliar with the exact device you have posted a picture of, but it could be used for the same purpose.
Ok thank you. If that is the case then I am wondering if this could be replaced with a modern frequency inverter.
 

mcardoso

Joined May 19, 2020
192
Ok thank you. If that is the case then I am wondering if this could be replaced with a modern frequency inverter.
This looks like an across-the-line arrangement with contactors. If you can give info on the motor that it is attached to then I could help would with the VFD end of things. Definitely adds a bit of complexity, but much better control and energy savings. You have to consider if it makes sense in the application.

VFDs are pretty cheap nowadays and you can get a basic 5HP unit for under $200. High end models still command a substantial price.

EDIT: I will add that VFD controls are becoming much more of the go-to for motor control applications. Small motors might still be started across the line, but prime movers in a system are often placed on a VFD. Most VFDs can be operated purely based on digital I/O making them easy to install in older systems.
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
22,403
If this is a set up where the motor is RPM controlled and you have a large bank of resistors that are switched in as needed, this will undoubtedly be a wound rotor AC induction motor, often used in older gantry cranes etc.
If this is verified, the output can be shorted and a VFD used.
Max.
 

R!f@@

Joined Apr 2, 2009
9,746
If this is a set up where the motor is RPM controlled and you have a large bank of resistors that are switched in as needed, this will undoubtedly be a wound rotor AC induction motor, often used in older gantry cranes etc.
If this is verified, the output can be shorted and a VFD used.
Max.
Come to think of it, what Max said makes more sense. As the photo shows a resistor bank with multiple connections. Not a single R use to stop the motor or brake it.
 
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