The Effect of Defective Motor Starting Circuits on Automatic Transfer Switch and Engine Performance
Engine generator set manufacturers have long been
aware of the effects of motor starting loads on overall performance. Manuals
like Onan's T-009 "Selecting Onan Electric Generating Sets for Electric Motor
Loads" explain in detail how to deal with motor starting circuits.
All
electric motors require extra power when starting. This extra requirement is
referred to as "locked rotor KVA" (kilovolt amperes) or "locked rotor amps." (It
is "locked rotor" because when an electric motor is stopped it needs extra force
to get moving again, and the rotating component within a motor is called a
rotor.) Assuming no load is present, the amount of power needed to start
a motor can be accurately estimated by comparing the motor horsepower and the
NEMA code from the motor's nameplate with a chart similar to the attached Table
2 from the Onan technical manual T-009. The resultant LrKVA value is compared to
available starting capabilities as in Onan's T-009 Table 4, also attached.
Through this method, an appropriate engine generator set may be chosen to drive
the load.
But what happens when the load changes after all the equipment is installed?
Whenever a load is applied to a generator set, the
voltage instantly drops. This drop is corrected by an application of torque from
the engine and electrical power from the voltage regulator (in the generator).
This correction to us seems instantaneous but may actually take many cycles in
the generator's reference of time. We try to size equipment so that the voltage
drop doesn't exceed 30% of the unit's rated voltage. If we exceed 30%, motor
contactors fall out, fuses blow, lights dim and a lot of other bad things
happen.
If an unplanned load hits the system, as when a start winding fails,
the 30% mark may be passed. This can throw the entire system out of sync.
A
typical air conditioning compressor uses a capacitor start motor like the
drawing below:
Typical Capacitor Start Motor
Let's say, for example, the
start winding has a normal impedance of 100 ohms but half of the windings are
shorted out of the circuit due to a breakdown in insulation. The impedance would
now be 50 ohms, or half its original value. The current now required to operate
this winding would have increased proportionally, right?
The start
winding is engaged only long enough to get the motor up to 75% of normal speed.
It is then disconnected by the centrifugal switch. But during the time it's
engaged, it may draw many times more power than the motor needs at its rated
speed, even under full load, because it is using this extra winding.
As the
starting motor nears its rated speed, a centrifugal switch disconnects the
starting winding. If the start winding is shorted or otherwise defective, it may
draw many times the usual requirement. But, since the start winding is used only
for a short while, if the running winding is intact, no problem may be apparent
when the motor is connected to the utility. (The utility has all the
power in the world. Well, at least the North American electrical grid. It is
actually referred to as an infinite bus.)
The utility power will support a
gross overload for a short duration without even opening a breaker because most
building codes rquire only thermal breakers. This breaker has to heat up before
it can open the circuit. The short time the start circuit is used may not allow
the breaker to trip. However, when a gross overload hits a generator,
excessive voltage drop may occur.
Most generators use a "volts per
hertz" type voltage regulator. This means as the engine slows down with load and
the hertz, or frequency, drops, the voltage regulator will allow the voltage to
fall off in order for the engine to get a chance to catch up with the extra
load. This voltage drop may be significant.
An automatic transfer switch is
designed to provide power to the load from any available source. It, therefore,
monitors both the normal and emergency sources. If, when it transfers tot he
emergency source, is senses a significant voltage drop in that source, and the
normal source is still available, it will immediately attempt to retransfer back
to normal. That retransfer action removes the load from the generator. When the
overload is removed from the generator its voltage instantly returns to the
normal level. The transfer switch will sense the restored voltage and again
issue a command to transfer to emergency. The transfer to emergency will once
again result in a severe voltage drop due to the gross overload and the cycle
will repeat. This may continue until a technician intercedes or a safety device
removes the overload from the circuit.
If the utility voltage is not
available, the transfer switch will not cycle back to normal. Under this
condition, when an overload is applied to the generator, the engine may slow
down, or the voltage may drop off to unusable levels, the voltage regulator may
be damaged or one of its protection circuits may open up.
All the while the
generator and automatic transfer switch are trying to figure out what to do with
the overload, the engine's governor is trying to handle load that is first
applied, then removed, then applied again. Each smaller system within the
overall emergency power system needs time, in milliseconds, to react to changing
conditions. It is not uncommon for an undesirable harmonic to develop between
two (or more!) systems, resulting in cycling, surging, or even mechanical
failure.
When overload is suspected, it is best to test with an ammeter
capable of holding a peak reading. Each suspect circuit, downstream from the
automatic transfer switch, should be examined.
The example I used of a
capacitor start motor is common in that most air conditioning compressors use
this type of motor. However, there are many other motor starting circuits
possible as well as other devices prone to failure. All circuits should be
checked under both peak load and starting conditions.
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