Remington HH-70T-KFA Service Manual download pdf (Page 10)

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DESA Service Information

Oil Fired Heaters

Component Description and Operation

Motors

The motors used on the low pressure heaters are fractional horsepower

motors ranging from 1/35 HP on the smallest heaters to 1/4 HP on the

largest heaters. The motors used can be grouped into two categories. The

first category is the shaded pole motor(FIG. 5.1). The shaded pole motor contains a single winding and does not require an

external start relay for operation.

These motors are used on the smaller heaters (30,000, 35,000 & 50,000 BTU) and are easily identified by observing that only two

wires (usually red and white) are necessary for operation.

The second category is the split-phase motor(FIG. 5.2). This motor contains two separate windings. The first winding being the

auxiliary or start winding and the second being the main or run winding. This motor requires an external means of switching the

power off to the auxiliary winding after the motor reaches a predetermined speed. This power off switching is accomplished by a

motor start relay or solid state relay. These motors are identified by observing that three wires (black, red and white) are routed

from the motor.

Motor Relay

The motor start relay is used to de-energize the start winding on split-phase motors. As

mentioned in the Motor Section, remember that split-phase motors have both an auxiliary

(start) and a main (run) winding. For proper motor operation, both the auxiliary and main

winding are energized during start-up. After the motor starts and reaches approximately

80% of rated speed, the auxiliary winding must be de-energized to prevent overheating. This

is accomplished by the use of a motor start relay. The operation of the relay functions

because the main winding amperage decreases as the motor speed increases. The coil of the

relay is wired in series with the main winding. The contacts of the relay are in series with the

auxiliary winding. When voltage is applied to the motor, the high inrush current through

the main winding creates a magnetic force in the relay coil and closes the contacts of the

relay, which energized the auxiliary winding. As the motor speed increases, the current

through the relay decreases and the contacts open, with gravity, de-energizing the auxiliary

winding.

Solid State Relay (Applicable models use 097061-01 Solid State Relay)

The solid state relay is used to de-energize the start winding on split-phase motors. As mentioned in the Motor Section,

remember that split-phase motors have both an auxiliary (start) and a main (run) winding. For proper motor operation,

both the auxiliary and main winding are energized during start up. After the motor starts and reaches

approximately 80% of rated speed, the auxiliary winding must be de-energized to prevent overheating.

This is accomplished by the use of a solid state relay. The operation of the relay functions as a positive

temperature coefficient resistor. When voltage is applied to the motor, the main and auxiliary windings

are energized as current flows through the solid state relay which is connected in series with the auxiliary

winding only. The resistance is increased over time in the solid state relay where it eventually rises to the point theoretically

no current flows, de-energizing the auxiliary winding.

Safety Control

The safety control circuit consists of a photocell (light sensitive cell) and a safety control. The photocell is used to sense the

presence of light inside the combustion chamber. The photocell varies its electrical resistance in relation to the light rays

which contact its surface. When the heater is operating properly, the photocell sees sufficient and the proper color light,

which keeps its resistance low. When the photocell does not sense the proper color or amount of light, the resistance of the

photocell increases and ceases current to flow to the solid state trigger circuit, which in turn sends current flows into the

circuit breaker and heats a (bimetal) bar causing the bar to warp. The warpage releases the spring loaded reset button,

which brakes all power to the heater. As a service technician, it is important to understand the operation of the safety

control. In the past, it has been the single most misdiagnosed part of the heater. It is suggested that all controls be checked

per instructions listed in the Troubleshooting Section, prior to replacement.

FIG. 5.1

FIG. 6

FIG. 5.2

Oil Heater Service Info.

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Remington HH-70T-KFA Service Manual Page 10

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