Electric motors drive industry, comprising 60-70% of the electrical energy consumed in an industrial environment. Calculating the cost of replacing even a small motor should include the labor to remove the failed motor and reinstall a new or repaired unit. Sometimes this operation can be completed easily and quickly but more often the motor is coupled to a gear reducer or the flange of a pump housing. Machine design engineers try to place motors and associated reduction systems in accessible locations, but that is not always possible and the motor ends up underneath a conveyor line in sometimes alarmingly adverse conditions, at least adverse to any electrical equipment.
Using standard motor starters with thermal or bimetallic overload protection downstream from a circuit breaker designed to protect the cabling will often be insufficient to keep damage to the motor to a level allowing the motor to be put back into service after a sustained over load occurs. Bimetallic and eutectic (solder pot) overloads are designed to trip and actuate the removal of the protected load in 10, 20 or 30 seconds when the current rises to 600% of the rating. At 150% of the target current level, class 10 overloads are designed to trip in 4 minutes, class 20 in 8 minutes and class 30 in 12 minutes. The heat created by the excessive current can be dissipated through the rotor and stator cores using the motor housing design and a fan attached to the shaft, but what operator will choose to allow the process to stop while the motor cools? In actual practice, after a over load condition the operator will likely press reset and start, repeatedly, to see if everything is back to working condition until the equipment is back up and running. Over-heating and winding failure would be imminent.
Protecting a motor from destruction and controlling a process are completely different requirements. If a conveyor jams, the current at the motor leads will climb immediately. Knowing this has occurred can keep the damage to a minimum. A current operated switch will react in 1/10 of a second or faster, creating a change in the output to set off an alarm or disconnect the drive from the power. As with most issues, the application drives the product selection. The more critical the operation, the fastest and most accurate protection available becomes a necessity.
Figure 1: A current monitoring control system using a current transformer connected to a separate, powered relay.
Monitoring AC current used to mean installing a current transformer (CT) and connecting the secondary to a separate box where an adjustable amplitude would trip a relay or produce an analog signal proportional to the current. While still common, the number of connection points creates the opportunity for low reliability and decreases the safety of the system. The simple “donut” type current transformer actually requires a substantial amount of forethought to be sure the needs of the application are fulfilled. Not only must the physical size and shape of the CT be considered to be certain the conductor will pass through the sensing window, the ratio of primary current to secondary current must be specified. In North America, most equipment is designed to accept a five amp secondary when the primary current rises to a set level. If current exceeds the CT ratio, the output will also increase until the magnetically permeable core saturates from the induced magnetic flux produced by the primary circuit. Depending on the core material and other construction details, the output can rise to ten or twenty times the normal magnitude and the secondary output will remain quite proportional to the current.
Also dependent on the CT design is how far the secondary can be taken from the installation point before the output exceeds the accuracy specifications. The most important issue with current transformers is safety. If the CT is energized,a load must be connected to the secondary leads at all times. If a wire becomes loose, the potential between the secondary lead or terminals will rise dramatically and instantly, impeded only by the resistance in the windings. This is the reason many installations include a termination block that allows the CT secondary to be shorted together. If access to the energized circuit is difficult or the power must remain on at all times, removal of the load connected to the CT, whether a panel meter, transducer, relay or power monitor; would be extremely dangerous. Over load conditions are quite costly in terms of repair of the equipment, but under load conditions cost in other ways. Under loaded motors can indicate a number of possible issues, these being the most common: broken or slipping drive belt, loose or sheared shaft coupling, open pump discharge, restricted pump intake, and clogged filters or screens.
There are other ways to detect these conditions instead of current sensitive relays. Most older motor starters do not have any type of under current condition monitoring capability, but many belt driven loads have current sensing added. The alternative method used most often to verify that a belt is driving the load is to visually inspect that the load is moving. However, to check for slippage the equipment must be stopped and the belt tightness must be manually checked. This can be very difficult to accomplish, especially with air handling fans in buildings. Having a broken belt or a belt which has come off the sheaves creates a situation where mechanical interlocks on a starter show that everything is fine, with the motor turning but driving no load.
While this condition may not be creating a dangerous situation in most applications, it would be potentially hazardous if the fan is relied upon to pressurize a stairwell or elevator to keep smoke accumulation minimized. If there was a fire, the stairwell could fill with smoke, making the escape route a dangerous place to be, while the fire control system indicates all is well.
Sewage lift stations pose more concern. When a discharge line opens, the current draw of the driver motor decreases but the pump continues to lift the raw sewage, depositing it over the surface of the ground, over the pump and controls and generally making a mess ideal for HAZMAT suits. Everything has to come to a grinding halt until the spill is contained. The addition of a current relay would immediately shut the pump down, reducing the spill size to a more manageable area.
Monitoring DC current levels traditionally has been even more complex than AC current measurement. With AC circuits, the magnetic field surrounding the conductor rises and falls 120 times each second (60 hertz), reversing polarity 60 times.
This expanding and contracting magnetic flux creates an induced voltage in the windings of a current transformer that is proportional to the primary circuit current. DC circuits have a magnetic field around the conductor too, but it rises and falls only as the current increases and decreases. Placing a current transformer over a wire carrying DC current will produce a brief burst of voltage in the CT secondary, but nothing more.
For many decades DC current measurement required terminating the conductor across a precision resistor (current shunt) and reading a small drop in voltage across the shunt. The standard voltage drop for a set current magnitude is 0.050 or 0.100 volts. This low level signal is then taken to a separate enclosure using twisted and shielded cable, where it is amplified and conditioned in a similar manner to the AC current relays.
Protecting the shunt output is very important as external electrical noise from contactors, relays and other control components can cause signal distortion. Another issue with shunts is they produce the most accurate representation of the primary circuit when the ambient temperature is stable. Environmentally controlled cabinets containing DC current shunts are quite common. The combination of shunts and relays has similar negative features compared to the AC monitoring approach of current transformers: Too many connections and possible interference with the output signal unless carefully installed.
One-piece DC current relays, using Hall effect elements rather than shunts, were introduced in the late 1990’s. One of the first applications was to monitor the field supply to DC shunt or compound wound motors, as a loss of the field would cause the motor to speed up uncontrollably. These type of motors are still quite common as the speed and torque are easily varied by increasing or decreasing the field current. What was being offered were precisely wound relay coils connected in series with the field supply power, rather than using shunts. The pickup and drop out current depended on the operating coil design, and was not adjustable.
Currently, the field source conductor is fed through a hole in the sensor, adding no burden to the field supply, and a solid-state or electromechanical relay contact is used to disconnect the armature supply if the field supply is lost. The point at which the output changes state is adjustable within a wide range. The product cost is a fraction of the traditional relay; this added to the time saved by not cutting and terminating the supply wire onto the relay make this a very attractive option.
Versions of the Hall-element based design can also be procured and can act as interlocks. In vehicles, an indicating light can be used to show when an electrical load such as defrosting elements are operating. The contacts are used to energize enclosure door locks, keeping personnel safe by limiting access to an energized cabinet. A company manufacturing truck pressure washers uses a DC current relay to energize a fuel supply solenoid only when the ignition system is operating.