3 Phase motor voltage drop when under load

Thread Starter

ABrosDesign

Joined Sep 6, 2022
10
Hello all,

I have a 10hp 3 phase 230V AC motor powering a dust collector that is experiencing a voltage drop/amp increase when running. The motor is rated at 30amp running current and the actual voltage being supplied is 208v. The wiring to the motor is 8/4 SO and runs about 50ft from the breaker panel to the motor. The motor is controlled by a starter and thermal overload relay.

I noticed the starter and relay getting very hot while the motor is running so I recently replaced both of them. After replacing the starter and relay, the motor continues to draw more amps and lower voltage while running. The measured voltage at the starter is 208v when the motor is off. After the motor has been running for a minute or so the measured voltage is around 202-204v and the amperage measured on each phase is around 34-35amps which exceeds the rating on the nameplate for the motor.

I am worried about continued overheating possibly damaging the motor and starter/relay. Is there anything else I can check for before removing the motor itself to have it checked for a winding issue?

Thank you
 

MaxHeadRoom

Joined Jul 18, 2013
25,979
Is this an radial impeller style dust collector?
If so it could be that the intake bore is too short and loading the motor, this happened to a 3ph water pump I installed until the intake was choked down slightly.
Also could be aggravated by the low supply voltage.
 

Thread Starter

ABrosDesign

Joined Sep 6, 2022
10
Is this an radial impeller style dust collector?
If so it could be that the intake bore is too short and loading the motor, this happened to a 3ph water pump I installed until the intake was choked down slightly.
Also could be aggravated by the low supply voltage.
Yes it is a radial impeller connected to to motor. What do you mean by the intake bore is too short? This is a cyclone dust collector, the motor shaft sits vertically with and impeller underneath and the intake is about 12” in diameter which is connected to a bunch of ducting beyond which extracts dust from other machines. Are you saying the intake possibly needs to be larger or smaller in diameter?

Yes I have also had problems with starters on other machines burning out, I believe because of the low supply voltage combined with excessive ambient heat during the summer.
 

MaxHeadRoom

Joined Jul 18, 2013
25,979
The pump I installed was brand new 5hp radial impeller style for industrial parts washer tank unit.
All the correct fusing and O/L disconnect was wired.
In use, the pump would run hot and eventually trip the O/L's.
In contacting the manuf. they suggested the outlet was too short and providing high load and to throttle the output slightly.
This cured the problem.
In your case, if you have a fair amount of ducting, it could then be a voltage issue.
 

GetDeviceInfo

Joined Jun 7, 2009
2,118
part of your problem is running a 230 rated motor on 208. That alone will increase the current above nameplate, at full load. You will need to throttle down the airflow which reduces the load. This could be done by closing down the slide gates, reducing duct sizing, or increasing duct lengths.
 

Thread Starter

ABrosDesign

Joined Sep 6, 2022
10
The pump I installed was brand new 5hp radial impeller style for industrial parts washer tank unit.
All the correct fusing and O/L disconnect was wired.
In use, the pump would run hot and eventually trip the O/L's.
In contacting the manuf. they suggested the outlet was too short and providing high load and to throttle the output slightly.
This cured the problem.
In your case, if you have a fair amount of ducting, it could then be a voltage issue.
Hm interesting, thanks for the information. I will mess around with throttling intake and output and see if that helps reduce load on the motor.
 

Thread Starter

ABrosDesign

Joined Sep 6, 2022
10
part of your problem is running a 230 rated motor on 208. That alone will increase the current above nameplate, at full load. You will need to throttle down the airflow which reduces the load. This could be done by closing down the slide gates, reducing duct sizing, or increasing duct lengths.
Yes I figured running on the lower end of the allowable voltage wasn't helping the overheating problem. I've considered switching the motor to 480v instead. Do you think that would help at all?
Interesting, that is what MaxHeadRoom above also recommended may help reduce load on the motor. This almost seems counterintuitive to me as reducing airflow would seem to make the motor work harder to pull more air, but I understand the reasoning I think. There is probably 150ft of ducting of various sizes connected to the dust collector. I will try to throttle down the intake and see if that helps to drop the amps drawn. Thanks for your reply
 

GetDeviceInfo

Joined Jun 7, 2009
2,118
I've considered switching the motor to 480v instead. Do you think that would help at all?
If you run it on 480, it will deliver the 10hp at nameplate current. We’ll assume this is matched to the blower, and not a deviation from original.

This almost seems counterintuitive to me as reducing airflow would seem to make the motor work harder to pull more air,
In similar fashion to the power being developed in your motor as a product of voltage and current, the blower as a load is determined by a product of flow and pressure differential. This is evidenced by a vacuum motor speeding up if you plug the suction, indicating a reduced load. You’re motor current will fall in response.
 

LowQCab

Joined Nov 6, 2012
2,506
The type of problem is commonly solved using 3 Buck-Boost-Transformers
which can bump the Voltage up by ~24-Volts.
The long Cable run could be an issue as well.

The Transformers are a pretty expensive solution, but cheaper than a new 480-Volt-Motor.
But one bonus with the 480-Volt-Motor is that the long Cable run will no longer be
a factor because You will be running at roughly half the Current.
This can actually save a noticeable amount on your Electric-Bill every month, ( ~$100+ ).

You might try looking for a "Re-Wound" 480-Volt Motor, substantially less Money.
.
.
.
 

MaxHeadRoom

Joined Jul 18, 2013
25,979
The type of problem is commonly solved using 3 Buck-Boost-Transformers
which can bump the Voltage up by ~24-Volts.
You might try looking for a "Re-Wound" 480-Volt Motor, substantially less Money.
I am assuming he already has the 480v version, if this is a typical star/delta 240v/480v motor.
If 460v/480v 3ph is available, this is the way to go.
Just switch the motor straps from delta to star.
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
25,979
" 480-Volt-MotorYou will be running at roughly half the Current."
"This can actually save a noticeable amount on your Electric-Bill every month, ( ~$100+ )."
Power is billed in Kw/hrs so the power consumption is the same for a 230v/480v motor ran under either voltage.
 
Last edited:

Thread Starter

ABrosDesign

Joined Sep 6, 2022
10
If you run it on 480, it will deliver the 10hp at nameplate current. We’ll assume this is matched to the blower, and not a deviation from original.



In similar fashion to the power being developed in your motor as a product of voltage and current, the blower as a load is determined by a product of flow and pressure differential. This is evidenced by a vacuum motor speeding up if you plug the suction, indicating a reduced load. You’re motor current will fall in response.
I tested the amp draw again today after reducing the intake as much as possible by closing blast gates in the dust collection system and it did drop slightly. Was previously at 34-35a per leg and after throttling went to 31-32a. So it did help a bit but is still above nameplate FLA. Thinking now that using a BB transformer to step up voltage or just running 480v would be best
 

Thread Starter

ABrosDesign

Joined Sep 6, 2022
10
The type of problem is commonly solved using 3 Buck-Boost-Transformers
which can bump the Voltage up by ~24-Volts.
The long Cable run could be an issue as well.

The Transformers are a pretty expensive solution, but cheaper than a new 480-Volt-Motor.
But one bonus with the 480-Volt-Motor is that the long Cable run will no longer be
a factor because You will be running at roughly half the Current.
This can actually save a noticeable amount on your Electric-Bill every month, ( ~$100+ ).

You might try looking for a "Re-Wound" 480-Volt Motor, substantially less Money.
.
.
.
The motor can also run on 480v. Using a BB transformer is a good idea as well. I will have to see if it is more economical to run new wiring for 480v or implementing a BB transformer as it seems like voltage needs to be increased to fix the issue. Thanks
 

Thread Starter

ABrosDesign

Joined Sep 6, 2022
10
I am assuming he already has the 480v version, if this is a typical star/delta 240v/480v motor.
If 460v/480v 3ph is available, this is the way to go.
Just switch the motor straps from delta to star.
Yes the motor can run on 480v as well and it is available in the building. That may be the best option
 

MisterBill2

Joined Jan 23, 2018
13,127
With that sort of voltage drop the very first thing to check is connections in the power feed. Although the current is more than it should be, so is the drop.
But the very first question is about the duct collection system: Is this a new installation, OR, has it been working well and then started having the problem?
That is because Max may have named the problem in his first response. There may be either a design error or an assembly error. Errors do happen. Or even an assembler made change that caused a problem. I have a sad tale about that.
 

MaxHeadRoom

Joined Jul 18, 2013
25,979
With that sort of voltage drop the very first thing to check is connections in the power feed.
I believe he may be running off of a 208 3ph transformer, usually used to implement lighting in industrial settings.
Each star phase is 120v to star neutral.
Not a good source for a 3ph motor. !
 

Thread Starter

ABrosDesign

Joined Sep 6, 2022
10
With that sort of voltage drop the very first thing to check is connections in the power feed. Although the current is more than it should be, so is the drop.
But the very first question is about the duct collection system: Is this a new installation, OR, has it been working well and then started having the problem?
That is because Max may have named the problem in his first response. There may be either a design error or an assembly error. Errors do happen. Or even an assembler made change that caused a problem. I have a sad tale about that.
The power supplied from the panel is 208v. The voltage drops to about 202-204v when the motor is running. The system has been in place for about two years with no issues but has always run warm. I think the problem was exacerbated by high ambient temperature this summer as I had other pieces of equipment overheat as well that have run fine for a couple years. I think the problem lies with the supply voltage of 208v being too low for the service factor of the motor which states 230v on the nameplate. Installation error is certainly a possibility as well but I’d like to hear your story!
 

Thread Starter

ABrosDesign

Joined Sep 6, 2022
10
I believe he may be running off of a 208 3ph transformer, usually used to implement lighting in industrial settings.
Each star phase is 120v to star neutral.
Not a good source for a 3ph motor. !
Yes the power supplied is: 480v panel > 45kva 480v to 208v transformer > 208v panel > 10hp motor. Maybe I could get a different transformer to increase the voltage to ~240v but that is a rather substantial investment. I have several other 3 phase machines powered by 208v as well. Is it bad for the motors long term to continue to run on 208? Would it be worth it in your opinion to look into using a different transformer?
 

MaxHeadRoom

Joined Jul 18, 2013
25,979
If you have several 240v loads and no requirement for the 120v star connections then, it may be prudent to look for a 230/240v transformer.
Although if you have quite a few devices, it will require a large unit.
I would look at hooking up the 480 volt devices direct the incoming power if there are a few such as the 10hp motor.
 
Top