|It is a good idea to start a generator and allow it to warm up without any
load. Indeed some generators will not come up to speed with any significant
load attached. Bogging the generator is not only hard on
the generator, it is also very hard on loads such as air conditioners which
suffer under low voltage, low frequency, high current conditions. The Onan
AJ series generator in my 82 Itasca motor home is an example of a generator
that will not start under significant load.
It is a pain to have to go around and turn off all the major loads before
cranking the generator. This is particularly annoying when one wants to
start the generator while underway in order to use the roof air conditioner
to aid in cooling the vehicle. I decided to do something about it.
also nice to have the power transfer automatically from shore power to
generator power when the generator is started. Like, for instance,
when the power goes off during a storm in the middle of the night.
Just crank the genny, roll over and go back to sleep. Sure beats
tromping out in the weather to move the power cord from the shore power
connector to the generator connector.
A system to automatically delay the application of the load until after the
generator is up and running and warm is an appropriate solution.
Fortunately this solution is simply implemented, costs under $50 and
involves only two components.
Already-manufactured generator delay/automatic transfer boxes are available. They're more
expensive than my design plus they're usually physically larger which makes
finding a mounting place more difficult. Besides, I like to roll my
My solution uses a Potter and Brumfield PRD series 30 amp single pole definite purpose contactor
and an HVAC-oriented time delay relay. A double pole contactor
is necessary because the neutral and the hot lead must be switched to make
sure GFI protectors work properly.
The particular relay I used is the PRD-11AGO-120, 30 amp contacts, 120
volt coil. For 50 amp service use the PRD-11AFO-120. The data sheet
for the PRD series of relays can be downloaded
This relay should cost no more than $20 and usually half that.
To implement the time delay, a simple, inexpensive time delay module is
used. This module is a delay-on-make relay. It is also known as an
anti-short-cycle relay because its most common use is to prevent short
cycling (energizing too soon after turning off) the compressor in an air
conditioner by delaying the power to the compressor by a set number of
The relay I selected is a MARS (Motors And Armature Service)
#32391. This relay can be set for a delay from between 0.1 minutes to
about 8 minutes. The relay is a 2 terminal device that simply
blocks the passage of current for the set number of minutes. Many companies
manufacture a compatible relay; the MARS unit was selected because my
friendly local electric motor repair shop stocks it. To the right is a photo of the relay.
The 30 amp relay should cost from $15 to $50 depending on whether or not
you buy it wholesale and the time delay relay should cost from between $12
to $24. In addition to electric motor repair shops, these components can be
had from appliance parts warehouses, HVAC supply houses and HVAC
contractors as well as direct sales operations such as Graingers.
The simple schematic for this device is shown at right.. The hot wire from the generator is connected to
the Normally Open (NO) side of
the contactor. Shore Power goes to the Normally Closed (NC) side. The house wiring is connected to the other. Power is
supplied to the 120 volt contactor coil from the generator lead through the
time delay relay (TDR).
How it works
When the generator is started, 120 volts AC appears on the generator "line"
lead. This powers the time delay relay. After a set amount of time,
the TDR energizes, passing the 120 volts to the contactor coil. This
energizes the contactor which passes the 120 volts to the house wiring.
When the generator is turned off, as soon as the voltage drops
significantly, the TDR de-energizes, dropping the contactor and cleanly
cutting off the power to the house. This keeps trash power from the slowing
generator out of the house wiring. When the generator is off, shore
power is fed to the RV via the normally closed contacts.
The contactor and the relay was mounted in a PVC plastic electrical pull
box, available from Home Depot. This box is waterproof. The leads were
brought out through a strain relief and caulked with silicone RTV. The
entire assembly is waterproof. The box is mounted in the generator
compartment away from any heat generating sources such as the exhaust. Set
the TDR to about 30-45 seconds and the job is complete.
Simply start the generator. Everything else is automatic. After the time
delay you set, the house will power up normally.
This article and graphics copyright 1999 by John De Armond. All rights
reserved. Private, personal use permitted. Other uses only by prior
explicit written permission. I may be contacted via the email button to the left
Compliments of Potter & Brumfield
Generators loaded since 05/23/07