Solid State Rotary Speed Control

Kevin Eldredge
2204 Timber Ridge, Newcastle, OK 73065

Here's a different twist for your next rotary speed controller, a solid state circuit using only a handful of junk box components. No large heavy variac needed. This circuit is very similar to a lamp dimmer circuit using only one triac and diac combination. The secret to make it live in a T.C. system is a good EMI filter and ground. This circuit has been used for about one year on induction and series (vacuum cleaner) motors with T.C.s running up to 10 Kilowatts with no problems. The circuit as shown uses a triac rated for 400 volts at 15 amperes, enough for most any rotary motor. The EMI filter should be rated for the voltage and current of the motor. The ground for the circuit is through the utility system and not connected to the T.C. secondary ground. This particular prototype was installed in a wooden cabinet with no side effects from the coil output.