A 250 VAC solid-state relay, a flyback diode, or a current rating equal to the motor nameplate value does not by itself produce a workable 115–130 VDC motor switch. The number that matters first is the output device’s DC blocking rating after the supply voltage and turn-off transient are added. The second number is starting or stalled current, followed by the heat generated during normal operation.
Incorrect first moves
Selecting by the AC voltage rating: A 250 VAC marking does not establish that an SSR can interrupt 130 VDC. Many AC SSRs use SCR or triac outputs. Once triggered on DC, these devices can remain latched because the current never passes naturally through zero. A zero-crossing input circuit also depends on an AC waveform and is the wrong switching function for a steady DC bus.
Adding only a flyback diode: A diode can control inductive voltage, but it does not correct an inadequate DC blocking rating, insufficient starting-current capacity, or excessive semiconductor temperature. A plain diode also produces slow current decay, which can make the motor coast or delay release of attached mechanisms.
Sizing from running current: A DC motor can draw a substantial starting surge because back EMF is initially zero. A stalled motor presents the same electrical condition for longer. The output device, suppression network, wiring, and protective device must tolerate the actual current profile.
Using the SSR as fault isolation: Semiconductor switches commonly fail shorted. Use a fuse or circuit breaker for short-circuit protection, and use an appropriate isolating device where a positive disconnection function is required.
DC switching physics
This is heat, not logic. A DC-rated SSR normally uses a transistor output such as a MOSFET or IGBT. A MOSFET is commonly characterized by on-resistance, so approximate conduction loss is P = I² × RDS(on). An IGBT is commonly characterized by saturation voltage, giving an approximate loss of P = Vsat × I. Use the manufacturer’s hot-device values rather than room-temperature typical values.
The motor stores magnetic energy according to E = ½ × L × I². Opening the circuit forces that current to continue. Without a controlled path, the switch voltage rises until parasitic capacitance, insulation, or the semiconductor absorbs the energy. Repeated avalanche events can damage an SSR even when the motor starts and stops correctly during an initial test.
A diode connected across a one-direction motor provides a recirculation path. Adding a series resistor raises the clamp voltage and shortens current decay, but it also raises SSR voltage stress. The peak switch voltage in a typical low-side arrangement is approximately the DC bus voltage plus the suppression-network clamp voltage and wiring overshoot.
Selection quantities and limits
| Quantity | Decision limit | Where to read or measure it |
|---|---|---|
| Maximum DC bus voltage | Use the highest credible value, not the nominal 115 VDC value; the stated range reaches 130 VDC | Supply specification and measurement at the motor feeder |
| SSR off-state voltage | Must exceed maximum bus voltage plus the measured or calculated turn-off transient | SSR DC output specification |
| Running current | Must remain within the thermally derated continuous-current curve | Motor nameplate and current measurement |
| Starting and stalled current | Must fit the SSR surge-current magnitude and duration limits | Captured current waveform and SSR surge curve |
| Conduction loss | Must keep junction temperature below the manufacturer’s limit at the actual ambient |
RDS(on) or Vsat, thermal resistance, heat-sink data |
| Inductive energy | Must remain within the clamp and switch energy capability on every stop | Motor inductance/current data or measured voltage-current waveforms |
| Input drive | Must exceed minimum input current without exceeding the allowed input voltage/current | SSR input specification and controller output data |
| Off-state leakage | Must not create an unsafe or functional residual voltage | SSR leakage specification and load-terminal measurement |
One identified product range, the Magnecraft 6DDX, was specified at 200 VDC and up to 40 A. Those two headline values are screening data, not a complete selection: verify the exact part’s motor-load rating, surge curve, transient capability, leakage, heat-sink requirement, and current product documentation.
Relay selection procedure
- Confirm that the load is a one-direction DC motor switched simply on and off. If a drive already controls the motor, use the drive’s approved stop or inhibit function when that function satisfies the required behavior.
- Record the highest DC bus voltage. Include supply tolerance, charging conditions, regeneration, and other operating states that can raise the bus above its nominal value.
- Capture motor current during normal running, cold starting, and the longest permitted acceleration. Treat stalled current separately if a mechanical jam is credible.
- Choose an SSR explicitly rated for DC output switching. Reject selection based only on an AC rating or a zero-crossing label.
- Select an off-state rating above the maximum bus plus the suppression clamp and measured overshoot. There is no universal voltage multiplier; the clamp design and waveform establish the required margin.
- Check the surge-current curve against the complete starting-current pulse. A single current headline does not establish suitability when pulse duration and repetition are missing.
- Calculate conduction loss at the highest operating current using hot-state output data. Apply the specified thermal resistance, ambient temperature, interface material, and heat-sink curve to calculate junction temperature.
- Coordinate the branch fuse or circuit breaker with conductor protection, motor starting current, and the SSR’s short-circuit limitations. The protective device must tolerate legitimate starts while clearing faults.
Suppression and drive topology
For a one-direction motor, place the flyback diode so it is reverse-biased while the motor is energized and conducts when the SSR opens. Rate its repetitive reverse voltage for the DC bus and its forward surge and energy capability for the motor current. Locate the suppression loop close to the motor and switch wiring to reduce inductive overshoot.
Choose the clamp from the stopping requirement. A plain diode minimizes switch voltage but produces the slowest current decay. A diode-resistor network dissipates energy faster, while a suitable voltage clamp can provide a defined turn-off voltage. Every increase in clamp voltage must remain inside the SSR off-state limit after bus voltage and overshoot are included.
Connection direction also matters. A single transistor output may include an intrinsic diode and may block only one polarity in its off state. Reversing loads or bidirectional current require an output topology designed for both polarities, commonly implemented with back-to-back devices.
A packaged SSR usually provides isolation between its control input and power output; verify that in its datasheet. A bare high-side MOSFET or IGBT requires a gate drive referenced to its moving source or emitter, so its driver supply may need to float. In either case, check minimum input drive and maximum permitted input voltage before connecting a controller output.
Waveform and thermal verification
- Test with a suitably rated differential voltage probe across the SSR and a current probe in the motor circuit. Capture turn-on, cold start, steady running, and turn-off.
- Confirm that peak off-state voltage remains below the selected device limit. Inspect the trace for additional high-frequency overshoot caused by wiring inductance.
- Compare starting-current magnitude and duration with the surge curve. Repeat at the maximum permitted switching frequency because repeated starts can accumulate heat.
- Measure SSR case or heat-sink temperature after thermal equilibrium at the highest ambient condition. Recalculate junction temperature using the specified case-to-junction or sink-to-junction path.
- Command the output off and measure residual load voltage. Off-state leakage may leave the motor terminal energized even when there is insufficient current to rotate the shaft.
- Remove control power and exercise relevant fault states. Verify that the fuse or circuit breaker handles short circuits and that the control scheme does not treat an SSR as a guaranteed open circuit.
Stop testing if the waveform exceeds the SSR voltage or surge envelope, the calculated junction temperature reaches its limit, or the device fails to turn off cleanly. Continued cycling can turn a marginal transient or thermal condition into a shorted output.
FAQ
What happens if I use a 250 VAC SSR on a 130 VDC motor?
An SCR- or triac-output AC SSR may turn on and then remain latched because DC has no natural current zero. Use an SSR with an explicit DC output rating and verify its blocking, surge, and thermal specifications.
What happens if I omit the flyback diode?
The motor’s stored energy drives the SSR voltage upward at turn-off, potentially causing avalanche stress or immediate breakdown. Measure the turn-off waveform and fit a diode, diode-resistor network, or defined clamp that stays inside the SSR voltage envelope.
When should I stop testing and contact official support?
Stop when the exact part’s DC motor rating, surge curve, clamp-energy capability, or thermal data cannot be obtained, or when measured voltage and current fall outside published curves. Send the manufacturer’s official support channel the exact part number, circuit topology, bus range, current captures, turn-off waveform, ambient temperature, switching frequency, and heat-sink details.