Why would thermal overload trip when low voltage is supplied.

The background story goes like this. Our gas turbine has an output rating of 11,000 volts but we had a problem with the Automatic Voltage Regulator (AVR) and it had to be replaced. After replacement, the HMI indicated that the output voltage was 11,000V but the actual voltage on the bus was about 10,400V and this had a ripple effect on the whole system as the output of our 3.3kV transformer was 3.1kV and the output of the 440v transformer was 400v.

 

After a while, the thermal overload relays on the well-pump motors began to trip one after the other. This prompted us to troubleshoot, and we discovered that the supply voltage to the motor was below the nameplate rating.

 

Why would a low supply voltage trip the thermal-overload protection relay, normally a when a motor is overloaded it draws more current from the supply and when the motor tries to pull current higher than its full load amperage (FLA) rating the overload opens the circuit to prevent the motor from damage due to overheating. 


I will give two reasons why the thermal-overload relay tripped.

This is a little bit oversimplified but here it goes

1)     (1) A motor must draw a fixed amount of power from the supply (Power = VI) so when a motor is supplied with a voltage less than its nameplate rating it will compensate for the lower voltage supplied to it by drawing more current to meet the Power Threshold. If the current pulled starts to exceed the FLA of the motor, heat begins to build up. This mimics the events that occur in the event of an actual overload of the motor so and the Thermal overload relay operates to protect the motor.

2)   (2) Low voltage supplied to the motor causes the torque developed to be reduced, this reduced torque results in an increased slip and lower speed, the motor tries to reduce the slip by drawing more current from the supply and this trips the thermal-overload relay.

 Proof that Low voltage supplied to the motor causes the torque developed to be reduced.

  1. Torque equation: For an AC induction motor, the torque (T) is proportional to the square of the applied voltage (V):

             T ∝ V²

      2. Power Equation: The power supplied to the motor (PP) is given by:

P=V×I×cos⁡(ϕ)
P = V \times I \times \cos(\phi)      3. Torque-Power relationship: Torque is also related to power and speed

P = T * ω 

Where ω is the angular velocity (speed) in radians per second.

  • The torque (TT) developed by the motor is proportional to the square of the voltage (VV). Therefore, a reduction in voltage significantly impacts the torque.

T∝V2T \propto V^2

  • For example, if the voltage is reduced to 90% of its rated value: Tnew∝(0.9V)2=0.81V2T_{new} \propto (0.9V)^2 = 0.81V^2
  • This shows that the torque is reduced to 81% of its original value when the voltage is reduced to 90%.


 

 

 

Comments

Post a Comment

Popular posts from this blog

How to check continuity over long distance

Image
When I began my career as an electrical technician my first challenge was the problem of checking the continuity of a wire that spans a long distance. My problem was compounded due to the physical layout of the wire under test. Let me use a simple picture to layout the nature of my problem. A bunch of Armoured cables outgoing from the  PCC (Power Control Center) in Building 1 feeds a group of distribution panels in Building 2 over 100ft away. Ideally, the cables should be tagged but due to aging and poor maintenance, most of the tags were not visible.  The problem was that some Distribution boards in building 2 had power while some others didn't.  The DB of interest didn't have power and thorough checks showed that there was no power on the cable feeding that DB.   All outgoing cables from Building one PCC had power so I suspected that the cable was no longer continuous. Anyway in the initial design of the facility 3 spare cables was laid underground but without tag
Read more

Making sense of a transformer nameplate data

Image
The nameplate of a power transformer contains the following details as per standard, then additional information could be provided varying from manufacturer to manufacturer. Name Of Manufacturer Serial Number Year Of Manufacture Connection Symbol – This tells you the HV winding and LV winding configuration of a transformer and difference in phase angle between them. Example DYN11 where the first letter represents the HV winding and indicates that it is connected in delta, the second letter represents the LV winding and indicates that it is connected in Star(wye)  and the third letter N indicates that the LV winding has a Neutral while 11 denotes a 30 degree lead in phase angle. This article explains more on the Vector group of a transformer and why it is important.  (Transformers connected in parallel must have the same vector group i.e. same phase angle shift to avoid circulating currents. This is a situation where one source will become load to the other sourc
Read more

Stall Rotor Protection Using Magnetic Overload Relays Oil Dashpot type

Image
  A Magnetic Overload relay works by detecting the magnetic field strength generated by the current flowing to the motor.  In most cases the relay input is a representation of the  current (through C.Ts)  flowing into the motor.   The Magnetic Overload Relay is built in such a way that the magnetic flux set up by current flowing through the  coil  drags (pulls) the core upward. A typical dashpot type magnetic overload relay is shown in the image below.                                    Construction. The relay consists of two main parts which is the trip mechanism at the top and the dashpot at the bottom.  The parts of the relay are further divided into a coil, a plunger (core), an oil-filled dashpot and switching contacts (trip contacts). The plunger is attached to a piston (disc). The piston is suspended in an oil filled chamber.  The relay coil is connected in series with the motor supply circuit. During normal operation magnetic flux induced by the coil is not great enough to c
Read more