2 - motors running through contacts won't start up

packerbacker · May 9, 2015

I just wired (2) 1 HP / 5.5 amp/230 volt/ single phase motors, powered through snap switch/ contactor control. I can get 1 of 2 motors to start no problem but when i add the 2nd motor neither will start. They just rattle and act like they are wired incorrectly. However when i bypass the contactor both motors start and operate just fine. My contactor is a 40-amp/ 120 volt coil square D dual purpose rated for 7.5 HP. (only using 2 HP) I feel like it is a contactor issue. Looking for any advice/help.

ptonsparky · May 9, 2015

Where are you getting the 120 v for your coils?

Draw us a schematic. Take a picture and post it

packerbacker · May 9, 2015

just getting 120v from closest available.? Same panel board/service. Why would that matter?

ronaldrc · May 9, 2015

If its wired right and you are using the snap switch for the contactor only its probably a voltage
drop problem causing the contactor to drop out,does your contactor buzz with the two motors?

It would be helpful to post a picture or diagram. I am trying a new website out Mediafire it is a
posting site one program is free and you could post it there,I would like to check this site out
from this user end. So far it seems like a good site. Of course you could just scan it,and post
it as a jpg.

Ronald :bye:

sparkyrick · May 10, 2015

What happens when you manually push in the contactor, do both motors start/run?

GoldDigger · May 10, 2015

sparkyrick said:
What happens when you manually push in the contactor, do both motors start/run?

Using appropriate insulated push stick and wearing PPE, of course!

kwired · May 10, 2015

packerbacker said:
I just wired (2) 1 HP / 5.5 amp/230 volt/ single phase motors, powered through snap switch/ contactor control. I can get 1 of 2 motors to start no problem but when i add the 2nd motor neither will start. They just rattle and act like they are wired incorrectly. However when i bypass the contactor both motors start and operate just fine. My contactor is a 40-amp/ 120 volt coil square D dual purpose rated for 7.5 HP. (only using 2 HP) I feel like it is a contactor issue. Looking for any advice/help.

Are you saying when you try to energize both contactor coils simultaneously you get "chatter" of the contactors but no solid pull - in and neither motor runs?

If so is the supply stout enough to handle starting current of both motors without significant voltage drop which may be the reason for contact chatter? Sounds like this may be a good possibility - what is the source and what is it's kVA rating, and/or is there a long conductor length involved somewhere between the load and the source?

packerbacker · May 10, 2015

My contacts were closed before I even turned on the circuit for my 2 fan motors still same problem. Both fans together pull just over 10 amps on full load and I pulled # 8 to disconnect.

kwired · May 10, 2015

packerbacker said:
My contacts were closed before I even turned on the circuit for my 2 fan motors still same problem. Both fans together pull just over 10 amps on full load and I pulled # 8 to disconnect.

Please answer the question of whether the contactors are "chattering" so to speak when your problem is occurring.

If the coils are supplied by same source as the motors (even though possibly on separate circuit) if the voltage drops enough when those motors instantly hit locked rotor/near locked rotor current they may drop out, then voltage recovers enough they pull in, then it drops enough they drop out.... and it could do this several times a second. Eventually the motor(s) may get started gaining a little momentum with each cycle of pull in/ drop out resulting in a little reduction in starting current each cycle, but this is much more destructive to the contacts then just one cycle of high starting current for a short duration.

packerbacker · May 10, 2015

the contactors themselves make no noise

packerbacker · May 10, 2015

all the noise comes from the fan motors themselves they are two speed motors high and low controlled by a snap switch

kwired · May 10, 2015

packerbacker said:
the contactors themselves make no noise

When I say "chatter" I mean rapid cycling of the contactor armature which results in rapid cycling of the motor circuit contacts, whether it makes much noise or not.

You said they work just fine when bypassing the contactors - which I assume means you either wire around them, or push the armature in with a foreign object rather then depend on the coil to pull it in, so it seems to me they likely are not pulling in and staying in or something of that nature when they are left to operate on their own. You also said one motor will start just fine but both will not. That also seems to indicate there is too much voltage drop when both are trying to start at same time to be able to pull and hold contactor armature in.

packerbacker · May 10, 2015

my supply conductors are number 8 they run approximately 75 feet to 400 amp MDP. I notice no noise coming from contactor as I stood right in front of them, all the noise appeared to be coming from the fan motors themselves.? I understand the point about the voltage drop with the contactor chattering but I don't think that is the case as they make no noise. thank you for your help so far. weird one

kwired · May 10, 2015

Maybe next step is to see what actual voltage is when trying to start these motors, Still don't quite understand the fact they work fine when bypassing the contactors.

Something else in the control circuit failing when trying to start both and messing with the control circuit operation?

GoldDigger · May 10, 2015

packerbacker said:
my supply conductors are number 8 they run approximately 75 feet to 400 amp MDP. I notice no noise coming from contactor as I stood right in front of them, all the noise appeared to be coming from the fan motors themselves.? I understand the point about the voltage drop with the contactor chattering but I don't think that is the case as they make no noise. thank you for your help so far. weird one

Is there any kind of low voltage protection in the motors themselves?
I suppose if the voltage drop is high enough (defective high resistance contacts in the contactor?) then the overcurrent protection in the motors might be getting involved.
Measuring the voltage at the output terminals of the contactor would tell you a lot about that.

ptonsparky · May 10, 2015

These motors are two speed.
They start and run with the contactors. One for each motor.
Problem arises when you attempt to change speeds with the snap switch.

Correct?

sparkyrick · May 10, 2015

Check the output voltage at the contactor. Our penny pinching shop queen recently switch from a quality name brand contactor to a Chinese POS contactor. I had a few with plastic fragments inside not allowing the contact to make. You might have some junk inside yours causing high resistance.

packerbacker · May 10, 2015

the problem happens at startup does not matter if it is on high or low they just won't start. I will check on the POS contactor Thanks

Jraef · May 11, 2015

I would suspect a VD problem, and when you are trying to start two motors, the loss of voltage causes a loss of torque which is not allowing them to fully accelerate and the start switches don't open up, so they never come out of start mode.

ActionDave · May 11, 2015

packerbacker said:
the problem happens at startup does not matter if it is on high or low they just won't start. I will check on the POS contactor Thanks

Sounds like the problem is not the contactor but the the wiring to the leads on the motors.

sparkyrick said:
What happens when you manually push in the contactor, do both motors start/run?

An answer to this question would help.

2 - motors running through contacts won't start up

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