Electric forklift scr problem

kendrickg

Member
Join Date
Sep 2007
Location
morgantown, PA
Posts
12
You all were very helpful when i asked about a VFD / dynamic brake problem so i had to ask the forum about this. I understand that this is a plc forum, but you all are some of the best diagnosticians in the world!!



Had a forklift repairman come down for $90 and hour for 10 hours and he couldn’t figure it out. We have an early 80s raymond Electric forklift that goes into full speed as soon as you tilt the joystick. Happens in forward and reverse. The drive motor contactor cycles on and off giving the motor full power during the one cycle (about 0.5s on and 0.5s off) . It is an SCR type drive system, with a "main SCR" and a "commutating SCR", and two main rectifiers. WE have a junk lift that i have been swapping parts from, so i swapped all the SCRs, rectifiers, control box and main capacitor. The repairman tested all the scrs and recs and said they were fine. What would cause the motor contactors to cycle on and off, it should be on all the time during motion and the scrs would vary the power to the motor. There is separate forward and reverse contactor, but they are both affected by the cycling problem.



The problem started when we had to push a machine up a slight grade into our building. The lift was straining for about a minute and we smelled a kind of burning resistor or transformer smell. When we got into the building the lift started doing this immediate full speed condition. We used the lift an hour later and it worked fine, and continued using it for two weeks. We stressed it again and a cable lug going to the rectifiers fried and the immediate full speed condition returned. This time it didn’t "go away". I replaced the cable and and rectifiers but the problem was not repaired what so ever. So, we replaced all kinds of parts on the lift and no results. We might as well buy another lift instead of having another idiot technician come out, but the lift has low hours for its age and has never given us any problems. Any thoughts?
 
I don't know much about forklifts but, did you consider calling the Raymond factory technical service? Explain the problem to them and if they don't answer, ask them for a "certified factory repair service dealer" in your area for that particular forklift.

Hope maybe this will help, good luck.
 
Check the field windings in the motor. It is possible that they are shorted and therefore you are running with a weak field.

Simply look at the nameplate field volts and amps. R = V/I. Use an ohmeter to check the resistance of the windings. It should be close to nameplate.
 
Does your Raymond have the vehicle manager installed if so does it report any error mesages? And do you have the equipment to access its menus? Also if you can supply the model of the Raymond I can talk to our power equipment tech here and see if he has any thoughts however he won't be in till Monday
 
I talked to another tech on your problem He has all the Raymond manuals here but he would need the model and serial number as well as age of the machine to help you out. He did state to check all the solenoids and mentioned that there is one in the mast. and to check your high low limit switch. another location on a solenoid is under the bottom plate on the machine near the front.
 
Mordred, that sounds great. I just figured that an odd symptom like a relay cycling would be an easy indicatior problem to an experienced tech. The model is 31-D-DR25TT , serial is 031D-DR25TT-4741. Thanks
 
Ok I'll be able to get more help on Monday the other tech just finished his shift I do beleive we have the same model here so should be able to give you some direction on Monday
 
Marius,


Don't know about the OP, but I had the same thing on a different make electric hilo, about 20 years ago - full speed as soon as the pedal was touched.


Fortunately in the building next to us was an electronics repair shop that tested all the components we gave them and it was a SCR bad. I don't remember if it was the main SCR or the trigger SCR, but they are not expensive so I suggest replacing both.
 
Just to add, if you think about it, if the SCR fails, and stays closed (which in my experience is extremely common) current will flow unrestricted to the winding.
 

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