Electrical fundamentals
Holding voltage on a potential relay is
The voltage require to keep the contacts open and leave the start capacitor out of the circuit.
Current imbalance calculation
% imbalance = (maximum deviation / average current) x 100 Average is adding the three currents and dividing by 3 Maximum deviation is calculated by subtracting each current from the calculated average. Use the greatest deviation. You can use the same formula to calculate voltage imbalance. current imbalance must NOT exceed 10% Voltage imbalance must NOT exceed 2%.
Electrically commutated motor ECM is
A brushless DC three-phase motor with a permanent rotor
When the split phase motor reaches 75% of its full speed
A centrifugal switch or exterior relay takes the start winding out of the circuit
Electrically commutated motor ECM consists of
A control module and a motor. The control module is a microprocessor that receives a single phase 120 or 240 volt AC and convert it to DC and outputs it as a three-phase Signal to regulate the current (torque) and frequency (speed) delivered to the motor
Conductors are rated by American wire gauge and size based on
A maximum current draw which prevent the temperature to reach unsafe conditions. (Ampacity) Ampacity depends on the size and insulation used, the ambient temperature and the number of conductors bundled together . Minimum Circuit Ampacity MCA is used for selecting proper size conductor. MCA = 1.25 x FLA1 + FLA2 +FLA3+... FLA1 represent the full load amperage of the largest motor in the circuit.
Capacitor start induction run motor (CSIR) is designed with
A start capacitor wired in series with the start winding and a centrifugal switch or external start relay
Starting methods for a three-phase motor are
Across line starting users only one contactor ( inrush current , locked-rotor current is low enough) Part winding start (softer start) uses two starting devices ( contactor/starter) the inrush current is reduced by first energizing part of the winding, and subsequently energizing the reminder of the winding. A time-delay relay is required between contactors.
Shaded pole motor can be configured with multiple speeds by
Adding multiple taps to the motor windings
In a three-phase motor
The voltage supplied are already out of phase with each other by 120 degrees. The rotation of a three phase motor can be reversed by changing two leads from the incoming power phases.
Pick up voltage on the potential relay is referred to
The voltage when the motor reaches 75 to 80 % of its full load speed. It is enough to energize the coil and open the NC contacts.
In Permanent Split Capacitor (PSC) motor
There is no switch/ or relay to take the capacitor from the circuit. The start winding and its capacitor stay in the circuit all the time. (Run capacitor)
Which of the following is used to energize the start winding in a split-phase motor A. A capacitor B. A centrifugal switch C. A shading ring D. A transformer
B. A centrifugal switch.
Capacitor start capacitor run CSCR motor uses
Both start capacitor to increase the starting torque and run capacitor to improve its running efficiency.
current imbalance can cause motor over heat because
Can be caused by the imbalance in the input voltage or when an electrical terminal or contact become loose or corroded, causing high resistance in one winding. The current should be balanced in all three winding. imbalance causes one winding temperature increase and winding working harder.
Small three-phase motor do not need a ............ to start efficiently
Capacitors ( the 3 winding are 120 degrees out of phase already)
To change the rotation of a three phase motor you must .............
Change two leads on the incoming power
Troubleshooting a potential relay using a clamp on ammeter
Clamp the ammeter around the wire connected to the 2 terminal. Apply voltage. Current should be read momentarily, if the current does not drop to zero after a few seconds. Disconnect the voltage and use an ohmmeter to check the relay.
Troubleshooting a current relay using a clump-on ammeter
Clump the ammeter on the wire from the terminal S. Apply voltage and watch the ammeter reads current momentarily and then drop to zero. If the current does not drop to zero after a few second, disconnect voltage and remove the relay and use a ohmmeter to check it.
Variable Frequency Drive (VFD) is used to ________ .
Control the speed of an AC motor.
Electrically commutated motor ECM can provide constant air flow by
Converting the desired CFM to a specific combination of speed and torque. Accounting the external static pressure the motor has to overcome ( there is only one proper CFM demanded at a specific total external static pressure)
A potential relay depends on _______ to release the contacts for the start windings?
Counter EMF
When replacing a capacitor you must
Determine the microfarad rating and match it. For a start capacitor a same rated one must be used or no more than 20% rating. For a run capacitor the replacement must have a 10% or the original's value Determine the voltage rating of the replacement capacitor. It should be equal or greater than the original's rating.
A bleed resistor is placed across the start capacitor to
Discharge the high voltage stored in the SC. If the SC discharges across the relay, it can damage the contacts and might weld them closed. The resistors are typically 15,000 to 18,000 0hm. 2 watts.
In a capacitor start capacitor run CSCR motor
The start capacitor and run capacitor are wired in parallel with each other and in series with the start windings. A relay is used to remove the start capacitor from the circuit, while leaving the run capacitor and start winding for continuous operation.
Reading a ladder diagram
Each load is drawn on a separate horizontal line, along with any switches that control that load. Each horizontal line is identified by a number to the left of the diagram. Number on the right represent the line number of the switches controlled by the load in that line. Underlined is normally closed. Normally open is not underlined.
The permanent magnet rotor in an ECM (electrically commutated motor) allows
For better electrical efficiency- that is, the ability to control speed and commutation more precisely
troubleshooting a single switch.
If yo place a voltmeter across a switch and read line voltage, the switch is open. when the voltmeter does not read voltage across the switch. The switch could be either open or closed. Use hopscotching to troubleshoot.
Grounding and ground in 115 v and 230 v single phase
In a 230 volt, 2 hot conductors plus a third ( white). In this case the white conductor is a true neutral since it does not carry current when the load is balanced, but when the load is unbalanced, a measurable current can be seen. In a 115 volt, the white is better referred to as a "grounded" conductor. It does curry current in normal operation. The ground ( green or bare) and the white are both connected to the earth terminal in the panel. The white conductor in 115 volt service should never be interrupted by a fuse, circuit breaker, switch, or other devices. The only exception to this rule is when both white and hot wires are interrupted at the same time.
A capacitor start capacitor run CSCR motor is used
In compressors where the system pressures do not equalize quickly during the off cycle (txv systems)
Permanent split capacitor PSC motor are found
In system where pressures equalize quickly during off cycle ( fixed orifice metering device, capillary tube or orifice)
The direction of rotation on a shaded pole motor is _________
In the direction of the shaded pole
A current relay depends on ________ to release the contacts for the start winding?
Inrush current
The maximum overcurrent protection (MOCP) of a system that contains a single motor compressor is _____________.
Is 175 % of the rated load
What is hopscotching
It is a technique used to isolate an open electrical component. Permanently attach one probe of the voltmeter to a common point of the circuit. Move the other probe around the circuit to determine which switches are open. if you read line voltage both before and after a switch, the switch is closed. If you read line voltage before a switch but not after the switch, the switch is closed.
The compressor data plate normally shows 2 current rating
LRA Locked Rotor Amperage which is the maximum current draw when the motor is in a locked position. Used to select the correct contactor. RLA Rated-Load Amperage represent a UL mathematical calculation (MCC (maximum continuous current) x 1.56). It is the current at which the overload trips , and it is used to select the current fuse/circuit breaker size and wire size.
The potential relay operates
On the counter electromotive force (CEMF) generated in the start winding.
Capacitor failure can be caused by
Overheating. Start capacitor is not designed to stay in the circuit. (Can't dissipate heat). Short cycling can cause the start capacitor to fail because it didn't get enough time to cool down. When a capacitor fails. It is a good practice to replace all starting component.
Troubleshooting a potential relay using a voltmeter
Place the leads on the 1 and 2 terminals. Apply voltage. The voltmeter should read Zero at start up, until the motor reaches 75% speed. And than it should read line voltage. If it reads zero all the time. Disconnect the voltage and use an ohmmeter to check.
Troubleshooting a current relay using ohmmeter
Place the probes on L and M terminals. If the ohmmeter reads approximately 1 Ohm. The coil is ok. If it reads an infinity, the coil is open. Need to be replaced. Place the probes on L and S terminals. hold the relay UPRIGHT. the ohmmeter reads infinity resistance, the contacts are open, good (NO contacts). If it shows low resistance, the contacts are stuck closed. Need to be replaced. By turning the relay UPSIDE DOWN and shaking it, the contact should be closed (zero resistance) and open (infinity resistance) as the relay is being shaken.
Troubleshooting a current relay using a voltmeter
Place the probs on the L and S terminals apply voltage. The voltmeter should read voltage momentarily and than drop to zero. If the voltage does not drop to zero after a few second. Disconnect power and used an ohmmeter to check.
The RLA on a compressor data plate indicates the ________
Rated Load Amperage
Capacitor can fail open short or weak to test it
Remove the capacitor from the circuit, discharge the capacitor with a bleed resistor. Check the capacitance using a digital multimeter. A run capacitor malformed or bulged in one area indicate defect. A start capacitor has a membrane on the top that will blow open if it is overheated.
Troubleshooting a potential relay using an ohmmeter
Remove the relay and place the leads on the 2 and 5 terminals. If the ohmmeter reads high resistance (6,000 to 18,000 ohm), the coil is OK. If infinity resistance is read, the coil is open. Relay must be replaced. Place the leads on 1 and 2 terminals. If the ohmmeter reads 0 resistance, the contacts are GOOD (NC contacts). If a high resistance or infinity is read. Replace the relay.
In a permanent split capacitor (PSC) motor a
Run capacitor is used ( it remains in the circuit all the time and it is designed to dissipate the heat generated by its continuous operation.
When testing a capacitor you should first remove it from the circuit and ...............
Safely discharge the capacitor with a bleed resistor
If you need to replace a PSC condenser fan motor, you must use a ________.
Similarly rated PSC motor
Dual three-phase motor has
Six stator windings, which pairs are wired together. The pairs wired in series allow for high voltage (460 v). The pairs wired in parallel allow for a lower voltage (230 v ).
Current relays are usually used in
Smaller motor since rapid cycling with high current can cause the contact to melt or weld them together. The contacts arc once opening and once closing under high current.
When voltage is applied to a split phase motor
The start winding is out of phase with The run winding ( producing enough torque to start the motor)
Variable frequency drives are
Solid state power conversion devices VFD converts the incoming AC power to DC power using a type of rectifier bridge. The DC power is converted to quasi-bridge sinusoidal AC power using an inverter switching circuit. VFD exist in three designs : Pulse Width Modulation (PWM); Current Source Inverter (CIS); Voltage Source Inverter (VSI). VFD allows a motor to develop 150% or its rated torque while the VFD is drawing less then 50 % of its rated current from the mains in the low speed range.
A bleed resistor is used to prevent damage to the ...............
Start relay
A capacitor is used to boost the ———- of a capacitor-start, induction- run (CSIR) motor
Starting torque
A start capacitor is designed to ?
Stay in the circuit momentarily
The direction of rotation on a shaded pole motor can be reversed by
Switching the rotor and bearing assembly around
When checking a potential relay, you measure 10,500 Ohm across terminal 2 and 5. What does that mean?
The coil is good.
In a potential relay
The coil is wired in parallel with the start winding, and the NC contacts are wired in series with the start capacitor.
The potential relay wiring is designed as
The coil terminals are 5 and 2 and are wired across the start winding. The NC contact are 2 and 1 and are wired in series with the start capacitor. The 5-2-1 Can She Run rhyming phrase is used signifying 5 is connected to common. 2 is connected to start and 1 is connected to run.
When troubleshooting a switch, you measure a voltage across the contacts equivalent to the voltage of the circuit. What does that mean?
The contacts are open
Potential relays are used on bigger compressor motors because
The contacts stay cool since they break the current only once.
Maximum overcurrent protection (MOCP) is the maximum current rating of the fuse or circuit breakers that are used to protect the system, it is normally determined by
The equipment manufacturer and listed in the data plate. The data plate specifies if fuses, circuit breaker, or either can be used. The rating of the short circuit or ground fault protective device must not exceed 175% of the motor compressor rated load current rating Standard protection size = (compressor RLA x 1.75) + FLA1 + FLA2 Maximum protection size =(compressor RLA X 2.25) + FLA1 + FLA2
Voltage ambulance in three phase motor can cause ___________.
The motor to overheat
Run capacitor failure can effect the motor by
The motor will fail to start or if started it will draw higher than normal current.
When the motor speed drops bellow 75%
The potential relay contacts close and bring back the start capacitor into the circuit. This can cause the capacitor to overheat and a fault can develop in the start winding.
A split phase motor has two separate windings
The run and start windings, which are wired in parallel with each other.
The start capacitor is used
To boost the starting torque. The start capacitor is in series with the start winding. SC cause the start winding to become out of phase (lag) with the run winding which cause higher starting torque (300 to 500 %). SC is designer to stay in circuit for short time (it can't dissipate the heat developed from continuous operation). SC are designed for no more than 20 starts in an hour and no more than 2 seconds per start.
run capacitor is wired in series with the start winding is used
To improve the the running efficiency( improve the power factor of the motor) and it needs to be able to dissipate the heat generated by being in continuous operation. The marked terminal (in older run capacitors) must be wired to the run terminal in the motor
Current relay coil is
Wired in series with the run winding. As the inrush current increase at start-up., it is sufficient to energize the coil and close the NO contacts bringing the start capacitor and/or start winding on the circuit to boost the motor.
When replacing a capacitor and can't find the same rating capacitor you can
You can wire two or more in parallel (Ct = C1+C2) or in series (1/Ct= 1/C1 + C2). The voltage has to be the same as the original capacitor or higher.
