Potential Relay

There are a lot of technicians out there that do not know the difference between a potential relay and a current relay. Do you know the difference??

Potential relays ; Most potential relays have 5 terminals and only 3 are for starting the motor it is connected to. terminals 1 and 2 are the contact terminals and 2 and 5 are the coil connections. The main function of the potential relay is to assist in starting the motor its attached to. These relays have a high resistance coil and a set of normally closed contacts. they also have terminals 4 and 6 which are sometimes called dummy terminals which are actually a tie point for capacitors or fan connections and nothing else, no internal connections to the starting part of the relay.
The relay in action ; When power is supplied through the control circuit both the run and start windings are energized. The capacitors cause a phase shift for starting torque because  being wired in parallel causes the capacitance to add. the potential starting relay works based on the EMF or electromotive force or back EMF, what we call a bucking voltage that is generated across the start winding as the motor increases in speed. this happens because of the mass of metal of the motors rotor spinning in close proximity to the motor windings which has a voltage generating effect. This generates a back EMF which opposes the line voltage which you can measure across terminals 2 and 5. The back EMF generated is usually a higher voltage than the line voltage which can reach as high as 400 volts depending on the design of the motor.
       The back EMF voltage generated across the start winding causes a small current to flow in the start winding and in the potential relay coil since they are in the same circuit. When the back EMF has built up to a high enough value, referred to as pick-up voltage, the contacts between terminals 1 and 2 will be picked-up or opened. This will take the start capacitor out of the circuit.

The pickup voltage usually occurs when the motor has reached about 3/4 speed. The start winding is still in the back EMF circuit keeping the relay's coil energized while the motor is running at full speed.

      When the cycling control opens, line voltage  is taken away from the motor. The motor's rotor decreases in speed and the back EMF generated across the start winding decreases. The relay now sees a lowering back EMF and no longer can generate enough magnetism in its iron core to keep contacts 1 and 2 open.

 

 

      Troubleshooting Potential Relays;



 

A simple ohmmeter is all that is needed to troubleshoot a potential relay. After taking all of the connecting wires off of the potential relay, measure the resistance across the 1 and 2 terminals. The resistance should read close to zero, since there are normally closed contacts between terminals 1 and 2. If the meter reads open or infinity, the contacts are stuck open, and the relay should be replaced. 

If the normally closed contacts between terminals 1 and 2 are arched and pitted, The ohmmeter should read a resistance no greater than 200 ohms. For example, if the contacts are dirty or pitted the relay should be replaced

       Open contacts between terminals 1 and 2 will prevent the start capacitor from being in the circuit. The start capacitor is a high microfarad capacitor, which is in series with the start winding. The start capacitor gives the start winding circuit more capacitance and gives the current a leading effect (phase shift) on the voltage.

The more the current leads the voltage wave, the more phase shift there will be, and the motor will have more starting power or torque. Without the start capacitor, the motor will usually lock its rotor and draw locked rotor amps (LRA). This will cause the motor’s overload to open and the motor will short-cycle on the overload. This short-cycling is detrimental to motor windings, starting relays, and capacitors because of its overheating effect.

   The relay contacts could be stuck or arched in the closed position. In this case, the start capacitor would never be taken out of the circuit. The motor would be stuck in its starting mode and draw high amps. The amp draw would be somewhere between running load amps (RLA) and locked rotor amps (LRA). The motor’s protective device would soon open. A short-cycling situation would occur.

If the contacts are stuck in the closed position, the relay would have to be checked with a voltmeter in the running mode, since the contacts between 1 and 2 are normally closed when not in operation. Once the motor is up and running, use a voltmeter to measure the voltage between terminals 1 and 2. A voltage reading of zero would prove that the contacts are not opening. Also, a high amp draw from the start capacitor and start winding circuit still being energized is a sign that the contacts have not opened.

After disconnecting all wires from the relay, ohm the coil between terminals 2 and 5. Since the coil should have a very high resistance, make sure you are using the proper scale on the ohmmeter. The (R x 100) scale is a good one to use. The R x 1 scale can fool a technician into believing that there is an open coil because of the coil’s extremely high resistance. It is not uncommon to have the resistance read in many thousand ohms.

If the ohm reading is infinity on the (R x 100) scale, the relay coil is opened. The relay should be discarded and a new one installed. An opened relay coil will prevent the contacts between 1 and 2 from opening. This is caused from no magnetism in the iron core that the coil is wrapped around. This, again, will cause high amp draws. Always use the model number of the old relay to cross reference to a new relay.