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ACS power supply, and fault-finding on electrical motors

Last updated: 11-18-2020

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ACS power supply, and fault-finding on electrical motors

To address these topics, we need to consider the SANS and a logical approach.

Hello Jaco, let us begin with the first question, here we need to look at regulations – namely SANS 10142. Item 6.16.4.1.1, which states: “Dedicated circuits shall be provided for fixed space heating and cooling (air-conditioning units) that are rated more than 16 Amps. There may be more than one unit on each circuit and the power supply to each unit shall be controlled by a switch disconnector.”

So, in essence if your unit(s) draw more than 16 Amps, then you have to run a dedicated circuit with an isolator to each unit. If your air conditioning unit is rated at less than 16 Amps, then you can use a wall plug. Take note: some users may prefer a dedicated circuit with isolator even on units rated lower than 16 Amps.

When approaching a fault, the first step is to ask questions. We need to refer our questions around our knowledge of symptoms. Questions need to be asked around how the fault occurred.

It is important to be logical in your approach.

There are seven-basic steps to fault-finding:

We need to ask questions about the symptoms of the fault. Gather any documentation, for example: technical manuals, service or fault reports, wiring diagrams, plant logbooks, operational manuals, maintenance history.

We need the necessary knowledge to understand the fault In what way is there a fault, how does the fault present itself? Analysis of the symptoms, what component would give these symptoms? What particular component or module could be faulty?

This is a very important aspect that is often neglected. Failure to do a root cause analysis could result in a repeat failure. Why did the fault occur? Why did the component fail? Did the component fail prematurely? Is the component failure the symptom or the cause? What can be done to prevent future failures?

Looking at electrical motors it is important to know that there are seven fault zones, namely:

By looking at the trouble-shooting charts, we can diagnose most faults with regards to an electric motor.

Let’s look at an example where a motor would not start. In gathering information, the client indicated that there was a burning smell.

On testing the windings, we found that the resistance readings on all windings were incorrect. In addition, the motor windings tested as down to earth. On opening the motor all was revealed.

In this case the motor supply was the cause. On testing, it was found that the supply voltage drop was excessive. This created a situation where the motor continued to run but in an overloaded state, drawing excessively high amperage. The overload on the power circuit was not operational and the result was a burnt-out motor. After all repairs were undertaken to the motor, we remembered to correct the root cause.

If an electric motor is subject to improper operating conditions, electrical, mechanical or environmental, the winding life will be reduced significantly.

Insulation defects can be caused by contaminants, abrasion or voltage fluctuation and overheating.

Complete insulation burn out on all phases of a three-phase motor is often caused by motor overload or under or over voltages. A locked rotor can also cause the windings to burn out.

Rapid start/stop scenarios will cause heat build-up in a motor – if temperatures become excessive, burnt insulation will result.

We may find that we have phase defect. This is a consequence of a power supply interruption in one phase. A burnt fuse, an open contactor, one power supply interrupted, or a poor connection normally causes this defect (also known as single phasing).

The insulation burn-out in one phase of the stator winding can be a result of uneven voltage between phases. Uneven voltages are usually caused by unbalanced loads in the power supply originated by poor connections at motor terminals or by bad contact. 1% of voltage unbalance can cause a current unbalance of 6% to 10%.

On a single-phase motor burn-out to the start winding is normally caused by the non-opening of the centrifugal or start relay where the coil remains switched on longer than the specified time.

Jaco, I hope that this assists you with your installations and motor issues. Remember to always determine and correct the root cause – this is often overlooked and will result in repeat failures.


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