Is it the Drive, the Motor or the Load?
By Colin PlastowMotion Control drives motors
Diagnosing tricky motor trouble requires a close inspection of power loads.
In troubleshooting situations involving a motor, more than half the battle is simply isolating the problem. Whenever there’s a working motor, there’s a load and there’s some sort of motor controller, which is increasingly going to be an adjustable speed drive (ASD).
So when problems arise, how can you tell if it is the drive, the motor or the load? Here are a few tips to tackle the problem in a quick, systematic way, making a few key measurements as you go.
A good place to start is with a measurement of current drawn by the motor. When we talk about motors here, we are referring to three-phase induction motors, the workhorse of industry.
Motors are balanced loads: The current they draw on each phase should be about the same with less than 6 percent as measured below being a general guide line for conventional, across-the-line motors. If they are not balanced, the cause could be internal to the motor (deteriorating stator insulation, for example), or it could be the result of voltage imbalance. A small voltage imbalance will cause a significantly larger current imbalance.
In the case of a ASD, the output voltages are electronically controlled and should be well balanced if the drive is functioning correctly. So, if there is any problem with current imbalance, compare the voltage imbalance at the output of the drive to that at the motor. A difference suggests a problem in the wiring, connections, etc. between the drive and motor.
If the voltage balance at the motor is good, a problem with the motor would be suspected. The voltage output waveform from an ASD is a sequence of pulses that has significant high frequency content. Accurate measurements usually require a DMM with a suitable low pass filter function.
Voltage and current imbalance measurements should be taken at the line side of the drive. Drives are generally not as sensitive to voltage imbalance as motors, since the incoming AC is rectified to DC which charges the capacitors on the DC bus. Voltage imbalance will usually show as a somewhat higher ripple voltage that most drives tolerate quite well. In fact, many drives will operate with a phase missing albeit with a severely de-rated (about 50 percent) load.
Over-voltage and Under-voltage
Drives have diagnostic codes that identify the cause of trip. Generally speaking, they can be classified as over-voltage, under-voltage or overload (over-current). Note that mechanical starters only have overload trips; they’re not concerned with over- or under-voltage.
Drives turn sine wave AC into DC (converter section), and then turn the DC back into AC (inverter section). However, the AC at the output is not a sine wave. It’s a special waveform known as the pulse-width modulated (PWM). The PWM, from the motor’s point of view, is accepted as if it were a sine wave — almost.
For now, though, let’s focus on the drive internals, specifically on what’s commonly referred to as the DC link. The DC link is nothing more than a capacitor bank, usually with a series link inductor (reactor) thrown in for filtering and protection. The DC link is carefully monitored by the drive; over-voltage or under-voltage refers to the voltage of the DC link. Under-voltage can be caused externally by voltage sags on the drive input. The Sags and Swells function on power quality analyzers can help identify line-related under-voltage problems occurring over time.
Problems could also exist internally with the DC link capacitors and/or reactor. In many drives, there are external test points to measure the DC link voltage. To check the voltage variations on the capacitors, use the min/max function of a digital multimeter or a power quality analyzer.
Check if voltage regulation is within the manufacturer’s specification. When troubleshooting a complete system, the tendency is to view the drive or PLC as the most susceptible to voltage sags. However, studies have shown that low cost ice-cube control relays are often the source of sag-related problems as they’re the first to drop out when voltage drops off. So don’t forget to look at any external control circuit while you’re troubleshooting intermittent system shutdowns.
Over-voltage could be caused by line-related voltage transients. At one point, utility capacitor switching transients were notorious for causing over-voltage trips in drives. Over-voltage could also be caused by regenerative loads. Loads such as cranes and elevators generate voltage when they decelerate. Dynamic braking circuits are installed to shunt off this energy from the drive, where they would otherwise be rectified by the output devices of the drive and show up as over-voltage on the DC link. Problems such as improper installation can result in over-voltage trips.
To troubleshoot the interaction between the load and the motor, you have to understand the relationship between torque and current. In essence, motor is a device to turn electrical energy (current) into rotational mechanical energy (torque), via the magical effects of magnetism.
What a load demands of a motor is torque. For all practical purposes, this torque is directly proportional to current used by the motor. This should make perfect sense, because we all know that for constant-speed motors—which include all motors with electro-mechanic starters—voltage is, or should be, stable and current is the variable.
When a load demands more torque and current than a motor can supply, the result is an overload condition. Overloading will cause overheating of the motor. Motor controllers will shut down the motor (and thereby the load) rather than allow permanent winding insulation.
Overloading is always relative to time: A high overload will trip the motor in a short time, while a lower level of overload will take longer to trip the motor. When we want to evaluate the impact of a load on the motor drive system, we have to measure the current it draws. Of course, this current draw typically varies over time as the load varies. The measurement of current over some period of time is called load profiling.
For load profiling, the power-record function of power quality analyzers is ideal for capturing a trend line of current consumption. A cursor enables you to identify the current values at different points on the trend line, along with a time stamp for those points.
Before load profiling, first make the current imbalance measurement to make sure the motor is healthy. If you don’t have a 3-phase instrument and your concern is nuisance tripping, then pick the highest current leg and measure that (an overload on one leg will trip all three legs).
When load profiling, we are looking for periods of especially high current, relative to the full load amps of the motor. Full load amp information is available on the nameplate of the motor. If there is a service factor, the range calculation should be made on the basis of full load amps multiplied by service factor. While high current is the main concern, low current should also be investigated.
A motor is most efficient, and has the best power factor, in the 60 to 80 percent range of its full load amps. There is no immediate penalty for under-loading — the motor will not trip. In fact, many motors are routinely oversized for the load, on the theory that the motor is less likely to trip from overload.
However, as is most often the case, there is no free lunch. In the case of under-loading an across-the-line motor, the power factor will be poor and the energy company sends a higher bill. An ASD isolates the power system from the effects of a lightly loaded motor.
To determine whether it’s the load, the motor or the drive that is causing problems, it helps to proceed systematically. Start with the basic motor measurements (imbalance) to check the health of the motor itself. Then do some simple drive measurements to check for causes of over- or under-voltage trips. Finally, profile the load to find the cause for intermittent overload trips.
Colin Plastow is Industrial Product Manager for Fluke Electronics Canada.