Page 52 - Boiler Operator’s Handbook

Operating Wisely 37

closed and it won’t necessarily open when the coil is de- that will also disconnect the control power but in oth-
energized. That’s when you’re running around trying to ers it will not. To access the starter it will normally pull
find another way to shut the damn motor down! out because they’re on a drawer assembly and that
If only two of the contacts fuse together or some- will disconnect the control power. Because the control
thing happens to one of the three circuit wires for a three power can still be on and the three phase power discon-
phase motor it runs on only one phase. We call that nected you can encounter a situation where everything
single phasing because current can only flow one way indicates the motor is running when it isn’t. Always be
at a time between two wires. Three phase motors can aware that indicating lights on any electric motor starter
operate on one phase if the load is low enough but it will or MCC can provide false indications. Also, a boiler can
destroy the motor in a short period of time. try to start without the fan running so disconnect and
Due to the considerable number of injuries from tag out control power for any boiler before disengaging
arc flashes there are now new regulations for maintain- the starter at the MCC.
ing electrical equipment to prevent injuries from arc The typical MCC indicating light arrangement
flashes. If you have an opportunity to take a course in provides a red light to indicate a motor is running and
arc flash protection it’s advisable to do so. You’ll dis- a green light to indicate it is shut down. I’ve always had
cover that there are requirements for (PPE) personal problems with that concept because I think of green to
protective equipment to be worn when working with mean it’s going. Accept the fact that the electrical engi-
energized electrical components. While it’s improbable neers use that color arrangement to indicate that you
that you’ll have a problem in normal operation it is very should stop when you see that red light before doing
advisable to leave the handling of high-voltage (600 V anything that would shut down that motor unless, of
and over) to electricians trained in arc flash prevention. course, you want to shut it down.
Three phase motors use three electrical currents That’s about all I know about three phase motors
that flow between the wires. If they aren’t balanced the that is worth telling an operator. The current has to flow
motors can run hot and fail early. Your motor starter ter- in all three wires for it to work and the current isn’t
minals should be checked regularly (every two or three flowing through each wire at the same rate and the volt-
years) and after any maintenance to be certain that the age isn’t the same in any wire at any particular instant
voltage is balanced. Use a meter to measure the voltage in time. Don’t do anything that could result in one wire
on each pair of leads, L1 to L2, L2 to L3, and L3 to L1. having an open circuit when the others don’t.
That big L, by the way, stands for “line” meaning line
voltage, the supply voltage. The difference between the
average difference and the lowest or highest measure-
ment shouldn’t exceed five percent. If there is a big dif-
ference in voltage you should get an electrician to check
everything in the plant.
Most boiler plants use MCCs (motor control cen-
ters, Figure 1-11) to control and distribute electrical
power around the plant. You may even have more than
one MCC in your plant, something I always recommend,
for improved reliability. These are more economical to
install than local motor starters at each motor but they
can also be a source of unique problems in operation. In-
dividual motor starters use their own power source for
control power. Control power is electricity used to feed
the motor starter coil that, when energized, pulls in the
contacts that close the circuit to feed power to the motor.
A typical MCC will have one control power transformer
that supplies control power for all the motor starters in
the MCC. It’s necessary to rotate or pull on the handle
on the door of a particular section of the MCC because
the handle operates disconnects for the three phase
power before the door can be opened. In some MCCs Figure 1-11. Motor control center