Solid-State Relays

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Three-Phase Reversing Motor Control

Three-phase reversing motor control can be implemented

with four SSRs as shown in the connection diagram. The SSRs

work in pairs with SSR1 and SSR3 operated for rotation in one

direction and SSR2 and SSR4 operated for rotation in the

reverse direction. The resistor R1 as shown in the connection

diagram protects against line-to-line shorts if SSR1 and SSR4

or SSR3 and SSR2 are on at the same time during the reversing

transition period. Use the following table as a guide to the

proper selection of an SSR for this application.

FAQ: SSR Applications

Q : What is a solid-state relay?

A: A solid-state relay (SSR) is a semiconductor device that can

be used in place of a mechanical relay to switch electricity to a

load in many applications. Solid-state relays are purely

electronic, normally composed of a low current “control” side

(equivalent to the coil on an electromechanical relay) and a

high-current load side (equivalent to the contact on a

conventional relay). SSRs typically also feature electrical

isolation to several thousand volts between the control and

load sides. Because of this isolation, the load side of the relay is

actually powered by the switched line; both line voltage and a

load (not to mention a control signal) must be present for the

relay to operate.

Q : What are the advantages of using an SSR over a

mechanical relay?

A: There are many applications that require a moderate

amount of power (W to kW) to be switched on and off fairly

rapidly. A good example would be the operation of a heater

element in a controlled-temperature system. Typically, the

amount of heat put into the system is regulated using pulse-

width modulation turning a fixed-power heating element on

and off for time periods ranging from seconds to minutes.

Mechanical relays have a finite cycle life, as their components

tend to wear out over thousands to millions of cycles. SSRs do

not have this problem; in the proper application, they could

be operated almost infinitely.

Q : What are the limitations of using an SSR?

A: SSRs have a few limitations when compared to the

capabilities of their mechanical counterparts. First, because

the relay is semiconductor-based, it will never turn all the way

on, nor off. This means that in the “on” state, the relay still has

some internal resistance to the flow of electricity, causing it to

get hot. When in the “off” state, the relay will exhibit a small

amount of leakage current, typically a few mA. This leakage

can conspire to keep some loads, especially ones with a high

impedance, from turning off! Additionally, SSRs are more

sensitive to voltage transients; while Opto 22 relays are very

well transient-protected, if a relay gets hit hard enough a

sufficient number of times, it will die or degrade. This makes

SSRs less ideal for driving highly inductive electromechanical

loads, such as some solenoids or motors. SSRs should also

never be used for applications such as safety power

disconnects, because even in the off state, leakage current is

present. Leakage current through an SSR also implies the

presence of a potentially high voltage. Even though the relay

is not conducting a large amount of current, the switched

terminal will still be “hot,” and thus dangerous.

480 Volt Three-Phase Motors

SSR MODEL

MOTOR

480D10-12

1-½ HP

480D15-12

1-½ HP

Opto 22

Relay

Motor Full

Load Rating

Resistor for

120V line

Resistor for

240V line

3-Amp

1.25-Amp

4 ohm 50 W

8 ohm 50 W

10-Amp

5-Amp

1 ohm 100 W

2 ohm 100 W

25-Amp

8-Amp

.5 ohm 100 W

1 ohm 100 W

45-Amp

16-Amp

.25 ohm 150 W

.5 ohm 150 W

15-Amp

5-Amp

1 ohm 100 W

2 ohm 100 W