# How can the current be rise to the required level?

#### krumplicukor

Joined Aug 6, 2018
8
Hi everyone,

I am working on my bachelor where my machine requires with two motors over 20A and my uni has a fuse with only 16A.
Can you give me please any suggestions how to solve my problem?

I need to run the machine asap and my teachers told me it is my responsibility to solve it.

#### jpanhalt

Joined Jan 18, 2008
11,088
There is no safe way to run a motor that "requires" 20A on a circuit that can only supply 16 A.

Is the 20 A for each motor or the combined? If combined, you may be able to start one motor. Then when it is not under load, start the other. You might consider using the running motor to get the second motor spinning before supplying current to the second motor. In any event, blowing that fuse will always be a concern.

Is it AC or DC?

#### krumplicukor

Joined Aug 6, 2018
8
There is no safe way to run a motor that "requires" 20A on a circuit that can only supply 16 A.

Is the 20 A for each motor or the combined? If combined, you may be able to start one motor. Then when it is not under load, start the other. You might consider using the running motor to get the second motor spinning before supplying current to the second motor. In any event, blowing that fuse will always be a concern.

Is it AC or DC?
We have 2 AC motors that are connected in delta 400V/10.3A each and they are controlled by 2 ABB frequency converters which are consuming 8.8A each (thats what it says on the FC label). The strange thing is that we were able to start only 1 frequency converter and the motor connected to it when combined current consumption is from the motor + FC= 10.3A+8.8A which is over the 16A fuse. I hope that our way of thinking is correct???

#### jpanhalt

Joined Jan 18, 2008
11,088
Can you show a picture of the label or a link to the datasheet/manual for the controller? From what you say, that is a very inefficient converter. More likely, I suspect you are misinterpreting some of the ratings, such as maximum input current, maximum continuous output, and so forth.

#### krumplicukor

Joined Aug 6, 2018
8
Can you show a picture of the label or a link to the datasheet/manual for the controller? From what you say, that is a very inefficient converter. More likely, I suspect you are misinterpreting some of the ratings, such as maximum input current, maximum continuous output, and so forth.

#### jpanhalt

Joined Jan 18, 2008
11,088
The motor requires 5.95 A at 400 volt, 50 Hz. The controller (second picture) will provide that with less than 8.8 A current. In other words, you do not simply add the current the motor requires to the current capability of the controller.

Controllers are usually quite efficient. So the critical value is the current drawn by the controller to supply 5.95 A to the motor at full load. That is not shown in on the data plate clearly. It is probably in the user manual. You might still be advised to start the motors sequentially. Your fuse may still be marginal with both motors at full load, but the situation is not as bad as you seemed to have imagined.

#### Yaakov

Joined Jan 27, 2019
3,735
The motor requires 5.95 A at 400 volt, 50 Hz. The controller (second picture) will provide that with less than 8.8 A current. In other words, you do not simply add the current the motor requires to the current capability of the controller.

Controllers are usually quite efficient. So the critical value is the current drawn by the controller to supply 5.95 A to the motor at full load. That is not shown in on the data plate clearly. It is probably in the user manual. You might still be advised to start the motors sequentially. Your fuse may still be marginal with both motors at full load, but the situation is not as bad as you seemed to have imagined.
It was my immediate surmise that sequential start was the thing they wanted to get out if it. It might not be the case but it's a pretty basic technique that people need to know, so...

#### jpanhalt

Joined Jan 18, 2008
11,088
I need to run the machine asap and my teachers told me it is my responsibility to solve it.
I interpreted that part of post #1 differently. That is, the teachers knew he was misinterpreting the ratings (see post #3) and considered sequential starting on the table as part of the solution.

#### Yaakov

Joined Jan 27, 2019
3,735
I interpreted that part of post #1 differently. That is, the teachers knew he was misinterpreting the ratings (see post #3) and considered sequential starting on the table as part of the solution.
Could be, but I would guess a naive approach of simply starting the motors would trip the breaker and that would compound the confusion caused by misunderstanding the labels.

#### jpanhalt

Joined Jan 18, 2008
11,088
@Yaakov
What other suggestions do you have for the TS? This is not my project.

#### Yaakov

Joined Jan 27, 2019
3,735
@Yaakov
What other suggestions do you have for the TS? This is not my project.
That's really it. With the information available I believe that sequential start is the answer to the (somewhat vague) problem.

#### kubeek

Joined Sep 20, 2005
5,768
I seem to remember that (at least in EU) the breaker should not ever trip below 1.13 times the rated current, so 18.08A in your case. So you might be ok, or it could last at least an hour or two under load.
Also remember the motors draw that much current only when they are loaded by the rated load, not when they are running freely. Also any machine will not likely use the full power all the time.